Spring AOP框架学习笔记（2）：AOP拦截器调用的实现

下面我们介绍一下JdkDynamicAopProxy的invoke拦截。JDK动态代理的生成调用为：

Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);

这里的this参数对应的是InvocationHandler对象，InvocationHandler是JDK定义的放射类的一个接口，这个接口定义了invoke方法，而这个方法是作为JDK Proxy代理对象进行拦截的回调入口出现的。在JdkDynamicAopProxy中实现了InvocationHandler接口，也就是说，当代理对象的代理方法被调用是，JdkDynamicProxy的invoke方法作为Proxy对象的回调函数被触发，从而通过invoke的具体实现，来完成对目标对象方法调用的拦截或者说功能增强的工作。

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

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

try {
if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
// The target does not implement the equals(Object) method itself.
return equals(args[0]);
}
if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
// The target does not implement the hashCode() method itself.
return hashCode();
}
if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
method.getDeclaringClass().isAssignableFrom(Advised.class)) {
// Service invocations on ProxyConfig with the proxy config...
return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
}

Object retVal;

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 = targetSource.getTarget();//取得目标对象的地方
if (target != null) {
targetClass = target.getClass();
}

// Get the interception chain for this method.取得为目标方法定义好的拦截器链
<strong>List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);</strong>

// 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.
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);//如果拦截器链为空，直接通过反射来调用target对应的方法。
}
else {
// We need to create a method invocation...
invocation = new <strong>ReflectiveMethodInvocation</strong>(proxy, target, method, args, targetClass, chain);
// Proceed to the joinpoint through the interceptor chain.
retVal = invocation.proceed();//沿着拦截器链继续前进
}

// Massage return value if necessary.
if (retVal != null && retVal == target && method.getReturnType().isInstance(proxy) &&
!RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {
// 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);
}
}
}
在ReflectiveMethodInvocation的proceed方法中，会逐个运行拦截器的拦截方法。在运行拦截器的拦截方法前，需要对代理方法完成一个匹配判断，通过这个匹配判断来决定拦截器是否满足切面增强的要求。在Pointcut切点中需要进行matches的匹配过程，来决定是否需要实行通知增强。

public Object proceed() throws Throwable {
//	We start with an index of -1 and increment early.
if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {//如果运行到拦截器的末尾，直接以反射方式调用目标对象的实现方法
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)) {
<strong>return dm.interceptor.invoke(this);//匹配则调用拦截器的增强方法，并迭代调用proceed方法</strong>
}
else {
// Dynamic matching failed.
// Skip this interceptor and invoke the next in the chain.
<strong>return proceed();//迭代调用proceed方法，直到拦截器链中的拦截器都完成以上的拦截过程</strong>
}
}
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);
}
}

从前面的代码，我们可以知道是这样取到的。

<pre name="code" class="java">List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);

这个advised是一个AdvisedSupport对象。

public List<Object> getInterceptorsAndDynamicInterceptionAdvice(Method method, Class targetClass) {
MethodCacheKey cacheKey = new MethodCacheKey(method);
List<Object> cached = this.methodCache.get(cacheKey);
if (cached == null) {
cached = this.advisorChainFactory.getInterceptorsAndDynamicInterceptionAdvice(
this, method, targetClass);
this.methodCache.put(cacheKey, cached);//为目的对象的目标方法的拦截器链建立
}
return cached;
}

advisorChainFactory被配置成一个DefaultAdvisorChainFactory对象，在该类中实现了interceptor链的获取过程。DefaultAdvisorChainFactory会通过一个AdvisorAdapterRegistry来实现拦截器的注册，AdvisorAdapterRegistry对advice通知的织入功能
4000
起了很大的作用。

public List<Object> getInterceptorsAndDynamicInterceptionAdvice(
Advised config, Method method, Class targetClass) {

