Spring原理（四）AOP的实现

落日映苍穹つ 2022-07-14 03:16 115阅读 0赞

1.  概述
    
    下面是一个简单的使用ProxyFactoryBean实现AOP的例子。

<?xml version="1.0" encoding="UTF-8"?>
    <beans xmlns="http://www.springframework.org/schema/beans"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context"
        xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.1.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.1.xsd"
            >
        <description>Spring公共配置 </description>
    
        <bean id = "testTarget" class = "com.sjj.aop.TestTarget"></bean>
        <bean id = "testAdvice1" class = "com.sjj.aop.TestAdvice1"></bean>
        <bean id = "testAdvice2" class = "com.sjj.aop.TestAdvice2"></bean>
    
        <bean id = "testPointcut" class = "org.springframework.aop.support.JdkRegexpMethodPointcut">
           <property name="pattern"  value = ".*itest"/>
        </bean>
    
        <bean id = "testAdvisor1" class = "org.springframework.aop.support.DefaultPointcutAdvisor">
           <property name="advice" ref="testAdvice1"/>
           <property name="pointcut" ref="testPointcut"/>
        </bean>
    
        <bean id = "testAdvisor2" class = "org.springframework.aop.support.DefaultPointcutAdvisor">
           <property name="advice" ref="testAdvice2"/>
           <property name="pointcut" ref="testPointcut"/>
        </bean>
    
        <bean id = "testAop" class = "org.springframework.aop.framework.ProxyFactoryBean" lazy-init="true">
           
           <property name="target" ref = "testTarget"/>
           <property name="interceptorNames" >
              <list>
                 <value>testAdvisor1</value>
                 <value>testAdvisor2</value>
              </list>
           </property>
        </bean>
    </beans>

public static void main(String[] args) {
            FileSystemXmlApplicationContext ctx = new 
            FileSystemXmlApplicationContext("D:/workspace/demo_spring_standalone/src/main/resources/applicationContext-aop-proxy.xml");
            TestTarget testTarget = (TestTarget) ctx.getBean("testAop");
            testTarget.itest();
        }

1.  源码分析  
    2.1 AOP初始化  
    `TestTarget testTarget = (TestTarget) ctx.getBean("testAop");`  
    上面这句代码看起来比较熟悉，没错，这句代码就是从spring上下文获取id=“testAop”的bean。从配置文件中看出，这是一个FactoryBean，这是spring专门设计为AOP服务的 ProxyFactoryBean，普通bean和FactoryBean的区别这里就不多说了。这里getBean方法直接会去调用getObject方法，
    
    这里跳过spring实例化bean的过程，直接进入到ProxyFactoryBean的方法getObject，

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

private synchronized void initializeAdvisorChain() throws AopConfigException, BeansException {
            //Advisor/Advice 链被初始化过的话，直接返回
            if (this.advisorChainInitialized) {
                return;
            }
    
            if (!ObjectUtils.isEmpty(this.interceptorNames)) {
    
                // Materialize interceptor chain from bean names.
                for (String name : this.interceptorNames) {
                    if (logger.isTraceEnabled()) {
                        logger.trace("Configuring advisor or advice '" + name + "'");
                    }
    
                    if (name.endsWith(GLOBAL_SUFFIX)) {
                        if (!(this.beanFactory instanceof ListableBeanFactory)) {
                            throw new AopConfigException(
                                    "Can only use global advisors or interceptors with a ListableBeanFactory");
                        }
                        addGlobalAdvisor((ListableBeanFactory) this.beanFactory,
                                name.substring(0, name.length() - GLOBAL_SUFFIX.length()));
                    }
    
                    else {
                        // If we get here, we need to add a named interceptor.
                        // We must check if it's a singleton or prototype.
                        Object advice;
                        if (this.singleton || this.beanFactory.isSingleton(name)) {
                            // Add the real Advisor/Advice to the chain.
                            //这里又看到了熟悉的方法，没错获取advice bean对象
                            advice = this.beanFactory.getBean(name);
                        }
                        else {
                            // It's a prototype Advice or Advisor: replace with a prototype.
                            // Avoid unnecessary creation of prototype bean just for advisor chain initialization.
                            advice = new PrototypePlaceholderAdvisor(name);
                        }
                        addAdvisorOnChainCreation(advice, name);
                    }
                }
            }
    
