生成代理对象后的问题是，代理对象生成了，那切面是如何织入的？
我们知道 InvocationHandler 是 JDK 动态代理的核心，生成的代理对象的方法调用都会委托到 InvocationHandler.invoke()方法。而从 JdkDynamicAopProxy 的源码我们可以看到这个类其实也实现了InvocationHandler，下面我们分析SpringAOP 是如何织入切面的，直接上源码看invoke()方法：
invoke()
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 {
// eqauls()方法，具目标对象未实现此方法
if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
// The target does not implement the equals(Object) method itself.
return equals(args[0]);
}
// hashCode()方法，具目标对象未实现此方法
else if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
// The target does not implement the hashCode() method itself.
return hashCode();
}
else if (method.getDeclaringClass() == DecoratingProxy.class) {
// There is only getDecoratedClass() declared -> dispatch to proxy config.
return AopProxyUtils.ultimateTargetClass(this.advised);
}
// Advised接口或者其父接口中定义的方法,直接反射调用,不应用通知
else 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;
}
// Get as late as possible to minimize the time we "own" the target,
// in case it comes from a pool.
// 获得目标对象的类
target = targetSource.getTarget();
Class<?> targetClass = (target != null ? target.getClass() : null);
// Get the interception chain for this method.
// 获取可以应用到此方法上的Interceptor列表
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.
// 如果没有可以应用到此方法的通知(Interceptor)，此直接反射调用 method.invoke(target, args)
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
Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
}
else {
// We need to create a method invocation...
// 创建MethodInvocation
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.
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);
}
}
}主要实现思路可以简述为：首先获取应用到此方法上的通知链（InterceptorChain）。如果有通知，则应用通知，并执行 JoinPoint；如果没有通知，则直接反射执行JoinPoint。而这里的关键是通知链是如何获取的以及它又是如何执行的呢？
1.获取通知链(DefaultAdvisorChainFactory)
首先，从上面的代码可以看到，通知链是通过Advised.getInterceptorsAndDynamicInterceptionAdvice()这个方法来获取的，我们来看下这个方法的实现逻辑：
getInterceptorsAndDynamicInterceptionAdvice()
// AdvisedSupport
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;
}通过上面的源码我们可以看到，实际获取通知的实现逻辑其实是由 AdvisorChainFactory 的
getInterceptorsAndDynamicInterceptionAdvice()方法来完成的，且获取到的结果会被缓存。下面来分析getInterceptorsAndDynamicInterceptionAdvice()方法的实现：
getInterceptorsAndDynamicInterceptionAdvice()
/**
* 从提供的配置实例config中获取advisor列表,遍历处理这些advisor.
* 如果是IntroductionAdvisor,则判断此Advisor能否应用到目标类targetClass上.
* 如果是PointcutAdvisor,则判断此Advisor能否应用到目标方法method上.将满足条件的Advisor通过AdvisorAdaptor转化成Interceptor列表返回.
*/
@Override
public List<Object> getInterceptorsAndDynamicInterceptionAdvice(
Advised config, Method method, @Nullable 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<>(config.getAdvisors().length);
Class<?> actualClass = (targetClass != null ? targetClass : method.getDeclaringClass());
// 查看是否包含IntroductionAdvisor
boolean hasIntroductions = hasMatchingIntroductions(config, actualClass);
//这 里实际上注册一系列AdvisorAdapter,用于将Advisor转化成MethodInterceptor
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(actualClass)) {
// 这个地方这两个方法的位置可以互换下,将Advisor转化成Interceptor
MethodInterceptor[] interceptors = registry.getInterceptors(advisor);
// 检查当前advisor的pointcut是否可以匹配当前方法
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;
}这个方法执行完成后，Advised 中配置能够应用到连接点（JoinPoint）或者目标类（Target Object）的Advisor全部被转化成了MethodInterceptor，接下来我们再看下得到的拦截器链是怎么起作用的（invoke方法中）
// 如果没有可以应用到此方法的通知(Interceptor)，此直接反射调用 method.invoke(target, args)
if (chain.isEmpty()) {
Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
}
else {
// 创建MethodInvocation
invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
// Proceed to the joinpoint through the interceptor chain.
retVal = invocation.proceed();
}从这段代码可以看出，如果得到的拦截器链为空，则直接反射调用目标方法，否则创建 MethodInvocation，调用其proceed()方法，触发拦截器链的执行，来看下具体代码:
2.执行通知链（ReflectiveMethodInvocation）
proceed()
public Object proceed() throws Throwable {
// We start with an index of -1 and increment early.
// 如果Interceptor执行完了，则执行joinPoint
if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1)
{
return invokeJoinpoint();
}
Object interceptorOrInterceptionAdvice = this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
// 如果要动态匹配joinPoint
if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
// Evaluate dynamic method matcher here: static part will already have been evaluated and found to match.
InterceptorAndDynamicMethodMatcher dm = (InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
//动态匹配：运行时参数是否满足匹配条件
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.
// 动态匹配失败时,略过当前Intercetpor,调用下一个Interceptor
return proceed();
}
}
else {
// It's an interceptor, so we just invoke it: The pointcut will have
// been evaluated statically before this object was constructed.
// 执行当前Intercetpor
return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
}
}至此，通知链就完美地形成了。我们再往下来看 invokeJoinpointUsingReflection()方法，其实就是反射调用：
3.反射调用（AopUtils）
invokeJoinpointUsingReflection()
public static Object invokeJoinpointUsingReflection(@Nullable Object target, Method method, Object[] args)
throws Throwable {
// Use reflection to invoke the method.
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);
}
}
本文章为转载内容，我们尊重原作者对文章享有的著作权。如有内容错误或侵权问题，欢迎原作者联系我们进行内容更正或删除文章。