// This is somewhat tricky... we have to process introductions first,
// but we need to preserve order in the ultimate list.
List<Object> interceptorList = new ArrayList<Object>(config.getAdvisors().length);
boolean hasIntroductions = hasMatchingIntroductions(config, targetClass);
AdvisorAdapterRegistry registry = GlobalAdvisorAdapterRegistry.getInstance();
for (Advisor advisor : config.getAdvisors()) {
if (advisor instanceof PointcutAdvisor) {
// Add it conditionally.
PointcutAdvisor pointcutAdvisor = (PointcutAdvisor) advisor;
if (config.isPreFiltered() || pointcutAdvisor.getPointcut().getClassFilter().matches(targetClass)) {
MethodInterceptor[] interceptors = registry.getInterceptors(advisor);
MethodMatcher mm = pointcutAdvisor.getPointcut().getMethodMatcher();
if (MethodMatchers.matches(mm, method, targetClass, hasIntroductions)) {
if (mm.isRuntime()) {
// Creating a new object instance in the getInterceptors() method
// isn't a problem as we normally cache created chains.
for (MethodInterceptor interceptor : interceptors) {
interceptorList.add(new InterceptorAndDynamicMethodMatcher(interceptor, mm));
}
}
else {
interceptorList.addAll(Arrays.asList(interceptors));
}
}
}
}
else if (advisor instanceof IntroductionAdvisor) {
IntroductionAdvisor ia = (IntroductionAdvisor) advisor;
if (config.isPreFiltered() || ia.getClassFilter().matches(targetClass)) {
Interceptor[] interceptors = registry.getInterceptors(advisor);
interceptorList.addAll(Arrays.asList(interceptors));
}
}
else {
Interceptor[] interceptors = registry.getInterceptors(advisor);
interceptorList.addAll(Arrays.asList(interceptors));
}
}
return interceptorList;
}

/**
* Determine whether the Advisors contain matching introductions.
*/
private static boolean hasMatchingIntroductions(Advised config, Class targetClass) {
for (int i = 0; i < config.getAdvisors().length; i++) {
Advisor advisor = config.getAdvisors()[i];
if (advisor instanceof IntroductionAdvisor) {
IntroductionAdvisor ia = (IntroductionAdvisor) advisor;
if (ia.getClassFilter().matches(targetClass)) {
return true;
}
}
}
return false;
}

看到这里，您会不会想这里的advisor是从哪里来的吗？实际上调用ProxyFactoryBean的getObject方法时，就初始化了一个advisor的list。
private synchronized void initializeAdvisorChain() throws AopConfigException, BeansException {
if (this.advisorChainInitialized) {
return;
}

if (!ObjectUtils.isEmpty(this.interceptorNames)) {
if (this.beanFactory == null) {
throw new IllegalStateException("No BeanFactory available anymore (probably due to serialization) " +
"- cannot resolve interceptor names " + Arrays.asList(this.interceptorNames));
}

// Globals can't be last unless we specified a targetSource using the property...
if (this.interceptorNames[this.interceptorNames.length - 1].endsWith(GLOBAL_SUFFIX) &&
this.targetName == null && this.targetSource == EMPTY_TARGET_SOURCE) {
throw new AopConfigException("Target required after globals");
}

// Materialize interceptor chain from bean names.
for (String name : this.interceptorNames) {
if (logger.isTraceEnabled()) {
logger.trace("Configuring advisor or advice '" + name + "'");
}

if (name.endsWith(GLOBAL_SUFFIX)) {
if (!(this.beanFactory instanceof ListableBeanFactory)) {
throw new AopConfigException(
"Can only use global advisors or interceptors with a ListableBeanFactory");
}
addGlobalAdvisor((ListableBeanFactory) this.beanFactory,
name.substring(0, name.length() - GLOBAL_SUFFIX.length()));
}

