Spring事务管理(4)-AOP创建代理

在上文中，我们分析了Spring创建Advisor的整个过程。不知道大家有没有注意到，在文章的开始阶段的wrapIfNecessary方法中，有createProxy的语句，这个就是创建代理的过程了。

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
if (beanName != null && this.targetSourcedBeans.containsKey(beanName)) {
return bean;
}
if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
return bean;
}
if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}

// Create proxy if we have advice.
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
if (specificInterceptors != DO_NOT_PROXY) {
this.advisedBeans.put(cacheKey, Boolean.TRUE);
// 创建代理对象
Object proxy = createProxy(bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
this.proxyTypes.put(cacheKey, proxy.getClass());
return proxy;
}

this.advisedBeans.put(cacheKey, Boolean.FALSE);
return bean;
}

protected Object createProxy(
Class<?> beanClass, String beanName, Object[] specificInterceptors, TargetSource targetSource) {

ProxyFactory proxyFactory = new ProxyFactory();
// Copy our properties (proxyTargetClass etc) inherited from ProxyConfig.
proxyFactory.copyFrom(this);

if (!shouldProxyTargetClass(beanClass, beanName)) {
// Must allow for introductions; can't just set interfaces to
// the target's interfaces only.
Class<?>[] targetInterfaces = ClassUtils.getAllInterfacesForClass(beanClass, this.proxyClassLoader);
for (Class<?> targetInterface : targetInterfaces) {
proxyFactory.addInterface(targetInterface);
}
}

// 添加commonAdvisors
Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
for (Advisor advisor : advisors) {
// 把advisors赋给proxyFactory
proxyFactory.addAdvisor(advisor);
}

proxyFactory.setTargetSource(targetSource);
customizeProxyFactory(proxyFactory);

proxyFactory.setFrozen(this.freezeProxy);
if (advisorsPreFiltered()) {
proxyFactory.setPreFiltered(true);
}

// 返回代理对象
return proxyFactory.getProxy(this.proxyClassLoader);
}

public Object getProxy(ClassLoader classLoader) {
return createAopProxy().getProxy(classLoader);
}

以上createAopProxy返回一个AopProxy对象，AopProxy的类层次结构如下图所示：



目前Spring支持CgilbAopProxy和JdkDynamicAopProxy。本文以JdkDynamicAopProxy代理对象的生成为例。

查看JdkDynamicAopProxy，可以发现其实现InvocationHandler的接口。这与我们自己定义一个代理对象已经没有什么区别了。

JdkDynamicAopProxy的getProxy返回一个代理对象，在invoke方法中则执行advisors进行增强。

public Object getProxy(ClassLoader classLoader) {
if (logger.isDebugEnabled()) {
logger.debug("Creating JDK dynamic proxy: target source is " + this.advised.getTargetSource());
}
Class[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised);
findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
// 返回代理对象
return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
}

/**
* Implementation of {@code InvocationHandler.invoke}.
* <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();
}

// 生成advisors的链
// 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...
// 生成ReflectiveMethodInvocation对象
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.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);
}
}
}

那么，如何按顺序执行目标对象的前置增强方法和后置增强方法呢？Spring将所有的信息封装到ReflectiveMethodInvocation中，然后执行proceed方法。

public Object proceed() throws Throwable {
//  We start with an index of -1 and increment early.
// 执行目标方法
if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
return invokeJoinpoint();
}

// 通过计数器提取当前要增强的方法
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.
// 执行当前要增强的方法
return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
}
}

proceed方法中通过计数器获得当前的Advisor对象，然后进入Advisor对象的invoke方法中进行增强处理。增强处理可以分为before、after、around、throw等。此处以before和after为例，讲解spring是如何按顺序执行对应的增强方法的。

1、获得第一个advisor，即beforeAdvisor

2、调用beforeAdvisor的invoke方法，同时将ReflectiveMethodInvocation自身对象this传给invoke方法。

3、在invoke方法中，执行beforeAdvisor的advice，并调用步骤2传递进来的ReflectiveMethodInvocation对象的proceed方法

4、回到proceed方法，获得第二个advisor，即afterAdvisor

5、调用afterAdvisor的invoke方法，在该方法中先不执行afterAdvisor的advice，而是直接调用传递进来ReflectiveMethodInvocation的proceed方法

6、回到proceed方法，此时发现没有advisor了，则执行目标方法

7、目标方法执行完后，退出procced方法，则回到了步骤5的afterAdvisor的invoke方法体中，此时执行afterAdvisor的advice

8、然后返回

通过以上过程，则完成了before、method、after的顺序过程。当存在多个before和after时，已经在获得advisors阶段进行排序了。