在前面的两篇文章中，分别介绍了 Spring AOP 是如何为目标 bean 筛选合适的通知器，以及如何创建代理对象的过程。现在得到了 bean 的代理对象，且通知也以合适的方式插在了目标方法的前后。接下来要做的事情，就是执行通知逻辑了。通知可能在目标方法前执行，也可能在目标方法后执行。具体的执行时机，取决于用户的配置。当目标方法被多个通知匹配到时，Spring 通过引入拦截器链来保证每个通知的正常执行。在本文中，我们将会通过源码了解到 Spring 是如何支持 expose-proxy 属性的，以及通知与拦截器之间的关系，拦截器链的执行过程等。
有时候目标对象内部的自我调用将无法实施切面中的增强：
public interface UserService{
public void a();
public void a();
}
public class UserServiceImpl implements UserService{
@Transactional(propagation = Propagation.REQUIRED)
public void a(){
this.b();
}
@Transactional(propagation = Propagation.REQUIRED_NEW)
public void b(){
System.out.println("b has been called");
}
}
a的事务会生效，b中不会有事务，因为a中调用b属于内部调用，没有通过代理，所以不会有事务产生（原因：SpringAOP对于最外层的函数只拦截public方法，不拦截protected和private方法，另外不会对最外层的public方法内部调用的其他方法也进行拦截，即只停留于代理对象所调用的方法。）。
为了解决这个问题，我们可以这样做：<aop:aspectj-autoproxy expose-proxy=“true”> ，设置expose-proxy属性为true，将代理暴露出来，使用AopContext.currentProxy()获取当前代理，将this.b()改为((UserService)AopContext.currentProxy()).b()。
本章所分析的源码来自 JdkDynamicAopProxy。
对于 JDK 动态代理，代理逻辑封装在 InvocationHandler 接口实现类的 invoke 方法中。JdkDynamicAopProxy 实现了 InvocationHandler 接口，下面我们就来分析一下 JdkDynamicAopProxy 的 invoke 方法。如下：
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
MethodInvocation invocation;
Object oldProxy = null;
boolean setProxyContext = false;
//获取原对象信息
TargetSource targetSource = this.advised.targetSource;
Class<?> targetClass = null;
Object target = null;
try {
//如果接口中定义了equals或hashCode方法，则进行专门处理
if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
// The target does not implement the equals(Object) method itself.
return equals(args[0]);
}
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);
}
//opaque属性，表示是否禁止将代理对象转换为Advised对象，默认是false
//如果调用的方法来自Advised接口
else if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
method.getDeclaringClass().isAssignableFrom(Advised.class)) {
//通过反射Method.invoke,调用advised（传入的ProxyFactory实例，该类实现了Advised接口）对应的方法
return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
}
Object retVal;
//为了解决目标对象内部的自我调用无法实施切面中的增强，需要暴露代理对象
if (this.advised.exposeProxy) {
oldProxy = AopContext.setCurrentProxy(proxy);
setProxyContext = true;
}
// 获取目标对象信息
target = targetSource.getTarget();
if (target != null) {
targetClass = target.getClass();
}
// 获取当前方法的拦截器链
List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
// 如果没有任何拦截器，则调用直接对原目标对象调用方法
if (chain.isEmpty()) {
Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
}
else {
// 将拦截器链封装到ReflectiveMethodInvocation，方便使用其proceed进行链式调用拦截器
invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
// 执行拦截器链
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())) {
// 特殊情况：为了防止方法返回"return this"，返回原目标对象,会将返回值替换为代理对象
retVal = proxy;
}
//如果返回类型是基本类型兵器，但是返回结果为null，抛出异常
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);
}
}
}下面来总结一下invoke方法的执行流程：
拦截器链是指对于目标方法的调用进行拦截的一种工具，下面以前置拦截器为例，如下：
public class MethodBeforeAdviceInterceptor implements MethodInterceptor, Serializable {
/** 前置通知 */
private MethodBeforeAdvice advice;
public MethodBeforeAdviceInterceptor(MethodBeforeAdvice advice) {
Assert.notNull(advice, "Advice must not be null");
this.advice = advice;
}
@Override
public Object invoke(MethodInvocation mi) throws Throwable {
// 执行前置通知逻辑
this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis());
// 通过 MethodInvocation 调用下一个拦截器，若所有拦截器均执行完，则调用目标方法
return mi.proceed();
}
}如源码所示，前置通知在目标方法执行前被执行。下来看看如何获取拦截器：
public List<Object> getInterceptorsAndDynamicInterceptionAdvice(Method method, Class<?> targetClass) {
//缓存的cacheKey，先尝试从缓存获取，不存在再从ProxyFactory中解析
MethodCacheKey cacheKey = new MethodCacheKey(method);
List<Object> cached = this.methodCache.get(cacheKey);
if (cached == null) {
//通过实例方法和ProxyFactory中保存的信息，解析出匹配的增强
cached = this.advisorChainFactory.getInterceptorsAndDynamicInterceptionAdvice(
this, method, targetClass);
this.methodCache.put(cacheKey, cached);
}
return cached;
}
public List<Object> getInterceptorsAndDynamicInterceptionAdvice(
Advised config, Method method, Class<?> targetClass) {
