最近新公司在用Spring MVC，跟踪Spring的Service发现是通过动态代理来实现的，而公司的事务是配置在Service层。所以想看下Spring 的AOP的具体实现。本文源码基于Spring 4.0。
我们可以使用debug跟踪一次Service调用的整体流程，可以清晰的看到一次流程处理：
CglibAopProxy.intercept方法，该方法中通过this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
获取一个List拦截链，然后通过
retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
方法执行对应的拦截链进行执行处理。
最后通过，processReturnType(proxy, target, method, retVal);
处理返回值，并返回。
整体过程完全采用动态代理模式来实现。最主要的代码如下：@Override 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 代码看起来很简单。
下面我们可以逐行对代码进行分析。 方法签名：public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable
可以看出，改方法传入代理对象，方法对象，参数以及方法代理。MethodProxy 包含我们的需要被代理的方法信息，包含方法完整签名。
if (this.advised.exposeProxy) { // Make invocation available if necessary. oldProxy = AopContext.setCurrentProxy(proxy); setProxyContext = true; } 此处代码是判断代理是否可用，大概是这个意思，此处的代码不是很明白。
target = getTarget(); if (target != null) { targetClass = target.getClass(); } 此处是获取目标类对象。
下来是获取对应的链对象。List
在这个方法中，比较重要的应该是cacheKey的获取。
/** * Simple wrapper class around a Method. Used as the key when * caching methods, for efficient equals and hashCode comparisons. */ private static class MethodCacheKey { private final Method method; private final int hashCode; public MethodCacheKey(Method method) { this.method = method; this.hashCode = method.hashCode(); } @Override public boolean equals(Object other) { if (other == this) { return true; } MethodCacheKey otherKey = (MethodCacheKey) other; return (this.method == otherKey.method); } @Override public int hashCode() { return this.hashCode; } }} 这个类比较简单，定义了对应的Method对象，以及Method对应的hashCode。
List
此处缓存了对应的拦截器链。如果为null，则通过factory实例化，并放入缓存。
if (cached == null) { cached = this.advisorChainFactory.getInterceptorsAndDynamicInterceptionAdvice( this, method, targetClass); this.methodCache.put(cacheKey, cached); }@Override public List 这个方法虽然很长，但是实际内容还是相对比较简单。
首先获取所有切点。config.getAdvisors()
判断不同的切点类型，放入List。
执行对应List方法。retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();@Override 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); } } 处理返回值：
retVal = processReturnType(proxy, target, method, retVal); return retVal;