[[aop]]
= Aspect Oriented Programming with Spring




[[aop-introduction]]
== Introduction

__Aspect-Oriented Programming__ (AOP) complements Object-Oriented Programming (OOP) by
providing another way of thinking about program structure. The key unit of modularity in
OOP is the class, whereas in AOP the unit of modularity is the __aspect__. Aspects
enable the modularization of concerns such as transaction management that cut across
multiple types and objects. (Such concerns are often termed __crosscutting__ concerns in
AOP literature.)

One of the key components of Spring is the __AOP framework__. While the Spring IoC
container does not depend on AOP, meaning you do not need to use AOP if you don't want
to, AOP complements Spring IoC to provide a very capable middleware solution.

.Spring 2.0+ AOP
****
Spring 2.0 introduced a simpler and more powerful way of writing custom aspects using
either a <<aop-schema,schema-based approach>> or the <<aop-ataspectj,@AspectJ annotation
style>>. Both of these styles offer fully typed advice and use of the AspectJ pointcut
language, while still using Spring AOP for weaving.

The Spring 2.0+ schema- and @AspectJ-based AOP support is discussed in this chapter.
The lower-level AOP support, as commonly exposed in Spring 1.2 applications, is
discussed in <<aop-api,the following chapter>>.
****

AOP is used in the Spring Framework to...

* ... provide declarative enterprise services, especially as a replacement for EJB
  declarative services. The most important such service is
  <<transaction-declarative,__declarative transaction management__>>.
* ... allow users to implement custom aspects, complementing their use of OOP with AOP.

[NOTE]
====
If you are interested only in generic declarative services or other pre-packaged
declarative middleware services such as pooling, you do not need to work directly with
Spring AOP, and can skip most of this chapter.
====



[[aop-introduction-defn]]
=== AOP concepts

Let us begin by defining some central AOP concepts and terminology. These terms are not
Spring-specific... unfortunately, AOP terminology is not particularly intuitive;
however, it would be even more confusing if Spring used its own terminology.

* __Aspect__: a modularization of a concern that cuts across multiple classes.
  Transaction management is a good example of a crosscutting concern in enterprise Java
  applications. In Spring AOP, aspects are implemented using regular classes
  (the <<aop-schema,schema-based approach>>) or regular classes annotated with the
  `@Aspect` annotation (the <<aop-ataspectj, `@AspectJ` style>>).
* __Join point__: a point during the execution of a program, such as the execution of a
  method or the handling of an exception. In Spring AOP, a join point __always__
  represents a method execution.
* __Advice__: action taken by an aspect at a particular join point. Different types of
  advice include "around", "before" and "after" advice. (Advice types are discussed
  below.) Many AOP frameworks, including Spring, model an advice as an __interceptor__,
  maintaining a chain of interceptors __around__ the join point.
* __Pointcut__: a predicate that matches join points. Advice is associated with a
  pointcut expression and runs at any join point matched by the pointcut (for example,
  the execution of a method with a certain name). The concept of join points as matched
  by pointcut expressions is central to AOP, and Spring uses the AspectJ pointcut
  expression language by default.
* __Introduction__: declaring additional methods or fields on behalf of a type. Spring
  AOP allows you to introduce new interfaces (and a corresponding implementation) to any
  advised object. For example, you could use an introduction to make a bean implement an
  `IsModified` interface, to simplify caching. (An introduction is known as an
  inter-type declaration in the AspectJ community.)
* __Target object__: object being advised by one or more aspects. Also referred to as
  the __advised__ object. Since Spring AOP is implemented using runtime proxies, this
  object will always be a __proxied__ object.
* __AOP proxy__: an object created by the AOP framework in order to implement the aspect
  contracts (advise method executions and so on). In the Spring Framework, an AOP proxy
  will be a JDK dynamic proxy or a CGLIB proxy.
* __Weaving__: linking aspects with other application types or objects to create an
  advised object. This can be done at compile time (using the AspectJ compiler, for
  example), load time, or at runtime. Spring AOP, like other pure Java AOP frameworks,
  performs weaving at runtime.

Types of advice:

* __Before advice__: Advice that executes before a join point, but which does not have
  the ability to prevent execution flow proceeding to the join point (unless it throws
  an exception).
* __After returning advice__: Advice to be executed after a join point completes
  normally: for example, if a method returns without throwing an exception.
* __After throwing advice__: Advice to be executed if a method exits by throwing an
  exception.
* __After (finally) advice__: Advice to be executed regardless of the means by which a
  join point exits (normal or exceptional return).
* __Around advice__: Advice that surrounds a join point such as a method invocation.
  This is the most powerful kind of advice. Around advice can perform custom behavior
  before and after the method invocation. It is also responsible for choosing whether to
  proceed to the join point or to shortcut the advised method execution by returning its
  own return value or throwing an exception.

Around advice is the most general kind of advice. Since Spring AOP, like AspectJ,
provides a full range of advice types, we recommend that you use the least powerful
advice type that can implement the required behavior. For example, if you need only to
update a cache with the return value of a method, you are better off implementing an
after returning advice than an around advice, although an around advice can accomplish
the same thing. Using the most specific advice type provides a simpler programming model
with less potential for errors. For example, you do not need to invoke the `proceed()`
method on the `JoinPoint` used for around advice, and hence cannot fail to invoke it.

In Spring 2.0, all advice parameters are statically typed, so that you work with advice
parameters of the appropriate type (the type of the return value from a method execution
for example) rather than `Object` arrays.

The concept of join points, matched by pointcuts, is the key to AOP which distinguishes
it from older technologies offering only interception. Pointcuts enable advice to be
targeted independently of the Object-Oriented hierarchy. For example, an around advice
providing declarative transaction management can be applied to a set of methods spanning
multiple objects (such as all business operations in the service layer).



[[aop-introduction-spring-defn]]
=== Spring AOP capabilities and goals

Spring AOP is implemented in pure Java. There is no need for a special compilation
process. Spring AOP does not need to control the class loader hierarchy, and is thus
suitable for use in a Servlet container or application server.

Spring AOP currently supports only method execution join points (advising the execution
of methods on Spring beans). Field interception is not implemented, although support for
field interception could be added without breaking the core Spring AOP APIs. If you need
to advise field access and update join points, consider a language such as AspectJ.

Spring AOP's approach to AOP differs from that of most other AOP frameworks. The aim is
not to provide the most complete AOP implementation (although Spring AOP is quite
capable); it is rather to provide a close integration between AOP implementation and
Spring IoC to help solve common problems in enterprise applications.

Thus, for example, the Spring Framework's AOP functionality is normally used in
conjunction with the Spring IoC container. Aspects are configured using normal bean
definition syntax (although this allows powerful "autoproxying" capabilities): this is a
crucial difference from other AOP implementations. There are some things you cannot do
easily or efficiently with Spring AOP, such as advise very fine-grained objects (such as
domain objects typically): AspectJ is the best choice in such cases. However, our
experience is that Spring AOP provides an excellent solution to most problems in
enterprise Java applications that are amenable to AOP.

Spring AOP will never strive to compete with AspectJ to provide a comprehensive AOP
solution. We believe that both proxy-based frameworks like Spring AOP and full-blown
frameworks such as AspectJ are valuable, and that they are complementary, rather than in
competition. Spring seamlessly integrates Spring AOP and IoC with AspectJ, to enable
all uses of AOP to be catered for within a consistent Spring-based application
architecture. This integration does not affect the Spring AOP API or the AOP Alliance
API: Spring AOP remains backward-compatible. See <<aop-api,the following chapter>> for a
discussion of the Spring AOP APIs.

[NOTE]
====
One of the central tenets of the Spring Framework is that of __non-invasiveness__; this
is the idea that you should not be forced to introduce framework-specific classes and
interfaces into your business/domain model. However, in some places the Spring Framework
does give you the option to introduce Spring Framework-specific dependencies into your
codebase: the rationale in giving you such options is because in certain scenarios it
might be just plain easier to read or code some specific piece of functionality in such
a way. The Spring Framework (almost) always offers you the choice though: you have the
freedom to make an informed decision as to which option best suits your particular use
case or scenario.

One such choice that is relevant to this chapter is that of which AOP framework (and
which AOP style) to choose. You have the choice of AspectJ and/or Spring AOP, and you
also have the choice of either the @AspectJ annotation-style approach or the Spring XML
configuration-style approach. The fact that this chapter chooses to introduce the
@AspectJ-style approach first should not be taken as an indication that the Spring team
favors the @AspectJ annotation-style approach over the Spring XML configuration-style.

See <<aop-choosing>> for a more complete discussion of the whys and wherefores of each
style.
====



[[aop-introduction-proxies]]
=== AOP Proxies

Spring AOP defaults to using standard JDK __dynamic proxies__ for AOP proxies. This
enables any interface (or set of interfaces) to be proxied.

Spring AOP can also use CGLIB proxies. This is necessary to proxy classes rather than
interfaces. CGLIB is used by default if a business object does not implement an
interface. As it is good practice to program to interfaces rather than classes; business
classes normally will implement one or more business interfaces. It is possible to
<<aop-proxying,force the use of CGLIB>>, in those (hopefully rare) cases where you
need to advise a method that is not declared on an interface, or where you need to
pass a proxied object to a method as a concrete type.

It is important to grasp the fact that Spring AOP is __proxy-based__. See
<<aop-understanding-aop-proxies>> for a thorough examination of exactly what this
implementation detail actually means.




[[aop-ataspectj]]
== @AspectJ support

@AspectJ refers to a style of declaring aspects as regular Java classes annotated with
annotations. The @AspectJ style was introduced by the
http://www.eclipse.org/aspectj[AspectJ project] as part of the AspectJ 5 release. Spring
interprets the same annotations as AspectJ 5, using a library supplied by AspectJ
for pointcut parsing and matching. The AOP runtime is still pure Spring AOP though, and
there is no dependency on the AspectJ compiler or weaver.

[NOTE]
====
Using the AspectJ compiler and weaver enables use of the full AspectJ language, and is
discussed in <<aop-using-aspectj>>.
====



[[aop-aspectj-support]]
=== Enabling @AspectJ Support

To use @AspectJ aspects in a Spring configuration you need to enable Spring support for
configuring Spring AOP based on @AspectJ aspects, and __autoproxying__ beans based on
whether or not they are advised by those aspects. By autoproxying we mean that if Spring
determines that a bean is advised by one or more aspects, it will automatically generate
a proxy for that bean to intercept method invocations and ensure that advice is executed
as needed.

The @AspectJ support can be enabled with XML or Java style configuration. In either
case you will also need to ensure that AspectJ's `aspectjweaver.jar` library is on the
classpath of your application (version 1.8 or later). This library is available in the
`'lib'` directory of an AspectJ distribution or via the Maven Central repository.


[[aop-enable-aspectj-java]]
==== Enabling @AspectJ Support with Java configuration

To enable @AspectJ support with Java `@Configuration` add the `@EnableAspectJAutoProxy`
annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableAspectJAutoProxy
	public class AppConfig {

	}
----


[[aop-enable-aspectj-xml]]
==== Enabling @AspectJ Support with XML configuration

To enable @AspectJ support with XML based configuration use the `aop:aspectj-autoproxy`
element:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:aspectj-autoproxy/>
----

This assumes that you are using schema support as described in
<<core.adoc#xsd-schemas, XML Schema-based configuration>>. See
<<core.adoc#xsd-schemas-aop, the AOP schema>> for how to import the tags in the `aop`
namespace.



[[aop-at-aspectj]]
=== Declaring an aspect

With the @AspectJ support enabled, any bean defined in your application context with a
class that is an @AspectJ aspect (has the `@Aspect` annotation) will be automatically
detected by Spring and used to configure Spring AOP. The following example shows the
minimal definition required for a not-very-useful aspect:

A regular bean definition in the application context, pointing to a bean class that has
the `@Aspect` annotation:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="myAspect" class="org.xyz.NotVeryUsefulAspect">
		<!-- configure properties of aspect here as normal -->
	</bean>
----

And the `NotVeryUsefulAspect` class definition, annotated with
`org.aspectj.lang.annotation.Aspect` annotation;

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	package org.xyz;
	import org.aspectj.lang.annotation.Aspect;

	@Aspect
	public class NotVeryUsefulAspect {

	}
----

Aspects (classes annotated with `@Aspect`) may have methods and fields just like any
other class. They may also contain pointcut, advice, and introduction (inter-type)
declarations.

.Autodetecting aspects through component scanning
[NOTE]
====
You may register aspect classes as regular beans in your Spring XML configuration, or
autodetect them through classpath scanning - just like any other Spring-managed bean.
However, note that the __@Aspect__ annotation is __not__ sufficient for autodetection in
the classpath: For that purpose, you need to add a separate __@Component__ annotation
(or alternatively a custom stereotype annotation that qualifies, as per the rules of
Spring's component scanner).
====

.Advising aspects with other aspects?
[NOTE]
====
In Spring AOP, it is __not__ possible to have aspects themselves be the target of advice
from other aspects. The __@Aspect__ annotation on a class marks it as an aspect, and
hence excludes it from auto-proxying.
====



[[aop-pointcuts]]
=== Declaring a pointcut

Recall that pointcuts determine join points of interest, and thus enable us to control
when advice executes. __Spring AOP only supports method execution join points for Spring
beans__, so you can think of a pointcut as matching the execution of methods on Spring
beans. A pointcut declaration has two parts: a signature comprising a name and any
parameters, and a pointcut expression that determines __exactly__ which method
executions we are interested in. In the @AspectJ annotation-style of AOP, a pointcut
signature is provided by a regular method definition, and the pointcut expression is
indicated using the `@Pointcut` annotation (the method serving as the pointcut signature
__must__ have a `void` return type).

An example will help make this distinction between a pointcut signature and a pointcut
expression clear. The following example defines a pointcut named `'anyOldTransfer'` that
will match the execution of any method named `'transfer'`:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Pointcut("execution(* transfer(..))")// the pointcut expression
	private void anyOldTransfer() {}// the pointcut signature
----

The pointcut expression that forms the value of the `@Pointcut` annotation is a regular
AspectJ 5 pointcut expression. For a full discussion of AspectJ's pointcut language, see
the http://www.eclipse.org/aspectj/doc/released/progguide/index.html[AspectJ
Programming Guide] (and for extensions, the
http://www.eclipse.org/aspectj/doc/released/adk15notebook/index.html[AspectJ 5
Developers Notebook]) or one of the books on AspectJ such as "Eclipse AspectJ" by Colyer
et. al. or "AspectJ in Action" by Ramnivas Laddad.


