AspectJ 处理多个匹配建议

Question

我在 Java 中使用 AspectJ 来记录对某些方法的调用。我在网上看过，但找不到答案：

当两个 @Around 建议匹配一个方法时会发生什么？

具体来说，我使用了两个@Around 建议，如下所示：

@Around("condition1() && condition2() && condition3()")
public Object around(ProceedingJoinPoint point) {
    return around(point, null);
}

@Around("condition1() && condition2() && condition3() && args(request)")
public Object around(ProceedingJoinPoint point, Object request) {
    ...
    result = (Result) point.proceed();
    ...
}

如果这两个建议都匹配，这会导致 point.proceed() 被调用两次（实际方法被调用两次）吗？

Answer 1

为了完整起见，将此留在这里。

我最终实现了一个虚拟项目来测试它。答案是该方法只会执行一次。我已经实施了以下内容：

Aspects.java:

package base;

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

@Aspect
public class Aspects {
    @Pointcut("@annotation(java.lang.Deprecated)")
    public void hasErrorResponseMethod() {
    }

    @Around("hasErrorResponseMethod()")
    public Object around(ProceedingJoinPoint point) throws Throwable {
        System.out.println("In CHILD advice.");
        return around(point, null);
    }

    @Around("hasErrorResponseMethod() && args(request)")
    public Object around(ProceedingJoinPoint point, Object request) throws Throwable {
        System.out.println("In PARENT advice with request " + request);
        return point.proceed();
    }
}

Methods.java:

package base;

public class Methods {
    private static int COUNT = 1;
    @Deprecated
    public int method1(String param) {
        System.out.println(">>> In method1! Param: " + param);
        return COUNT++;
    }
}

applicationContext.xml:

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:context="http://www.springframework.org/schema/context"
       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-3.0.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context.xsd
    http://www.springframework.org/schema/aop
    http://www.springframework.org/schema/aop/spring-aop-3.0.xsd">

    <context:component-scan base-package="base" annotation-config="true" />

    <aop:aspectj-autoproxy />

    <bean id="logAspect" class="base.Aspects"/>

    <bean id="methods" class="base.Methods"/>
    <bean id="main" class="base.Main"/>

</beans>

Main.java:

package base;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Main {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");

        Methods methods = (Methods) context.getBean("methods");
        System.out.println("<<< Result: " + Methods.method1("REQUEST_VALUE"));
    }
}

输出如下：

In PARENT advice with request REQUEST_VALUE
In CHILD advice.
In PARENT advice with request null
>>> In method1! Param: REQUEST_VALUE
<<< Result: 1

如您所见，该方法只被调用一次，由看起来更具体的建议调用。知道 AspectJ 如何决定两者中的哪一个调用它仍然很棒。

Answer 2

您的方法存在很大问题，因为您手动从另一个建议中调用一个建议。这不是应用 AOP 的方式。请让 AspectJ 根据各自的切入点决定执行哪些建议。从一个建议委托给另一个建议的方式，您甚至可以调用一个本身不匹配的建议。在没有 Spring 的普通 AspectJ 中的示例（尽管在 Spring AOP 中工作相同）：

Java 驱动申请：

package de.scrum_master.app;

public class Application {
    private static void doSomething() {
        System.out.println("Doing something");
    }

    public static void main(String[] args) {
        doSomething();
    }
}

看点：

package de.scrum_master.aspect;

import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;

@Aspect
public class MyBogusAspect {
    @Around("execution(* doSomething(..))")
    public Object matchingAdvice(ProceedingJoinPoint thisJoinPoint) {
        System.out.println("matching advice called on joinpoint " + thisJoinPoint);
        return nonMatchingAdvice(thisJoinPoint);
    }

    @Around("execution(* doSomethingElse(..))")
    public Object nonMatchingAdvice(ProceedingJoinPoint thisJoinPoint) {
        System.out.println("non-matching advice called on joinpoint " + thisJoinPoint);
        return thisJoinPoint.proceed();
    }
}

控制台日志：

matching advice called on joinpoint execution(void de.scrum_master.app.Application.doSomething())
non-matching advice called on joinpoint execution(void de.scrum_master.app.Application.doSomething())
Doing something

