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