[[aop-pointcuts-designators]]
==== Supported Pointcut Designators

Spring AOP supports the following AspectJ pointcut designators (PCD) for use in pointcut
expressions:

.Other pointcut types
****
The full AspectJ pointcut language supports additional pointcut designators that are not
supported in Spring. These are: `call, get, set, preinitialization,
staticinitialization, initialization, handler, adviceexecution, withincode, cflow,
cflowbelow, if, @this`, and `@withincode`. Use of these pointcut designators in pointcut
expressions interpreted by Spring AOP will result in an `IllegalArgumentException` being
thrown.

The set of pointcut designators supported by Spring AOP may be extended in future
releases to support more of the AspectJ pointcut designators.
****

* __execution__ - for matching method execution join points, this is the primary
  pointcut designator you will use when working with Spring AOP
* __within__ - limits matching to join points within certain types (simply the execution
  of a method declared within a matching type when using Spring AOP)
* __this__ - limits matching to join points (the execution of methods when using Spring
  AOP) where the bean reference (Spring AOP proxy) is an instance of the given type
* __target__ - limits matching to join points (the execution of methods when using
  Spring AOP) where the target object (application object being proxied) is an instance
  of the given type
* __args__ - limits matching to join points (the execution of methods when using Spring
  AOP) where the arguments are instances of the given types
* __@target__ - limits matching to join points (the execution of methods when using
  Spring AOP) where the class of the executing object has an annotation of the given type
* __@args__ - limits matching to join points (the execution of methods when using Spring
  AOP) where the runtime type of the actual arguments passed have annotations of the
  given type(s)
* __@within__ - limits matching to join points within types that have the given
  annotation (the execution of methods declared in types with the given annotation when
  using Spring AOP)
* __@annotation__ - limits matching to join points where the subject of the join point
  (method being executed in Spring AOP) has the given annotation

Because Spring AOP limits matching to only method execution join points, the discussion
of the pointcut designators above gives a narrower definition than you will find in the
AspectJ programming guide. In addition, AspectJ itself has type-based semantics and at
an execution join point both `this` and `target` refer to the same object - the
object executing the method. Spring AOP is a proxy-based system and differentiates
between the proxy object itself (bound to `this`) and the target object behind the
proxy (bound to `target`).

[NOTE]
====
Due to the proxy-based nature of Spring's AOP framework, calls within the target object
are by definition __not__ intercepted. For JDK proxies, only public interface method
calls on the proxy can be intercepted. With CGLIB, public and protected method calls on
the proxy will be intercepted, and even package-visible methods if necessary. However,
common interactions through proxies should always be designed through public signatures.

Note that pointcut definitions are generally matched against any intercepted method.
If a pointcut is strictly meant to be public-only, even in a CGLIB proxy scenario with
potential non-public interactions through proxies, it needs to be defined accordingly.

If your interception needs include method calls or even constructors within the target
class, consider the use of Spring-driven <<aop-aj-ltw,native AspectJ weaving>> instead
of Spring's proxy-based AOP framework. This constitutes a different mode of AOP usage
with different characteristics, so be sure to make yourself familiar with weaving first
before making a decision.
====

Spring AOP also supports an additional PCD named `bean`. This PCD allows you to limit
the matching of join points to a particular named Spring bean, or to a set of named
Spring beans (when using wildcards). The `bean` PCD has the following form:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	bean(idOrNameOfBean)
----

The `idOrNameOfBean` token can be the name of any Spring bean: limited wildcard
support using the `*` character is provided, so if you establish some naming
conventions for your Spring beans you can quite easily write a `bean` PCD expression
to pick them out. As is the case with other pointcut designators, the `bean` PCD can
be &&'ed, ||'ed, and ! (negated) too.

[NOTE]
====
Please note that the `bean` PCD is __only__ supported in Spring AOP - and __not__ in
native AspectJ weaving. It is a Spring-specific extension to the standard PCDs that
AspectJ defines and therefore not available for aspects declared in the `@Aspect` model.

The `bean` PCD operates at the __instance__ level (building on the Spring bean name
concept) rather than at the type level only (which is what weaving-based AOP is limited
to). Instance-based pointcut designators are a special capability of Spring's
proxy-based AOP framework and its close integration with the Spring bean factory, where
it is natural and straightforward to identify specific beans by name.
====


[[aop-pointcuts-combining]]
==== Combining pointcut expressions

Pointcut expressions can be combined using '&&', '||' and '!'. It is also possible to
refer to pointcut expressions by name. The following example shows three pointcut
expressions: `anyPublicOperation` (which matches if a method execution join point
represents the execution of any public method); `inTrading` (which matches if a method
execution is in the trading module), and `tradingOperation` (which matches if a method
execution represents any public method in the trading module).

[source,java,indent=0]
[subs="verbatim"]
----
	@Pointcut("execution(public * *(..))")
	private void anyPublicOperation() {}

	@Pointcut("within(com.xyz.someapp.trading..*)")
	private void inTrading() {}

	@Pointcut("anyPublicOperation() && inTrading()")
	private void tradingOperation() {}
----

It is a best practice to build more complex pointcut expressions out of smaller named
components as shown above. When referring to pointcuts by name, normal Java visibility
rules apply (you can see private pointcuts in the same type, protected pointcuts in the
hierarchy, public pointcuts anywhere and so on). Visibility does not affect pointcut
__matching__.


[[aop-common-pointcuts]]
==== Sharing common pointcut definitions

When working with enterprise applications, you often want to refer to modules of the
application and particular sets of operations from within several aspects. We recommend
defining a "SystemArchitecture" aspect that captures common pointcut expressions for
this purpose. A typical such aspect would look as follows:

[source,java,indent=0]
[subs="verbatim"]
----
	package com.xyz.someapp;

	import org.aspectj.lang.annotation.Aspect;
	import org.aspectj.lang.annotation.Pointcut;

	@Aspect
	public class SystemArchitecture {

		/**
		 * A join point is in the web layer if the method is defined
		 * in a type in the com.xyz.someapp.web package or any sub-package
		 * under that.
		 */
		@Pointcut("within(com.xyz.someapp.web..*)")
		public void inWebLayer() {}

		/**
		 * A join point is in the service layer if the method is defined
		 * in a type in the com.xyz.someapp.service package or any sub-package
		 * under that.
		 */
		@Pointcut("within(com.xyz.someapp.service..*)")
		public void inServiceLayer() {}

		/**
		 * A join point is in the data access layer if the method is defined
		 * in a type in the com.xyz.someapp.dao package or any sub-package
		 * under that.
		 */
		@Pointcut("within(com.xyz.someapp.dao..*)")
		public void inDataAccessLayer() {}

		/**
		 * A business service is the execution of any method defined on a service
		 * interface. This definition assumes that interfaces are placed in the
		 * "service" package, and that implementation types are in sub-packages.
		 *
		 * If you group service interfaces by functional area (for example,
		 * in packages com.xyz.someapp.abc.service and com.xyz.someapp.def.service) then
		 * the pointcut expression "execution(* com.xyz.someapp..service.*.*(..))"
		 * could be used instead.
		 *
		 * Alternatively, you can write the expression using the 'bean'
		 * PCD, like so "bean(*Service)". (This assumes that you have
		 * named your Spring service beans in a consistent fashion.)
		 */
		@Pointcut("execution(* com.xyz.someapp..service.*.*(..))")
		public void businessService() {}

		/**
		 * A data access operation is the execution of any method defined on a
		 * dao interface. This definition assumes that interfaces are placed in the
		 * "dao" package, and that implementation types are in sub-packages.
		 */
		@Pointcut("execution(* com.xyz.someapp.dao.*.*(..))")
		public void dataAccessOperation() {}

	}
----

The pointcuts defined in such an aspect can be referred to anywhere that you need a
pointcut expression. For example, to make the service layer transactional, you could
write:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:config>
		<aop:advisor
			pointcut="com.xyz.someapp.SystemArchitecture.businessService()"
			advice-ref="tx-advice"/>
	</aop:config>

	<tx:advice id="tx-advice">
		<tx:attributes>
			<tx:method name="*" propagation="REQUIRED"/>
		</tx:attributes>
	</tx:advice>
----

The `<aop:config>` and `<aop:advisor>` elements are discussed in <<aop-schema>>. The
transaction elements are discussed in <<data-access.adoc#transaction, Transaction Management>>.


[[aop-pointcuts-examples]]
==== Examples

Spring AOP users are likely to use the `execution` pointcut designator the most often.
The format of an execution expression is:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	execution(modifiers-pattern? ret-type-pattern declaring-type-pattern?name-pattern(param-pattern)
				throws-pattern?)
----

All parts except the returning type pattern (ret-type-pattern in the snippet above),
name pattern, and parameters pattern are optional. The returning type pattern determines
what the return type of the method must be in order for a join point to be matched. Most
frequently you will use `{asterisk}` as the returning type pattern, which matches any return
type. A fully-qualified type name will match only when the method returns the given
type. The name pattern matches the method name. You can use the `{asterisk}` wildcard as all or
part of a name pattern. If specifying a declaring type pattern
then include a trailing `.` to join it to the name pattern component.
The parameters pattern is slightly more complex: `()` matches a
method that takes no parameters, whereas `(..)` matches any number of parameters (zero
or more). The pattern `({asterisk})` matches a method taking one parameter of any type,
`(*,String)` matches a method taking two parameters, the first can be of any type, the
second must be a String. Consult the
http://www.eclipse.org/aspectj/doc/released/progguide/semantics-pointcuts.html[Language
Semantics] section of the AspectJ Programming Guide for more information.

Some examples of common pointcut expressions are given below.

* the execution of any public method:

[source,java,indent=0]
[subs="verbatim"]
----
	execution(public * *(..))
----

* the execution of any method with a name beginning with "set":

[source,java,indent=0]
[subs="verbatim"]
----
	execution(* set*(..))
----

* the execution of any method defined by the `AccountService` interface:

[source,java,indent=0]
[subs="verbatim"]
----
	execution(* com.xyz.service.AccountService.*(..))
----

* the execution of any method defined in the service package:

[source,java,indent=0]
[subs="verbatim"]
----
	execution(* com.xyz.service.*.*(..))
----

* the execution of any method defined in the service package or a sub-package:

[source,java,indent=0]
[subs="verbatim"]
----
	execution(* com.xyz.service..*.*(..))
----

* any join point (method execution only in Spring AOP) within the service package:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	within(com.xyz.service.*)
----

* any join point (method execution only in Spring AOP) within the service package or a
  sub-package:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	within(com.xyz.service..*)
----

* any join point (method execution only in Spring AOP) where the proxy implements the
  `AccountService` interface:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	this(com.xyz.service.AccountService)
----

[NOTE]
====
'this' is more commonly used in a binding form :- see the following section on advice
for how to make the proxy object available in the advice body.
====

* any join point (method execution only in Spring AOP) where the target object
  implements the `AccountService` interface:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	target(com.xyz.service.AccountService)
----

[NOTE]
====
'target' is more commonly used in a binding form :- see the following section on advice
for how to make the target object available in the advice body.
====

* any join point (method execution only in Spring AOP) which takes a single parameter,
  and where the argument passed at runtime is `Serializable`:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	args(java.io.Serializable)
----

[NOTE]
====
'args' is more commonly used in a binding form :- see the following section on advice
for how to make the method arguments available in the advice body.
====

Note that the pointcut given in this example is different to `execution(*
*(java.io.Serializable))`: the args version matches if the argument passed at runtime is
Serializable, the execution version matches if the method signature declares a single
parameter of type `Serializable`.

* any join point (method execution only in Spring AOP) where the target object has an
  `@Transactional` annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@target(org.springframework.transaction.annotation.Transactional)
----

[NOTE]
====
'@target' can also be used in a binding form :- see the following section on advice for
how to make the annotation object available in the advice body.
====

* any join point (method execution only in Spring AOP) where the declared type of the
  target object has an `@Transactional` annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@within(org.springframework.transaction.annotation.Transactional)
----

[NOTE]
====
'@within' can also be used in a binding form :- see the following section on advice for
how to make the annotation object available in the advice body.
====

* any join point (method execution only in Spring AOP) where the executing method has an
  `@Transactional` annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@annotation(org.springframework.transaction.annotation.Transactional)
----

[NOTE]
====
'@annotation' can also be used in a binding form :- see the following section on advice
for how to make the annotation object available in the advice body.
====

* any join point (method execution only in Spring AOP) which takes a single parameter,
  and where the runtime type of the argument passed has the `@Classified` annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@args(com.xyz.security.Classified)
----

[NOTE]
====
'@args' can also be used in a binding form :- see the following section on advice for
how to make the annotation object(s) available in the advice body.
====

* any join point (method execution only in Spring AOP) on a Spring bean named
  `tradeService`:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	bean(tradeService)
----

* any join point (method execution only in Spring AOP) on Spring beans having names that
  match the wildcard expression `*Service`:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	bean(*Service)
----


[[writing-good-pointcuts]]
==== Writing good pointcuts

During compilation, AspectJ processes pointcuts in order to try and optimize matching
performance. Examining code and determining if each join point matches (statically or
dynamically) a given pointcut is a costly process. (A dynamic match means the match
cannot be fully determined from static analysis and a test will be placed in the code to
determine if there is an actual match when the code is running). On first encountering a
pointcut declaration, AspectJ will rewrite it into an optimal form for the matching
process. What does this mean? Basically pointcuts are rewritten in DNF (Disjunctive
Normal Form) and the components of the pointcut are sorted such that those components
that are cheaper to evaluate are checked first. This means you do not have to worry
about understanding the performance of various pointcut designators and may supply them
in any order in a pointcut declaration.

However, AspectJ can only work with what it is told, and for optimal performance of
matching you should think about what they are trying to achieve and narrow the search
space for matches as much as possible in the definition. The existing designators
naturally fall into one of three groups: kinded, scoping and context:

* Kinded designators are those which select a particular kind of join point. For
  example: execution, get, set, call, handler
* Scoping designators are those which select a group of join points of interest (of
  probably many kinds). For example: within, withincode
* Contextual designators are those that match (and optionally bind) based on context.
  For example: this, target, @annotation

A well written pointcut should try and include at least the first two types (kinded and
scoping), whilst the contextual designators may be included if wishing to match based on
join point context, or bind that context for use in the advice. Supplying either just a
kinded designator or just a contextual designator will work but could affect weaving
performance (time and memory used) due to all the extra processing and analysis. Scoping
designators are very fast to match and their usage means AspectJ can very quickly
dismiss groups of join points that should not be further processed - that is why a good
pointcut should always include one if possible.