            this.advisorChainInitialized = true;
        }

private void addAdvisorOnChainCreation(Object next, String name) {
            // We need to convert to an Advisor if necessary so that our source reference
            // matches what we find from superclass interceptors.
            Advisor advisor = namedBeanToAdvisor(next);
            if (logger.isTraceEnabled()) {
                logger.trace("Adding advisor with name '" + name + "'");
            }
            addAdvisor(advisor);
        }

advisors的类型是linkedList，这是一个advisor链，所有的advisor都放在这个链表里面。

private List<Advisor> advisors = new LinkedList<Advisor>();

public void addAdvisor(int pos, Advisor advisor) throws AopConfigException {
            if (advisor instanceof IntroductionAdvisor) {
                validateIntroductionAdvisor((IntroductionAdvisor) advisor);
            }
            addAdvisorInternal(pos, advisor);
        }

接下来继续看方法getObject下面的逻辑。

private synchronized Object getSingletonInstance() {
            if (this.singletonInstance == null) {
                //获取target bean的对象
                this.targetSource = freshTargetSource();
                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");
                    }
                    //递归的获取目标类的及其父类的接口信息，并添加到AdvisedSupport的一个全局的列表中去
                    setInterfaces(ClassUtils.getAllInterfacesForClass(targetClass, this.proxyClassLoader));
                }
                // Initialize the shared singleton instance.
                super.setFrozen(this.freezeProxy);
                this.singletonInstance = getProxy(createAopProxy());
            }
            return this.singletonInstance;
        }

protected final synchronized AopProxy createAopProxy() {
            if (!this.active) {
                activate();
            }
            /*这里的方法getAopProxyFactory，得到的是DefaultAopProxyFactory，ProxyFactoryBean继承了ProxyCreatorSupport， *在ProxyCreatorSupport的构造函数中设置了的 this.aopProxyFactory = new DefaultAopProxyFactory(); *所以在实例化ProxyFactoryBean的时候就已经设置了这个aopProxyFactory */
            return getAopProxyFactory().createAopProxy(this);
        }

/* * 这里写明了spring内部创建代理对象的两种方式： * （1）jdk动态代理：适用于目标类实现了接口，且目标方法在接口中有定义。 * （2）cglib动态代理，适用于目标类没有实现任何接口的情况。 */
            public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
            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.");
                }
                if (targetClass.isInterface()) {
                    return new JdkDynamicAopProxy(config);
                }
                return new ObjenesisCglibAopProxy(config);
            }
            else {
                return new JdkDynamicAopProxy(config);
            }
        }

protected Object getProxy(AopProxy aopProxy) {
            /*ProxyFactoryBean的getProxy方法，传入的参数是实际的proxy对象，也就是上一步创建的proxy， *也就是说实际调用的是具体的proxy对象的getProxy方法， */
            return aopProxy.getProxy(this.proxyClassLoader);
        }

//这是jdk动态代理的方式
        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);
        }

//这是cglib字节码增强的方式
        public Object getProxy(ClassLoader classLoader) {
            if (logger.isDebugEnabled()) {
                logger.debug("Creating CGLIB proxy: target source is " + this.advised.getTargetSource());
            }
    
            try {
                Class<?> rootClass = this.advised.getTargetClass();
                Assert.state(rootClass != null, "Target class must be available for creating a CGLIB proxy");
    
                Class<?> proxySuperClass = rootClass;
                if (ClassUtils.isCglibProxyClass(rootClass)) {
                    proxySuperClass = rootClass.getSuperclass();
                    Class<?>[] additionalInterfaces = rootClass.getInterfaces();
                    for (Class<?> additionalInterface : additionalInterfaces) {
                        this.advised.addInterface(additionalInterface);
                    }
                }
    
                // Validate the class, writing log messages as necessary.
                validateClassIfNecessary(proxySuperClass, classLoader);
    
                // Configure CGLIB Enhancer...
                Enhancer enhancer = createEnhancer();
                if (classLoader != null) {
                    enhancer.setClassLoader(classLoader);
                    if (classLoader instanceof SmartClassLoader &&
                            ((SmartClassLoader) classLoader).isClassReloadable(proxySuperClass)) {
                        enhancer.setUseCache(false);
                    }
                }
                enhancer.setSuperclass(proxySuperClass);
                enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
                enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
                enhancer.setStrategy(new UndeclaredThrowableStrategy(UndeclaredThrowableException.class));
    