else {
// If we get here, we need to add a named interceptor.
// We must check if it's a singleton or prototype.
Object advice;
if (this.singleton || this.beanFactory.isSingleton(name)) {
// Add the real Advisor/Advice to the chain.
<strong>advice = this.beanFactory.getBean(name);</strong>
}
else {
// It's a prototype Advice or Advisor: replace with a prototype.
// Avoid unnecessary creation of prototype bean just for advisor chain initialization.
advice = new PrototypePlaceholderAdvisor(name);
}
addAdvisorOnChainCreation(advice, name);
}
}
}

this.advisorChainInitialized = true;
}

private void addAdvisorOnChainCreation(Object next, String name) {
// We need to convert to an Advisor if necessary so that our source reference
// matches what we find from superclass interceptors.
<strong>Advisor advisor = namedBeanToAdvisor(next);</strong>
if (logger.isTraceEnabled()) {
logger.trace("Adding advisor with name '" + name + "'");
}
addAdvisor(advisor);
}

advisor通知器的取得是委托给IoC容器完成的，ProxyFactoryBean实现了BeanFactoryAware接口，在自身实现时，就能得到它所在的IoC容器。取出来Advice对象会在DefaultAdvisorAdapterRegistry中被封装为Advisor对象。
public class DefaultAdvisorAdapterRegistry implements AdvisorAdapterRegistry, Serializable {

private final List<AdvisorAdapter> adapters = new ArrayList<AdvisorAdapter>(3);

/**
* Create a new DefaultAdvisorAdapterRegistry, registering well-known adapters.
*/
public DefaultAdvisorAdapterRegistry() {
registerAdvisorAdapter(new MethodBeforeAdviceAdapter());
registerAdvisorAdapter(new AfterReturningAdviceAdapter());
registerAdvisorAdapter(new ThrowsAdviceAdapter());
}

public Advisor wrap(Object adviceObject) throws UnknownAdviceTypeException {
if (adviceObject instanceof Advisor) {
return (Advisor) adviceObject;
}
if (!(adviceObject instanceof Advice)) {
throw new UnknownAdviceTypeException(adviceObject);
}
Advice advice = (Advice) adviceObject;
if (advice instanceof MethodInterceptor) {
// So well-known it doesn't even need an adapter.
return new DefaultPointcutAdvisor(advice);
}
for (AdvisorAdapter adapter : this.adapters) {
// Check that it is supported.
if (adapter.supportsAdvice(advice)) {
return new DefaultPointcutAdvisor(advice);
}
}
throw new UnknownAdviceTypeException(advice);
}

public MethodInterceptor[] getInterceptors(Advisor advisor) throws UnknownAdviceTypeException {
List<MethodInterceptor> interceptors = new ArrayList<MethodInterceptor>(3);
Advice advice = advisor.getAdvice();
if (advice instanceof MethodInterceptor) {
interceptors.add((MethodInterceptor) advice);
}
for (AdvisorAdapter adapter : this.adapters) {
if (adapter.supportsAdvice(advice)) {
interceptors.add(adapter.getInterceptor(advisor));
}
}
if (interceptors.isEmpty()) {
throw new UnknownAdviceTypeException(advisor.getAdvice());
}
return interceptors.toArray(new MethodInterceptor[interceptors.size()]);
}

public void registerAdvisorAdapter(AdvisorAdapter adapter) {
this.adapters.add(adapter);
}

}
DefaultAdvisorAdapterRegistry作为单件被GlobalAdvisorAdapterRegistry所用。在DefaultAdvisorAdapterRegistry中设置了一系列的adapter适配器，这些适配器为Spring AOP的advice提供了编织能力。这里Spring使用了Adapter模式，通过调用adapter的support方法，来判断取得的advice属于什么类型的advice通知，从而根据不同的advice类型来注册不同的AdviceInterceptor，这些AdviceInterceptor都是Spring
AOP框架设计好的，为实现不同的advice功能提供服务。正式这些AdviceInterceptor最终实现了advice通知在AopProxy代理对象中的织入功能。
  具体的Adapter是如何生成对应的Interceptor的，这里就不再详细介绍，感兴趣的读者可以查看Spring的相应源码。