本节一起学习Spring AOP拦截器调用的实现
在Spring AOP通过JDK的代理方式或者CGLIB方式生成代理对象的时候，相关的拦截器已经配置到代理对象中了，拦截器在代理对象中起作用是通过对这些方法的回调完成的。
如果是JDK的Proxy生成代理对象，那么需要通过InvocationHandler来设置拦截器回调，如果是使用CGLIB，则通过DynamicAdvisedInterceptor完成回调。
下面先看一下JdkDynamicAopProxy的invoke拦截
@Override
@Nullable
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
MethodInvocation invocation;
Object oldProxy = null;
boolean setProxyContext = false;
TargetSource targetSource = this.advised.targetSource;
Object target = null;
try {
if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
// 如果目标对象没有实现Object类的基本方法 equals
return equals(args[0]);
}
else if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
// 如果目标对象没有实现Object类的基本方法 hashcode
return hashCode();
}
else if (method.getDeclaringClass() == DecoratingProxy.class) {
// There is only getDecoratedClass() declared -> dispatch to proxy config.
return AopProxyUtils.ultimateTargetClass(this.advised);
}
else if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
method.getDeclaringClass().isAssignableFrom(Advised.class)) {
// 根据代理对象的配置调用服务
return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
}
Object retVal;
if (this.advised.exposeProxy) {
// Make invocation available if necessary.
oldProxy = AopContext.setCurrentProxy(proxy);
setProxyContext = true;
}
// 得到目标对象
target = targetSource.getTarget();
Class<?> targetClass = (target != null ? target.getClass() : null);
// 获取定义好的拦截器链
List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
// 如果没有定义拦截器链，则直接调用target的对应方法
if (chain.isEmpty()) {
Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
}
else {
// 如果有拦截器链，需要调用拦截器链后才调用目标对象的相应方法
invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
// 沿着拦截器链继续前进
retVal = invocation.proceed();
}
// Massage return value if necessary.
Class<?> returnType = method.getReturnType();
if (retVal != null && retVal == target &&
returnType != Object.class && returnType.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;
}
else if (retVal == null && returnType != Void.TYPE && returnType.isPrimitive()) {
throw new AopInvocationException(
"Null return value from advice does not match primitive return type for: " + method);
}
return retVal;
}
finally {
if (target != null && !targetSource.isStatic()) {
// Must have come from TargetSource.
targetSource.releaseTarget(target);
}
if (setProxyContext) {
// Restore old proxy.
AopContext.setCurrentProxy(oldProxy);
}
}
}
下面看一下CglibAopProxy的intercept拦截
@Override
@Nullable
public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
Object oldProxy = null;
boolean setProxyContext = false;
Object target = null;
TargetSource targetSource = this.advised.getTargetSource();
try {
if (this.advised.exposeProxy) {
// Make invocation available if necessary.
oldProxy = AopContext.setCurrentProxy(proxy);
setProxyContext = true;
}
// Get as late as possible to minimize the time we "own" the target, in case it comes from a pool...
target = targetSource.getTarget();
Class<?> targetClass = (target != null ? target.getClass() : null);
List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
Object retVal;
// Check whether we only have one InvokerInterceptor: that is,
// no real advice, but just reflective invocation of the target.
if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) {
// 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.
Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
retVal = methodProxy.invoke(target, argsToUse);
}
else {
// 通过CglibMethodInvocation启动advice通知
retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
}
retVal = processReturnType(proxy, target, method, retVal);
return retVal;
}
finally {
if (target != null && !targetSource.isStatic()) {
targetSource.releaseTarget(target);
}
if (setProxyContext) {
// Restore old proxy.
