Spring AOP深入理解之拦截器调用

Posted 2020-09-09 brucemengbm

Spring AOP深入理解之拦截器调用

Spring AOP代理对象生成回想

上一篇博客中：深入理解Spring AOP之二代理对象生成介绍了Spring代理对象是怎样生成的，当中重点介绍了JDK动态代理方式，简单回想下代理对象生成过程：

1、上面讲到了两种生成代理对象的方法，一种是通过ProxyFactory，一种是通过ProxyFactoryBean。

第一种获取比較简单，可是须要手工的进行写代码。而另外一种是通过Spring的IOC机制来控制Bean的生成。

2、不管是ProxyFactory或者ProxyFactoryBean都是要通过createAopProxy().getProxy()来获取对应的代理对象，而通过Proxyfactory比較直接，上面重点介绍的是通过ProxyFactoryBean获得proxy。

3、首先，找到ProxyFactoryBean的getObject方法，为什么？（主要是跟Bean容器中getObject能返回代理对象）

4、其次调用getSingletonInstance（），在getSingletonInstance方法中引入了super中的方法，super是指ProxyCreatorSupport。这里ProxyCreatorSupport是ProxyFactoryBean和ProxyFactory的父类。已经做了非常多工作，仅仅需在ProxyFactoryBean的getObject()方法中通过父类的createAopProxy()取得对应的AopProxy。

5、跟踪createAopProxy方法，追踪到了ProxyCreatorSupport中，然后，借助了AopProxyFactory。此时得到的aopProxyFactory。在构造函数中已经定义为DefaultAopProxyFactory

6、进入DefaultAopProxyFactory中，找到createAopProxy方法，在这里推断是调用JDK动态或者CGlib动态中的一种。

既然代理对象已经生成，那么拦截器是怎样被调用的呢

回想下JdkDynamicAopProxy中生成proxy方法

@Override
    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);
    }

看最后一句return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this)。这个地方的三个參数在博客动态代理中做过具体介绍。再回想下

• classloader类载入器，定义了由哪个ClassLoader对象来对生成的代理对象进行载入

• proxiedInterfaces，一个Interface对象的数组，表示的是我将要给我须要代理的对象提供一组什么接口

• this，表示JdkDynamicAopProxy自身，由于JdkDynamicAopProxy实现了InvocationHandler接口

在动态代理这篇博客中我们分析过。生成的proxy代理类被调用时。会调用super.h.method()方法，这里的super一般指的是Proxy。而Proxy中有一个InvocationHandler，即h。所以InvocationHandler的invoke方法会被作为回调函数调用

@Override
    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)) {
                //目标对象未实现equals方法
                return equals(args[0]);
            }
            if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
                //目标对象未实现hashcode方法
                return hashCode();
            }
            if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
                    method.getDeclaringClass().isAssignableFrom(Advised.class)) {
            //这里就是目标对象方法的调用
                return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
            }

            Object retVal;

            if (this.advised.exposeProxy) {
                // 获得当前的对象
                oldProxy = AopContext.setCurrentProxy(proxy);
                setProxyContext = true;
            }

            //目标对象可能源自一个池或者一个简单的方法
            target = targetSource.getTarget();
            if (target != null) {
                targetClass = target.getClass();
            }

            // 得到拦截器链
            List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);

            // 检查是否定义了拦截器方法，假设没有的话直接调用目标方法

            if (chain.isEmpty()) {

                retVal = AopUtils.invokeJoinpointUsingReflection(target, method, args);
            }
            else {
                //我们须要创建一个调用方法
                invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
                // proceed内部实现了递归调用遍历拦截器链
                retVal = invocation.proceed();
            }

            Class<?
> returnType = method.getReturnType();
            if (retVal != null && retVal == target && returnType.isInstance(proxy) &&
                    !RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {

