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：
[code]
return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
[/code]
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：
[code]
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 null. 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);
}
}
}
[/code]
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：
[code]
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);
}
}
[/code]
对拦截器链的调用处理是在ReflectiveMethodInvocation里实现的：
[code]
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());
}
}
[/code]
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:
[code]
private ReflectiveMethodInvocation nextInvocation() throws CloneNotSupportedException {
ReflectiveMethodInvocation invocation = (ReflectiveMethodInvocation) clone();
invocation.currentInterceptorIndex = this.currentInterceptorIndex + 1;
invocation.parent = this;
return invocation;
}
[/code]
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：
[code]
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
}
[/code]
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。