之前在开发过程中使用 @AfterReturning ，想要在切面方法中改变目标方法的返回值，但是却发现这样做不行。当时没有想到原因，后来猜测 @AfterReturing 标记的切面方法可能在 finnally 块中被调用所以不能改变返回值。今天经过测试发现这个猜测是错误的，在 finally 块中是可以改变返回值的，因为此时方法还没有返回。

public class FinallyAlterReturnValDemo {    public static String saySomething(){        return "say something ...";    }    public static Object invoke(){        String val;        try {            val =  saySomething();            return val;        }catch (Exception e){        }finally {            val = "changed return value";            return val;        }    }    public static void main(String[] args) {        Object invoke = invoke();        System.out.println(invoke);        // 执行结果是 changed return value    }}

    对Spring AOP 的 @AfterReturning 进行测试

public interface Say {    String saySomething();}

@Componentpublic class Target implements Say{    @Override    public String saySomething(){        return "say something ... ";    }}

import org.aspectj.lang.JoinPoint;import org.aspectj.lang.annotation.AfterReturning;import org.aspectj.lang.annotation.Pointcut;import org.springframework.context.annotation.EnableAspectJAutoProxy;import org.springframework.stereotype.Component;/** * Created by j4love on 2016/12/22. * Description : */@Component@org.aspectj.lang.annotation.Aspect@EnableAspectJAutoProxy()public class Aspect {    @Pointcut(value = "execution(* kernel.spring.AOP.bean.*.*(..))")    public void pointcut(){}    @AfterReturning(pointcut = "pointcut()",returning = "retVal")    public Object afterReturning(JoinPoint jp,Object retVal){        System.out.println("afterReturning ---- " + retVal);        return "change ... " + retVal;    }}

import org.junit.Test;import org.junit.runner.RunWith;import org.springframework.aop.support.AopUtils;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.test.context.ContextConfiguration;import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;/** * Created by j4love on 2016/12/22. * Description : */@RunWith(SpringJUnit4ClassRunner.class)@ContextConfiguration(locations = "classpath*:conf/spring/applicationContext-aop.xml")public class TargetTest {    @Autowired    private Say target;    @Test    public void saySomething() throws Exception {//        Object o = AopContext.currentProxy();        System.out.println("isJdkProxy = " + AopUtils.isJdkDynamicProxy(this.target));        System.out.println("isCglibProxy = " + AopUtils.isCglibProxy(this.target));        String s = this.target.saySomething();        System.out.println(s);    }}

AfterReturning 确实是无法改变返回值的，查看源码发现原因，下面贴出关键源码。

package org.springframework.aop.framework.adapter;import java.io.Serializable;import org.aopalliance.intercept.MethodInterceptor;import org.aopalliance.intercept.MethodInvocation;import org.springframework.aop.AfterAdvice;import org.springframework.aop.AfterReturningAdvice;import org.springframework.util.Assert;/** * Interceptor to wrap am {@link org.springframework.aop.AfterReturningAdvice}. * Used internally by the AOP framework; application developers should not need * to use this class directly. * * @author Rod Johnson */@SuppressWarnings("serial")public class AfterReturningAdviceInterceptor implements MethodInterceptor, AfterAdvice, Serializable {	private final AfterReturningAdvice advice;	/**	 * Create a new AfterReturningAdviceInterceptor for the given advice.	 * @param advice the AfterReturningAdvice to wrap	 */	public AfterReturningAdviceInterceptor(AfterReturningAdvice advice) {		Assert.notNull(advice, "Advice must not be null");		this.advice = advice;	}	@Override	public Object invoke(MethodInvocation mi) throws Throwable {		Object retVal = mi.proceed();		this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());		return retVal;	}}

程序运行，目标方法被调用后，会调用 invoke(MethodInvocation mi) throws Throwable 方法，这里就会去调用目标方法，也就是说目标方法在这里就执行完了。retVal 就是目标方法的返回值。而 afterReturing 是在目标方法执行返回之后才去调用的，并且 aftreReturing 方法返回 void 所以，尽管我在@AfterReturing 标记的切面方法中有返回值也没什么作用。整个 invoke 方法最终返回的是目标方法的返回值 retVal 。这下把 @afterReturing 为什么没法改变返回值搞清楚了。