[[aop-advice]]
=== Declaring advice

Advice is associated with a pointcut expression, and runs before, after, or around
method executions matched by the pointcut. The pointcut expression may be either a
simple reference to a named pointcut, or a pointcut expression declared in place.


[[aop-advice-before]]
==== Before advice

Before advice is declared in an aspect using the `@Before` annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.aspectj.lang.annotation.Aspect;
	import org.aspectj.lang.annotation.Before;

	@Aspect
	public class BeforeExample {

		@Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
		public void doAccessCheck() {
			// ...
		}

	}
----

If using an in-place pointcut expression we could rewrite the above example as:

[source,java,indent=0]
[subs="verbatim"]
----
	import org.aspectj.lang.annotation.Aspect;
	import org.aspectj.lang.annotation.Before;

	@Aspect
	public class BeforeExample {

		@Before("execution(* com.xyz.myapp.dao.*.*(..))")
		public void doAccessCheck() {
			// ...
		}

	}
----


[[aop-advice-after-returning]]
==== After returning advice

After returning advice runs when a matched method execution returns normally. It is
declared using the `@AfterReturning` annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.aspectj.lang.annotation.Aspect;
	import org.aspectj.lang.annotation.AfterReturning;

	@Aspect
	public class AfterReturningExample {

		@AfterReturning("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
		public void doAccessCheck() {
			// ...
		}

	}
----

[NOTE]
====
Note: it is of course possible to have multiple advice declarations, and other members
as well, all inside the same aspect. We're just showing a single advice declaration in
these examples to focus on the issue under discussion at the time.
====

Sometimes you need access in the advice body to the actual value that was returned. You
can use the form of `@AfterReturning` that binds the return value for this:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.aspectj.lang.annotation.Aspect;
	import org.aspectj.lang.annotation.AfterReturning;

	@Aspect
	public class AfterReturningExample {

		@AfterReturning(
			pointcut="com.xyz.myapp.SystemArchitecture.dataAccessOperation()",
			returning="retVal")
		public void doAccessCheck(Object retVal) {
			// ...
		}

	}
----

The name used in the `returning` attribute must correspond to the name of a parameter in
the advice method. When a method execution returns, the return value will be passed to
the advice method as the corresponding argument value. A `returning` clause also
restricts matching to only those method executions that return a value of the specified
type ( `Object` in this case, which will match any return value).

Please note that it is __not__ possible to return a totally different reference when
using after-returning advice.


[[aop-advice-after-throwing]]
==== After throwing advice

After throwing advice runs when a matched method execution exits by throwing an
exception. It is declared using the `@AfterThrowing` annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.aspectj.lang.annotation.Aspect;
	import org.aspectj.lang.annotation.AfterThrowing;

	@Aspect
	public class AfterThrowingExample {

		@AfterThrowing("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
		public void doRecoveryActions() {
			// ...
		}

	}
----

Often you want the advice to run only when exceptions of a given type are thrown, and
you also often need access to the thrown exception in the advice body. Use the
`throwing` attribute to both restrict matching (if desired, use `Throwable` as the
exception type otherwise) and bind the thrown exception to an advice parameter.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.aspectj.lang.annotation.Aspect;
	import org.aspectj.lang.annotation.AfterThrowing;

	@Aspect
	public class AfterThrowingExample {

		@AfterThrowing(
			pointcut="com.xyz.myapp.SystemArchitecture.dataAccessOperation()",
			throwing="ex")
		public void doRecoveryActions(DataAccessException ex) {
			// ...
		}

	}
----

The name used in the `throwing` attribute must correspond to the name of a parameter in
the advice method. When a method execution exits by throwing an exception, the exception
will be passed to the advice method as the corresponding argument value. A `throwing`
clause also restricts matching to only those method executions that throw an exception
of the specified type ( `DataAccessException` in this case).


[[aop-advice-after-finally]]
==== After (finally) advice

After (finally) advice runs however a matched method execution exits. It is declared
using the `@After` annotation. After advice must be prepared to handle both normal and
exception return conditions. It is typically used for releasing resources, etc.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.aspectj.lang.annotation.Aspect;
	import org.aspectj.lang.annotation.After;

	@Aspect
	public class AfterFinallyExample {

		@After("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
		public void doReleaseLock() {
			// ...
		}

	}
----


[[aop-ataspectj-around-advice]]
==== Around advice

The final kind of advice is around advice. Around advice runs "around" a matched method
execution. It has the opportunity to do work both before and after the method executes,
and to determine when, how, and even if, the method actually gets to execute at all.
Around advice is often used if you need to share state before and after a method
execution in a thread-safe manner (starting and stopping a timer for example). Always
use the least powerful form of advice that meets your requirements (i.e. don't use
around advice if simple before advice would do).

Around advice is declared using the `@Around` annotation. The first parameter of the
advice method must be of type `ProceedingJoinPoint`. Within the body of the advice,
calling `proceed()` on the `ProceedingJoinPoint` causes the underlying method to
execute. The `proceed` method may also be called passing in an `Object[]` - the values
in the array will be used as the arguments to the method execution when it proceeds.

[NOTE]
====
The behavior of proceed when called with an Object[] is a little different than the
behavior of proceed for around advice compiled by the AspectJ compiler. For around
advice written using the traditional AspectJ language, the number of arguments passed to
proceed must match the number of arguments passed to the around advice (not the number
of arguments taken by the underlying join point), and the value passed to proceed in a
given argument position supplants the original value at the join point for the entity
the value was bound to (Don't worry if this doesn't make sense right now!). The approach
taken by Spring is simpler and a better match to its proxy-based, execution only
semantics. You only need to be aware of this difference if you are compiling @AspectJ
aspects written for Spring and using proceed with arguments with the AspectJ compiler
and weaver. There is a way to write such aspects that is 100% compatible across both
Spring AOP and AspectJ, and this is discussed in the following section on advice
parameters.
====

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.aspectj.lang.annotation.Aspect;
	import org.aspectj.lang.annotation.Around;
	import org.aspectj.lang.ProceedingJoinPoint;

	@Aspect
	public class AroundExample {

		@Around("com.xyz.myapp.SystemArchitecture.businessService()")
		public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable {
			// start stopwatch
			Object retVal = pjp.proceed();
			// stop stopwatch
			return retVal;
		}

	}
----

The value returned by the around advice will be the return value seen by the caller of
the method. A simple caching aspect for example could return a value from a cache if it
has one, and invoke proceed() if it does not. Note that proceed may be invoked once,
many times, or not at all within the body of the around advice, all of these are quite
legal.


[[aop-ataspectj-advice-params]]
==== Advice parameters

Spring offers fully typed advice - meaning that you declare the parameters you need
in the advice signature (as we saw for the returning and throwing examples above) rather
than work with `Object[]` arrays all the time. We'll see how to make argument and other
contextual values available to the advice body in a moment. First let's take a look at
how to write generic advice that can find out about the method the advice is currently
advising.

[[aop-ataspectj-advice-params-the-joinpoint]]
===== Access to the current JoinPoint

Any advice method may declare as its first parameter, a parameter of type
`org.aspectj.lang.JoinPoint` (please note that around advice is __required__ to declare
a first parameter of type `ProceedingJoinPoint`, which is a subclass of `JoinPoint`. The
`JoinPoint` interface provides a number of useful methods such as `getArgs()` (returns
the method arguments), `getThis()` (returns the proxy object), `getTarget()` (returns
the target object), `getSignature()` (returns a description of the method that is being
advised) and `toString()` (prints a useful description of the method being advised).
Please do consult the javadocs for full details.

[[aop-ataspectj-advice-params-passing]]
===== Passing parameters to advice

We've already seen how to bind the returned value or exception value (using after
returning and after throwing advice). To make argument values available to the advice
body, you can use the binding form of `args`. If a parameter name is used in place of a
type name in an args expression, then the value of the corresponding argument will be
passed as the parameter value when the advice is invoked. An example should make this
clearer. Suppose you want to advise the execution of dao operations that take an Account
object as the first parameter, and you need access to the account in the advice body.
You could write the following:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation() && args(account,..)")
	public void validateAccount(Account account) {
		// ...
	}
----

The `args(account,..)` part of the pointcut expression serves two purposes: firstly, it
restricts matching to only those method executions where the method takes at least one
parameter, and the argument passed to that parameter is an instance of `Account`;
secondly, it makes the actual `Account` object available to the advice via the `account`
parameter.

Another way of writing this is to declare a pointcut that "provides" the `Account`
object value when it matches a join point, and then just refer to the named pointcut
from the advice. This would look as follows:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Pointcut("com.xyz.myapp.SystemArchitecture.dataAccessOperation() && args(account,..)")
	private void accountDataAccessOperation(Account account) {}

	@Before("accountDataAccessOperation(account)")
	public void validateAccount(Account account) {
		// ...
	}
----

The interested reader is once more referred to the AspectJ programming guide for more
details.

The proxy object ( `this`), target object ( `target`), and annotations ( `@within,
@target, @annotation, @args`) can all be bound in a similar fashion. The following
example shows how you could match the execution of methods annotated with an
`@Auditable` annotation, and extract the audit code.

First the definition of the `@Auditable` annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Retention(RetentionPolicy.RUNTIME)
	@Target(ElementType.METHOD)
	public @interface Auditable {
		AuditCode value();
	}
----

And then the advice that matches the execution of `@Auditable` methods:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Before("com.xyz.lib.Pointcuts.anyPublicMethod() && @annotation(auditable)")
	public void audit(Auditable auditable) {
		AuditCode code = auditable.value();
		// ...
	}
----

[[aop-ataspectj-advice-params-generics]]
===== Advice parameters and generics

Spring AOP can handle generics used in class declarations and method parameters. Suppose
you have a generic type like this:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public interface Sample<T> {
		void sampleGenericMethod(T param);
		void sampleGenericCollectionMethod(Collection<T> param);
	}
----

You can restrict interception of method types to certain parameter types by simply
typing the advice parameter to the parameter type you want to intercept the method for:

[source,java,indent=0]
[subs="verbatim"]
----
	@Before("execution(* ..Sample+.sampleGenericMethod(*)) && args(param)")
	public void beforeSampleMethod(MyType param) {
		// Advice implementation
	}
----

That this works is pretty obvious as we already discussed above. However, it's worth
pointing out that this won't work for generic collections. So you cannot define a
pointcut like this:

[source,java,indent=0]
[subs="verbatim"]
----
	@Before("execution(* ..Sample+.sampleGenericCollectionMethod(*)) && args(param)")
	public void beforeSampleMethod(Collection<MyType> param) {
		// Advice implementation
	}
----

To make this work we would have to inspect every element of the collection, which is not
reasonable as we also cannot decide how to treat `null` values in general. To achieve
something similar to this you have to type the parameter to `Collection<?>` and manually
check the type of the elements.

[[aop-ataspectj-advice-params-names]]
===== Determining argument names

The parameter binding in advice invocations relies on matching names used in pointcut
expressions to declared parameter names in (advice and pointcut) method signatures.
Parameter names are __not__ available through Java reflection, so Spring AOP uses the
following strategies to determine parameter names:

* If the parameter names have been specified by the user explicitly, then the specified
  parameter names are used: both the advice and the pointcut annotations have
  an optional "argNames" attribute which can be used to specify the argument names of
  the annotated method - these argument names __are__ available at runtime. For example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Before(value="com.xyz.lib.Pointcuts.anyPublicMethod() && target(bean) && @annotation(auditable)",
			argNames="bean,auditable")
	public void audit(Object bean, Auditable auditable) {
		AuditCode code = auditable.value();
		// ... use code and bean
	}
----

If the first parameter is of the `JoinPoint`, `ProceedingJoinPoint`, or
`JoinPoint.StaticPart` type, you may leave out the name of the parameter from the value
of the "argNames" attribute. For example, if you modify the preceding advice to receive
the join point object, the "argNames" attribute need not include it:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Before(value="com.xyz.lib.Pointcuts.anyPublicMethod() && target(bean) && @annotation(auditable)",
			argNames="bean,auditable")
	public void audit(JoinPoint jp, Object bean, Auditable auditable) {
		AuditCode code = auditable.value();
		// ... use code, bean, and jp
	}
----

The special treatment given to the first parameter of the `JoinPoint`,
`ProceedingJoinPoint`, and `JoinPoint.StaticPart` types is particularly convenient for
advice that do not collect any other join point context. In such situations, you may
simply omit the "argNames" attribute. For example, the following advice need not declare
the "argNames" attribute:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Before("com.xyz.lib.Pointcuts.anyPublicMethod()")
	public void audit(JoinPoint jp) {
		// ... use jp
	}
----

* Using the `'argNames'` attribute is a little clumsy, so if the `'argNames'` attribute
  has not been specified, then Spring AOP will look at the debug information for the
  class and try to determine the parameter names from the local variable table. This
  information will be present as long as the classes have been compiled with debug
  information ( `'-g:vars'` at a minimum). The consequences of compiling with this flag
  on are: (1) your code will be slightly easier to understand (reverse engineer), (2)
  the class file sizes will be very slightly bigger (typically inconsequential), (3) the
  optimization to remove unused local variables will not be applied by your compiler. In
  other words, you should encounter no difficulties building with this flag on.

[NOTE]
====
If an @AspectJ aspect has been compiled by the AspectJ compiler (ajc) even without the
debug information then there is no need to add the argNames attribute as the compiler
will retain the needed information.
====

* If the code has been compiled without the necessary debug information, then Spring AOP
  will attempt to deduce the pairing of binding variables to parameters (for example, if
  only one variable is bound in the pointcut expression, and the advice method only
  takes one parameter, the pairing is obvious!). If the binding of variables is
  ambiguous given the available information, then an `AmbiguousBindingException` will be
  thrown.
* If all of the above strategies fail then an `IllegalArgumentException` will be thrown.

