AOP动态代理解析5-cglib代理的实现

CGLIB是一个强大的高性能的代码生成包。它广泛地被许多AOP的框架使用，例如Spring AOP和dynaop，为他们提供方法的Interception（拦截）。EasyMock和jMock是通过使用模仿（moke）对象来测试Java代码的包。它们都通过使用CGLIB来为那些没有接口的类创建模仿（moke）对象。

CGLIB包的底层通过使用一个小而快的字节码处理框架ASM，来转换字节码并生成新的类。除了CGLIB包，脚本语言例如Groovy和BeanShell，也是使用ASM来生成Java的字节码。当然不鼓励直接使用ASM，因为它要求你必须对JVM内部结构包括class文件的格式和指令集都很熟悉。

CGLIB代理的类是委托给CglibAopProxy类去实现的。Cglib2AopProxy的入口是在getProxy，也就是说在CglibAopProxy类的getProxy方法中实现了Enhancer的创建及接口封装。

    public Object getProxy() {
        return getProxy(null);
    }

    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);
                }
            }
            //验证Class
            validateClassIfNecessary(proxySuperClass);
            //创建及配置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 MemorySafeUndeclaredThrowableStrategy(UndeclaredThrowableException.class));
            enhancer.setInterceptDuringConstruction(false);
　　　　　　　//设置拦截器
            Callback[] callbacks = getCallbacks(rootClass);
            Class<?>[] types = new Class<?>[callbacks.length];
            for (int x = 0; x < types.length; x++) {
                types[x] = callbacks[x].getClass();
            }
            enhancer.setCallbackFilter(new ProxyCallbackFilter(
                    this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
            enhancer.setCallbackTypes(types);
            enhancer.setCallbacks(callbacks);
            //生成代理类以及创建代理
            Object proxy;
            if (this.constructorArgs != null) {
                proxy = enhancer.create(this.constructorArgTypes, this.constructorArgs);
            }
            else {
                proxy = enhancer.create();
            }

            return proxy;
        }
        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) {
            throw new AopConfigException("Unexpected AOP exception", ex);
        }
    }

以上函数完整地阐述了一个创建Spring中的Enhancer的过程，参考Enhancer的文档查看每个步骤的含义，这里最重要的是通过getCallbacks方法设置拦截器链。

    private Callback[] getCallbacks(Class<?> rootClass) throws Exception {
        //对于expose-proxy属性的处理
        boolean exposeProxy = this.advised.isExposeProxy();
        boolean isFrozen = this.advised.isFrozen();
        boolean isStatic = this.advised.getTargetSource().isStatic();
        //将拦截器封装在DynamicAdvisedInterceptor中
        Callback aopInterceptor = new DynamicAdvisedInterceptor(this.advised);
        // Choose a "straight to target" interceptor. (used for calls that are
        // unadvised but can return this). May be required to expose the proxy.
        Callback targetInterceptor;
        if (exposeProxy) {
            targetInterceptor = isStatic ?
                    new StaticUnadvisedExposedInterceptor(this.advised.getTargetSource().getTarget()) :
                    new DynamicUnadvisedExposedInterceptor(this.advised.getTargetSource());
        }
        else {
            targetInterceptor = isStatic ?
                    new StaticUnadvisedInterceptor(this.advised.getTargetSource().getTarget()) :
                    new DynamicUnadvisedInterceptor(this.advised.getTargetSource());
        }
        // Choose a "direct to target" dispatcher (used for
        // unadvised calls to static targets that cannot return this).
        Callback targetDispatcher = isStatic ?
                new StaticDispatcher(this.advised.getTargetSource().getTarget()) : new SerializableNoOp();
　　　　　//将拦截器链加入Callback中
        Callback[] mainCallbacks = new Callback[]{
            aopInterceptor, // for normal advice
            targetInterceptor, // invoke target without considering advice, if optimized
            new SerializableNoOp(), // no override for methods mapped to this
            targetDispatcher, this.advisedDispatcher,
            new EqualsInterceptor(this.advised),
            new HashCodeInterceptor(this.advised)
        };
        Callback[] callbacks;
        // If the target is a static one and the advice chain is frozen,
        // then we can make some optimisations by sending the AOP calls
        // direct to the target using the fixed chain for that method.
        if (isStatic && isFrozen) {
            Method[] methods = rootClass.getMethods();
            Callback[] fixedCallbacks = new Callback[methods.length];
            this.fixedInterceptorMap = new HashMap<String, Integer>(methods.length);

            // TODO: small memory optimisation here (can skip creation for methods with no advice)
            for (int x = 0; x < methods.length; x++) {
                List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(methods[x], rootClass);
                fixedCallbacks[x] = new FixedChainStaticTargetInterceptor(
                        chain, this.advised.getTargetSource().getTarget(), this.advised.getTargetClass());
                this.fixedInterceptorMap.put(methods[x].toString(), x);
            }
            // Now copy both the callbacks from mainCallbacks
            // and fixedCallbacks into the callbacks array.
            callbacks = new Callback[mainCallbacks.length + fixedCallbacks.length];
            System.arraycopy(mainCallbacks, 0, callbacks, 0, mainCallbacks.length);
            System.arraycopy(fixedCallbacks, 0, callbacks, mainCallbacks.length, fixedCallbacks.length);
            this.fixedInterceptorOffset = mainCallbacks.length;
        }
        else {
            callbacks = mainCallbacks;
        }
        return callbacks;
    }

在getCallback中Spring考虑了很多情况，对于我们来说，只需要理解最常用的就可以了，比如将advised属性封装在DynamicAdvisedInterceptor并加入在callbacks中，这么做的目的是什么呢，如何调用呢？CGLIB中对于方法的拦截是通过将自定义的拦截器（实现MethodInterceptor接口）加入Callback中并在调用代理时直接激活拦截器中的intercept方法来实现的，那么在getCallback中正是实现了这样一个目的，DynamicAdvisedInterceptor继承自MethodInterceptor，加入Callback中后，在再次调用代理时会直接调用DynamicAdvisedInterceptor中的intercept方法，由此推断，对于CGLIB方式实现的代理，其核心逻辑必然在DynamicAdvisedInterceptor中的intercept中。

        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();
                }
　　　　　　　　　 //获取拦截器链
                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())) {
                    //如果拦截器链为空则直接激活原方法
                    retVal = methodProxy.invoke(target, args);
                }
                else {
                    //进入链
                    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);
                }
            }
        }

上述的实现与JDK方式实现代理中的invoke方法大同小异，都是首先构造链，然后封装此链进行串联调用，稍有些区别就是在JDK中直接构造ReflectiveMethodInvocation，而在cglib中使用CglibMethodInvocation。CglibMethodInvocation继承自ReflectiveMethodInvocation，但是proceed方法并没有重写。