您能看出您的方法有多不健康吗？否则不会匹配的建议由匹配的建议调用。这会产生一些非常意想不到的行为 IMO。 请不要这样做！！！

现在关于你原来关于多重匹配建议的问题，你应该这样做：

修改后的方面：

package de.scrum_master.aspect;

import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;

@Aspect
public class MyBetterAspect {
    @Around("execution(* doSomething(..))")
    public Object matchingAdvice(ProceedingJoinPoint thisJoinPoint) {
        System.out.println(">>> matching advice on " + thisJoinPoint);
        Object result = thisJoinPoint.proceed();
        System.out.println("<<< matching advice on " + thisJoinPoint);
        return result;
    }

    @Around("execution(* doSomething(..))")
    public Object anotherMatchingAdvice(ProceedingJoinPoint thisJoinPoint) {
        System.out.println(">>> another matching advice on " + thisJoinPoint);
        Object result = thisJoinPoint.proceed();
        System.out.println("<<< another matching advice on " + thisJoinPoint);
        return result;
    }
}

新控制台日志：

>>> matching advice on execution(void de.scrum_master.app.Application.doSomething())
>>> another matching advice on execution(void de.scrum_master.app.Application.doSomething())
Doing something
<<< another matching advice on execution(void de.scrum_master.app.Application.doSomething())
<<< matching advice on execution(void de.scrum_master.app.Application.doSomething())

如您所见，AspectJ 或 Spring AOP 将多个匹配建议像洋葱皮一样包裹在连接点周围，只有最内层的 proceed() 调用实际连接点，而外层调用内部连接点，使得确保每个连接点只执行一次。你没有必要试图比 AOP 框架更聪明，可能会造成损害（见我的第一个例子）。

还有一件事：如果多个方面有匹配的切入点，您可以通过 AspectJ 中的 @DeclarePrecedence 影响它们的执行顺序，但在单个方面内您对执行顺序没有影响，或者至少您应该不要依赖它。在 Spring AOP 中，您可以使用 @Order 注释来确定方面优先级，但是对于来自同一方面的多个建议，顺序也是未定义的，另请参见 Spring manual.

更新2016-02-28，18:30 CET，经过评论讨论：

好的，我们稍微扩展驱动程序 class 以便我们可以测试更多：

package de.scrum_master.app;

public class Application {
    private static void doSomething() {
        System.out.println("Doing something");
    }

    private static String doSomethingElse(String text) {
        System.out.println("Doing something else");
        return text;
    }

    private static int doAnotherThing(int i, int j, int k) {
        System.out.println("Doing another thing");
        return (i + j) * k;
    }

    public static void main(String[] args) {
        doSomething();
        doSomethingElse("foo");
        doAnotherThing(11, 22, 33);
    }
}

现在，在 AspectJ 中绑定第一个参数就像 args(request, ..) 一样简单，后者适用于一个或多个参数。唯一的例外是零参数，在这种情况下不会触发切入点。所以要么我最终得到类似于你所做的事情：

package de.scrum_master.aspect;

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

@Aspect
public class BoundFirstParameterAspect {
    @Pointcut("execution(* do*(..))")
    public static void myPointcut() {}

    @Around("myPointcut()")
    public Object matchingAdvice(ProceedingJoinPoint thisJoinPoint) {
        return anotherMatchingAdvice(thisJoinPoint, null);
    }

    @Around("myPointcut() && args(request, ..)")
    public Object anotherMatchingAdvice(ProceedingJoinPoint thisJoinPoint, Object request) {
        System.out.println(">>> another matching advice on " + thisJoinPoint);
        Object result = thisJoinPoint.proceed();
        System.out.println("<<< another matching advice on " + thisJoinPoint);
        return result;
    }
}

这使得相同的建议触发两次并因此导致开销，即使原始方法只被调用一次，但您可以在日志中看到开销：

>>> another matching advice on execution(void de.scrum_master.app.Application.doSomething())
Doing something
<<< another matching advice on execution(void de.scrum_master.app.Application.doSomething())
>>> another matching advice on execution(String de.scrum_master.app.Application.doSomethingElse(String))
>>> another matching advice on execution(String de.scrum_master.app.Application.doSomethingElse(String))
Doing something else
<<< another matching advice on execution(String de.scrum_master.app.Application.doSomethingElse(String))
<<< another matching advice on execution(String de.scrum_master.app.Application.doSomethingElse(String))
>>> another matching advice on execution(int de.scrum_master.app.Application.doAnotherThing(int, int, int))
>>> another matching advice on execution(int de.scrum_master.app.Application.doAnotherThing(int, int, int))
Doing another thing
<<< another matching advice on execution(int de.scrum_master.app.Application.doAnotherThing(int, int, int))
<<< another matching advice on execution(int de.scrum_master.app.Application.doAnotherThing(int, int, int))

