我們知道Spring是通過(guò)JDK或者CGLib實(shí)現(xiàn)動(dòng)態(tài)代理的，今天我們討論一下JDK實(shí)現(xiàn)動(dòng)態(tài)代理的原理。

一、簡(jiǎn)述

Spring在解析Bean的定義之后會(huì)將Bean的定義生成一個(gè)BeanDefinition對(duì)象并且由BeanDefinitionHolder對(duì)象持有。在這個(gè)過(guò)程中，如果Bean需要被通知切入，BeanDefinition會(huì)被重新轉(zhuǎn)換成一個(gè)proxyDefinition（其實(shí)也是一個(gè)BeanDefinition對(duì)象，只不過(guò)描述的是一個(gè)ProxyFactoryBean）。ProxyFactoryBean是一個(gè)實(shí)現(xiàn)了FactoryBean的接口，用來(lái)生成被被切入的對(duì)象。Spring AOP的實(shí)現(xiàn)基本上是通過(guò)ProxyFactoryBean實(shí)現(xiàn)的。我們今天討論的重點(diǎn)也是這個(gè)類。
　　在討論P(yáng)roxyFactoryBean之前，我們先看一下一個(gè)BeanDefinition轉(zhuǎn)換成proxyDefintion的過(guò)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 這里創(chuàng)建proxyDefinition對(duì)象，并且從原來(lái)的BeanDefinition對(duì)象中復(fù)制屬性
            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<String>());
            // 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
我們先來(lái)看一下ProxyFactoryBean的繼承關(guān)系：

ProxyFactoryBean實(shí)現(xiàn)了FactoryBean、BeanClassLoaderAware、BeanFactoryAware接口，這里就不多說(shuō)了。ProxyCreatorSupport這個(gè)類則是創(chuàng)建代理對(duì)象的關(guān)鍵所在?！　∥覀兿葋?lái)看看產(chǎn)生代理對(duì)象的方法：

public Object getObject() throws BeansException {
        initializeAdvisorChain();
        if (isSingleton()) {
                        //單例
            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.");
            }
                        //非單例
            return newPrototypeInstance();
        }
    }

initializeAdvisorChain() 方法是將通知鏈實(shí)例化。然后判斷對(duì)象是否要生成單例而選擇調(diào)用不同的方法，這里我們只看生成單例對(duì)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對(duì)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");
                }
                      //獲取目標(biāo)類實(shí)現(xiàn)的所有接口，并注冊(cè)給父類的interfaces屬性，為jdk動(dòng)態(tài)代理做準(zhǔn)備
                setInterfaces(ClassUtils.getAllInterfacesForClass(targetClass, this.proxyClassLoader));
            }
            // Initialize the shared singleton instance.
            super.setFrozen(this.freezeProxy);
                   //這里產(chǎn)生代理對(duì)象
            this.singletonInstance = getProxy(createAopProxy());
        }
        return this.singletonInstance;
    }

我們可以看到，產(chǎn)生代理對(duì)象是通過(guò)getProxy()方法實(shí)現(xiàn)的，這個(gè)方法我們看一下：

protected Object getProxy(AopProxy aopProxy) {
        return aopProxy.getProxy(this.proxyClassLoader);
    }

AopProxy對(duì)象的getProxy()方法產(chǎn)生我們需要的代理對(duì)象，究竟AopProxy這個(gè)類是什么，我們接下來(lái)先看一下產(chǎn)生這個(gè)對(duì)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 {
                //目標(biāo)對(duì)象不是接口類的實(shí)現(xiàn)或者沒(méi)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.");
            }  
            //代理對(duì)象自身是接口
            if (targetClass.isInterface()) {
                return new JdkDynamicAopProxy(config);
            }
            return new ObjenesisCglibAopProxy(config);
        }
        else {
            return new JdkDynamicAopProxy(config);
        }
    }

在這里我們只看JdkDynamicAopProxy這個(gè)類的實(shí)現(xiàn)，我們前面提到，真正代理對(duì)象的生成是由AopProxy的getProxy方法完成的，這里我們看一下JdkDynamicAopProxy的getProxy方法，這也是本文討論的重點(diǎn)：

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);
    }

我們看可以很清楚的看到，代理對(duì)象的生成直接使用了jdk動(dòng)態(tài)代理:Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);而代理邏輯是通過(guò)實(shí)現(xiàn)了InvocationHandler接口的invoke方法實(shí)現(xiàn)的。而這里用到的實(shí)現(xiàn)了InvocationHandler接口的類就是JdkDynamicAopProxy自身。JdkDynamicAopProxy自身實(shí)現(xiàn)了InvocationHandler接口，完成了Spring AOP攔截器鏈攔截等一系列邏輯，我們看一下JdkDynamicAopProxy的invoke方法的具體實(shí)現(xiàn)：