List<Object> interceptorList = new ArrayList<Object>(config.getAdvisors().length);
Class<?> actualClass = (targetClass != null ? targetClass : method.getDeclaringClass());
//方法是否匹配引介增强
boolean hasIntroductions = hasMatchingIntroductions(config, actualClass);
AdvisorAdapterRegistry registry = GlobalAdvisorAdapterRegistry.getInstance();
//遍历增强器
for (Advisor advisor : config.getAdvisors()) {
//PointcutAdvisor类型的增强器
//通过@Aspect加入的增强器类型为InstantiationModelAwarePointcutAdvisorImpl，实现了PointcutAdvisor
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));
}
}
}
}
//对于引介增强的处理
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;
}
public MethodInterceptor[] getInterceptors(Advisor advisor) throws UnknownAdviceTypeException {
List<MethodInterceptor> interceptors = new ArrayList<MethodInterceptor>(3);
//获取Advice
Advice advice = advisor.getAdvice();
//如果Advice实例同时已经实现MethodInterceptor接口，则直接使用
if (advice instanceof MethodInterceptor) {
interceptors.add((MethodInterceptor) advice);
}
//需要使用适配器来转换Advice接口
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()]);
}
以上就是获取拦截器的过程，这里简单总结一下以上源码的执行过程，如下：
在前面的getInterceptors方法中将Advice转换为MethodInterceptor的工作是交给this.adapters来完成的，该变量定义如下：
List<MethodInterceptor> interceptors = new ArrayList<MethodInterceptor>(3);并且在DefaultAdvisorAdapterRegistry的构造函数中，对该变量进行了初始化填充：
public DefaultAdvisorAdapterRegistry() {
registerAdvisorAdapter(new MethodBeforeAdviceAdapter());
registerAdvisorAdapter(new AfterReturningAdviceAdapter());
registerAdvisorAdapter(new ThrowsAdviceAdapter());
}可以看到，Spring实现会加入3个默认的是Advice适配器：MethodBeforeAdviceAdapter、AfterReturningAdviceAdapter、ThrowsAdviceAdapter。
这三个适配器通过support方法，验证是否是否支持传入的Advice对象，如果支持，会将Advice实例封装为对应的MethodInterceptor实例：
class MethodBeforeAdviceAdapter implements AdvisorAdapter, Serializable {
@Override
public boolean supportsAdvice(Advice advice) {
return (advice instanceof MethodBeforeAdvice);
}
@Override
public MethodInterceptor getInterceptor(Advisor advisor) {
MethodBeforeAdvice advice = (MethodBeforeAdvice) advisor.getAdvice();
return new MethodBeforeAdviceInterceptor(advice);
}
}Spring会在获取到方法匹配的拦截器后，将代理对象、目标对象、调用方法、参数、拦截器等信息封装ReflectiveMethodInvocation中：
protected ReflectiveMethodInvocation(
Object proxy, Object target, Method method, Object[] arguments,
Class<?> targetClass, List<Object> interceptorsAndDynamicMethodMatchers) {
this.proxy = proxy;
this.target = target;
this.targetClass = targetClass;
this.method = BridgeMethodResolver.findBridgedMethod(method);
this.arguments = AopProxyUtils.adaptArgumentsIfNecessary(method, arguments);
this.interceptorsAndDynamicMethodMatchers = interceptorsAndDynamicMethodMatchers;
}然后调用ReflectiveMethodInvocation的proceed方法：
public Object proceed() throws Throwable {
// 执行完所有增强后，执行切点方法
if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
return invokeJoinpoint();
}
//获取下一个拦截器
Object interceptorOrInterceptionAdvice =
this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
//动态匹配
InterceptorAndDynamicMethodMatcher dm =
(InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
if (dm.methodMatcher.matches(this.method, this.targetClass, this.arguments)) {
return dm.interceptor.invoke(this);
}
else {
//不匹配则不执行拦截器
return proceed();
}
}
else {
// 普通拦截器。比如：MethodBeforeAdviceInterceptor、AfterReturningAdviceInterceptor、ThrowsAdviceInterceptor
return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
}
}通过currentInterceptorIndex记录当前拦截器的索引，每调用一个拦截器就+1，再次调用proceed方法时就会获取下一个拦截器调用。
在之前一节中，我们以前置拦截器为例，向大家展示拦截器的样子，现在又向大家展示下后置拦截器：
public class AspectJAfterAdvice extends AbstractAspectJAdvice
implements MethodInterceptor, AfterAdvice, Serializable {
public AspectJAfterAdvice(
Method aspectJBeforeAdviceMethod, AspectJExpressionPointcut pointcut, AspectInstanceFactory aif) {
super(aspectJBeforeAdviceMethod, pointcut, aif);
}
@Override
public Object invoke(MethodInvocation mi) throws Throwable {
try {
// 调用 proceed
return mi.proceed();
}
finally {
// 调用后置通知逻辑
invokeAdviceMethod(getJoinPointMatch(), null, null);
}
}
//...