您可以很容易地认识到如何为每个连接点触发双重建议。

或者，您可以在运行时绑定参数，这不是很优雅并且会产生一点运行时损失，但效果非常好：

package de.scrum_master.aspect;

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

@Aspect
public class BoundFirstParameterAspect {
    @Pointcut("execution(* do*(..))")
    public static void myPointcut() {}

    @Around("myPointcut()")
    public Object matchingAdvice(ProceedingJoinPoint thisJoinPoint) {
        System.out.println(">>> matching advice on " + thisJoinPoint);
        Object[] args = thisJoinPoint.getArgs();
        Object request =  args.length > 0 ? args[0] : null;
        System.out.println("First parameter = " + request);
        Object result = thisJoinPoint.proceed();
        System.out.println("<<< matching advice on " + thisJoinPoint);
        return result;
    }
}

这避免了双重建议执行以及代码重复，并产生以下控制台输出：

>>> matching advice on execution(void de.scrum_master.app.Application.doSomething())
First parameter = null
Doing something
<<< matching advice on execution(void de.scrum_master.app.Application.doSomething())
>>> matching advice on execution(String de.scrum_master.app.Application.doSomethingElse(String))
First parameter = foo
Doing something else
<<< matching advice on execution(String de.scrum_master.app.Application.doSomethingElse(String))
>>> matching advice on execution(int de.scrum_master.app.Application.doAnotherThing(int, int, int))
First parameter = 11
Doing another thing
<<< matching advice on execution(int de.scrum_master.app.Application.doAnotherThing(int, int, int))

最后但同样重要的是，您可以有两个略有不同的切入点 - 一个空 args() 和一个 args(request, ..) - 两者都可以将参数处理、日志记录和异常处理委托给辅助方法以避免重复，正如我在其中一条评论中所说：

package de.scrum_master.aspect;

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

@Aspect
public class BoundFirstParameterAspect {
    @Pointcut("execution(* do*(..))")
    public static void myPointcut() {}

    @Around("myPointcut() && args()")
    public Object myAdvice(ProceedingJoinPoint thisJoinPoint) {
        return myAdviceHelper(thisJoinPoint, null);
    }

    @Around("myPointcut() && args(request, ..)")
    public Object myAdviceWithParams(ProceedingJoinPoint thisJoinPoint, Object request) {
        return myAdviceHelper(thisJoinPoint, request);
    }

    private Object myAdviceHelper(ProceedingJoinPoint thisJoinPoint, Object request) {
        System.out.println(">>> matching advice on " + thisJoinPoint);
        System.out.println("First parameter = " + request);
        Object result = thisJoinPoint.proceed();
        System.out.println("<<< matching advice on " + thisJoinPoint);
        return result;
    }
}

控制台日志应该和上一个完全一样。

更新二：

好吧，我刚刚意识到空的 args() 技巧也适用于您最初的想法并避免双重执行以及辅助方法：

package de.scrum_master.aspect;

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

@Aspect
public class BoundFirstParameterAspect {
    @Pointcut("execution(* do*(..))")
    public static void myPointcut() {}

    @Around("myPointcut() && args()")
    public Object myAdvice(ProceedingJoinPoint thisJoinPoint) {
        return myAdviceWithParams(thisJoinPoint, null);
    }

    @Around("myPointcut() && args(request, ..)")
    public Object myAdviceWithParams(ProceedingJoinPoint thisJoinPoint, Object request) {
        System.out.println(">>> matching advice on " + thisJoinPoint);
        System.out.println("First parameter = " + request);
        Object result = thisJoinPoint.proceed();
        System.out.println("<<< matching advice on " + thisJoinPoint);
        return result;
    }
}

这是可以接受的，也是优雅的，因为它不会为每个连接点生成两次字节码。这两个切入点是互斥的，所以这是一件好事。我推荐这个解决方案。

AspectJ 处理多个匹配建议

AspectJ Handling of Multiple Matching Advices

java

aop

aspectj

pointcut

aspects