@AspectJ support （good）

AspectJ类型匹配的通配符：

*：匹配任何数量字符；

..：匹配任何数量字符的重复，如在类型模式中匹配任何数量子包；而在方法参数模式中匹配任何数量参数。

+：匹配指定类型的子类型；仅能作为后缀放在类型模式后边。

java.lang.String 匹配String类型；

java.*.String 匹配java包下的任何“一级子包”下的String类型；//*只能匹配一个子包

如匹配java.lang.String，但不匹配java.lang.ss.String

java..* 匹配java包及任何子包下的任何类型;

如匹配java.lang.String、java.lang.annotation.Annotation

java.lang.*ing 匹配任何java.lang包下的以ing结尾的类型；

java.lang.Number+ 匹配java.lang包下的任何Number的子类型；

如匹配java.lang.Integer，也匹配java.math.BigInteger

eg:

任意公共方法的执行：
execution(public * *(..))
任何一个以“set”开始的方法的执行：
execution(* set*(..))
AccountService 接口的任意方法的执行：
execution(* com.xyz.service.AccountService.*(..))
定义在service包里的任意方法的执行：
execution(* com.xyz.service.*.*(..))
定义在service包和所有子包里的任意类的任意方法的执行：
execution(* com.xyz.service..*.*(..))
定义在pointcutexp包和所有子包里的JoinPointObjP2类的任意方法的执行：
execution(* com.test.spring.aop.pointcutexp..JoinPointObjP2.*(..))")

***>    
最靠近方法名(..)的为方法名,   //括号左边是方法名
最靠近类名.*(..))的为类名或者接口名,如上例的JoinPointObjP2.*(..))   //点号左边是类名

pointcutexp包里的任意类.
within(com.test.spring.aop.pointcutexp.*)
pointcutexp包和所有子包里的任意类.
within(com.test.spring.aop.pointcutexp..*)
实现了Intf接口的所有类,如果Intf不是接口,限定Intf单个类.
this(com.test.spring.aop.pointcutexp.Intf)
***> 当一个实现了接口的类被AOP的时候,用getBean方法必须cast为接口类型,不能为该类的类型.

带有@Transactional标注的所有类的任意方法.
@within(org.springframework.transaction.annotation.Transactional)
@target(org.springframework.transaction.annotation.Transactional)
带有@Transactional标注的任意方法.
@annotation(org.springframework.transaction.annotation.Transactional)
***> @within和@target针对类的注解,@annotation是针对方法的注解

参数带有@Transactional标注的方法.
@args(org.springframework.transaction.annotation.Transactional)
参数为String类型(运行是决定)的方法.

​​http://www.mekau.com/4880.html​​ https://github.com/doc-spring/AspectJ

​​http://si加shuo加k.com/forum/posts/list/281.html​​

注解？ 修饰符? 返回值类型 类型声明?方法名(参数列表) 异常列表？

注解：可选，方法上持有的注解，如@Deprecated；
修饰符：可选，如public、protected；
返回值类型：必填，可以是任何类型模式；“*”表示所有类型；
类型声明：可选，可以是任何类型模式；
方法名：必填，可以使用“*”进行模式匹配；
参数列表：“()”表示方法没有任何参数；“(..)”表示匹配接受任意个参数的方法，“(..,java.lang.String)”表示匹配接受java.lang.String类型的参数结束，且其前边可以接受有任意个参数的方法；“(java.lang.String,..)” 表示匹配接受java.lang.String类型的参数开始，且其后边可以接受任意个参数的方法；“(*,java.lang.String)” 表示匹配接受java.lang.String类型的参数结束，且其前边接受有一个任意类型参数的方法；
异常列表：可选，以“throws 异常全限定名列表”声明，异常全限定名列表如有多个以“，”分割，如throws java.lang.IllegalArgumentException, java.lang.ArrayIndexOutOfBoundsException。
匹配Bean名称：可以使用Bean的id或name进行匹配，并且可使用通配符“*”；
6.5.3 组合切入点表达式

AspectJ使用 且（&&）、或（||）、非（！）来组合切入点表达式。
在Schema风格下，由于在XML中使用“&&”需要使用转义字符“&&”来代替之，所以很不方便，因此Spring AOP 提供了and、or、not来代替&&、||、！。

​​https://docs.spring.io/spring/docs/current/spring-framework-reference/html/aop.html​​

11.2 @AspectJ support

@AspectJ refers to a style of declaring aspects as regular Java classes annotated with annotations. The @AspectJ style was introduced by the ​​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.

11.2.1 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 1.6.8 or later). This library is available in the ​​'lib'​​ directory of an AspectJ distribution or via the Maven Central repository.

Enabling @AspectJ Support with Java configuration

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

@Configuration
@EnableAspectJAutoProxy
public class AppConfig {

}

Enabling @AspectJ Support with XML configuration

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

<aop:aspectj-autoproxy/>

This assumes that you are using schema support as described in ​​Chapter 41, XML Schema-based configuration​​​. See ​​Section 41.2.7, “the aop schema”​​​ for how to import the tags in the ​​aop​​ namespace.

11.2.2 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:

<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;

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.

11.2.3 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'​​:

@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 ​​AspectJ Programming Guide​​​ (and for extensions, the ​​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.

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​​).

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:

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.

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).

@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.

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:

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:

<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 ​​Section 11.3, “Schema-based AOP support”​​​. The transaction elements are discussed in ​​Chapter 17, Transaction Management​​.

Examples

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

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 ​​*​​​ 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 ​​*​​​ 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 ​​(*)​​​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 ​​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:

execution(public * *(..))

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

execution(* set*(..))

• the execution of any method defined by the ​​AccountService​​ interface:

execution(* com.xyz.service.AccountService.*(..))

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

execution(* com.xyz.service.*.*(..))

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

execution(* com.xyz.service..*.*(..))

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

within(com.xyz.service.*)

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

within(com.xyz.service..*)

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

this(com.xyz.service.AccountService)

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

target(com.xyz.service.AccountService)

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

args(java.io.Serializable)

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:

@target(org.springframework.transaction.annotation.Transactional)

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

@within(org.springframework.transaction.annotation.Transactional)

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

@annotation(org.springframework.transaction.annotation.Transactional)

• 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:

@args(com.xyz.security.Classified)

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

bean(tradeService)

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

bean(*Service)

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.

11.2.4 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.

Before advice

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

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:

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() {
        // ...
    }

}

After returning advice

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

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() {
        // ...
    }

}

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:

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.

After throwing advice

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

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.

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).

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.

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() {
        // ...
    }

}

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.

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.

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.

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.

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:

@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:

@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:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Auditable {
    AuditCode value();
}

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

@Before("com.xyz.lib.Pointcuts.anyPublicMethod() && @annotation(auditable)")
public void audit(Auditable auditable) {
    AuditCode code = auditable.value();
    // ...
}

Advice parameters and generics

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

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:

@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:

@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.

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:

@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:

@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:

@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.

• 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.

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:

@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).

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.

11.2.5 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.)

@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:

UsageTracked usageTracked = (UsageTracked) context.getBean("myService");

11.2.6 Aspect instantiation models

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.

@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.

11.2.7 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:

@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:

<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:

@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:

@Around("com.xyz.myapp.SystemArchitecture.businessService() && " +
        "@annotation(com.xyz.myapp.service.Idempotent)")
public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable {
    ...
}

​​http://docs.spring.io/spring/docs/current/spring-framework-reference/html/aop.html​​