[[aop-ataspectj-advice-proceeding-with-the-call]]
===== Proceeding with arguments

We remarked earlier that we would describe how to write a proceed call __with
arguments__ that works consistently across Spring AOP and AspectJ. The solution is
simply to ensure that the advice signature binds each of the method parameters in order.
For example:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Around("execution(List<Account> find*(..)) && " +
			"com.xyz.myapp.SystemArchitecture.inDataAccessLayer() && " +
			"args(accountHolderNamePattern)")
	public Object preProcessQueryPattern(ProceedingJoinPoint pjp,
			String accountHolderNamePattern) throws Throwable {
		String newPattern = preProcess(accountHolderNamePattern);
		return pjp.proceed(new Object[] {newPattern});
	}
----

In many cases you will be doing this binding anyway (as in the example above).


[[aop-ataspectj-advice-ordering]]
==== Advice ordering

What happens when multiple pieces of advice all want to run at the same join point?
Spring AOP follows the same precedence rules as AspectJ to determine the order of advice
execution. The highest precedence advice runs first "on the way in" (so given two pieces
of before advice, the one with highest precedence runs first). "On the way out" from a
join point, the highest precedence advice runs last (so given two pieces of after
advice, the one with the highest precedence will run second).

When two pieces of advice defined in __different__ aspects both need to run at the same
join point, unless you specify otherwise the order of execution is undefined. You can
control the order of execution by specifying precedence. This is done in the normal
Spring way by either implementing the `org.springframework.core.Ordered` interface in
the aspect class or annotating it with the `Order` annotation. Given two aspects, the
aspect returning the lower value from `Ordered.getValue()` (or the annotation value) has
the higher precedence.

When two pieces of advice defined in __the same__ aspect both need to run at the same
join point, the ordering is undefined (since there is no way to retrieve the declaration
order via reflection for javac-compiled classes). Consider collapsing such advice
methods into one advice method per join point in each aspect class, or refactor the
pieces of advice into separate aspect classes - which can be ordered at the aspect level.



[[aop-introductions]]
=== Introductions

Introductions (known as inter-type declarations in AspectJ) enable an aspect to declare
that advised objects implement a given interface, and to provide an implementation of
that interface on behalf of those objects.

An introduction is made using the `@DeclareParents` annotation. This annotation is used
to declare that matching types have a new parent (hence the name). For example, given an
interface `UsageTracked`, and an implementation of that interface `DefaultUsageTracked`,
the following aspect declares that all implementors of service interfaces also implement
the `UsageTracked` interface. (In order to expose statistics via JMX for example.)

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Aspect
	public class UsageTracking {

		@DeclareParents(value="com.xzy.myapp.service.*+", defaultImpl=DefaultUsageTracked.class)
		public static UsageTracked mixin;

		@Before("com.xyz.myapp.SystemArchitecture.businessService() && this(usageTracked)")
		public void recordUsage(UsageTracked usageTracked) {
			usageTracked.incrementUseCount();
		}

	}
----

The interface to be implemented is determined by the type of the annotated field. The
`value` attribute of the `@DeclareParents` annotation is an AspectJ type pattern :- any
bean of a matching type will implement the UsageTracked interface. Note that in the
before advice of the above example, service beans can be directly used as
implementations of the `UsageTracked` interface. If accessing a bean programmatically
you would write the following:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	UsageTracked usageTracked = (UsageTracked) context.getBean("myService");
----



[[aop-instantiation-models]]
=== Aspect instantiation models

[NOTE]
====
(This is an advanced topic, so if you are just starting out with AOP you can safely skip
it until later.)
====

By default there will be a single instance of each aspect within the application
context. AspectJ calls this the singleton instantiation model. It is possible to define
aspects with alternate lifecycles :- Spring supports AspectJ's `perthis` and `pertarget`
instantiation models ( `percflow, percflowbelow,` and `pertypewithin` are not currently
supported).

A "perthis" aspect is declared by specifying a `perthis` clause in the `@Aspect`
annotation. Let's look at an example, and then we'll explain how it works.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Aspect("perthis(com.xyz.myapp.SystemArchitecture.businessService())")
	public class MyAspect {

		private int someState;

		@Before(com.xyz.myapp.SystemArchitecture.businessService())
		public void recordServiceUsage() {
			// ...
		}

	}
----

The effect of the `'perthis'` clause is that one aspect instance will be created for
each unique service object executing a business service (each unique object bound to
'this' at join points matched by the pointcut expression). The aspect instance is
created the first time that a method is invoked on the service object. The aspect goes
out of scope when the service object goes out of scope. Before the aspect instance is
created, none of the advice within it executes. As soon as the aspect instance has been
created, the advice declared within it will execute at matched join points, but only
when the service object is the one this aspect is associated with. See the AspectJ
programming guide for more information on per-clauses.

The `'pertarget'` instantiation model works in exactly the same way as perthis, but
creates one aspect instance for each unique target object at matched join points.



[[aop-ataspectj-example]]
=== Example

Now that you have seen how all the constituent parts work, let's put them together to do
something useful!

The execution of business services can sometimes fail due to concurrency issues (for
example, deadlock loser). If the operation is retried, it is quite likely to succeed
next time round. For business services where it is appropriate to retry in such
conditions (idempotent operations that don't need to go back to the user for conflict
resolution), we'd like to transparently retry the operation to avoid the client seeing a
`PessimisticLockingFailureException`. This is a requirement that clearly cuts across
multiple services in the service layer, and hence is ideal for implementing via an
aspect.

Because we want to retry the operation, we will need to use around advice so that we can
call proceed multiple times. Here's how the basic aspect implementation looks:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Aspect
	public class ConcurrentOperationExecutor implements Ordered {

		private static final int DEFAULT_MAX_RETRIES = 2;

		private int maxRetries = DEFAULT_MAX_RETRIES;
		private int order = 1;

		public void setMaxRetries(int maxRetries) {
			this.maxRetries = maxRetries;
		}

		public int getOrder() {
			return this.order;
		}

		public void setOrder(int order) {
			this.order = order;
		}

		@Around("com.xyz.myapp.SystemArchitecture.businessService()")
		public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable {
			int numAttempts = 0;
			PessimisticLockingFailureException lockFailureException;
			do {
				numAttempts++;
				try {
					return pjp.proceed();
				}
				catch(PessimisticLockingFailureException ex) {
					lockFailureException = ex;
				}
			} while(numAttempts <= this.maxRetries);
			throw lockFailureException;
		}

	}
----

Note that the aspect implements the `Ordered` interface so we can set the precedence of
the aspect higher than the transaction advice (we want a fresh transaction each time we
retry). The `maxRetries` and `order` properties will both be configured by Spring. The
main action happens in the `doConcurrentOperation` around advice. Notice that for the
moment we're applying the retry logic to all `businessService()s`. We try to proceed,
and if we fail with an `PessimisticLockingFailureException` we simply try again unless
we have exhausted all of our retry attempts.

The corresponding Spring configuration is:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:aspectj-autoproxy/>

	<bean id="concurrentOperationExecutor" class="com.xyz.myapp.service.impl.ConcurrentOperationExecutor">
		<property name="maxRetries" value="3"/>
		<property name="order" value="100"/>
	</bean>
----

To refine the aspect so that it only retries idempotent operations, we might define an
`Idempotent` annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Retention(RetentionPolicy.RUNTIME)
	public @interface Idempotent {
		// marker annotation
	}
----

and use the annotation to annotate the implementation of service operations. The change
to the aspect to only retry idempotent operations simply involves refining the pointcut
expression so that only `@Idempotent` operations match:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Around("com.xyz.myapp.SystemArchitecture.businessService() && " +
			"@annotation(com.xyz.myapp.service.Idempotent)")
	public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable {
		...
	}
----




[[aop-schema]]
== Schema-based AOP support

If you prefer an XML-based format, then Spring also offers support for defining aspects
using the new "aop" namespace tags. The exact same pointcut expressions and advice kinds
are supported as when using the @AspectJ style, hence in this section we will focus on
the new __syntax__ and refer the reader to the discussion in the previous section
(<<aop-ataspectj>>) for an understanding of writing pointcut expressions and the binding
of advice parameters.

To use the aop namespace tags described in this section, you need to import the
`spring-aop` schema as described in <<core.adoc#xsd-schemas,
XML Schema-based configuration>>.
See <<core.adoc#xsd-schemas-aop, the AOP schema>>
for how to import the tags in the `aop` namespace.

Within your Spring configurations, all aspect and advisor elements must be placed within
an `<aop:config>` element (you can have more than one `<aop:config>` element in an
application context configuration). An `<aop:config>` element can contain pointcut,
advisor, and aspect elements (note these must be declared in that order).

[WARNING]
====
The `<aop:config>` style of configuration makes heavy use of Spring's
<<aop-autoproxy,auto-proxying>> mechanism. This can cause issues (such as advice not
being woven) if you are already using explicit auto-proxying via the use of
`BeanNameAutoProxyCreator` or suchlike. The recommended usage pattern is to use either
just the `<aop:config>` style, or just the `AutoProxyCreator` style.
====



[[aop-schema-declaring-an-aspect]]
=== Declaring an aspect

Using the schema support, an aspect is simply a regular Java object defined as a bean in
your Spring application context. The state and behavior is captured in the fields and
methods of the object, and the pointcut and advice information is captured in the XML.

An aspect is declared using the <aop:aspect> element, and the backing bean is referenced
using the `ref` attribute:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:config>
		<aop:aspect id="myAspect" ref="aBean">
			...
		</aop:aspect>
	</aop:config>

	<bean id="aBean" class="...">
		...
	</bean>
----

The bean backing the aspect (`"aBean"` in this case) can of course be configured and
dependency injected just like any other Spring bean.



[[aop-schema-pointcuts]]
=== Declaring a pointcut

A named pointcut can be declared inside an <aop:config> element, enabling the pointcut
definition to be shared across several aspects and advisors.

A pointcut representing the execution of any business service in the service layer could
be defined as follows:

[source,xml,indent=0]
[subs="verbatim"]
----
	<aop:config>

		<aop:pointcut id="businessService"
			expression="execution(* com.xyz.myapp.service.*.*(..))"/>

	</aop:config>
----

Note that the pointcut expression itself is using the same AspectJ pointcut expression
language as described in <<aop-ataspectj>>. If you are using the schema based
declaration style, you can refer to named pointcuts defined in types
(@Aspects) within the pointcut expression. Another way of defining the above pointcut
would be:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:config>

		<aop:pointcut id="businessService"
			expression="com.xyz.myapp.SystemArchitecture.businessService()"/>

	</aop:config>
----

Assuming you have a `SystemArchitecture` aspect as described in <<aop-common-pointcuts>>.

Declaring a pointcut inside an aspect is very similar to declaring a top-level pointcut:

[source,xml,indent=0]
[subs="verbatim"]
----
	<aop:config>

		<aop:aspect id="myAspect" ref="aBean">

			<aop:pointcut id="businessService"
				expression="execution(* com.xyz.myapp.service.*.*(..))"/>

			...

		</aop:aspect>

	</aop:config>
----

Much the same way in an @AspectJ aspect, pointcuts declared using the schema based
definition style may collect join point context. For example, the following pointcut
collects the 'this' object as the join point context and passes it to advice:

[source,xml,indent=0]
[subs="verbatim"]
----
	<aop:config>

		<aop:aspect id="myAspect" ref="aBean">

			<aop:pointcut id="businessService"
				expression="execution(* com.xyz.myapp.service.*.*(..)) &amp;&amp; this(service)"/>

			<aop:before pointcut-ref="businessService" method="monitor"/>

			...

		</aop:aspect>

	</aop:config>
----

The advice must be declared to receive the collected join point context by including
parameters of the matching names:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public void monitor(Object service) {
		...
	}
----

When combining pointcut sub-expressions, `&&` is awkward within an XML document, and so
the keywords `and`, `or`, and `not` can be used in place of `&&`, `||`, and `!`
respectively. For example, the previous pointcut may be better written as:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:config>

		<aop:aspect id="myAspect" ref="aBean">

			<aop:pointcut id="businessService"
				expression="execution(* com.xyz.myapp.service.*.*(..)) **and** this(service)"/>

			<aop:before pointcut-ref="businessService" method="monitor"/>

			...
		</aop:aspect>
	</aop:config>
----

Note that pointcuts defined in this way are referred to by their XML id and cannot be
used as named pointcuts to form composite pointcuts. The named pointcut support in the
schema based definition style is thus more limited than that offered by the @AspectJ
style.



[[aop-schema-advice]]
=== Declaring advice

The same five advice kinds are supported as for the @AspectJ style, and they have
exactly the same semantics.


[[aop-schema-advice-before]]
==== Before advice

Before advice runs before a matched method execution. It is declared inside an
`<aop:aspect>` using the <aop:before> element.

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:aspect id="beforeExample" ref="aBean">

		<aop:before
			pointcut-ref="dataAccessOperation"
			method="doAccessCheck"/>

		...

	</aop:aspect>
----

Here `dataAccessOperation` is the id of a pointcut defined at the top ( `<aop:config>`)
level. To define the pointcut inline instead, replace the `pointcut-ref` attribute with
a `pointcut` attribute:

[source,xml,indent=0]
[subs="verbatim"]
----
	<aop:aspect id="beforeExample" ref="aBean">

		<aop:before
			pointcut="execution(* com.xyz.myapp.dao.*.*(..))"
			method="doAccessCheck"/>

		...

	</aop:aspect>
----

As we noted in the discussion of the @AspectJ style, using named pointcuts can
significantly improve the readability of your code.

The method attribute identifies a method ( `doAccessCheck`) that provides the body of
the advice. This method must be defined for the bean referenced by the aspect element
containing the advice. Before a data access operation is executed (a method execution
join point matched by the pointcut expression), the "doAccessCheck" method on the aspect
bean will be invoked.


[[aop-schema-advice-after-returning]]
==== After returning advice

After returning advice runs when a matched method execution completes normally. It is
declared inside an `<aop:aspect>` in the same way as before advice. For example:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:aspect id="afterReturningExample" ref="aBean">

		<aop:after-returning
			pointcut-ref="dataAccessOperation"
			method="doAccessCheck"/>

		...

	</aop:aspect>
----

Just as in the @AspectJ style, it is possible to get hold of the return value within the
advice body. Use the returning attribute to specify the name of the parameter to which
the return value should be passed:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:aspect id="afterReturningExample" ref="aBean">

		<aop:after-returning
			pointcut-ref="dataAccessOperation"
			returning="retVal"
			method="doAccessCheck"/>

		...

