Spring AOP源码分析(八)SpringAOP要注意的地方
Spring AOP源码分析(八)SpringAOP要注意的地方
乒乓狂魔 发表于3年前
Spring AOP源码分析(八)SpringAOP要注意的地方
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SpringAOP要注意的地方有很多,下面就举一个,之后想到了再列出来:
(1)SpringAOP对于最外层的函数只拦截public方法,不拦截protected和private方法,另外不会对最外层的public方法内部调用的其他方法也进行拦截,即只停留于代理对象所调用的方法。如下案例:
B类有两个public方法,foo1()和foo2(),foo1内部调用了foo2,简单如下:

public void foo2() {  
        System.out.println("foo2");  
    }  
    
    public void foo1(){
    	System.out.println("foo1");
    	this.foo2();
    }

假如都对这两个方法进行拦截。当你调用,B对象.foo1()仅仅对foo1整个方法拦截,对于它内部调用的foo2()方法不会进行拦截。
源码分析:
判断上述this.foo2()方法是否被拦截的最本质的东西是看this到底是谁?有如下对象B类的对象b,和cglib生成的代理对象bProxy,代理对象bProxy内部拥有b。如果调用b对象的任何方法,肯定不会发生任何拦截,当调用bProxy的方法则都会进入拦截函数。

当我们调用bProxy对象的foo1()方法时,先执行cglib之前设置的callback对象的intercept拦截函数,如下:

public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
			Object oldProxy = null;
			boolean setProxyContext = false;
			Class<?> targetClass = null;
			Object target = null;
			try {
				if (this.advised.exposeProxy) {
					// Make invocation available if necessary.
					oldProxy = AopContext.setCurrentProxy(proxy);
					setProxyContext = true;
				}
				// May be null. Get as late as possible to minimize the time we
				// "own" the target, in case it comes from a pool...
				target = getTarget();
				if (target != null) {
					targetClass = target.getClass();
				}
				List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
				Object retVal;
				// Check whether we only have one InvokerInterceptor: that is,
				// no real advice, but just reflective invocation of the target.
				if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) {
					// We can skip creating a MethodInvocation: just invoke the target directly.
					// Note that the final invoker must be an InvokerInterceptor, so we know
					// it does nothing but a reflective operation on the target, and no hot
					// swapping or fancy proxying.
					retVal = methodProxy.invoke(target, args);
				}
				else {
					// We need to create a method invocation...
					retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
				}
				retVal = processReturnType(proxy, target, method, retVal);
				return retVal;
			}
			finally {
				if (target != null) {
					releaseTarget(target);
				}
				if (setProxyContext) {
					// Restore old proxy.
					AopContext.setCurrentProxy(oldProxy);
				}
			}
		}

这个过程之前的文章已经分析过,这里就是首先取出拦截器链List<Object> chain,当foo1方法不符合我们所配置的pointcut时,拦截器链必然为空,然后就是直接执行目标对象的方法。
当foo1方法符合所配置的pointcut时,拦截器链不为空,执行相应的通知advice,currentInterceptorIndex 从-1开始,如下:

public Object proceed() throws Throwable {
		//	We start with an index of -1 and increment early.
		if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
			return invokeJoinpoint();
		}

		Object interceptorOrInterceptionAdvice =
				this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
		if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
			// Evaluate dynamic method matcher here: static part will already have
			// been evaluated and found to match.
			InterceptorAndDynamicMethodMatcher dm =
					(InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
			if (dm.methodMatcher.matches(this.method, this.targetClass, this.arguments)) {
				return dm.interceptor.invoke(this);
			}
			else {
				// Dynamic matching failed.
				// Skip this interceptor and invoke the next in the chain.
				return proceed();
			}
		}
		else {
			// It's an interceptor, so we just invoke it: The pointcut will have
			// been evaluated statically before this object was constructed.
			return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
		}
	}

随着通知不断的传递执行,最终this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1将会满足条件,将会来到执行目标对象的方法invokeJoinpoint():
protected Object invokeJoinpoint() throws Throwable {
			if (this.publicMethod) {
				return this.methodProxy.invoke(this.target, this.arguments);
			}
			else {
				return super.invokeJoinpoint();
			}
		}

在这里不管要拦截的目标方法是不是public方法,最终所传递的对象都是this.target,他是目标对象而不是代理对象,即执行上述foo1()函数的对象是目标对象而不是代理对象,所以它内部所调用的this.foo2()也是目标对象,因此不会发生拦截,如果是执行的是代理对象.foo2()则必然会进入intercept拦截过程。所以上述调用foo1函数,其内部调用的foo2函数是不会发生拦截的,因为this指的是目标对象,不是代理对象。

如果你想实现foo1调用时内部的foo2也进行拦截,就必须把this换成代理对象。这时就要用到了,xml配置中的expose-proxy="true",即暴露出代理对象,它使用的是ThreadLocal设计模式,我们可以这样获取代理对象,AopContext.currentProxy()就是代理对象,然后转换成目标对象或者目标接口,执行相应的方法:

public void foo1(){
    	System.out.println("run foo1");
    	BServiceImpl proxy=(BServiceImpl) AopContext.currentProxy();
    	proxy.foo2();
    }


最后再给出Spring的文档说明:
Due to the proxy-based nature of Spring’s AOP framework, protected methods are by definition not intercepted, neither for JDK proxies (where this isn’t applicable) nor for CGLIB proxies (where this is technically possible but not recommendable for AOP purposes). As a consequence, any given pointcut will be matched against public methods only!

If your interception needs include protected/private methods or even constructors, consider the use of Spring-driven 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.

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作者:乒乓狂魔
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