Spring AOP --JDK动态代理方式

我们知道Spring是通过JDK或者CGLib实现动态代理的，今天我们讨论一下JDK实现动态代理的原理。

一、简述

Spring在解析Bean的定义之后会将Bean的定义生成一个BeanDefinition对象并且由BeanDefinitionHolder对象持有。在这个过程中，如果Bean需要被通知切入，BeanDefinition会被重新转换成一个proxyDefinition（其实也是一个BeanDefinition对象，只不过描述的是一个ProxyFactoryBean）。ProxyFactoryBean是一个实现了FactoryBean的接口，用来生成被被切入的对象。Spring AOP的实现基本上是通过ProxyFactoryBean实现的。我们今天讨论的重点也是这个类。
　　在讨论ProxyFactoryBean之前，我们先看一下一个BeanDefinition转换成proxyDefintion的过程。

public final BeanDefinitionHolder decorate(Node node, BeanDefinitionHolder definitionHolder, ParserContext parserContext) {
BeanDefinitionRegistry registry = parserContext.getRegistry();

    // get the root bean name - will be the name of the generated proxy factory bean
    String existingBeanName = definitionHolder.getBeanName();
    BeanDefinition targetDefinition = definitionHolder.getBeanDefinition();
    BeanDefinitionHolder targetHolder = new BeanDefinitionHolder(targetDefinition, existingBeanName + ".TARGET");

    // delegate to subclass for interceptor definition
    BeanDefinition interceptorDefinition = createInterceptorDefinition(node);

    // generate name and register the interceptor
    String interceptorName = existingBeanName + "." + getInterceptorNameSuffix(interceptorDefinition);
    BeanDefinitionReaderUtils.registerBeanDefinition(
            new BeanDefinitionHolder(interceptorDefinition, interceptorName), registry);

    BeanDefinitionHolder result = definitionHolder;

    if (!isProxyFactoryBeanDefinition(targetDefinition)) {
        // create the proxy definition 这里创建proxyDefinition对象，并且从原来的BeanDefinition对象中复制属性
        RootBeanDefinition proxyDefinition = new RootBeanDefinition();
        // create proxy factory bean definition
        proxyDefinition.setBeanClass(ProxyFactoryBean.class);
        proxyDefinition.setScope(targetDefinition.getScope());
        proxyDefinition.setLazyInit(targetDefinition.isLazyInit());
        // set the target
        proxyDefinition.setDecoratedDefinition(targetHolder);
        proxyDefinition.getPropertyValues().add("target", targetHolder);
        // create the interceptor names list
        proxyDefinition.getPropertyValues().add("interceptorNames", new ManagedList());
        // copy autowire settings from original bean definition.
        proxyDefinition.setAutowireCandidate(targetDefinition.isAutowireCandidate());
        proxyDefinition.setPrimary(targetDefinition.isPrimary());
        if (targetDefinition instanceof AbstractBeanDefinition) {
            proxyDefinition.copyQualifiersFrom((AbstractBeanDefinition) targetDefinition);
        }
        // wrap it in a BeanDefinitionHolder with bean name
        result = new BeanDefinitionHolder(proxyDefinition, existingBeanName);
    }

    addInterceptorNameToList(interceptorName, result.getBeanDefinition());
    return result;
}

二、ProxyFactoryBean的原理
我们先来看一下ProxyFactoryBean的继承关系：

ProxyFactoryBean类图
ProxyFactoryBean实现了FactoryBean、BeanClassLoaderAware、BeanFactoryAware接口，这里就不多说了。ProxyCreatorSupport这个类则是创建代理对象的关键所在。　　我们先来看看产生代理对象的方法：

public Object getObject() throws BeansException {
initializeAdvisorChain();
if (isSingleton()) {
//单例
return getSingletonInstance();
}
else {
if (this.targetName == null) {
logger.warn("Using non-singleton proxies with singleton targets is often undesirable. " +

                    "Enable prototype proxies by setting the 'targetName' property.");
        }
                    //非单例
        return newPrototypeInstance();
    }
}

initializeAdvisorChain() 方法是将通知链实例化。然后判断对象是否要生成单例而选择调用不同的方法，这里我们只看生成单例对象的方法。

private synchronized Object getSingletonInstance() {
if (this.singletonInstance == null) {
this.targetSource = freshTargetSource();
//如果以接口的方式代理对象
if (this.autodetectInterfaces && getProxiedInterfaces().length == 0 && !isProxyTargetClass()) {
// Rely on AOP infrastructure to tell us what interfaces to proxy.
Class targetClass = getTargetClass();
if (targetClass == null) {
throw new FactoryBeanNotInitializedException(“Cannot determine target class for proxy”);
}
//获取目标类实现的所有接口，并注册给父类的interfaces属性，为jdk动态代理做准备
setInterfaces(ClassUtils.getAllInterfacesForClass(targetClass, this.proxyClassLoader));
}
// Initialize the shared singleton instance.
super.setFrozen(this.freezeProxy);
//这里产生代理对象
this.singletonInstance = getProxy(createAopProxy());
}
return this.singletonInstance;
}
我们可以看到，产生代理对象是通过getProxy()方法实现的，这个方法我们看一下：

protected Object getProxy(AopProxy aopProxy) {
return aopProxy.getProxy(this.proxyClassLoader);
}
AopProxy对象的getProxy()方法产生我们需要的代理对象，究竟AopProxy这个类是什么，我们接下来先看一下产生这个对象的方法createAopProxy()：

protected final synchronized AopProxy createAopProxy() {
if (!this.active) {
activate();
}
return getAopProxyFactory().createAopProxy(this);
}

createAopProxy方法：
public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
//目标对象不是接口类的实现或者没有提供代理接口
if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) {
Class targetClass = config.getTargetClass();
if (targetClass == null) {
throw new AopConfigException("TargetSource cannot determine target class: " +
“Either an interface or a target is required for proxy creation.”);
}
//代理对象自身是接口
if (targetClass.isInterface()) {
return new JdkDynamicAopProxy(config);
}
return new ObjenesisCglibAopProxy(config);
}
else {
return new JdkDynamicAopProxy(config);
}
}
在这里我们只看JdkDynamicAopProxy这个类的实现，我们前面提到，真正代理对象的生成是由AopProxy的getProxy方法完成的，这里我们看一下JdkDynamicAopProxy的getProxy方法，这也是本文讨论的重点：

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);
return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
}
我们看可以很清楚的看到，代理对象的生成直接使用了jdk动态代理:Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);而代理逻辑是通过实现了InvocationHandler接口的invoke方法实现的。而这里用到的实现了InvocationHandler接口的类就是JdkDynamicAopProxy自身。JdkDynamicAopProxy自身实现了InvocationHandler接口，完成了Spring AOP拦截器链拦截等一系列逻辑，我们看一下JdkDynamicAopProxy的invoke方法的具体实现：

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 {
        //没有重写equals方法
        if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
            // The target does not implement the equals(Object) method itself.
            return equals(args[0]);
        }
        //没有重写hashCode方法
        if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
            // The target does not implement the hashCode() method itself.
            return hashCode();
        }
      //代理的类是Advised，这里直接执行，不做任何代理
        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.
        //获得代理对象
        target = targetSource.getTarget();
        if (target != null) {
            targetClass = target.getClass();
        }

        // Get the interception chain for this method.
        //获得已经定义的拦截器链
        List