AopContext.setCurrentProxy(oldProxy);
}
}
}
下面分析一下对目标对象的直接调用方式：
如果没有设置拦截器，那么会对目标对象的方法直接进行调用。对于JdkDynamicAopProxy代理对象，，这个对目标对象的方法调用是通过AopUtils使用反射机制在invokeJoinpointUsingReflection方法实现的。
@Nullable
public static Object invokeJoinpointUsingReflection(@Nullable Object target, Method method, Object[] args)
throws Throwable {
// 使用反射调用target对象方法
try {
ReflectionUtils.makeAccessible(method);
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("Could not access method [" + method + "]", ex);
}
}
对于CGLIB ，对目标对象的调用是通过CGLIB的MethodProxy对象完成的
retVal = methodProxy.invoke(target, argsToUse);
下面分析一下AOP拦截器的调用
对拦截器的调用，JDK使用的是ReflectiveMethodInvocation的proceed方法实现的，下面看一下源码：
@Override
@Nullable
public Object proceed() throws Throwable {
// 从索引为-1的拦截器开始调用，按顺序递增，如果拦截器链中拦截器迭代调用完毕，这里开始调用target的函数
if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
return invokeJoinpoint();
}
Object interceptorOrInterceptionAdvice =
this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
// 这里对拦截器进行动态匹配的判断
InterceptorAndDynamicMethodMatcher dm =
(InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
if (dm.methodMatcher.matches(this.method, this.targetClass, this.arguments)) {
return dm.interceptor.invoke(this);
}
else {
// 如果不匹配，则递归调用，直到所有的拦截器都被运行过为止
return proceed();
}
}
else {
// 如果是一个interceptor，直接调用这个interceptor对应的方法
return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
}
}
先进行判断，如果已经运行到拦截器链的末尾，那么会直接调用目标对象的实现方法。否则，沿着拦截器链继续进行，得到下一个拦截器，通过这个拦截器进行match判断，判断是否适用于横切增强的场合，如果是，从拦截器中得到通知器，并启动通知器的invoke方法进行切面增强。这个过程结束后会迭代调用，直到拦截器链中的拦截器都完成以上的拦截过程为止。
下面看一下如何获取的拦截器链
List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
public List<Object> getInterceptorsAndDynamicInterceptionAdvice(Method method, @Nullable 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;
}可以看到获取拦截器链，是由advisorChainFactory完成的，它是一个生成通知器链的工厂。
@Override
public List<Object> getInterceptorsAndDynamicInterceptionAdvice(
Advised config, Method method, @Nullable Class<?> targetClass) {
// 首先创建了一个list，长度由配置的通知器的个数决定
List<Object> interceptorList = new ArrayList<>(config.getAdvisors().length);
Class<?> actualClass = (targetClass != null ? targetClass : method.getDeclaringClass());
boolean hasIntroductions = hasMatchingIntroductions(config, actualClass);
AdvisorAdapterRegistry registry = GlobalAdvisorAdapterRegistry.getInstance();
//使用AdvisorAdapterRegistry实现拦截器的注册
for (Advisor advisor : config.getAdvisors()) {
if (advisor instanceof PointcutAdvisor) {
// Add it conditionally.
PointcutAdvisor pointcutAdvisor = (PointcutAdvisor) advisor;
if (config.isPreFiltered() || pointcutAdvisor.getPointcut().getClassFilter().matches(actualClass)) {
MethodInterceptor[] interceptors = registry.getInterceptors(advisor);
MethodMatcher mm = pointcutAdvisor.getPointcut().getMethodMatcher();
if (MethodMatchers.matches(mm, method, actualClass, 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(actualClass)) {
Interceptor[] interceptors = registry.getInterceptors(advisor);
interceptorList.addAll(Arrays.asList(interceptors));
}
}
else {
Interceptor[] interceptors = registry.getInterceptors(advisor);
interceptorList.addAll(Arrays.asList(interceptors));
}
}
return interceptorList;
}
实际上，这里的advisor通知器是从AdvisorSupport取得的。
在ProxyFactoryBean的getObject方法中对advisor进行初始化时，从XML配置中获取了advisor通知器。在proxyFactoryBean中，可以看到对IOC容器的一个getBean回调，通过这个回调得到配置好的advisor通知器。
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.
advice = this.beanFactory.getBean(name);
}
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;
}
下面看一下Advice通知的实现
在为AopProxy代理对象配置拦截器的实现中，有一个取得拦截器的配置过程，这个过程是由DefaultAdvisorChainFactory实现的，这个工厂类负责生成拦截器链，在它的getInterceptorsAndDynamicInterceptionAdvice方法中，有一个适配器和注册过程，通过配置Spring 预先设计好的拦截器，Spring 加入了它对AOP实现的处理。
GlobalAdvisorAdapterRegistry负责拦截器的适配和注册过程。
public abstract class GlobalAdvisorAdapterRegistry {
/**
* Keep track of a single instance so we can return it to classes that request it.
