Spring AOP的大致流程

我们在之前的2章中，知晓了Spring AOP在BeanPostProcessor的几个关键接口做了介入,分为几个关键点:

1. 将切面类进行解析成advisors,其中包括解析XML和被@Aspect注解标注的Java切面类.
2. 在初始化Bean的阶段，将advisors对当前Bean进行筛选，获取到当前Bean匹配的advisors.
3. 进行动态代理类的创建.

今天，我们就来看看动态代理类在Spring AOP中是怎样去创建出来的.

创建动态代理的入口:wrapIfNecessary

• org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator#wrapIfNecessary

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
    // beanName不为空，并且存在于targetSourcedBeans中，也就是自定义的TargetSource被解析过了
    if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
        return bean;
    }
    // 如果Bean为advisedBeans，也不需要被代理
    if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
        return bean;
    }
    // isInfrastructureClass和shouldSkip的作用:
    // 识别切面类，加载切面类成advisors
    // 为什么又执行一次是因为存在循环依赖的情况下无法加载advisor
    if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
        this.advisedBeans.put(cacheKey, Boolean.FALSE);
        return bean;
    }

    // Create proxy if we have advice.
    // 返回匹配当前Bean的所有Advice、Advisor、Interceptor
    Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
    if (specificInterceptors != DO_NOT_PROXY) {
        this.advisedBeans.put(cacheKey, Boolean.TRUE);
        // 创建Bean对应的代理，SingletonTargetSource用于封装实现类的信息
        Object proxy = createProxy(
                bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
        this.proxyTypes.put(cacheKey, proxy.getClass());
        return proxy;
    }
    // 下次代理的时候直接从缓存拿出判断，不需要重复代理
    this.advisedBeans.put(cacheKey, Boolean.FALSE);
    return bean;
}

1. targetSourcedBeans存储了用户自定义的targetSource，这部分的bean在org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator#postProcessBeforeInstantiation中已经进行了createProxy的处理，所以在自动化代理的环节中，要过滤掉这部分的bean.
2. advisedBeans存储了已经被Spring AOP处理过的bean，这部分的bean也是需要进行过滤的.
3. 进行isInfrastructureClass和shouldSkip的处理，这两个方法在之前的文章已经做了详细的解析，它们的主要作用是:识别切面类、解析切面类。之所以再重复调用一次，是为了收尾的工作.
4. getAdvicesAndAdvisorsForBean获取当前bean匹配的advisors.
5. 将当前bean缓存到advisedBeans,创建动态代理-createProxy,缓存proxyType. 这里无论是否能获取到bean的advisors，都会做缓存到advisedBeans的步骤.已确保不重复处理同一个bean.

创建动态代理:createProxy

• org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator#createProxy

protected Object createProxy(Class<?> beanClass, @Nullable String beanName,
        @Nullable Object[] specificInterceptors, TargetSource targetSource) {
    // 如果beanFactory是ConfigurableListableBeanFactory的类型,暴露目标类
    if (this.beanFactory instanceof ConfigurableListableBeanFactory) {
        AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);
    }
    // 创建一个ProxyFactory,当前ProxyCreator在创建代理时，将需要用到的字段赋值到ProxyFactory中去
    ProxyFactory proxyFactory = new ProxyFactory();
    // 将当前的AnnotationAwareAspectJAutoProxyCreator对象的属性赋值给ProxyFactory对象
    // 加载一些ProxyConfig
    proxyFactory.copyFrom(this);
    // 处理proxyTargetClass属性
    // 如果希望使用CGLIB进行代理，配置proxyTargetClass为true
    if (!proxyFactory.isProxyTargetClass()) {
        // 检查相应BeanDefinition的“ preserveTargetClass”属性
        if (shouldProxyTargetClass(beanClass, beanName)) {
            proxyFactory.setProxyTargetClass(true);
        }
        else {
            // 1. 有接口的，调用一次或者多次：proxyFactory.addInterface(ifc);
            // 2. 没有接口的,调用: proxyFactory.setProxyTargetClass(true);
            evaluateProxyInterfaces(beanClass, proxyFactory);
        }
    }
    // 这个方法主要是对前面传递进来的横切逻辑实例进行包装
    // specificInterceptors中有Advice和Interceptor，它们都会被包装成Advisor
    // 调用的先后顺序，通过方法中的applyCommonInterceptorsFirst参数进行设置
    Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
    proxyFactory.addAdvisors(advisors);
    proxyFactory.setTargetSource(targetSource);
    // 扩展实现，子类可以定制proxyFactory
    customizeProxyFactory(proxyFactory);