                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()) {
                // 释放target对象
                targetSource.releaseTarget(target);
            }
            if (setProxyContext) {
                // 替换回原来的proxy
                AopContext.setCurrentProxy(oldProxy);
            }
        }
    }

上面的invoke（）主要分为三部分

拦截器链的获取， this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass)

目标对象方法的调用，即invokeJoinpointUsingReflection（）方法

拦截器对象方法的调用，即ReflectiveMethodInvocation中proceed()

首先看拦截器链是如获得的。

进入AdvisedSupport（advised为AdvisedSupport实例）的getInterceptorsAndDynamicInterceptionAdvice()方法

public List<Object> getInterceptorsAndDynamicInterceptionAdvice(Method method, Class<?> targetClass) {
        MethodCacheKey cacheKey = new MethodCacheKey(method);
        List<Object> cached = this.methodCache.get(cacheKey);
        if (cached == null) {
            cached = this.advisorChainFactory.getInterceptorsAndDynamicInterceptionAdvice(
                    this, method, targetClass);
            this.methodCache.put(cacheKey, cached);
        }
        return cached;
    }

能够看出上面方法用到缓存的, 如果如今缓存没有对应的拦截器, 则到 AdvisorChainFactory 的对应方法中获取，继续跟踪AdvisorChainFactory，发现定义为new DefaultAdvisorChainFactory()，这里是不是有一点眼熟，对的，在上一篇中讲到动态代理是也有一个DefaultAopProxy。进入DefaultAdvisorChainFactory中的对应方法。比較长

/** 
    * 从提供的配置实例config中获取advisor列表,遍历处理这些advisor.假设是IntroductionAdvisor, 
    * 则推断此Advisor是否能应用到目标类targetClass上.假设是PointcutAdvisor,则推断 
    * 此Advisor是否能应用到目标方法method上.将满足条件的Advisor通过AdvisorAdaptor转化成Interceptor列表返回. 
    */  

    public List getInterceptorsAndDynamicInterceptionAdvice(Advised config, Methodmethod, Class targetClass) {

       List interceptorList = new ArrayList(config.getAdvisors().length);

       //查看是否包括IntroductionAdvisor
       boolean hasIntroductions = hasMatchingIntroductions(config,targetClass);

       //这里实际上注冊一系列AdvisorAdapter,用于将Advisor转化成MethodInterceptor
       AdvisorAdapterRegistry registry = GlobalAdvisorAdapterRegistry.getInstance();

       Advisor[] advisors = config.getAdvisors();
        for (int i = 0; i <advisors.length; i++) {
           Advisor advisor = advisors[i];
           if (advisor instanceof PointcutAdvisor) {
                // Add it conditionally.
                PointcutAdvisor pointcutAdvisor= (PointcutAdvisor) advisor;
                if(config.isPreFiltered() ||pointcutAdvisor.getPointcut().getClassFilter().matches(targetClass)) {
                    //TODO: 这个地方这两个方法的位置能够互换下
                    //将Advisor转化成Interceptor
                    MethodInterceptor[]interceptors = registry.getInterceptors(advisor);

                    //检查当前advisor的pointcut能否够匹配当前方法
                    MethodMatcher mm =pointcutAdvisor.getPointcut().getMethodMatcher();

                    if (MethodMatchers.matches(mm,method, targetClass, hasIntroductions)) {
                        if(mm.isRuntime()) {
                            // Creating a newobject instance in the getInterceptors() method
                            // isn‘t a problemas we normally cache created chains.
                            for (intj = 0; j < interceptors.length; j++) {
                               interceptorList.add(new InterceptorAndDynamicMethodMatcher(interceptors[j],mm));
                            }
                        } else {
                            interceptorList.addAll(Arrays.asList(interceptors));
                        }
                    }
                }
           } else if (advisor instanceof IntroductionAdvisor){
                IntroductionAdvisor ia =(IntroductionAdvisor) advisor;
                if(config.isPreFiltered() || ia.getClassFilter().matches(targetClass)) {
                    Interceptor[] interceptors= registry.getInterceptors(advisor);
                    interceptorList.addAll(Arrays.asList(interceptors));
                }
           } else {
                Interceptor[] interceptors =registry.getInterceptors(advisor);
                interceptorList.addAll(Arrays.asList(interceptors));
           }
       }
       return interceptorList;
}