                Callback[] callbacks = getCallbacks(rootClass);
                Class<?>[] types = new Class<?>[callbacks.length];
                for (int x = 0; x < types.length; x++) {
                    types[x] = callbacks[x].getClass();
                }
                // fixedInterceptorMap only populated at this point, after getCallbacks call above
                enhancer.setCallbackFilter(new ProxyCallbackFilter(
                        this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
                enhancer.setCallbackTypes(types);
    
                // Generate the proxy class and create a proxy instance.
                return createProxyClassAndInstance(enhancer, callbacks);
            }
            catch (CodeGenerationException ex) {
                throw new AopConfigException("Could not generate CGLIB subclass of class [" +
                        this.advised.getTargetClass() + "]: " +
                        "Common causes of this problem include using a final class or a non-visible class",
                        ex);
            }
            catch (IllegalArgumentException ex) {
                throw new AopConfigException("Could not generate CGLIB subclass of class [" +
                        this.advised.getTargetClass() + "]: " +
                        "Common causes of this problem include using a final class or a non-visible class",
                        ex);
            }
            catch (Exception ex) {
                // TargetSource.getTarget() failed
                throw new AopConfigException("Unexpected AOP exception", ex);
            }
        }

2.2 切面（Advice）方法触发

testTarget.itest();

testTarget是spring封装后的目标类的代理对象实例，在平常的spring AOP的使用中，咋看起来好像和这里的用法不太一样，这里是直接从上下文中直接获取封装后的bean对象  
testAOP，而正常的spring AOP的使用中是使用aop标签在命令行中定义aop相关的元素，或者使用aop相关的注解，而且是直接调用目标方法，只要符合定义的aop切面的拦截规  
则，该方法就会被拦截，并且织入advice逻辑。我们并没有去给目标对象生产代理，看似是这样的，其实封装代理这一步spring已经帮我们做了，也就是说我们运行程序的时候实际  
上调用的是spring创建的目标对象的代理对象，只是创建代理对象这一步对用户来说是透明的。

下面我们看下相关的源码：

如果熟悉jdk动态代理的话，应该知道invoke方法，是通过jdk动态代理的方式实现代理需要重写的接口InvocationHandler的方法。

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(); }
    
                // 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... 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); }
            }
        }

//当拦截器链为空的话，会调用方法AopUtils.invokeJoinpointUsingReflection，直接调用目标对象的目标方法
        public static Object invokeJoinpointUsingReflection(Object target, Method method, Object[] args)
                throws Throwable {
    
            // Use reflection to invoke the method.
            try {
                ReflectionUtils.makeAccessible(method);
                return method.invoke(target, args);
            }
            catch (InvocationTargetException ex) {
                // Invoked method threw a checked exception.
                // We must rethrow it. The client won't see the interceptor.
                throw ex.getTargetException();
            }
            catch (IllegalArgumentException ex) {
                throw new AopInvocationException("AOP configuration seems to be invalid: tried calling method [" +
                        method + "] on target [" + target + "]", ex);
            }
            catch (IllegalAccessException ex) {
                throw new AopInvocationException("Could not access method [" + method + "]", ex);
            }
        }

熟悉cglib动态代理的应该熟悉下面这个方法，方法intercept就是cglib方式实现动态代理时需要重写接口MethodInterceptor的方法。

public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable { Object oldProxy = null; boolean setProxyContext = false; Class<?> targetClass = null; Object target = null; try { 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 = getTarget();
                    if (target != null) { targetClass = target.getClass(); }
                    //获取所有的拦截器及advice
                    List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
                    Object retVal;
                    // Check whether we only have one InvokerInterceptor: that is,
                    // no real advice, but just reflective invocation of the target.
                    if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) { // 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 = methodProxy.invoke(target, args); } else { // We need to create a method invocation... retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed(); } retVal = processReturnType(proxy, target, method, retVal);
                    return retVal;
                }
                finally { if (target != null) { releaseTarget(target); }
                    if (setProxyContext) { // Restore old proxy. AopContext.setCurrentProxy(oldProxy); }
                }
            }

如果你debug调试的话，代码会进入上面提到的两个方法之一（视目标类而定，如果目标类实现了接口且目标方法在接口中有定义，spring会使用jdk动态代理来实现代理，此时则会调用invoke方法；否则使用cglib来实现代理，则会调用intercept方法）。那到底为什么会调用这两个方法（或者到底什么原理才调用的这两个方法呢？），为了弄清楚这一点，这里使用ProxyGenerator来生成一个代理类对象，并把对象写到文件中，通过反编译工具查看生成的代码。

byte[] classFile = sun.misc.ProxyGenerator.generateProxyClass("$Proxy11", TestTarget.class.getInterfaces());

