Spring源码解析 -- AOP原理(1)

Spring源码解析 -- 读取bean元数据
spring源码解析 -- 构造bean
spring源码解析 -- 注入属性
spring源码解析 -- Spring Context
Spring源码解析 -- AOP原理(1)
Spring源码解析 -- AOP原理(2)
Spring源码解析 -- SpringMvc原理

源码分析基于spring 4.3.x
本文通过阅读Spring源码，分析Spring AOP的功能是如何实现的。
关于阅读源码的思路，可参考 -- 如何阅读java源码

首先，明确几个概念
pointcut：切入点，根据方法名或者正则表达式去拦截一个方法。
advice：通知，在拦截到方法执行前或执行后的增强操作。
aspect：切面，一个切面可以包括多个切入点和通知，spring内部的切入点和通知是无序的。
advisor：只有一个通知和一个切入点的单元切面，可以看做一种特殊的aspect。

开始阅读源码前，本文主要关注的几个问题：

1. xml中<aop:aspectj-autoproxy/>元素的作用
2. spring如何根据Aspect注解创建切面
3. spring如何根据切面信息创建代理对象
4. spring是如何对多个通知进行链式调用的

<aop:aspectj-autoproxy/> 的作用

aspectj-autoproxy是spring内部定义的标签。
前面解析Spring读取bean元数据的文章说过，在spring中除了<beans>标签，其他标签需要编写一个NamespaceHandlerSupport实现类来完成标签解析工作。
在spring源码中搜索一下，就可以发现aspectj-autoproxy的解析类AopNamespaceHandler

public void init() {
    this.registerBeanDefinitionParser("config", new ConfigBeanDefinitionParser());
    this.registerBeanDefinitionParser("aspectj-autoproxy", new AspectJAutoProxyBeanDefinitionParser());
    this.registerBeanDefinitionDecorator("scoped-proxy", new ScopedProxyBeanDefinitionDecorator());
    this.registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser());
}

而AspectJAutoProxyBeanDefinitionParser类负责对aspectj-autoproxy标签进行解析：
AspectJAutoProxyBeanDefinitionParser#parse

public BeanDefinition parse(Element element, ParserContext parserContext) {
    AopNamespaceUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(parserContext, element);
    extendBeanDefinition(element, parserContext);
    return null;
}

AopNamespaceUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary -> AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary

public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(
        BeanDefinitionRegistry registry, Object source) {
    return registerOrEscalateApcAsRequired(AnnotationAwareAspectJAutoProxyCreator.class, registry, source);
}

这里将AnnotationAwareAspectJAutoProxyCreator注入到spring上下文环境中。
AnnotationAwareAspectJAutoProxyCreator是一个关键的类，继承了BeanPostProcessor接口，它负责完成创建切面工作。

创建advisor

AbstractAutoProxyCreator#postProcessAfterInitialization -> AbstractAutoProxyCreator#wrapIfNecessary
（AbstractAutoProxyCreator是AnnotationAwareAspectJAutoProxyCreator的父类）

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
    ...
    
    Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);    // #1
    if (specificInterceptors != DO_NOT_PROXY) {
        this.advisedBeans.put(cacheKey, Boolean.TRUE);
        Object proxy = createProxy(
                bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));    // #2
        this.proxyTypes.put(cacheKey, proxy.getClass());
        return proxy;
    }

    this.advisedBeans.put(cacheKey, Boolean.FALSE);
    return bean;
}

#1 根据bean元数据构造通知和单元切面
#2 创建代理对象

getAdvicesAndAdvisorsForBean -> AbstractAdvisorAutoProxyCreator#getAdvicesAndAdvisorsForBean -> AbstractAdvisorAutoProxyCreator#findEligibleAdvisors

protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
    List<Advisor> candidateAdvisors = findCandidateAdvisors();    // #1
    List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);    // #2
    extendAdvisors(eligibleAdvisors);    // #3
    if (!eligibleAdvisors.isEmpty()) {
        eligibleAdvisors = sortAdvisors(eligibleAdvisors);    // #4
    }
    return eligibleAdvisors;
}

这是一个非常重要的方法，实现了几个关键步骤
#1 findCandidateAdvisors -- 查找所有的单元切面(第一次会创建)
#2 findAdvisorsThatCanApply -- 根据目标bean的class过滤一部分的单元切面
#3 extendAdvisors -- 扩充单元切面列表，spring会根据需要添加一些内部使用的单元切面
#4 sortAdvisors -- 对单元切面排序

查找所有的单元切面

findCandidateAdvisors -> AnnotationAwareAspectJAutoProxyCreator#findCandidateAdvisors -> BeanFactoryAspectJAdvisorsBuilder#buildAspectJAdvisors

public List<Advisor> buildAspectJAdvisors() {
    List<String> aspectNames = this.aspectBeanNames;

