我们继续上一节的讨论来看看在Spring的AOP中，拦截器链是如何调用的。
一:首先我们来看一下JdkDynamicAopProxy是如何生成代理对象的.JdkDynamicAopProxy实现了InvocationHandler,invoke方法的源码如下:
/**
* Implementation of <code>InvocationHandler.invoke</code>.
* <p>Callers will see exactly the exception thrown by the target,
* unless a hook method throws an exception.
*/
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();
if (target != null) {
targetClass = target.getClass();
}
// Get the interception chain for this method.
List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(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.
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 = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
// Proceed to the joinpoint through the interceptor chain.
retVal = invocation.proceed();
}
// 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);
}
}
}
从这个方法的源码中，我们看出如果没有配置拦截器链，会直接利用反射调用的目标对象的method，详见代码AopUtils.invokeJoinpointUsingReflection(target, method, args)。如果配置了拦截器链，则把处理逻辑转交给ReflectiveMethodInvocation的process方法。Cglib2AopProxy的intercept拦截的方式跟JdkDynamicAopProxy是相似的，这里不多做介绍，我们继续来看看ReflectiveMethodInvocation的process的源码。
public Object proceed() throws Throwable {
// We start with an index of -1 and increment early.
if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
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)) {
return dm.interceptor.invoke(this);
}
else {
// Dynamic matching failed.
// Skip this interceptor and invoke the next in the chain.
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(this);
}
}
看到这个变种责任链模式的应用是不是很熟悉哈，其实这个跟长软的基础框架中拦截器的实现原理是一致的。非常巧妙的一种处理方式，自己慢慢去领会吧。
二：我们再来看看拦截器链的配置和获取，从JdkDynamicAopProxy的invoke方法的源码List chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);我们可以逆向追踪chain的来源。
我们进入到AdvisedSupport类中来看看getInterceptorsAndDynamicInterceptionAdvice的源码
/**
* Determine a list of {@link org.aopalliance.intercept.MethodInterceptor} objects
* for the given method, based on this configuration.
* @param method the proxied method
* @param targetClass the target class
* @return List of MethodInterceptors (may also include InterceptorAndDynamicMethodMatchers)
*/
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);
}
return cached;
}
这里很明显使用了缓存技术，读取一次便存储在methodCache中，第一次读取的操作是DefaultAdvisorChainFactory的getInterceptorsAndDynamicInterceptionAdvice方法中完成的。
public class DefaultAdvisorChainFactory implements AdvisorChainFactory, Serializable {
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;
}
}
在getInterceptorsAndDynamicInterceptionAdvice中，最核心的操作是registry.getInterceptors(advisor).这个registery是通过AdvisorAdapterRegistry registry = GlobalAdvisorAdapterRegistry.getInstance();来获取的。我们再来看一下GlobalAdvisorAdapterRegistry的源码：
public abstract class GlobalAdvisorAdapterRegistry {
/**
* Keep track of a single instance so we can return it to classes that request it.
*/
private static final AdvisorAdapterRegistry instance = new DefaultAdvisorAdapterRegistry();
/**
* Return the singleton DefaultAdvisorAdapterRegistry instance.
*/
public static AdvisorAdapterRegistry getInstance() {
return instance;
}
}
很显然这是一个单例模式，真正得到的registery是DefaultAdvisorAdapterRegistry类型的.DefaultAdvisorAdapterRegistry的源码解析如下：
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);
}
}
在DefaultAdvisorAdapterRegistry的初始化方法中，我们可以看到注册了三个适配器，从MethodBeforeAdviceAdapter、AfterReturningAdviceAdapter以及ThrowsAdviceAdapter的名字就可以看出来他们对应的Advice类型。
以MethodBeforeAdviceAdapter为例，我们来看看源码：
class MethodBeforeAdviceAdapter implements AdvisorAdapter, Serializable {
public boolean supportsAdvice(Advice advice) {
return (advice instanceof MethodBeforeAdvice);
}
public MethodInterceptor getInterceptor(Advisor advisor) {
MethodBeforeAdvice advice = (MethodBeforeAdvice) advisor.getAdvice();
return new MethodBeforeAdviceInterceptor(advice);
}
}
接下来我们来看一下MethodBeforeAdviceInterceptor的源码实现:
public class MethodBeforeAdviceInterceptor implements MethodInterceptor, Serializable {
private MethodBeforeAdvice advice;
/**
* Create a new MethodBeforeAdviceInterceptor for the given advice.
* @param advice the MethodBeforeAdvice to wrap
*/
public MethodBeforeAdviceInterceptor(MethodBeforeAdvice advice) {
Assert.notNull(advice, "Advice must not be null");
this.advice = advice;
}
public Object invoke(MethodInvocation mi) throws Throwable {
this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis() );
return mi.proceed();
}
}
这里重点关注一下mi.proceed()方法.
AfterReturningAdviceInterceptor和ThrowAdviceInterceptor的实现类似于MethodBeforeAdviceInterceptor，这里不多介绍。
至此，我们再来回顾一下ReflectiveMethodInvocation的process方法,
public Object proceed() throws Throwable {
// We start with an index of -1 and increment early.
if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
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)) {
return dm.interceptor.invoke(this);
}
else {
// Dynamic matching failed.
// Skip this interceptor and invoke the next in the chain.
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(this);
}
}
我们来关注一下interceptor.invoke(this)，其实在这个interceptor的invoke方法里又返过来调用这个process方法。
正是这两个地方的交互，形成一个巧妙和责任链处理模式。这里需要用心去体会一下。