这是生成的代理类的代码，可以看到把接口的方法全部重写了一遍（包括隐式继承的Object的方法toString,hashCode,equals）,最重要的是itest方法，也就是代理的目标方法。

import com.sjj.aop.TestInterface;
    import java.lang.reflect.InvocationHandler;
    import java.lang.reflect.Method;
    import java.lang.reflect.Proxy;
    import java.lang.reflect.UndeclaredThrowableException;
    
    public final class $Proxy11 extends Proxy implements TestInterface { 
      private static Method m1;
      private static Method m0;
      private static Method m3;
      private static Method m2;
    
      public $Proxy11(InvocationHandler paramInvocationHandler)
        throws 
      {
        super(paramInvocationHandler);
      }
    
      public final boolean equals(Object paramObject)
        throws 
      {
        try
        {
          return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
        }
        catch (Error|RuntimeException localError)
        {
          throw localError;
        }
        catch (Throwable localThrowable)
        {
          throw new UndeclaredThrowableException(localThrowable);
        }
      }
    
      public final int hashCode()
        throws 
      {
        try
        {
          return ((Integer)this.h.invoke(this, m0, null)).intValue();
        }
        catch (Error|RuntimeException localError)
        {
          throw localError;
        }
        catch (Throwable localThrowable)
        {
          throw new UndeclaredThrowableException(localThrowable);
        }
      }
    
      public final void itest()
        throws 
      {
        try
        {
          /* * 这里调用了invoke方法，解释开始的疑问，cglib的方式应该与此类似 */
          this.h.invoke(this, m3, null);
          return;
        }
        catch (Error|RuntimeException localError)
        {
          throw localError;
        }
        catch (Throwable localThrowable)
        {
          throw new UndeclaredThrowableException(localThrowable);
        }
      }
    
      public final String toString()
        throws 
      {
        try
        {
          return (String)this.h.invoke(this, m2, null);
        }
        catch (Error|RuntimeException localError)
        {
          throw localError;
        }
        catch (Throwable localThrowable)
        {
          throw new UndeclaredThrowableException(localThrowable);
        }
      }
    
      static
      {
        try
        {
          m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
          m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
          m3 = Class.forName("com.sjj.aop.TestInterface").getMethod("itest", new Class[0]);
          m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
          return;
        }
        catch (NoSuchMethodException localNoSuchMethodException)
        {
          throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
        }
        catch (ClassNotFoundException localClassNotFoundException)
        {
          throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
        }
      }
    }

最终会调用到DefaultAdvisorChainFactory的getInterceptorsAndDynamicInterceptionAdvice

public List<Object> getInterceptorsAndDynamicInterceptionAdvice(
                Advised config, Method method, Class<?> targetClass) {
    
            // This is somewhat tricky... We have to process introductions first,
            // but we need to preserve order in the ultimate list.
            List<Object> interceptorList = new ArrayList<Object>(config.getAdvisors().length);
            Class<?> actualClass = (targetClass != null ? targetClass : method.getDeclaringClass());
            boolean hasIntroductions = hasMatchingIntroductions(config, actualClass);
            /* *这里的GlobalAdvisorAdapterRegistry使用了单例的设计模式，具体可参考其源码 *采用饿汉模式直接创建了一个DefaultAdvisorAdapterRegistry对象,并把三种类型的拦截器注册进去 *具体内容参考DefaultAdvisorAdapterRegistry的构造方法 */
            AdvisorAdapterRegistry registry = GlobalAdvisorAdapterRegistry.getInstance();
    
            for (Advisor advisor : config.getAdvisors()) {
                if (advisor instanceof PointcutAdvisor) {
                    // Add it conditionally.
                    PointcutAdvisor pointcutAdvisor = (PointcutAdvisor) advisor;
                    if (config.isPreFiltered() || pointcutAdvisor.getPointcut().getClassFilter().matches(actualClass)) {
                        /*根据advisor关联的advice的类型，为advice创建相应的Interceptor, *AfterReturningAdviceAdapter --- AfterReturningAdviceInterceptor *MethodBeforeAdviceAdapter --- MethodBeforeAdviceInterceptor *ThrowsAdviceAdapter --- ThrowsAdviceInterceptor */
                        MethodInterceptor[] interceptors = registry.getInterceptors(advisor);
                        MethodMatcher mm = pointcutAdvisor.getPointcut().getMethodMatcher();
                        //根据方法的签名和advisor的拦截规则判断方法method是否需要拦截，如果需要则把advisor下的所有advice添加到拦截器列表里去
                        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));
                            }
                        }
                    }
                }
                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 abstract class GlobalAdvisorAdapterRegistry { 
    
