Spring技术内幕：Spring AOP的实现原理（三）

生成SingleTon代理对象在getSingleTonInstance方法中完成，这个方法时ProxyFactoryBean生成AopProxy对象的入口。代理对象会封装对target目标对象的调用，也就是说针对target对象的方法调用行为会被这里生成的代理对象所拦截。具体的生成过程是首先读取ProxyFactoryBean配置，为生成代理对象做好准备。Spring通过AopProxy类来具体生成代理对象。对于getSingleTonInstance方法中生成代理对象的过程如下：

/**
     * Return the singleton instance of this class‘s proxy object,
     * lazily creating it if it hasn‘t been created already.
     * @return the shared singleton proxy
     */
    private synchronized Object getSingletonInstance() {
        if (this.singletonInstance == null) {
            this.targetSource = freshTargetSource();
            if (this.autodetectInterfaces && getProxiedInterfaces().length == 0 && !isProxyTargetClass()) {
                // Rely on AOP infrastructure to tell us what interfaces to proxy.
                // 根据AOP框架来判断需要代理的接口
                Class targetClass = getTargetClass();
                if (targetClass == null) {
                    throw new FactoryBeanNotInitializedException("Cannot determine target class for proxy");
                }
                // 这里设置代理对象的接口
                setInterfaces(ClassUtils.getAllInterfacesForClass(targetClass, this.proxyClassLoader));
            }
            // Initialize the shared singleton instance.
            super.setFrozen(this.freezeProxy);
            // 这里会使用ProxyFactoryBean来生成需要的proxy对象
            this.singletonInstance = getProxy(createAopProxy());
        }
        return this.singletonInstance;
    }

/**
     * Return the proxy object to expose.
     * <p>The default implementation uses a {@code getProxy} call with
     * the factory‘s bean class loader. Can be overridden to specify a
     * custom class loader.
     * @param aopProxy the prepared AopProxy instance to get the proxy from
     * @return the proxy object to expose
     * @see AopProxy#getProxy(ClassLoader)
     */
    protected Object getProxy(AopProxy aopProxy) {
        return aopProxy.getProxy(this.proxyClassLoader);
    }

这里出现了AopProxy对象类型，Spring利用AOPProxy接口类把AOP代理对象的实现与框架其他部分有效隔离开来。AopProxy接口有两个子类实现，一个Cglib2AopProxy，另一个是JdkDynamicProxy。
具体代理对象的生成是在ProxyFactoryBean的基类AdvisedSupport中实现，借助AopProxyFactory完成，这个对象要么从JDK中生成，要么借助CGLIB获得。下面看看ProxyCreatorSupport中是如何生成代理对象的。

    /**
     * Subclasses should call this to get a new AOP proxy. They should <b>not</b>
     * create an AOP proxy with {@code this} as an argument.
     */
    protected final synchronized AopProxy createAopProxy() {
        if (!this.active) {
            activate();
        }
        // 通过AopProxyFactory取得AopProxy，这个AopProxyFactory是在初始化函数中定义的，使用的是DefaultAopProxyFactory
        return getAopProxyFactory().createAopProxy(this);
    }

AopProxy代理对象的生成有两种方式，如果目标对象是接口类使用JDK来生成，否则Spring会使用CGLIB来生成目标的代理对象。下面看看在DefaultAopProxyFactory是如何生成AopProxy目标代理对象的：

    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 CglibProxyFactory.createCglibProxy(config);
        }
        else {
            return new JdkDynamicAopProxy(config);
        }
    }

在AopProxy代理对象的生成过程中，首先要从AdviseSupport对象中取得配置的目标对象，AOP完成的是切面应用对目标应用对象的增强。如果这里没有配置目标对象会直接抛出异常。一般而言，默认方式是使用JDK来产生AopProxy代理对象，但如果配置的目标对象不是接口类的实现，会使用CGLIB来产生AopProxy代理对象；在使用CGLIB来产生AopProxy代理对象时，因为CGLIB是第三方类库，本身不在JDK基类库中，所有需要在classPath中正确配置，以便能够加载和利用。在Spring中，使用JDK和CGLIB来生成AopProxy代理对象的工作，是由JdkDynamicAopProxy和CglibProxyFactory来完成。
4、JDK生成AopProxy对象（接口实现类）
通过上面我们已经知道生成AopProxy对象有两种方式，下面看下类图：

我们先看下JdkDynamicAopProxy是如何生成AopProxy对象的：

    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);
        // 调用JDK生成Proxy
        return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
    }

5、CGLIB生成AopProxy对象（非接口实现类）

    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);
            // Configure CGLIB Enhancer...
            // 来自advised的IOC配置，比如使用AOP的DynamicAdvisedInterceptor
            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);
            // 通过设置DynamicAdvisedInterceptor拦截器来完成AOP功能，getCallBacks方法如下：
            // Callback aopInterceptor = new DynamicAdvisedInterceptor(this.advised)
            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);
            // Generate the proxy class and create a proxy instance.
            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) {
            // TargetSource.getTarget() failed
            throw new AopConfigException("Unexpected AOP exception", ex);
        }
    }

四、Spring AOP拦截器调用的实现
1、设计原理
在Spring AOP通过JDK的Proxy方式或CGLIB方式生成代理对象的时候，相关的拦截器已经配置到代理对象中去了，拦截器在代理对象中起作用是通过对这些方法的回调来完成的。
如果使用JDK的Proxy来生成代理对象，那么需要InvocationHandler来设置拦截器回调，而如果使用CGLIB来生成代理对象，通过DynamicAdvisedInterceptor来完成回调。
2、JdkDynamicAopProxy的invoke拦截
在JDKDynamicAopProxy生成代理对象时，他的AopProxy代理对象生成调用：

Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);

this指的是InvocationHandler对象，InvocationHandler是JDK定义反射类的一个接口，这个接口定义了invoke方法，此方法为回调方法。通过invoke的具体实现，来完成对目标对象方法调用的拦截器或者功能增强工作。在这个方法中，包含一个完整的拦截器链对目标对象的拦截过程，比如获取拦截器链中的拦截器进行配置，逐个运行拦截器链里的拦截器增强，知道最后的目标对象方法的运行。下面看下invoke的源码

    /**
     * Implementation of {@code InvocationHandler.invoke}.
     * <p>Callers will see exactly the exception thrown by the target,
     * unless a hook method throws an exception.
     */
    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.
            // 获得定义好的拦截器
            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.
            // 如果没有拦截器直接调用target的对应方法
            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.
                // 如果有拦截器设定，那么需要调用拦截器之后才调用目标对象相应的方法
                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);
            }
        }
    }

3、CglibAopProxy的intercept拦截器
使用CglibAopProxy生成AopProxy对象时候，对于AOP拦截器调用，回调的是DynamicAdvisedInterceptor对象生成的。回调的方法时intercept，下面看看回调方法的源码：

        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();
                }
                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);
                }
            }
        }