	</aop:aspect>
----

The doAccessCheck method must declare a parameter named `retVal`. The type of this
parameter constrains matching in the same way as described for @AfterReturning. For
example, the method signature may be declared as:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public void doAccessCheck(Object retVal) {...
----


[[aop-schema-advice-after-throwing]]
==== After throwing advice

After throwing advice executes when a matched method execution exits by throwing an
exception. It is declared inside an `<aop:aspect>` using the after-throwing element:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:aspect id="afterThrowingExample" ref="aBean">

		<aop:after-throwing
			pointcut-ref="dataAccessOperation"
			method="doRecoveryActions"/>

		...

	</aop:aspect>
----

Just as in the @AspectJ style, it is possible to get hold of the thrown exception within
the advice body. Use the throwing attribute to specify the name of the parameter to
which the exception should be passed:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:aspect id="afterThrowingExample" ref="aBean">

		<aop:after-throwing
			pointcut-ref="dataAccessOperation"
			throwing="dataAccessEx"
			method="doRecoveryActions"/>

		...

	</aop:aspect>
----

The doRecoveryActions method must declare a parameter named `dataAccessEx`. The type of
this parameter constrains matching in the same way as described for @AfterThrowing. For
example, the method signature may be declared as:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public void doRecoveryActions(DataAccessException dataAccessEx) {...
----


[[aop-schema-advice-after-finally]]
==== After (finally) advice

After (finally) advice runs however a matched method execution exits. It is declared
using the `after` element:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:aspect id="afterFinallyExample" ref="aBean">

		<aop:after
			pointcut-ref="dataAccessOperation"
			method="doReleaseLock"/>

		...

	</aop:aspect>
----


[[aop-schema-advice-around]]
==== Around advice

The final kind of advice is around advice. Around advice runs "around" a matched method
execution. It has the opportunity to do work both before and after the method executes,
and to determine when, how, and even if, the method actually gets to execute at all.
Around advice is often used if you need to share state before and after a method
execution in a thread-safe manner (starting and stopping a timer for example). Always
use the least powerful form of advice that meets your requirements; don't use around
advice if simple before advice would do.

Around advice is declared using the `aop:around` element. The first parameter of the
advice method must be of type `ProceedingJoinPoint`. Within the body of the advice,
calling `proceed()` on the `ProceedingJoinPoint` causes the underlying method to
execute. The `proceed` method may also be calling passing in an `Object[]` - the values
in the array will be used as the arguments to the method execution when it proceeds. See
<<aop-ataspectj-around-advice>> for notes on calling proceed with an `Object[]`.

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:aspect id="aroundExample" ref="aBean">

		<aop:around
			pointcut-ref="businessService"
			method="doBasicProfiling"/>

		...

	</aop:aspect>
----

The implementation of the `doBasicProfiling` advice would be exactly the same as in the
@AspectJ example (minus the annotation of course):

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable {
		// start stopwatch
		Object retVal = pjp.proceed();
		// stop stopwatch
		return retVal;
	}
----


[[aop-schema-params]]
==== Advice parameters

The schema based declaration style supports fully typed advice in the same way as
described for the @AspectJ support - by matching pointcut parameters by name against
advice method parameters. See <<aop-ataspectj-advice-params>> for details. If you wish
to explicitly specify argument names for the advice methods (not relying on the
detection strategies previously described) then this is done using the `arg-names`
attribute of the advice element, which is treated in the same manner to the "argNames"
attribute in an advice annotation as described in <<aop-ataspectj-advice-params-names>>.
For example:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:before
		pointcut="com.xyz.lib.Pointcuts.anyPublicMethod() and @annotation(auditable)"
		method="audit"
		arg-names="auditable"/>
----

The `arg-names` attribute accepts a comma-delimited list of parameter names.

Find below a slightly more involved example of the XSD-based approach that illustrates
some around advice used in conjunction with a number of strongly typed parameters.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	package x.y.service;

	public interface FooService {

		Foo getFoo(String fooName, int age);
	}

	public class DefaultFooService implements FooService {

		public Foo getFoo(String name, int age) {
			return new Foo(name, age);
		}
	}
----

Next up is the aspect. Notice the fact that the `profile(..)` method accepts a number of
strongly-typed parameters, the first of which happens to be the join point used to
proceed with the method call: the presence of this parameter is an indication that the
`profile(..)` is to be used as `around` advice:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	package x.y;

	import org.aspectj.lang.ProceedingJoinPoint;
	import org.springframework.util.StopWatch;

	public class SimpleProfiler {

		public Object profile(ProceedingJoinPoint call, String name, int age) throws Throwable {
			StopWatch clock = new StopWatch("Profiling for '" + name + "' and '" + age + "'");
			try {
				clock.start(call.toShortString());
				return call.proceed();
			} finally {
				clock.stop();
				System.out.println(clock.prettyPrint());
			}
		}
	}
----

Finally, here is the XML configuration that is required to effect the execution of the
above advice for a particular join point:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:aop="http://www.springframework.org/schema/aop"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd">

		<!-- this is the object that will be proxied by Spring's AOP infrastructure -->
		<bean id="fooService" class="x.y.service.DefaultFooService"/>

		<!-- this is the actual advice itself -->
		<bean id="profiler" class="x.y.SimpleProfiler"/>

		<aop:config>
			<aop:aspect ref="profiler">

				<aop:pointcut id="theExecutionOfSomeFooServiceMethod"
					expression="execution(* x.y.service.FooService.getFoo(String,int))
					and args(name, age)"/>

				<aop:around pointcut-ref="theExecutionOfSomeFooServiceMethod"
					method="profile"/>

			</aop:aspect>
		</aop:config>

	</beans>
----

If we had the following driver script, we would get output something like this on
standard output:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	import org.springframework.beans.factory.BeanFactory;
	import org.springframework.context.support.ClassPathXmlApplicationContext;
	import x.y.service.FooService;

	public final class Boot {

		public static void main(final String[] args) throws Exception {
			BeanFactory ctx = new ClassPathXmlApplicationContext("x/y/plain.xml");
			FooService foo = (FooService) ctx.getBean("fooService");
			foo.getFoo("Pengo", 12);
		}
	}
----

[literal]
[subs="verbatim,quotes"]
----
StopWatch 'Profiling for 'Pengo' and '12'': running time (millis) = 0
-----------------------------------------
ms     %     Task name
-----------------------------------------
00000  ?  execution(getFoo)
----


[[aop-ordering]]
==== Advice ordering

When multiple advice needs to execute at the same join point (executing method) the
ordering rules are as described in <<aop-ataspectj-advice-ordering>>. The precedence
between aspects is determined by either adding the `Order` annotation to the bean
backing the aspect or by having the bean implement the `Ordered` interface.



[[aop-schema-introductions]]
=== Introductions

Introductions (known as inter-type declarations in AspectJ) enable an aspect to declare
that advised objects implement a given interface, and to provide an implementation of
that interface on behalf of those objects.

An introduction is made using the `aop:declare-parents` element inside an `aop:aspect`
This element is used to declare that matching types have a new parent (hence the name).
For example, given an interface `UsageTracked`, and an implementation of that interface
`DefaultUsageTracked`, the following aspect declares that all implementors of service
interfaces also implement the `UsageTracked` interface. (In order to expose statistics
via JMX for example.)

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:aspect id="usageTrackerAspect" ref="usageTracking">

		<aop:declare-parents
			types-matching="com.xzy.myapp.service.*+"
			implement-interface="com.xyz.myapp.service.tracking.UsageTracked"
			default-impl="com.xyz.myapp.service.tracking.DefaultUsageTracked"/>

		<aop:before
			pointcut="com.xyz.myapp.SystemArchitecture.businessService()
				and this(usageTracked)"
				method="recordUsage"/>

	</aop:aspect>
----

The class backing the `usageTracking` bean would contain the method:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public void recordUsage(UsageTracked usageTracked) {
		usageTracked.incrementUseCount();
	}
----

The interface to be implemented is determined by `implement-interface` attribute. The
value of the `types-matching` attribute is an AspectJ type pattern :- any bean of a
matching type will implement the `UsageTracked` interface. Note that in the before
advice of the above example, service beans can be directly used as implementations of
the `UsageTracked` interface. If accessing a bean programmatically you would write the
following:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	UsageTracked usageTracked = (UsageTracked) context.getBean("myService");
----



[[aop-schema-instatiation-models]]
=== Aspect instantiation models

The only supported instantiation model for schema-defined aspects is the singleton
model. Other instantiation models may be supported in future releases.



[[aop-schema-advisors]]
=== Advisors

The concept of "advisors" is brought forward from the AOP support defined in Spring
and does not have a direct equivalent in AspectJ. An advisor is like a small
self-contained aspect that has a single piece of advice. The advice itself is
represented by a bean, and must implement one of the advice interfaces described in
<<aop-api-advice-types>>. Advisors can take advantage of AspectJ pointcut expressions
though.

Spring supports the advisor concept with the `<aop:advisor>` element. You will most
commonly see it used in conjunction with transactional advice, which also has its own
namespace support in Spring. Here's how it looks:

[source,xml,indent=0]
[subs="verbatim"]
----
	<aop:config>

		<aop:pointcut id="businessService"
			expression="execution(* com.xyz.myapp.service.*.*(..))"/>

		<aop:advisor
			pointcut-ref="businessService"
			advice-ref="tx-advice"/>

	</aop:config>

	<tx:advice id="tx-advice">
		<tx:attributes>
			<tx:method name="*" propagation="REQUIRED"/>
		</tx:attributes>
	</tx:advice>
----

As well as the `pointcut-ref` attribute used in the above example, you can also use the
`pointcut` attribute to define a pointcut expression inline.

To define the precedence of an advisor so that the advice can participate in ordering,
use the `order` attribute to define the `Ordered` value of the advisor.



[[aop-schema-example]]
=== Example

Let's see how the concurrent locking failure retry example from
<<aop-ataspectj-example>> looks when rewritten using the schema support.

The execution of business services can sometimes fail due to concurrency issues (for
example, deadlock loser). If the operation is retried, it is quite likely it will
succeed next time round. For business services where it is appropriate to retry in such
conditions (idempotent operations that don't need to go back to the user for conflict
resolution), we'd like to transparently retry the operation to avoid the client seeing a
`PessimisticLockingFailureException`. This is a requirement that clearly cuts across
multiple services in the service layer, and hence is ideal for implementing via an
aspect.

Because we want to retry the operation, we'll need to use around advice so that we can
call proceed multiple times. Here's how the basic aspect implementation looks (it's just
a regular Java class using the schema support):

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class ConcurrentOperationExecutor implements Ordered {

		private static final int DEFAULT_MAX_RETRIES = 2;

		private int maxRetries = DEFAULT_MAX_RETRIES;
		private int order = 1;

		public void setMaxRetries(int maxRetries) {
			this.maxRetries = maxRetries;
		}

		public int getOrder() {
			return this.order;
		}

		public void setOrder(int order) {
			this.order = order;
		}

		public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable {
			int numAttempts = 0;
			PessimisticLockingFailureException lockFailureException;
			do {
				numAttempts++;
				try {
					return pjp.proceed();
				}
				catch(PessimisticLockingFailureException ex) {
					lockFailureException = ex;
				}
			} while(numAttempts <= this.maxRetries);
			throw lockFailureException;
		}

	}
----

Note that the aspect implements the `Ordered` interface so we can set the precedence of
the aspect higher than the transaction advice (we want a fresh transaction each time we
retry). The `maxRetries` and `order` properties will both be configured by Spring. The
main action happens in the `doConcurrentOperation` around advice method. We try to
proceed, and if we fail with a `PessimisticLockingFailureException` we simply try again
unless we have exhausted all of our retry attempts.

[NOTE]
====
This class is identical to the one used in the @AspectJ example, but with the
annotations removed.
====

The corresponding Spring configuration is:

[source,xml,indent=0]
[subs="verbatim"]
----
	<aop:config>

		<aop:aspect id="concurrentOperationRetry" ref="concurrentOperationExecutor">

			<aop:pointcut id="idempotentOperation"
				expression="execution(* com.xyz.myapp.service.*.*(..))"/>

			<aop:around
				pointcut-ref="idempotentOperation"
				method="doConcurrentOperation"/>

		</aop:aspect>

	</aop:config>

	<bean id="concurrentOperationExecutor"
		class="com.xyz.myapp.service.impl.ConcurrentOperationExecutor">
			<property name="maxRetries" value="3"/>
			<property name="order" value="100"/>
	</bean>
----

Notice that for the time being we assume that all business services are idempotent. If
this is not the case we can refine the aspect so that it only retries genuinely
idempotent operations, by introducing an `Idempotent` annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Retention(RetentionPolicy.RUNTIME)
	public @interface Idempotent {
		// marker annotation
	}
----

and using the annotation to annotate the implementation of service operations. The
change to the aspect to retry only idempotent operations simply involves refining the
pointcut expression so that only `@Idempotent` operations match:

[source,xml,indent=0]
[subs="verbatim"]
----
	<aop:pointcut id="idempotentOperation"
			expression="execution(* com.xyz.myapp.service.*.*(..)) and
			@annotation(com.xyz.myapp.service.Idempotent)"/>
----




[[aop-choosing]]
== Choosing which AOP declaration style to use

Once you have decided that an aspect is the best approach for implementing a given
requirement, how do you decide between using Spring AOP or AspectJ, and between the
Aspect language (code) style, @AspectJ annotation style, or the Spring XML style? These
decisions are influenced by a number of factors including application requirements,
development tools, and team familiarity with AOP.



[[aop-spring-or-aspectj]]
=== Spring AOP or full AspectJ?

Use the simplest thing that can work. Spring AOP is simpler than using full AspectJ as
there is no requirement to introduce the AspectJ compiler / weaver into your development
and build processes. If you only need to advise the execution of operations on Spring
beans, then Spring AOP is the right choice. If you need to advise objects not managed by
the Spring container (such as domain objects typically), then you will need to use
AspectJ. You will also need to use AspectJ if you wish to advise join points other than
simple method executions (for example, field get or set join points, and so on).