        /** * Keep track of a single instance so we can return it to classes that request it. */
        private static AdvisorAdapterRegistry instance = new DefaultAdvisorAdapterRegistry();
    
        /** * Return the singleton {@link DefaultAdvisorAdapterRegistry} instance. */
        public static AdvisorAdapterRegistry getInstance() {
            return instance;
        }
    
        /** * Reset the singleton {@link DefaultAdvisorAdapterRegistry}, removing any * {@link AdvisorAdapterRegistry#registerAdvisorAdapter(AdvisorAdapter) registered} * adapters. */
        static void reset() {
            instance = new DefaultAdvisorAdapterRegistry();
        }
    
    }

public class DefaultAdvisorAdapterRegistry implements AdvisorAdapterRegistry, Serializable { 
    
        private final List<AdvisorAdapter> adapters = new ArrayList<AdvisorAdapter>(3);
    
    
        /** * Create a new DefaultAdvisorAdapterRegistry, registering well-known adapters. */
        public DefaultAdvisorAdapterRegistry() {
            registerAdvisorAdapter(new MethodBeforeAdviceAdapter());
            registerAdvisorAdapter(new AfterReturningAdviceAdapter());
            registerAdvisorAdapter(new ThrowsAdviceAdapter());
        }

CglibMethodInvocation是CglibAopProxy的内部类，继承了ReflectiveMethodInvocation，下面看下其proceed方法：

public Object proceed() throws Throwable {
            // We start with an index of -1 and increment early.
            //当遍历完整个拦截器链时，代码进入这个分支，主要功能是调用真正的目标方法
            if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
                return invokeJoinpoint();
            }
    
            /* * 这里interceptorsAndDynamicMethodMatchers和currentInterceptorIndex都是全局变量， * 这样设计的巧妙之处就在于，当具体的interceptor里的invoke方法递归调用这里proceed方法的时候， * 其实是对拦截器链做顺序访问，即依次调用拦截器链里的拦截器。 */
            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.
                    // 如果拦截器和方法不匹配，则递归调用proceed方法，遍历拦截器链
                    return proceed();
                }
            }
            else {
                // It's an interceptor, so we just invoke it: The pointcut will have
                // been evaluated statically before this object was constructed.
                /*这里的设计比较巧妙，把当前对象作为参数传进去，这样在相应的Interceptor的invoke方法里就可以递归调用 *proceed方法，从而达到遍历拦截器链的目的 */
                return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
            }
        }

/* *这是AfterReturningAdviceInterceptor的invoke方法，注意看两者调用proceed方法、advice方法（before、afterReturning）的顺序 */
        public Object invoke(MethodInvocation mi) throws Throwable {
            // 这里调用proceed方法，而不是目标方法，这样可以递归调用整个拦截器链的拦截器
            Object retVal = mi.proceed();
            this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());
            return retVal;
        }

/* *这是MethodBeforeAdviceInterceptor的invoke方法 */
        public Object invoke(MethodInvocation mi) throws Throwable {
            this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis() );
            // 这里调用proceed方法，而不是目标方法，这样可以递归调用整个拦截器链的拦截器
            return mi.proceed();
        }

protected Object invokeJoinpoint() throws Throwable {
                if (this.publicMethod) {
                    //调用真正的目标方法，在整个拦截器链最后才调用这个方法
                    return this.methodProxy.invoke(this.target, this.arguments);
                }
                else {
                    return super.invokeJoinpoint();
                }
            }

1.  之前对spring一直停留在用的层面，知道springAOP是通过动态代理实现，动态代理有两种实现方式：jdk dynamic proxy和cglib字节码增强，但并不知道具体的实  
    现细节，分析了AOP的实现源码之后才对如何使用者两种方式实现动态代理以及AOP的实现细节有了清晰的认识。