Spring源代码解析（七）：Spring AOP中对拦截器调用的实现

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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： 

Java代码  

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： 

Java代码  

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： 

Java代码  

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： 

Java代码  

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: 

Java代码  

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： 

Java代码  

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。