    proxyFactory.setFrozen(this.freezeProxy);
    // 设置preFiltered的属性值,默认为false.子类:AbstractAdvisorAutoProxyCreator修改为true
    // preFiltered字段的意思是:是否为特定目标类筛选Advisor
    // 该字段和DefaultAdvisorChainFactory.getInterceptorsAndDynamicInterceptionAdvice获取所有的Advisors
    // CglibAopProxy和JdkDynamicAopProxy都会调用此方法，然后递归执行所有的advisor
    if (advisorsPreFiltered()) {
        proxyFactory.setPreFiltered(true);
    }

    return proxyFactory.getProxy(getProxyClassLoader());
}

1. 如果当前的beanFactory属于ConfigurableListableBeanFactory类型,给当前bean对应的BeanDefinition设置上属性"originalTargetClass"为targetClass.
2. 先new一个ProxyFactory实例，并根据AnnotationAwareAspectJAutoProxyCreator的配置进行proxyTargetClass、optimize、exposeProxy、frozen、opaque的属性设置.
3. Spring不仅提供了对所有bean生效的proxyTargetClass设置，也提供了对单个bean的动态代理配置preserveTargetClass.
4. 调用buildAdvisors对specificInterceptors进行适配，封装成advisor.有兴趣的可以看看org.springframework.aop.framework.adapter.DefaultAdvisorAdapterRegistry#wrap.
5. 配置advisors、targetSource、frozen等属性后,进行动态代理的创建.
6. proxyFactory.getProxy(getProxyClassLoader()),进行动态代理生成.

getProxy: 根据当前bean选择JDK或者CGLIB动态代理

• org.springframework.aop.framework.ProxyFactory#getProxy(java.lang.ClassLoader)

public Object getProxy(@Nullable ClassLoader classLoader) {
    // 首先获取AopProxy对象，其主要有两个实现:JdkDynamicAopProxy和ObjenesisCglibAopProxy
    // 分别用于JDK和CGLIB代理类的生成，其getProxy方法则用于获取具体的代理对象
    return createAopProxy().getProxy(classLoader);
}

ProxyFactory是ProxyCreatorSupport的子类，在ProxyCreatorSupport中有一个createAopProxy方法，从这里可以获取到具体的代理工厂类.

• org.springframework.aop.framework.ProxyCreatorSupport#createAopProxy

protected final synchronized AopProxy createAopProxy() {
    // 激活AdvisedSupportListener监听器
    if (!this.active) {
        activate();
    }
    return getAopProxyFactory().createAopProxy(this);
}

1. activate方法主要是用来激活AdvisedSupportListener的.
2. 重点方法是这个:getAopProxyFactory().createAopProxy(this);其中,getAopProxyFactory中存储了一个DefaultAopProxyFactory,它实现了AopProxyFactory接口,接口方法createAopProxy才是真正获取代理工厂的地方.

• org.springframework.aop.framework.DefaultAopProxyFactory#createAopProxy

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.");
        }
        // 如果要代理的类本身就是接口
        // 或者它已经是JDK的代理类(Proxy子类)
        // 使用JDK动态代理
        if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
            return new JdkDynamicAopProxy(config);
        }
        // 使用CGLIB动态代理
        return new ObjenesisCglibAopProxy(config);
    }
    else {
        // 接口->JDK动态代理
        return new JdkDynamicAopProxy(config);
    }
}

这里就会看到我们很熟悉的一个逻辑:实现了接口的bean,Spring会使用JDK动态代理;否则使用CGLIB代理.

工厂模式:JdkDynamicAopProxy和ObjenesisCglibAopProxy

代理策略选择

由于篇幅的原因，我选择讲解JdkDynamicAopProxy这种模式生成代理的源码解析.

• org.springframework.aop.framework.JdkDynamicAopProxy#getProxy(java.lang.ClassLoader)

public Object getProxy(@Nullable ClassLoader classLoader) {
    if (logger.isTraceEnabled()) {
        logger.trace("Creating JDK dynamic proxy: " + this.advised.getTargetSource());
    }
    // 获取完整的代理接口
    Class<?>[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised, true);
    findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
    // JdkDynamicAopProxy本身实现了InvocationHandler接口
    return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
}

关于JDK如何生成动态代理的原理，这里不做过多的分析了，这在我之前的博客中有关于JDK和CGLIB生成Proxy的文章，不了解的读者可以去看看:
点我前往
在生成代理之后，Proxy就会调用InvocationHandler.invoke方法，所以关于Spring AOP的代理拦截链执行流程，我们直接去到invoke解析即可。

• org.springframework.aop.framework.JdkDynamicAopProxy#invoke

final class JdkDynamicAopProxy implements AopProxy, InvocationHandler

首先,JdkDynamicAopProxy实现了InvocationHandler接口,动态代理类最终会执行InvocationHandler.invoke.