When using AspectJ, you have the choice of the AspectJ language syntax (also known as
the "code style") or the @AspectJ annotation style. Clearly, if you are not using Java
5+ then the choice has been made for you... use the code style. If aspects play a large
role in your design, and you are able to use the http://www.eclipse.org/ajdt/[AspectJ
Development Tools (AJDT)] plugin for Eclipse, then the AspectJ language syntax is the
preferred option: it is cleaner and simpler because the language was purposefully
designed for writing aspects. If you are not using Eclipse, or have only a few aspects
that do not play a major role in your application, then you may want to consider using
the @AspectJ style and sticking with a regular Java compilation in your IDE, and adding
an aspect weaving phase to your build script.



[[aop-ataspectj-or-xml]]
=== @AspectJ or XML for Spring AOP?

If you have chosen to use Spring AOP, then you have a choice of @AspectJ or XML style.
There are various tradeoffs to consider.

The XML style will be most familiar to existing Spring users and it is backed by genuine
POJOs. When using AOP as a tool to configure enterprise services then XML can be a good
choice (a good test is whether you consider the pointcut expression to be a part of your
configuration you might want to change independently). With the XML style arguably it is
clearer from your configuration what aspects are present in the system.

The XML style has two disadvantages. Firstly it does not fully encapsulate the
implementation of the requirement it addresses in a single place. The DRY principle says
that there should be a single, unambiguous, authoritative representation of any piece of
knowledge within a system. When using the XML style, the knowledge of __how__ a
requirement is implemented is split across the declaration of the backing bean class,
and the XML in the configuration file. When using the @AspectJ style there is a single
module - the aspect - in which this information is encapsulated. Secondly, the XML style
is slightly more limited in what it can express than the @AspectJ style: only the
"singleton" aspect instantiation model is supported, and it is not possible to combine
named pointcuts declared in XML. For example, in the @AspectJ style you can write
something like:

[source,java,indent=0]
[subs="verbatim"]
----
	@Pointcut(execution(* get*()))
	public void propertyAccess() {}

	@Pointcut(execution(org.xyz.Account+ *(..))
	public void operationReturningAnAccount() {}

	@Pointcut(propertyAccess() && operationReturningAnAccount())
	public void accountPropertyAccess() {}
----

In the XML style I can declare the first two pointcuts:

[source,xml,indent=0]
[subs="verbatim"]
----
	<aop:pointcut id="propertyAccess"
			expression="execution(* get*())"/>

	<aop:pointcut id="operationReturningAnAccount"
			expression="execution(org.xyz.Account+ *(..))"/>
----

The downside of the XML approach is that you cannot define the
`accountPropertyAccess` pointcut by combining these definitions.

The @AspectJ style supports additional instantiation models, and richer pointcut
composition. It has the advantage of keeping the aspect as a modular unit. It also has
the advantage the @AspectJ aspects can be understood (and thus consumed) both by Spring
AOP and by AspectJ - so if you later decide you need the capabilities of AspectJ to
implement additional requirements then it is very easy to migrate to an AspectJ-based
approach. On balance the Spring team prefer the @AspectJ style whenever you have aspects
that do more than simple "configuration" of enterprise services.




[[aop-mixing-styles]]
== Mixing aspect types

It is perfectly possible to mix @AspectJ style aspects using the autoproxying support,
schema-defined `<aop:aspect>` aspects, `<aop:advisor>` declared advisors and even
proxies and interceptors defined using the Spring 1.2 style in the same configuration.
All of these are implemented using the same underlying support mechanism and will
co-exist without any difficulty.




[[aop-proxying]]
== Proxying mechanisms

Spring AOP uses either JDK dynamic proxies or CGLIB to create the proxy for a given
target object. (JDK dynamic proxies are preferred whenever you have a choice).

If the target object to be proxied implements at least one interface then a JDK dynamic
proxy will be used. All of the interfaces implemented by the target type will be
proxied. If the target object does not implement any interfaces then a CGLIB proxy will
be created.

If you want to force the use of CGLIB proxying (for example, to proxy every method
defined for the target object, not just those implemented by its interfaces) you can do
so. However, there are some issues to consider:

* `final` methods cannot be advised, as they cannot be overridden.
* As of Spring 3.2, it is no longer necessary to add CGLIB to your project classpath, as
  CGLIB classes are repackaged under org.springframework and included directly in the
  spring-core JAR. This means that CGLIB-based proxy support 'just works' in the same
  way that JDK dynamic proxies always have.
* As of Spring 4.0, the constructor of your proxied object will NOT be called twice
  anymore since the CGLIB proxy instance will be created via Objenesis. Only if your
  JVM does not allow for constructor bypassing, you might see double invocations and
  corresponding debug log entries from Spring's AOP support.

To force the use of CGLIB proxies set the value of the `proxy-target-class` attribute of
the `<aop:config>` element to true:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:config proxy-target-class="true">
		<!-- other beans defined here... -->
	</aop:config>
----

To force CGLIB proxying when using the @AspectJ autoproxy support, set the
`'proxy-target-class'` attribute of the `<aop:aspectj-autoproxy>` element to `true`:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:aspectj-autoproxy proxy-target-class="true"/>
----

[NOTE]
====
Multiple `<aop:config/>` sections are collapsed into a single unified auto-proxy creator
at runtime, which applies the __strongest__ proxy settings that any of the
`<aop:config/>` sections (typically from different XML bean definition files) specified.
This also applies to the `<tx:annotation-driven/>` and `<aop:aspectj-autoproxy/>`
elements.

To be clear: using `proxy-target-class="true"` on `<tx:annotation-driven/>`,
`<aop:aspectj-autoproxy/>` or `<aop:config/>` elements will force the use of CGLIB
proxies __for all three of them__.
====



[[aop-understanding-aop-proxies]]
=== Understanding AOP proxies

Spring AOP is __proxy-based__. It is vitally important that you grasp the semantics of
what that last statement actually means before you write your own aspects or use any of
the Spring AOP-based aspects supplied with the Spring Framework.

Consider first the scenario where you have a plain-vanilla, un-proxied,
nothing-special-about-it, straight object reference, as illustrated by the following
code snippet.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class SimplePojo implements Pojo {

		public void foo() {
			// this next method invocation is a direct call on the 'this' reference
			this.bar();
		}

		public void bar() {
			// some logic...
		}
	}
----

If you invoke a method on an object reference, the method is invoked __directly__ on
that object reference, as can be seen below.

image::images/aop-proxy-plain-pojo-call.png[]

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class Main {

		public static void main(String[] args) {

			Pojo pojo = new SimplePojo();

			// this is a direct method call on the 'pojo' reference
			pojo.foo();
		}
	}
----

Things change slightly when the reference that client code has is a proxy. Consider the
following diagram and code snippet.

image::images/aop-proxy-call.png[]

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class Main {

		public static void main(String[] args) {

			ProxyFactory factory = new ProxyFactory(new SimplePojo());
			factory.addInterface(Pojo.class);
			factory.addAdvice(new RetryAdvice());

			Pojo pojo = (Pojo) factory.getProxy();

			// this is a method call on the proxy!
			pojo.foo();
		}
	}
----

The key thing to understand here is that the client code inside the `main(..)` of the
`Main` class __has a reference to the proxy__. This means that method calls on that
object reference will be calls on the proxy, and as such the proxy will be able to
delegate to all of the interceptors (advice) that are relevant to that particular method
call. However, once the call has finally reached the target object, the `SimplePojo`
reference in this case, any method calls that it may make on itself, such as
`this.bar()` or `this.foo()`, are going to be invoked against the __this__ reference,
and __not__ the proxy. This has important implications. It means that self-invocation is
__not__ going to result in the advice associated with a method invocation getting a
chance to execute.

Okay, so what is to be done about this? The best approach (the term best is used loosely
here) is to refactor your code such that the self-invocation does not happen. For sure,
this does entail some work on your part, but it is the best, least-invasive approach.
The next approach is absolutely horrendous, and I am almost reticent to point it out
precisely because it is so horrendous. You can (choke!) totally tie the logic within
your class to Spring AOP by doing this:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class SimplePojo implements Pojo {

		public void foo() {
			// this works, but... gah!
			((Pojo) AopContext.currentProxy()).bar();
		}

		public void bar() {
			// some logic...
		}
	}
----

This totally couples your code to Spring AOP, __and__ it makes the class itself aware of
the fact that it is being used in an AOP context, which flies in the face of AOP. It
also requires some additional configuration when the proxy is being created:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public class Main {

		public static void main(String[] args) {

			ProxyFactory factory = new ProxyFactory(new SimplePojo());
			factory.adddInterface(Pojo.class);
			factory.addAdvice(new RetryAdvice());
			factory.setExposeProxy(true);

			Pojo pojo = (Pojo) factory.getProxy();

			// this is a method call on the proxy!
			pojo.foo();
		}
	}
----

Finally, it must be noted that AspectJ does not have this self-invocation issue because
it is not a proxy-based AOP framework.




[[aop-aspectj-programmatic]]
== Programmatic creation of @AspectJ Proxies

In addition to declaring aspects in your configuration using either `<aop:config>` or
`<aop:aspectj-autoproxy>`, it is also possible programmatically to create proxies that
advise target objects. For the full details of Spring's AOP API, see the next chapter.
Here we want to focus on the ability to automatically create proxies using @AspectJ
aspects.

The class `org.springframework.aop.aspectj.annotation.AspectJProxyFactory` can be used
to create a proxy for a target object that is advised by one or more @AspectJ aspects.
Basic usage for this class is very simple, as illustrated below. See the javadocs for
full information.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	// create a factory that can generate a proxy for the given target object
	AspectJProxyFactory factory = new AspectJProxyFactory(targetObject);

	// add an aspect, the class must be an @AspectJ aspect
	// you can call this as many times as you need with different aspects
	factory.addAspect(SecurityManager.class);

	// you can also add existing aspect instances, the type of the object supplied must be an @AspectJ aspect
	factory.addAspect(usageTracker);

	// now get the proxy object...
	MyInterfaceType proxy = factory.getProxy();
----




[[aop-using-aspectj]]
== Using AspectJ with Spring applications

Everything we've covered so far in this chapter is pure Spring AOP. In this section,
we're going to look at how you can use the AspectJ compiler/weaver instead of, or in
addition to, Spring AOP if your needs go beyond the facilities offered by Spring AOP
alone.

Spring ships with a small AspectJ aspect library, which is available standalone in your
distribution as `spring-aspects.jar`; you'll need to add this to your classpath in order
to use the aspects in it. <<aop-atconfigurable>> and <<aop-ajlib-other>> discuss the
content of this library and how you can use it. <<aop-aj-configure>> discusses how to
dependency inject AspectJ aspects that are woven using the AspectJ compiler. Finally,
<<aop-aj-ltw>> provides an introduction to load-time weaving for Spring applications
using AspectJ.



[[aop-atconfigurable]]
=== Using AspectJ to dependency inject domain objects with Spring

The Spring container instantiates and configures beans defined in your application
context. It is also possible to ask a bean factory to configure a __pre-existing__
object given the name of a bean definition containing the configuration to be applied.
The `spring-aspects.jar` contains an annotation-driven aspect that exploits this
capability to allow dependency injection of __any object__. The support is intended to
be used for objects created __outside of the control of any container__. Domain objects
often fall into this category because they are often created programmatically using the
`new` operator, or by an ORM tool as a result of a database query.

The `@Configurable` annotation marks a class as eligible for Spring-driven
configuration. In the simplest case it can be used just as a marker annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	package com.xyz.myapp.domain;

	import org.springframework.beans.factory.annotation.Configurable;

	@Configurable
	public class Account {
		// ...
	}
----

When used as a marker interface in this way, Spring will configure new instances of the
annotated type ( `Account` in this case) using a bean definition (typically
prototype-scoped) with the same name as the fully-qualified type name (
`com.xyz.myapp.domain.Account`). Since the default name for a bean is the
fully-qualified name of its type, a convenient way to declare the prototype definition
is simply to omit the `id` attribute:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean class="com.xyz.myapp.domain.Account" scope="prototype">
		<property name="fundsTransferService" ref="fundsTransferService"/>
	</bean>
----

If you want to explicitly specify the name of the prototype bean definition to use, you
can do so directly in the annotation:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	package com.xyz.myapp.domain;

	import org.springframework.beans.factory.annotation.Configurable;

	@Configurable("account")
	public class Account {
		// ...
	}
----

Spring will now look for a bean definition named "account" and use that as the
definition to configure new `Account` instances.

You can also use autowiring to avoid having to specify a dedicated bean definition at
all. To have Spring apply autowiring use the `autowire` property of the
`@Configurable` annotation: specify either `@Configurable(autowire=Autowire.BY_TYPE)` or
`@Configurable(autowire=Autowire.BY_NAME` for autowiring by type or by name
respectively. As an alternative, as of Spring 2.5 it is preferable to specify explicit,
annotation-driven dependency injection for your `@Configurable` beans by using
`@Autowired` or `@Inject` at the field or method level (see <<beans-annotation-config>>
for further details).

Finally you can enable Spring dependency checking for the object references in the newly
created and configured object by using the `dependencyCheck` attribute (for example:
`@Configurable(autowire=Autowire.BY_NAME,dependencyCheck=true)`). If this attribute is
set to true, then Spring will validate after configuration that all properties (__which
are not primitives or collections__) have been set.

Using the annotation on its own does nothing of course. It is the
`AnnotationBeanConfigurerAspect` in `spring-aspects.jar` that acts on the presence of
the annotation. In essence the aspect says "after returning from the initialization of a
new object of a type annotated with `@Configurable`, configure the newly created object
using Spring in accordance with the properties of the annotation". In this context,
__initialization__ refers to newly instantiated objects (e.g., objects instantiated with
the `new` operator) as well as to `Serializable` objects that are undergoing
deserialization (e.g., via
http://docs.oracle.com/javase/6/docs/api/java/io/Serializable.html[readResolve()]).