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 {
            //沒(méi)有重寫equals方法
            if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
                // The target does not implement the equals(Object) method itself.
                return equals(args[0]);
            }
            //沒(méi)有重寫hashCode方法
            if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
                // The target does not implement the hashCode() method itself.
                return hashCode();
            }
          //代理的類是Advised，這里直接執(zhí)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.
            //獲得代理對(duì)象
            target = targetSource.getTarget();
            if (target != null) {
                targetClass = target.getClass();
            }

            // Get the interception chain for this method.
            //獲得已經(jīng)定義的攔截器鏈
            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.
                //攔截器鏈?zhǔn)强盏?，直接?zhí)行需要代理的方法
                retVal = AopUtils.invokeJoinpointUsingReflection(target, method, args);
            }
            else {
                // We need to create a method invocation...
                //這里是調(diào)用攔截器鏈的地方，先創(chuàng)建一個(gè)MethodInvocation對(duì)象，然后調(diào)用該對(duì)象的proceed方法完成攔截器鏈調(diào)用
                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();
            //這里處理返回值，判斷返回值和方法需要的返回是否一致
            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);
            }
        }
    }

攔截器鏈的調(diào)用

從上面的代碼和注釋中我們可以看到spring實(shí)現(xiàn)aop的主要流程，具體如何調(diào)用攔截器鏈，我們來(lái)看一下MethodInvocation的proceed方法

public Object proceed() throws Throwable {
        //  We start with an index of -1 and increment early.
        //   currentInterceptorIndex是從-1開(kāi)始的，所以攔截器鏈調(diào)用結(jié)束的時(shí)候index是 this.interceptorsAndDynamicMethodMatchers.size() - 1
        //  調(diào)用鏈結(jié)束后執(zhí)行目標(biāo)方法
        if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
            return invokeJoinpoint();
        }
        //  獲得當(dāng)前處理到的攔截器
        Object interceptorOrInterceptionAdvice =
                this.interceptorsAndDynamicMethodMatchers.get(  this.currentInterceptorIndex);
        //  這里判斷是否是InterceptorAndDynamicMethodMatcher，如果是，這要判斷是否匹配methodMatcher，不匹配則此攔截器不生效
        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.
            return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
        }
    }

proceed()方法是一個(gè)遞歸方法，我們可以根據(jù)代碼的注釋知道大體邏輯，InterceptorAndDynamicMethodMatcher的代碼如下，我們可以看到，InterceptorAndDynamicMethodMatcher 持有一個(gè)MethodInterceptor 對(duì)象和一個(gè)MethodMatcher 對(duì)象，在攔截器鏈調(diào)用過(guò)程中，如果攔截器是InterceptorAndDynamicMethodMatcher ，則會(huì)先根據(jù)MethodMatcher 判斷是否匹配，匹配MethodInterceptor 才會(huì)生效。

class InterceptorAndDynamicMethodMatcher {

    final MethodInterceptor interceptor;

    final MethodMatcher methodMatcher;

    public InterceptorAndDynamicMethodMatcher(MethodInterceptor interceptor, MethodMatcher methodMatcher) {
        this.interceptor = interceptor;
        this.methodMatcher = methodMatcher;
    }

}

至于MethodInterceptor 是什么，MethodInterceptor 的邏輯是怎么樣的，我們可以看一下MethodInterceptor 的一個(gè)子類AfterReturningAdviceInterceptor的實(shí)現(xiàn)：

public class AfterReturningAdviceInterceptor implements MethodInterceptor, AfterAdvice, Serializable {    private final AfterReturningAdvice advice;    /**     * Create a new AfterReturningAdviceInterceptor for the given advice.     * @param advice the AfterReturningAdvice to wrap     */    public AfterReturningAdviceInterceptor(AfterReturningAdvice advice) {        Assert.notNull(advice, "Advice must not be null");        this.advice = advice;    }    @Override    public Object invoke(MethodInvocation mi) throws Throwable {        Object retVal = mi.proceed();        this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());        return retVal;    }}

AfterReturningAdviceInterceptor的作用是在被代理的方法返回結(jié)果之后添加我們需要的處理邏輯，其實(shí)現(xiàn)方式我們可以看到，先調(diào)用MethodInvocation 的proceed，也就是先繼續(xù)處理攔截器鏈，等調(diào)用完成后執(zhí)行我們需要的邏輯：this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());
到這里，spring使用jdk動(dòng)態(tài)代理實(shí)現(xiàn)aop的分析基本上結(jié)束，其中攔截器鏈的調(diào)用比較難懂而且比較重要，需要的同學(xué)可以多看看這一塊。