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    Object oldProxy = null;
    boolean setProxyContext = false;

    TargetSource targetSource = this.advised.targetSource;
    Object target = null;

    try {
        // equals方法不需要代理
        if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
            // The target does not implement the equals(Object) method itself.
            return equals(args[0]);
        }
        // hashcode方法不需要代理
        else if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
            // The target does not implement the hashCode() method itself.
            return hashCode();
        }
        // 如果当前方法是Spring织入的DecoratingProxy接口中的方法，返回目标对象的class类型
        else if (method.getDeclaringClass() == DecoratingProxy.class) {
            // There is only getDecoratedClass() declared -> dispatch to proxy config.
            return AopProxyUtils.ultimateTargetClass(this.advised);
        }
        // 如果被代理的对象本身实现了Advised接口，则证明该类里面的方法已经被代理了，直接执行即可
        else 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;
        // 是否暴露代理引用
        if (this.advised.exposeProxy) {
            // Make invocation available if necessary.
            oldProxy = AopContext.setCurrentProxy(proxy);
            setProxyContext = true;
        }

        // Get as late as possible to minimize the time we "own" the target,
        // in case it comes from a pool.
        // 获取目标类
        target = targetSource.getTarget();
        Class<?> targetClass = (target != null ? target.getClass() : null);

        // Get the interception chain for this method.
        // 获取方法的拦截链
        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.
        // 如果该方法上没有匹配的拦截器，直接反射调用Method.invoke(target,args)
        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.
            Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
            retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
        }
        else {
            // We need to create a method invocation...
            // 创建ReflectiveMethodInvocation
            MethodInvocation 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 != Object.class && 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);
        }
    }
}

1. Spring AOP不对equals和hashCode做代理.
2. 如果当前方法是Spring织入的DecoratingProxy接口中的方法，返回目标对象的class类型.
3. 如果被代理的对象本身实现了Advised接口，则证明该类里面的方法已经被代理了，直接执行即可
4. 查看当前配置判断是否暴露代理引用，可以来解决this调用引起的代理失效等问题.
5. 从targetSource中获取target信息.获取当前method的拦截链路.getInterceptorsAndDynamicInterceptionAdvice会对method的拦截链进行缓存，如果没有缓存，会执行org.springframework.aop.framework.DefaultAdvisorChainFactory#getInterceptorsAndDynamicInterceptionAdvice重新对method进行match.
6. 从getInterceptorsAndDynamicInterceptionAdvice获取到chain之后,如果chain为空，那么直接激活method.
7. 如果chain不为空,生成ReflectiveMethodInvocation,从JavaDoc可以知道，这是一个Spring实现了AOP Alliance MethodInvocation接口的类，关键方法:invokeJoinpoint(激活连接点方法)、proceed(执行拦截链逻辑).
8. 执行proceed.
9. 处理返回值。包装此返回值（如果有必要作为代理），并验证没有将null作为原语返回。这是什么意思呢:假设方法返回的是int,增强around方法返回了null,这就会报错,因为基础数据类型都有默认值.
10. 释放资源、重新设置一次代理引用.

这里最关键的的看chain的执行过程，整个Spring AOP拦截链执行的过程采用了一种递归的方式，值得一品.

代理类执行链路

• org.springframework.aop.framework.ReflectiveMethodInvocation#proceed

public Object proceed() throws Throwable {
    // We start with an index of -1 and increment early.
    // 如果拦截器执行完了，则执行连接点
    if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
        return invokeJoinpoint();
    }
    // 从0开始执行,每次递归进来都会+1
    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;
        Class<?> targetClass = (this.targetClass != null ? this.targetClass : this.method.getDeclaringClass());
        // 动态匹配:运行时参数是否满足匹配条件
        if (dm.methodMatcher.matches(this.method, targetClass, this.arguments)) {
            return dm.interceptor.invoke(this);
        }
        else {
            // Dynamic matching failed.
            // Skip this interceptor and invoke the next in the chain.
            // 动态匹配失败，进入下一个拦截器
            return proceed();
        }
    }
    else {
        // It's an interceptor, so we just invoke it: The pointcut will have
        // been evaluated statically before this object was constructed.
        // 执行当前拦截器
        return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
    }
}

1. 在遇到AfterThrowing、AfterRetruning、After这类advice的时候，它们会执行mi.proceed回到chain中.
2. 回到chain中后，列表索引自增，往下执行下一个advice.
3. 依次递归，直到遇到Around通知，执行around.
4. 在around中执行joinPoint.proceed又会回到chain中.
5. 递归到Before,执行before,然后回到around的后置代码块中.
6. 回溯到after、afterReturning,如果有异常，执行afterThrowing.

执行流程图

proceed

总结

• 1. 生成代理类通过工厂模式进行生成,实现接口默认使用JDK动态代理，否则使用CGLIB
• 2. Spring AOP通过后置处理器的postProcessAfterInitialization方法调用wrapIfNessary对Bean进行代理并替换.
• 3. 代理类执行拦截链通过chain进行递归proceed来执行每个通知的方法