[NOTE]
====
One of the key phrases in the above paragraph is '__in essence__'. For most cases, the
exact semantics of '__after returning from the initialization of a new object__' will be
fine... in this context, '__after initialization__' means that the dependencies will be
injected __after__ the object has been constructed - this means that the dependencies
will not be available for use in the constructor bodies of the class. If you want the
dependencies to be injected __before__ the constructor bodies execute, and thus be
available for use in the body of the constructors, then you need to define this on the
`@Configurable` declaration like so:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configurable(preConstruction=true)
----

You can find out more information about the language semantics of the various pointcut
types in AspectJ
http://www.eclipse.org/aspectj/doc/next/progguide/semantics-joinPoints.html[in this
appendix] of the http://www.eclipse.org/aspectj/doc/next/progguide/index.html[AspectJ
Programming Guide].
====

For this to work the annotated types must be woven with the AspectJ weaver - you can
either use a build-time Ant or Maven task to do this (see for example the
http://www.eclipse.org/aspectj/doc/released/devguide/antTasks.html[AspectJ Development
Environment Guide]) or load-time weaving (see <<aop-aj-ltw>>). The
`AnnotationBeanConfigurerAspect` itself needs configuring by Spring (in order to obtain
a reference to the bean factory that is to be used to configure new objects). If you are
using Java based configuration simply add `@EnableSpringConfigured` to any
`@Configuration` class.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableSpringConfigured
	public class AppConfig {

	}
----

If you prefer XML based configuration, the Spring
<<core.adoc#xsd-schemas-context, `context` namespace>>
defines a convenient `context:spring-configured` element:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<context:spring-configured/>
----

Instances of `@Configurable` objects created __before__ the aspect has been configured
will result in a message being issued to the debug log and no configuration of the
object taking place. An example might be a bean in the Spring configuration that creates
domain objects when it is initialized by Spring. In this case you can use the
"depends-on" bean attribute to manually specify that the bean depends on the
configuration aspect.

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="myService"
			class="com.xzy.myapp.service.MyService"
			depends-on="org.springframework.beans.factory.aspectj.AnnotationBeanConfigurerAspect">

		<!-- ... -->

	</bean>
----

[NOTE]
====
Do not activate `@Configurable` processing through the bean configurer aspect unless you
really mean to rely on its semantics at runtime. In particular, make sure that you do
not use `@Configurable` on bean classes which are registered as regular Spring beans
with the container: You would get double initialization otherwise, once through the
container and once through the aspect.
====


[[aop-configurable-testing]]
==== Unit testing @Configurable objects

One of the goals of the `@Configurable` support is to enable independent unit testing of
domain objects without the difficulties associated with hard-coded lookups. If
`@Configurable` types have not been woven by AspectJ then the annotation has no affect
during unit testing, and you can simply set mock or stub property references in the
object under test and proceed as normal. If `@Configurable` types __have__ been woven by
AspectJ then you can still unit test outside of the container as normal, but you will
see a warning message each time that you construct an `@Configurable` object indicating
that it has not been configured by Spring.


[[aop-configurable-container]]
==== Working with multiple application contexts

The `AnnotationBeanConfigurerAspect` used to implement the `@Configurable` support is an
AspectJ singleton aspect. The scope of a singleton aspect is the same as the scope of
`static` members, that is to say there is one aspect instance per classloader that
defines the type. This means that if you define multiple application contexts within the
same classloader hierarchy you need to consider where to define the
`@EnableSpringConfigured` bean and where to place `spring-aspects.jar` on the classpath.

Consider a typical Spring web-app configuration with a shared parent application context
defining common business services and everything needed to support them, and one child
application context per servlet containing definitions particular to that servlet. All
of these contexts will co-exist within the same classloader hierarchy, and so the
`AnnotationBeanConfigurerAspect` can only hold a reference to one of them. In this case
we recommend defining the `@EnableSpringConfigured` bean in the shared (parent)
application context: this defines the services that you are likely to want to inject
into domain objects. A consequence is that you cannot configure domain objects with
references to beans defined in the child (servlet-specific) contexts using the
@Configurable mechanism (probably not something you want to do anyway!).

When deploying multiple web-apps within the same container, ensure that each
web-application loads the types in `spring-aspects.jar` using its own classloader (for
example, by placing `spring-aspects.jar` in `'WEB-INF/lib'`). If `spring-aspects.jar` is
only added to the container wide classpath (and hence loaded by the shared parent
classloader), all web applications will share the same aspect instance which is probably
not what you want.



[[aop-ajlib-other]]
=== Other Spring aspects for AspectJ

In addition to the `@Configurable` aspect, `spring-aspects.jar` contains an AspectJ
aspect that can be used to drive Spring's transaction management for types and methods
annotated with the `@Transactional` annotation. This is primarily intended for users who
want to use the Spring Framework's transaction support outside of the Spring container.

The aspect that interprets `@Transactional` annotations is the
`AnnotationTransactionAspect`. When using this aspect, you must annotate the
__implementation__ class (and/or methods within that class), __not__ the interface (if
any) that the class implements. AspectJ follows Java's rule that annotations on
interfaces are __not inherited__.

A `@Transactional` annotation on a class specifies the default transaction semantics for
the execution of any __public__ operation in the class.

A `@Transactional` annotation on a method within the class overrides the default
transaction semantics given by the class annotation (if present). Methods of any
visibility may be annotated, including private methods. Annotating non-public methods
directly is the only way to get transaction demarcation for the execution of such methods.

[TIP]
====
Since Spring Framework 4.2, `spring-aspects` provides a similar aspect that offers the
exact same features for the standard `javax.transaction.Transactional` annotation. Check
`JtaAnnotationTransactionAspect` for more details.
====

For AspectJ programmers that want to use the Spring configuration and transaction
management support but don't want to (or cannot) use annotations, `spring-aspects.jar`
also contains `abstract` aspects you can extend to provide your own pointcut
definitions. See the sources for the `AbstractBeanConfigurerAspect` and
`AbstractTransactionAspect` aspects for more information. As an example, the following
excerpt shows how you could write an aspect to configure all instances of objects
defined in the domain model using prototype bean definitions that match the
fully-qualified class names:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	public aspect DomainObjectConfiguration extends AbstractBeanConfigurerAspect {

		public DomainObjectConfiguration() {
			setBeanWiringInfoResolver(new ClassNameBeanWiringInfoResolver());
		}

		// the creation of a new bean (any object in the domain model)
		protected pointcut beanCreation(Object beanInstance) :
			initialization(new(..)) &&
			SystemArchitecture.inDomainModel() &&
			this(beanInstance);

	}
----



[[aop-aj-configure]]
=== Configuring AspectJ aspects using Spring IoC

When using AspectJ aspects with Spring applications, it is natural to both want and
expect to be able to configure such aspects using Spring. The AspectJ runtime itself is
responsible for aspect creation, and the means of configuring the AspectJ created
aspects via Spring depends on the AspectJ instantiation model (the `per-xxx` clause)
used by the aspect.

The majority of AspectJ aspects are __singleton__ aspects. Configuration of these
aspects is very easy: simply create a bean definition referencing the aspect type as
normal, and include the bean attribute `'factory-method="aspectOf"'`. This ensures that
Spring obtains the aspect instance by asking AspectJ for it rather than trying to create
an instance itself. For example:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<bean id="profiler" class="com.xyz.profiler.Profiler"
			**factory-method="aspectOf"**>

		<property name="profilingStrategy" ref="jamonProfilingStrategy"/>
	</bean>
----

Non-singleton aspects are harder to configure: however it is possible to do so by
creating prototype bean definitions and using the `@Configurable` support from
`spring-aspects.jar` to configure the aspect instances once they have bean created by
the AspectJ runtime.

If you have some @AspectJ aspects that you want to weave with AspectJ (for example,
using load-time weaving for domain model types) and other @AspectJ aspects that you want
to use with Spring AOP, and these aspects are all configured using Spring, then you will
need to tell the Spring AOP @AspectJ autoproxying support which exact subset of the
@AspectJ aspects defined in the configuration should be used for autoproxying. You can
do this by using one or more `<include/>` elements inside the `<aop:aspectj-autoproxy/>`
declaration. Each `<include/>` element specifies a name pattern, and only beans with
names matched by at least one of the patterns will be used for Spring AOP autoproxy
configuration:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<aop:aspectj-autoproxy>
		<aop:include name="thisBean"/>
		<aop:include name="thatBean"/>
	</aop:aspectj-autoproxy>
----

[NOTE]
====
Do not be misled by the name of the `<aop:aspectj-autoproxy/>` element: using it will
result in the creation of __Spring AOP proxies__. The @AspectJ style of aspect
declaration is just being used here, but the AspectJ runtime is __not__ involved.
====



[[aop-aj-ltw]]
=== Load-time weaving with AspectJ in the Spring Framework

Load-time weaving (LTW) refers to the process of weaving AspectJ aspects into an
application's class files as they are being loaded into the Java virtual machine (JVM).
The focus of this section is on configuring and using LTW in the specific context of the
Spring Framework: this section is not an introduction to LTW though. For full details on
the specifics of LTW and configuring LTW with just AspectJ (with Spring not being
involved at all), see the
http://www.eclipse.org/aspectj/doc/released/devguide/ltw.html[LTW section of the AspectJ
Development Environment Guide].

The value-add that the Spring Framework brings to AspectJ LTW is in enabling much
finer-grained control over the weaving process. 'Vanilla' AspectJ LTW is effected using
a Java (5+) agent, which is switched on by specifying a VM argument when starting up a
JVM. It is thus a JVM-wide setting, which may be fine in some situations, but often is a
little too coarse. Spring-enabled LTW enables you to switch on LTW on a
__per-ClassLoader__ basis, which obviously is more fine-grained and which can make more
sense in a 'single-JVM-multiple-application' environment (such as is found in a typical
application server environment).

Further, <<aop-aj-ltw-environments,in certain environments>>, this support enables
load-time weaving __without making any modifications to the application server's launch
script__ that will be needed to add `-javaagent:path/to/aspectjweaver.jar` or (as we
describe later in this section)
`-javaagent:path/to/org.springframework.instrument-{version}.jar` (previously named
`spring-agent.jar`). Developers simply modify one or more files that form the
application context to enable load-time weaving instead of relying on administrators who
typically are in charge of the deployment configuration such as the launch script.

Now that the sales pitch is over, let us first walk through a quick example of AspectJ
LTW using Spring, followed by detailed specifics about elements introduced in the
following example. For a complete example, please see the
https://github.com/spring-projects/spring-petclinic[Petclinic sample application].


[[aop-aj-ltw-first-example]]
==== A first example

Let us assume that you are an application developer who has been tasked with diagnosing
the cause of some performance problems in a system. Rather than break out a profiling
tool, what we are going to do is switch on a simple profiling aspect that will enable us
to very quickly get some performance metrics, so that we can then apply a finer-grained
profiling tool to that specific area immediately afterwards.

[NOTE]
====
The example presented here uses XML style configuration, it is also possible to
configure and use @AspectJ with <<beans-java,Java Configuration>>.
Specifically the `@EnableLoadTimeWeaving` annotation can be used as an alternative to
`<context:load-time-weaver/>` (see <<aop-aj-ltw-spring,below>> for details).
====

Here is the profiling aspect. Nothing too fancy, just a quick-and-dirty time-based
profiler, using the @AspectJ-style of aspect declaration.

[source,java,indent=0]
[subs="verbatim"]
----
	package foo;

	import org.aspectj.lang.ProceedingJoinPoint;
	import org.aspectj.lang.annotation.Aspect;
	import org.aspectj.lang.annotation.Around;
	import org.aspectj.lang.annotation.Pointcut;
	import org.springframework.util.StopWatch;
	import org.springframework.core.annotation.Order;

	@Aspect
	public class ProfilingAspect {

		@Around("methodsToBeProfiled()")
		public Object profile(ProceedingJoinPoint pjp) throws Throwable {
			StopWatch sw = new StopWatch(getClass().getSimpleName());
			try {
				sw.start(pjp.getSignature().getName());
				return pjp.proceed();
			} finally {
				sw.stop();
				System.out.println(sw.prettyPrint());
			}
		}

		@Pointcut("execution(public * foo..*.*(..))")
		public void methodsToBeProfiled(){}
	}
----

We will also need to create an `META-INF/aop.xml` file, to inform the AspectJ weaver
that we want to weave our `ProfilingAspect` into our classes. This file convention,
namely the presence of a file (or files) on the Java classpath called
`META-INF/aop.xml` is standard AspectJ.

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<!DOCTYPE aspectj PUBLIC "-//AspectJ//DTD//EN" "http://www.eclipse.org/aspectj/dtd/aspectj.dtd">
	<aspectj>

		<weaver>
			<!-- only weave classes in our application-specific packages -->
			<include within="foo.*"/>
		</weaver>

		<aspects>
			<!-- weave in just this aspect -->
			<aspect name="foo.ProfilingAspect"/>
		</aspects>

	</aspectj>
----

Now to the Spring-specific portion of the configuration. We need to configure a
`LoadTimeWeaver` (all explained later, just take it on trust for now). This load-time
weaver is the essential component responsible for weaving the aspect configuration in
one or more `META-INF/aop.xml` files into the classes in your application. The good
thing is that it does not require a lot of configuration, as can be seen below (there
are some more options that you can specify, but these are detailed later).

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:context="http://www.springframework.org/schema/context"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/context
			http://www.springframework.org/schema/context/spring-context.xsd">

		<!-- a service object; we will be profiling its methods -->
		<bean id="entitlementCalculationService"
				class="foo.StubEntitlementCalculationService"/>

		<!-- this switches on the load-time weaving -->
		**<context:load-time-weaver/>**
	</beans>
----

Now that all the required artifacts are in place - the aspect, the `META-INF/aop.xml`
file, and the Spring configuration -, let us create a simple driver class with a
`main(..)` method to demonstrate the LTW in action.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	package foo;

	import org.springframework.context.support.ClassPathXmlApplicationContext;

	public final class Main {

		public static void main(String[] args) {

			ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml", Main.class);

			EntitlementCalculationService entitlementCalculationService
				= (EntitlementCalculationService) ctx.getBean("entitlementCalculationService");

			// the profiling aspect is 'woven' around this method execution
			entitlementCalculationService.calculateEntitlement();
		}
	}
----

There is one last thing to do. The introduction to this section did say that one could
switch on LTW selectively on a per- `ClassLoader` basis with Spring, and this is true.
However, just for this example, we are going to use a Java agent (supplied with Spring)
to switch on the LTW. This is the command line we will use to run the above `Main` class:

[literal]
[subs="verbatim,quotes"]
----
java -javaagent:C:/projects/foo/lib/global/spring-instrument.jar foo.Main
----

The `-javaagent` is a flag for specifying and enabling
http://docs.oracle.com/javase/6/docs/api/java/lang/instrument/package-summary.html[agents
to instrument programs running on the JVM]. The Spring Framework ships with such an
agent, the `InstrumentationSavingAgent`, which is packaged in the
`spring-instrument.jar` that was supplied as the value of the `-javaagent` argument in
the above example.

