Spring源代码解析（七）：Spring AOP中对拦截器调用的实现

前面我们分析了Spring AOP实现中得到Proxy对象的过程，下面我们看看在Spring AOP中拦截器链是怎样被调用的，也就是Proxy模式是怎样起作用的，或者说Spring是怎样为我们提供AOP功能的；
在JdkDynamicAopProxy中生成Proxy对象的时候：

        return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);

这里的this参数对应的是InvocationHandler对象,这里我们的JdkDynamicAopProxy实现了这个接口，也就是说当Proxy对象的函数被调用的时候，这个InvocationHandler的invoke方法会被作为回调函数调用，下面我们看看这个方法的实现：

    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        MethodInvocation invocation = null;
        Object oldProxy = null;
        boolean setProxyContext = false;

        TargetSource targetSource = this.advised.targetSource;
        Class targetClass = null;
        Object target = null;

        try {
            // Try special rules for equals() method and implementation of the
            // Advised AOP configuration interface.

            if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
                // What if equals throws exception!?
                // This class implements the equals(Object) method itself.
                return equals(args[0]) ? Boolean.TRUE : Boolean.FALSE;
            }
            if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
                // This class implements the hashCode() method itself.
                return new Integer(hashCode());
            }
            if (Advised.class == method.getDeclaringClass()) {
                // service invocations on ProxyConfig with the proxy config
                return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
            }

            Object retVal = null;

            if (this.advised.exposeProxy) {
                // make invocation available if necessary
                oldProxy = AopContext.setCurrentProxy(proxy);
                setProxyContext = true;
            }

            // May be <code>null</code>. Get as late as possible to minimize the time we "own" the target,
            // in case it comes from a pool.
            // 这里是得到目标对象的地方，当然这个目标对象可能来自于一个实例池或者是一个简单的JAVA对象
            target = targetSource.getTarget();
            if (target != null) {
                targetClass = target.getClass();
            }

            // get the interception chain for this method
            // 这里获得定义好的拦截器链
            List chain = this.advised.advisorChainFactory.getInterceptorsAndDynamicInterceptionAdvice(
                    this.advised, proxy, method, targetClass);

            // Check whether we have any advice. If we don't, we can fallback on direct
            // reflective invocation of the target, and avoid creating a MethodInvocation.
            // 如果没有设定拦截器，那么我们就直接调用目标的对应方法
            if (chain.isEmpty()) {
                // 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 = AopUtils.invokeJoinpointUsingReflection(target, method, args);
            }
            else {
                // We need to create a method invocation...
                // invocation = advised.getMethodInvocationFactory().getMethodInvocation(
                //         proxy, method, targetClass, target, args, chain, advised);
                // 如果有拦截器的设定，那么需要调用拦截器之后才调用目标对象的相应方法
                // 这里通过构造一个ReflectiveMethodInvocation来实现，下面我们会看这个ReflectiveMethodInvocation类
                invocation = new ReflectiveMethodInvocation(
                        proxy, target, method, args, targetClass, chain);

                // proceed to the joinpoint through the interceptor chain
                // 这里通过ReflectiveMethodInvocation来调用拦截器链和相应的目标方法
                retVal = invocation.proceed();
            }

            // massage return value if necessary
            if (retVal != null && retVal == target && method.getReturnType().isInstance(proxy)) {
                // Special case: it returned "this" and the return type of the method is type-compatible
                // Note that we can't help if the target sets
                // a reference to itself in another returned object.
                retVal = proxy;
            }
            return retVal;
        }
        finally {
            if (target != null && !targetSource.isStatic()) {
                // must have come from TargetSource
                targetSource.releaseTarget(target);
            }

            if (setProxyContext) {
                // restore old proxy
                AopContext.setCurrentProxy(oldProxy);
            }
        }
    }

我们先看看目标对象方法的调用，这里是通过AopUtils的方法调用 - 使用反射机制来对目标对象的方法进行调用：

    public static Object invokeJoinpointUsingReflection(Object target, Method method, Object[] args)
        throws Throwable {