注意上面代码中的一行，注冊一系列AdvisorAdapter。找到GlobalAdvisorAdapterRegistry，事实上定义为 new DefaultAdvisorAdapterRegistry()，又是default~~~

AdvisorAdapterRegistry registry = GlobalAdvisorAdapterRegistry.getInstance();

instance = new DefaultAdvisorAdapterRegistry();

public DefaultAdvisorAdapterRegistry() {
        registerAdvisorAdapter(new MethodBeforeAdviceAdapter());
        registerAdvisorAdapter(new AfterReturningAdviceAdapter());
        registerAdvisorAdapter(new ThrowsAdviceAdapter());
    }

所以registry事实上为调用了DefaultAdvisorAdapterRegistry的构造函数，然后才干在将Advisor转化成Interceptor时。调用DefaultAdvisorAdapterRegistry的getInterceptors（）方法。

    @Override
    public MethodInterceptor[] getInterceptors(Advisor advisor) throws UnknownAdviceTypeException {
        List<MethodInterceptor> interceptors = new ArrayList<MethodInterceptor>(3);
        Advice advice = advisor.getAdvice();
        if (advice instanceof MethodInterceptor) {
            interceptors.add((MethodInterceptor) advice);
        }
        for (AdvisorAdapter adapter : this.adapters) {
            if (adapter.supportsAdvice(advice)) {
                interceptors.add(adapter.getInterceptor(advisor));
            }
        }
        if (interceptors.isEmpty()) {
            throw new UnknownAdviceTypeException(advisor.getAdvice());
        }
        return interceptors.toArray(new MethodInterceptor[interceptors.size()]);
    }

这里的MethodInterceptor[]返回值，正好能够匹配上面代码中的 MethodInterceptor[]interceptors = registry.getInterceptors(advisor);

目标对象方法的调用

来看看invokeJoinpointUsingReflection（）。即上面所说的目标对象方法的调用，事实上是通过AopUtils的方法调用。使用反射机制来对目标对象的方法进行的：

    public static Object invokeJoinpointUsingReflection(Object target, Method method, Object[] args)
            throws Throwable {

        // 通过反射机制来获得对应的方法。并调用invoke
        try {
            ReflectionUtils.makeAccessible(method);
            return method.invoke(target, args);
        }
        catch (InvocationTargetException ex) {

            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);
        }
    }

拦截器方法运行

看看拦截器对象，即ReflectiveMethodInvocation中proceed()方法的调用

invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);

retVal = invocation.proceed();

invocation是一个new ReflectiveMethodInvocation（）实例，找到ReflectiveMethodInvocation的proceed()方法

private int currentInterceptorIndex = -1;

    @Override
    public Object proceed() throws Throwable {
        //  currentInterceptorIndex初始化的长度为-1，以下就就是推断          //interceptorsAndDynamicMethodMatchers长度是否为0
        if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
            return invokeJoinpoint();
        }

        Object interceptorOrInterceptionAdvice =
                this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
        if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
            //匹配是否为正确的方法逻辑，（MethodMatcher）能够看出为匹配假设不是就调用下一个
            InterceptorAndDynamicMethodMatcher dm =
                    (InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
            if (dm.methodMatcher.matches(this.method, this.targetClass, this.arguments)) {
                return dm.interceptor.invoke(this);
            }
            else {
                //匹配失败，跳过这个拦截器，继续下一个
                return proceed();
            }
        }
        else {
            // 假设是interceptor。则调用invoke方法。这是为了兼容

            return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
        }
    }