The output from the execution of the `Main` program will look something like that below.
(I have introduced a `Thread.sleep(..)` statement into the `calculateEntitlement()`
implementation so that the profiler actually captures something other than 0
milliseconds - the `01234` milliseconds is __not__ an overhead introduced by the AOP :) )

[literal]
[subs="verbatim,quotes"]
----
Calculating entitlement

StopWatch 'ProfilingAspect': running time (millis) = 1234
------ ----- ----------------------------
ms     %     Task name
------ ----- ----------------------------
01234  100%  calculateEntitlement
----

Since this LTW is effected using full-blown AspectJ, we are not just limited to advising
Spring beans; the following slight variation on the `Main` program will yield the same
result.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	package foo;

	import org.springframework.context.support.ClassPathXmlApplicationContext;

	public final class Main {

		public static void main(String[] args) {

			new ClassPathXmlApplicationContext("beans.xml", Main.class);

			EntitlementCalculationService entitlementCalculationService =
				new StubEntitlementCalculationService();

			// the profiling aspect will be 'woven' around this method execution
			entitlementCalculationService.calculateEntitlement();
		}
	}
----

Notice how in the above program we are simply bootstrapping the Spring container, and
then creating a new instance of the `StubEntitlementCalculationService` totally outside
the context of Spring... the profiling advice still gets woven in.

The example admittedly is simplistic... however the basics of the LTW support in Spring
have all been introduced in the above example, and the rest of this section will explain
the 'why' behind each bit of configuration and usage in detail.

[NOTE]
====
The `ProfilingAspect` used in this example may be basic, but it is quite useful. It is a
nice example of a development-time aspect that developers can use during development (of
course), and then quite easily exclude from builds of the application being deployed
into UAT or production.
====


[[aop-aj-ltw-the-aspects]]
==== Aspects

The aspects that you use in LTW have to be AspectJ aspects. They can be written in
either the AspectJ language itself or you can write your aspects in the @AspectJ-style.
It means that your aspects are then both valid AspectJ __and__ Spring AOP aspects.
Furthermore, the compiled aspect classes need to be available on the classpath.


[[aop-aj-ltw-aop_dot_xml]]
==== 'META-INF/aop.xml'

The AspectJ LTW infrastructure is configured using one or more `META-INF/aop.xml`
files, that are on the Java classpath (either directly, or more typically in jar files).

The structure and contents of this file is detailed in the main AspectJ reference
documentation, and the interested reader is
http://www.eclipse.org/aspectj/doc/released/devguide/ltw-configuration.html[referred to
that resource]. (I appreciate that this section is brief, but the `aop.xml` file is
100% AspectJ - there is no Spring-specific information or semantics that apply to it,
and so there is no extra value that I can contribute either as a result), so rather than
rehash the quite satisfactory section that the AspectJ developers wrote, I am just
directing you there.)


[[aop-aj-ltw-libraries]]
==== Required libraries (JARS)

At a minimum you will need the following libraries to use the Spring Framework's support
for AspectJ LTW:

* `spring-aop.jar` (version 2.5 or later, plus all mandatory dependencies)
* `aspectjweaver.jar` (version 1.6.8 or later)

If you are using the <<aop-aj-ltw-environment-generic,Spring-provided agent to enable
instrumentation>>, you will also need:

* `spring-instrument.jar`


[[aop-aj-ltw-spring]]
==== Spring configuration

The key component in Spring's LTW support is the `LoadTimeWeaver` interface (in the
`org.springframework.instrument.classloading` package), and the numerous implementations
of it that ship with the Spring distribution. A `LoadTimeWeaver` is responsible for
adding one or more `java.lang.instrument.ClassFileTransformers` to a `ClassLoader` at
runtime, which opens the door to all manner of interesting applications, one of which
happens to be the LTW of aspects.

[TIP]
====
If you are unfamiliar with the idea of runtime class file transformation, you are
encouraged to read the javadoc API documentation for the `java.lang.instrument` package
before continuing. This is not a huge chore because there is - rather annoyingly -
precious little documentation there... the key interfaces and classes will at least be
laid out in front of you for reference as you read through this section.
====

Configuring a `LoadTimeWeaver` for a particular `ApplicationContext` can be as easy as
adding one line. (Please note that you almost certainly will need to be using an
`ApplicationContext` as your Spring container - typically a `BeanFactory` will not be
enough because the LTW support makes use of `BeanFactoryPostProcessors`.)

To enable the Spring Framework's LTW support, you need to configure a `LoadTimeWeaver`,
which typically is done using the `@EnableLoadTimeWeaving` annotation.

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableLoadTimeWeaving
	public class AppConfig {

	}
----

Alternatively, if you prefer XML based configuration, use the
`<context:load-time-weaver/>` element. Note that the element is defined in the
`context` namespace.

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:context="http://www.springframework.org/schema/context"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/context
			http://www.springframework.org/schema/context/spring-context.xsd">

		<context:load-time-weaver/>

	</beans>
----

The above configuration will define and register a number of LTW-specific infrastructure
beans for you automatically, such as a `LoadTimeWeaver` and an `AspectJWeavingEnabler`.
The default `LoadTimeWeaver` is the `DefaultContextLoadTimeWeaver` class, which attempts
to decorate an automatically detected `LoadTimeWeaver`: the exact type of
`LoadTimeWeaver` that will be 'automatically detected' is dependent upon your runtime
environment (summarized in the following table).

[[aop-aj-ltw-spring-env-impls]]
.DefaultContextLoadTimeWeaver LoadTimeWeavers
|===
| Runtime Environment| `LoadTimeWeaver` implementation

| Running in Oracle's
  http://www.oracle.com/technetwork/middleware/weblogic/overview/index-085209.html[WebLogic]
| `WebLogicLoadTimeWeaver`

| Running in Oracle's http://glassfish.dev.java.net/[GlassFish]
| `GlassFishLoadTimeWeaver`

| Running in http://tomcat.apache.org/[Apache Tomcat]
| `TomcatLoadTimeWeaver`

| Running in Red Hat's http://www.jboss.org/jbossas/[JBoss AS] or http://www.wildfly.org/[WildFly]
| `JBossLoadTimeWeaver`

| Running in IBM's http://www-01.ibm.com/software/webservers/appserv/was/[WebSphere]
| `WebSphereLoadTimeWeaver`

| JVM started with Spring `InstrumentationSavingAgent` __(java
  -javaagent:path/to/spring-instrument.jar)__
| `InstrumentationLoadTimeWeaver`

| Fallback, expecting the underlying ClassLoader to follow common conventions (e.g.
  applicable to `TomcatInstrumentableClassLoader` and http://www.caucho.com/[Resin])
| `ReflectiveLoadTimeWeaver`
|===

Note that these are just the `LoadTimeWeavers` that are autodetected when using the
`DefaultContextLoadTimeWeaver`: it is of course possible to specify exactly which
`LoadTimeWeaver` implementation that you wish to use.

To specify a specific `LoadTimeWeaver` with Java configuration implement the
`LoadTimeWeavingConfigurer` interface and override the `getLoadTimeWeaver()` method:

[source,java,indent=0]
[subs="verbatim,quotes"]
----
	@Configuration
	@EnableLoadTimeWeaving
	public class AppConfig implements LoadTimeWeavingConfigurer {

		@Override
		public LoadTimeWeaver getLoadTimeWeaver() {
			return new ReflectiveLoadTimeWeaver();
		}
	}
----

If you are using XML based configuration you can specify the fully-qualified classname
as the value of the `weaver-class` attribute on the `<context:load-time-weaver/>`
element:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<?xml version="1.0" encoding="UTF-8"?>
	<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns:context="http://www.springframework.org/schema/context"
		xsi:schemaLocation="
			http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans.xsd
			http://www.springframework.org/schema/context
			http://www.springframework.org/schema/context/spring-context.xsd">

		<context:load-time-weaver
				weaver-class="org.springframework.instrument.classloading.ReflectiveLoadTimeWeaver"/>

	</beans>
----

The `LoadTimeWeaver` that is defined and registered by the configuration can be later
retrieved from the Spring container using the well-known name `loadTimeWeaver`.
Remember that the `LoadTimeWeaver` exists just as a mechanism for Spring's LTW
infrastructure to add one or more `ClassFileTransformers`. The actual
`ClassFileTransformer` that does the LTW is the `ClassPreProcessorAgentAdapter` (from
the `org.aspectj.weaver.loadtime` package) class. See the class-level javadocs of the
`ClassPreProcessorAgentAdapter` class for further details, because the specifics of how
the weaving is actually effected is beyond the scope of this section.

There is one final attribute of the configuration left to discuss: the
`aspectjWeaving` attribute (or `aspectj-weaving` if you are using XML). This is a
simple attribute that controls whether LTW is enabled or not; it is as simple as that.
It accepts one of three possible values, summarized below, with the default value being
`autodetect` if the attribute is not present.

[[aop-aj-ltw-ltw-tag-attrs]]
.AspectJ weaving attribute values
|===
| Annotation Value| XML Value| Explanation

| `ENABLED`
| `on`
| AspectJ weaving is on, and aspects will be woven at load-time as appropriate.

| `DISABLED`
| `off`
| LTW is off... no aspect will be woven at load-time.

| `AUTODETECT`
| `autodetect`
| If the Spring LTW infrastructure can find at least one `META-INF/aop.xml` file,
  then AspectJ weaving is on, else it is off. This is the default value.
|===


[[aop-aj-ltw-environments]]
==== Environment-specific configuration

This last section contains any additional settings and configuration that you will need
when using Spring's LTW support in environments such as application servers and web
containers.

[[aop-aj-ltw-environment-tomcat]]
===== Tomcat

Historically, http://tomcat.apache.org/[Apache Tomcat]'s default class loader did not
support class transformation which is why Spring provides an enhanced implementation
that addresses this need. Named `TomcatInstrumentableClassLoader`, the loader works on
Tomcat 6.0 and above.

[TIP]
====
Do not define `TomcatInstrumentableClassLoader` anymore on Tomcat 8.0 and higher.
Instead, let Spring automatically use Tomcat's new native `InstrumentableClassLoader`
facility through the `TomcatLoadTimeWeaver` strategy.
====

If you still need to use `TomcatInstrumentableClassLoader`, it can be registered
individually for __each__ web application as follows:

* Copy `org.springframework.instrument.tomcat.jar` into __$CATALINA_HOME__/lib, where
  __$CATALINA_HOME__ represents the root of the Tomcat installation)
* Instruct Tomcat to use the custom class loader (instead of the default) by editing the
  web application context file:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<Context path="/myWebApp" docBase="/my/webApp/location">
		<Loader
			loaderClass="org.springframework.instrument.classloading.tomcat.TomcatInstrumentableClassLoader"/>
	</Context>
----

Apache Tomcat (6.0+) supports several context locations:

* server configuration file - __$CATALINA_HOME/conf/server.xml__
* default context configuration - __$CATALINA_HOME/conf/context.xml__ - that affects all
  deployed web applications
* per-web application configuration which can be deployed either on the server-side at
  __$CATALINA_HOME/conf/[enginename]/[hostname]/[webapp]-context.xml__ or embedded
  inside the web-app archive at __META-INF/context.xml__

For efficiency, the embedded per-web-app configuration style is recommended because it
will impact only applications that use the custom class loader and does not require any
changes to the server configuration. See the Tomcat 6.0.x
http://tomcat.apache.org/tomcat-6.0-doc/config/context.html[documentation] for more
details about available context locations.

Alternatively, consider the use of the Spring-provided generic VM agent, to be specified
in Tomcat's launch script (see above). This will make instrumentation available to all
deployed web applications, no matter what ClassLoader they happen to run on.

[[aop-aj-ltw-environments-weblogic-oc4j-resin-glassfish-jboss]]
===== WebLogic, WebSphere, Resin, GlassFish, JBoss

Recent versions of WebLogic Server (version 10 and above), IBM WebSphere Application
Server (version 7 and above), Resin (3.1 and above) and JBoss (6.x or above) provide a
ClassLoader that is capable of local instrumentation. Spring's native LTW leverages such
ClassLoaders to enable AspectJ weaving. You can enable LTW by simply activating
load-time weaving as described earlier. Specifically, you do __not__ need to modify the
launch script to add `-javaagent:path/to/spring-instrument.jar`.

Note that GlassFish instrumentation-capable ClassLoader is available only in its EAR
environment. For GlassFish web applications, follow the Tomcat setup instructions as
outlined above.

Note that on JBoss 6.x, the app server scanning needs to be disabled to prevent it from
loading the classes before the application actually starts. A quick workaround is to add
to your artifact a file named `WEB-INF/jboss-scanning.xml` with the following content:

[source,xml,indent=0]
[subs="verbatim,quotes"]
----
	<scanning xmlns="urn:jboss:scanning:1.0"/>
----

[[aop-aj-ltw-environment-generic]]
===== Generic Java applications

When class instrumentation is required in environments that do not support or are not
supported by the existing `LoadTimeWeaver` implementations, a JDK agent can be the only
solution. For such cases, Spring provides `InstrumentationLoadTimeWeaver`, which
requires a Spring-specific (but very general) VM agent,
`org.springframework.instrument-{version}.jar` (previously named `spring-agent.jar`).

To use it, you must start the virtual machine with the Spring agent, by supplying the
following JVM options:

[literal]
[subs="verbatim,quotes"]
----
-javaagent:/path/to/org.springframework.instrument-{version}.jar
----

Note that this requires modification of the VM launch script which may prevent you from
using this in application server environments (depending on your operation policies).
Additionally, the JDK agent will instrument the __entire__ VM which can prove expensive.

For performance reasons, it is recommended to use this configuration only if your target
environment (such as http://www.eclipse.org/jetty/[Jetty]) does not have (or does not
support) a dedicated LTW.




[[aop-resources]]
== Further Resources

More information on AspectJ can be found on the http://www.eclipse.org/aspectj[AspectJ
website].

The book __Eclipse AspectJ__ by Adrian Colyer et. al. (Addison-Wesley, 2005) provides a
comprehensive introduction and reference for the AspectJ language.

The book __AspectJ in Action, Second Edition__ by Ramnivas Laddad (Manning, 2009) comes highly
recommended; the focus of the book is on AspectJ, but a lot of general AOP themes are
explored (in some depth).