*/
private static AdvisorAdapterRegistry instance = new DefaultAdvisorAdapterRegistry();
/**
* Return the singleton {@link DefaultAdvisorAdapterRegistry} instance.
*/
public static AdvisorAdapterRegistry getInstance() {
return instance;
}
/**
* Reset the singleton {@link DefaultAdvisorAdapterRegistry}, removing any
* {@link AdvisorAdapterRegistry#registerAdvisorAdapter(AdvisorAdapter) registered}
* adapters.
*/
static void reset() {
instance = new DefaultAdvisorAdapterRegistry();
}
}
GlobalAdvisorAdapterRegistry起到了适配器和单例模式的作用，提供了一个DefaultAdvisorAdapterRegistry，它用来完成各种通知的适配和注册过程。
public class DefaultAdvisorAdapterRegistry implements AdvisorAdapterRegistry, Serializable {
//持有一个AdvisorAdapter的List
private final List<AdvisorAdapter> adapters = new ArrayList<>(3);
/**
* 这里把已有的advice实现的Adapter加入进来
*/
public DefaultAdvisorAdapterRegistry() {
registerAdvisorAdapter(new MethodBeforeAdviceAdapter());
registerAdvisorAdapter(new AfterReturningAdviceAdapter());
registerAdvisorAdapter(new ThrowsAdviceAdapter());
}
@Override
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);
}
@Override
public MethodInterceptor[] getInterceptors(Advisor advisor) throws UnknownAdviceTypeException {
List<MethodInterceptor> interceptors = new ArrayList<>(3);
Advice advice = advisor.getAdvice();//从advisor通知器中获得advice通知
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[0]);
}
@Override
public void registerAdvisorAdapter(AdvisorAdapter adapter) {
this.adapters.add(adapter);
}
}
DefaultAdvisorAdapterRegistry设置了一系列的是配置，正是这些适配器的实现，为Spring AOP提供了编织能力。
下面以MethodBeforeAdviceAdapter为例子，看具体的实现：
class MethodBeforeAdviceAdapter implements AdvisorAdapter, Serializable {
@Override
public boolean supportsAdvice(Advice advice) {//对advice的类型判断
return (advice instanceof MethodBeforeAdvice);
}
@Override
public MethodInterceptor getInterceptor(Advisor advisor) {//把advice通知从通知器中取出，返回一个包装了advice的类
MethodBeforeAdvice advice = (MethodBeforeAdvice) advisor.getAdvice();
return new MethodBeforeAdviceInterceptor(advice);
}
}
Spring AOP为了实现advice的织入，设计了特定的拦截器对这些功能进行了封装。
我们接着看MethodBeforeAdviceInterceptor如何完成封装的
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;
}
@Override
public Object invoke(MethodInvocation mi) throws Throwable {
this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis());
return mi.proceed();
}
}可以看到，invoke方法中，首先触发了advice的before回调，然后才是proceed。
AfterReturningAdviceInterceptor的源码：
public class AfterReturningAdviceInterceptor implements MethodInterceptor, AfterAdvice, Serializable {
private final AfterReturningAdvice advice;
/**
* Create a new AfterReturningAdviceInterceptor for the given advice.
* @param advice the AfterReturningAdvice to wrap
*/
public AfterReturningAdviceInterceptor(AfterReturningAdvice advice) {
Assert.notNull(advice, "Advice must not be null");
this.advice = advice;
}
@Override
public Object invoke(MethodInvocation mi) throws Throwable {
Object retVal = mi.proceed();
this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());
return retVal;
}
}
ThrowsAdviceInterceptor的源码
@Override
public Object invoke(MethodInvocation mi) throws Throwable {
try {
return mi.proceed();
}
catch (Throwable ex) {
Method handlerMethod = getExceptionHandler(ex);
if (handlerMethod != null) {
invokeHandlerMethod(mi, ex, handlerMethod);
}
throw ex;
}
}
至此，我们知道了对目标对象的增强是通过拦截器实现的。