java - 2008-5-23 17:51:00
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：
1. 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：
1. public static Object invokeJoinpointUsingReflection(Object target, Method method, Object[] args)
2. throws Throwable {
3.
4. // Use reflection to invoke the method.
5. // 利用放射机制得到相应的方法，并且调用invoke
6. try {
7. if (!Modifier.isPublic(method.getModifiers()) ||
8. !Modifier.isPublic(method.getDeclaringClass().getModifiers())) {
9. method.setAccessible(true);
10. }
11. return method.invoke(target, args);
12. }
13. catch (InvocationTargetException ex) {
14. // Invoked method threw a checked exception.
15. // We must rethrow it. The client won't see the interceptor.
16. throw ex.getTargetException();
17. }
18. catch (IllegalArgumentException ex) {
19. throw new AopInvocationException("AOP configuration seems to be invalid: tried calling method [" +
20. method + "] on target [" + target + "]", ex);
21. }
22. catch (IllegalAccessException ex) {
23. throw new AopInvocationException("Couldn't access method: " + method, ex);
24. }
25. }
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：
1. public Object invoke(final MethodInvocation invocation) throws Throwable {
2. ......//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
3. try {
4. //这里是对配置的拦截器链进行迭代处理的调用
5. retVal = invocation.proceed();
6. }
7. ......//省略了和事务处理的异常处理代码 ，也是TransactionInterceptor插入的处理
8. else {
9. try {
10. Object result = ((CallbackPreferringPlatformTransactionManager) getTransactionManager()).execute(txAttr,
11. new TransactionCallback() {
12. public Object doInTransaction(TransactionStatus status) {
13. //这里是TransactionInterceptor插入对事务处理的代码
14. TransactionInfo txInfo = prepareTransactionInfo(txAttr, joinpointIdentification, status);
15. //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
16. try {
17. return invocation.proceed();
18. }
19. ......//省略了和事务处理的异常处理代码 ，也是TransactionInterceptor插入的处理
20. }
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。