}和前置拦截器不同的是，由于后置通知需要在目标方法返回后执行，所以 AspectJAfterAdvice 先调用 mi.proceed() 执行下一个拦截器逻辑，等下一个拦截器返回后，再执行后置通知逻辑。
这里假设目标方法 method 在执行前，需要执行一个前置通知和一个后置通知。下面我们看一下由三个拦截器组成的拦截器链是如何执行的，如下：
protected Object invokeJoinpoint() throws Throwable {
return AopUtils.invokeJoinpointUsingReflection(this.target, this.method, this.arguments);
}
public abstract class AopUtils {
public static Object invokeJoinpointUsingReflection(Object target, Method method, Object[] args)
throws Throwable {
try {
ReflectionUtils.makeAccessible(method);
// 通过反射执行目标方法
return method.invoke(target, args);
}
catch (InvocationTargetException ex) {...}
catch (IllegalArgumentException ex) {...}
catch (IllegalAccessException ex) {...}
}
}在之前的筛选合适通知器的一文中，介绍了extendAdvisors，其中有一个点没有详细说明，现在重新进行解析：
protected void extendAdvisors(List<Advisor> candidateAdvisors) {
AspectJProxyUtils.makeAdvisorChainAspectJCapableIfNecessary(candidateAdvisors);
}
public static boolean makeAdvisorChainAspectJCapableIfNecessary(List<Advisor> advisors) {
if (!advisors.isEmpty()) {
// 省略部分代码
if (foundAspectJAdvice && !advisors.contains(ExposeInvocationInterceptor.ADVISOR)) {
// 向通知器列表中添加 ExposeInvocationInterceptor.ADVISOR
advisors.add(0, ExposeInvocationInterceptor.ADVISOR);
return true;
}
}
return false;
}如上，extendAdvisors 所调用的方法会向通知器列表首部添加 ExposeInvocationInterceptor.ADVISOR。现在我们再来看看 ExposeInvocationInterceptor 的源码，如下：
public class ExposeInvocationInterceptor implements MethodInterceptor, PriorityOrdered, Serializable {
public static final ExposeInvocationInterceptor INSTANCE = new ExposeInvocationInterceptor();
// 创建 DefaultPointcutAdvisor 匿名对象
public static final Advisor ADVISOR = new DefaultPointcutAdvisor(INSTANCE) {
@Override
public String toString() {
return ExposeInvocationInterceptor.class.getName() +".ADVISOR";
}
};
private static final ThreadLocal<MethodInvocation> invocation =
new NamedThreadLocal<MethodInvocation>("Current AOP method invocation");
public static MethodInvocation currentInvocation() throws IllegalStateException {
MethodInvocation mi = invocation.get();
if (mi == null)
throw new IllegalStateException(
"No MethodInvocation found: Check that an AOP invocation is in progress, and that the " +
"ExposeInvocationInterceptor is upfront in the interceptor chain. Specifically, note that " +
"advices with order HIGHEST_PRECEDENCE will execute before ExposeInvocationInterceptor!");
return mi;
}
// 私有构造方法
private ExposeInvocationInterceptor() {
}
@Override
public Object invoke(MethodInvocation mi) throws Throwable {
MethodInvocation oldInvocation = invocation.get();
// 将 mi 设置到 ThreadLocal 中
invocation.set(mi);
try {
// 调用下一个拦截器
return mi.proceed();
}
finally {
invocation.set(oldInvocation);
}
}
//...
}如上，ExposeInvocationInterceptor.ADVISOR 经过 registry.getInterceptors 方法（前面已分析过）处理后，即可得到 ExposeInvocationInterceptor。ExposeInvocationInterceptor 的作用是用于暴露 MethodInvocation 对象到 ThreadLocal 中，其名字也体现出了这一点。如果其他地方需要当前的 MethodInvocation 对象，直接通过调用 currentInvocation 方法取出。
参考文章：