        // Use reflection to invoke the method.
        // 利用放射机制得到相应的方法，并且调用invoke
        try {
            if (!Modifier.isPublic(method.getModifiers()) ||
                    !Modifier.isPublic(method.getDeclaringClass().getModifiers())) {
                method.setAccessible(true);
            }
            return method.invoke(target, args);
        }
        catch (InvocationTargetException ex) {
            // Invoked method threw a checked exception.
            // We must rethrow it. The client won't see the interceptor.
            throw ex.getTargetException();
        }
        catch (IllegalArgumentException ex) {
            throw new AopInvocationException("AOP configuration seems to be invalid: tried calling method [" +
                    method + "] on target [" + target + "]", ex);
        }
        catch (IllegalAccessException ex) {
            throw new AopInvocationException("Couldn't access method: " + method, ex);
        }
    }

对拦截器链的调用处理是在ReflectiveMethodInvocation里实现的：

    public Object proceed() throws Throwable {
        //    We start with an index of -1 and increment early.
        // 这里直接调用目标对象的方法，没有拦截器的调用或者拦截器已经调用完了，这个currentInterceptorIndex的初始值是0
        if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size()) {
            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.
            // 这里获得相应的拦截器，如果拦截器可以匹配的上的话，那就调用拦截器的invoke方法
            InterceptorAndDynamicMethodMatcher dm =
                (InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
            if (dm.methodMatcher.matches(this.method, this.targetClass, this.arguments)) {
                return dm.interceptor.invoke(nextInvocation());
            }
            else {
                // Dynamic matching failed.
                // Skip this interceptor and invoke the next in the chain.
                // 如果拦截器匹配不上，那就调用下一个拦截器，这个时候拦截器链的位置指示后移并迭代调用当前的proceed方法
                this.currentInterceptorIndex++;
                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(nextInvocation());
        }
    }

这里把当前的拦截器链以及在拦截器链的位置标志都clone到一个MethodInvocation对象了，作用是当前的拦截器执行完之后，会继续沿着得到这个拦截器链执行下面的拦截行为，也就是会迭代的调用上面这个proceed:

    private ReflectiveMethodInvocation nextInvocation() throws CloneNotSupportedException {
        ReflectiveMethodInvocation invocation = (ReflectiveMethodInvocation) clone();
        invocation.currentInterceptorIndex = this.currentInterceptorIndex + 1;
        invocation.parent = this;
        return invocation;
    }

这里的nextInvocation就已经包含了当前的拦截链的基本信息，我们看到在Interceptor中的实现比如TransactionInterceptor的实现中：

    public Object invoke(final MethodInvocation invocation) throws Throwable {
       ......//这里是TransactionInterceptor插入的事务处理代码，我们会在后面分析事务处理实现的时候进行分析
            try {
                //这里是对配置的拦截器链进行迭代处理的调用
                retVal = invocation.proceed();
            }
       ......//省略了和事务处理的异常处理代码 ，也是TransactionInterceptor插入的处理
          else {
            try {
                Object result = ((CallbackPreferringPlatformTransactionManager) getTransactionManager()).execute(txAttr,
                        new TransactionCallback() {
                            public Object doInTransaction(TransactionStatus status) {
                                 //这里是TransactionInterceptor插入对事务处理的代码
                                TransactionInfo txInfo = prepareTransactionInfo(txAttr, joinpointIdentification, status);
                                //这里是对配置的拦截器链进行迭代处理的调用，接着顺着拦截器进行处理
                                try {                        
                                    return invocation.proceed();
                                }
       ......//省略了和事务处理的异常处理代码 ，也是TransactionInterceptor插入的处理
       }

从上面的分析我们看到了Spring AOP的基本实现，比如Spring怎样得到Proxy,怎样利用JAVA Proxy以及反射机制对用户定义的拦截器链进行处理。