    if (aspectNames == null) {
        synchronized (this) {
            aspectNames = this.aspectBeanNames;
            if (aspectNames == null) {    // #1
                List<Advisor> advisors = new LinkedList<Advisor>();
                aspectNames = new LinkedList<String>();
                String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                        this.beanFactory, Object.class, true, false);
                for (String beanName : beanNames) {    // #2
                    if (!isEligibleBean(beanName)) {    // #3
                        continue;
                    }
                    // We must be careful not to instantiate beans eagerly as in this case they
                    // would be cached by the Spring container but would not have been weaved.
                    Class<?> beanType = this.beanFactory.getType(beanName);
                    if (beanType == null) {
                        continue;
                    }
                    if (this.advisorFactory.isAspect(beanType)) {    // #4
                        aspectNames.add(beanName);
                        AspectMetadata amd = new AspectMetadata(beanType, beanName);    // #5
                        if (amd.getAjType().getPerClause().getKind() == PerClauseKind.SINGLETON) {
                            MetadataAwareAspectInstanceFactory factory =
                                    new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);
                            List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);    // #6
                            if (this.beanFactory.isSingleton(beanName)) {    // #7
                                this.advisorsCache.put(beanName, classAdvisors);
                            }
                            else {
                                this.aspectFactoryCache.put(beanName, factory);
                            }
                            advisors.addAll(classAdvisors);    // #8
                        }
                        else {
                            // Per target or per this.
                            if (this.beanFactory.isSingleton(beanName)) {
                                throw new IllegalArgumentException("Bean with name '" + beanName +
                                        "' is a singleton, but aspect instantiation model is not singleton");
                            }
                            MetadataAwareAspectInstanceFactory factory =
                                    new PrototypeAspectInstanceFactory(this.beanFactory, beanName);
                            this.aspectFactoryCache.put(beanName, factory);
                            advisors.addAll(this.advisorFactory.getAdvisors(factory));
                        }
                    }
                }
                this.aspectBeanNames = aspectNames;
                return advisors;
            }
        }
    }

    ... #9
}        

#1 典型的double check
#2 遍历所有的bean
#3 判断是否为合格的切面类
#4 是否为切面类(是否有Aspect注解)
#5 获取切面信息
#6 构建一系列的单元切面
#7 加入缓存
#8 加入结果
#9 缓存不为空，从缓存中获取数据返回

步骤#6调用ReflectiveAspectJAdvisorFactory.getAdvisors

public List<Advisor> getAdvisors(MetadataAwareAspectInstanceFactory aspectInstanceFactory) {
    Class<?> aspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
    String aspectName = aspectInstanceFactory.getAspectMetadata().getAspectName();
    validate(aspectClass);

    MetadataAwareAspectInstanceFactory lazySingletonAspectInstanceFactory =
            new LazySingletonAspectInstanceFactoryDecorator(aspectInstanceFactory);    // #1

    List<Advisor> advisors = new LinkedList<Advisor>();
    for (Method method : getAdvisorMethods(aspectClass)) {    // #2
        Advisor advisor = getAdvisor(method, lazySingletonAspectInstanceFactory, advisors.size(), aspectName);    // #3
        if (advisor != null) {
            advisors.add(advisor);
        }
    }
    ...
    return advisors;
}

#1 lazySingletonAspectInstanceFactory对getAspectInstance进行了缓存，保证getAspectInstance方法返回单例
#2 getAdvisorMethods获取所有没有Pointcut注解的方法（有Pointcut注解的方法不可能是Advisor）
#3 使用方法元数据构造单元切面

public Advisor getAdvisor(Method candidateAdviceMethod, MetadataAwareAspectInstanceFactory aspectInstanceFactory,
        int declarationOrderInAspect, String aspectName) {

    validate(aspectInstanceFactory.getAspectMetadata().getAspectClass());

    AspectJExpressionPointcut expressionPointcut = getPointcut(
            candidateAdviceMethod, aspectInstanceFactory.getAspectMetadata().getAspectClass());    // #1
    if (expressionPointcut == null) {
        return null;
    }

    return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
            this, aspectInstanceFactory, declarationOrderInAspect, aspectName);    // #2
}    

#1 获取切入点Pointcut，并构建了AspectJExpressionPointcut
#2 创建单元切面InstantiationModelAwarePointcutAdvisorImpl，该类中包含属性 --
被拦截类declaringClass，被拦截方法aspectJAdviceMethod，切入点declaredPointcut和通知instantiatedAdvice。

InstantiationModelAwarePointcutAdvisorImpl#构造方法 -> InstantiationModelAwarePointcutAdvisorImpl#instantiateAdvice -> ReflectiveAspectJAdvisorFactory#getAdvice

public Advice getAdvice(Method candidateAdviceMethod, AspectJExpressionPointcut expressionPointcut,
        MetadataAwareAspectInstanceFactory aspectInstanceFactory, int declarationOrder, String aspectName) {

    ... // #1

    AbstractAspectJAdvice springAdvice;

    switch (aspectJAnnotation.getAnnotationType()) {    // #2
        case AtBefore:
            springAdvice = new AspectJMethodBeforeAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            break;
        case AtAfter:
            springAdvice = new AspectJAfterAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            break;
        case AtAfterReturning:
            springAdvice = new AspectJAfterReturningAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            AfterReturning afterReturningAnnotation = (AfterReturning) aspectJAnnotation.getAnnotation();
            if (StringUtils.hasText(afterReturningAnnotation.returning())) {
                springAdvice.setReturningName(afterReturningAnnotation.returning());
            }
            break;
        case AtAfterThrowing:
            springAdvice = new AspectJAfterThrowingAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            AfterThrowing afterThrowingAnnotation = (AfterThrowing) aspectJAnnotation.getAnnotation();
            if (StringUtils.hasText(afterThrowingAnnotation.throwing())) {
                springAdvice.setThrowingName(afterThrowingAnnotation.throwing());
            }
            break;
        case AtAround:
            springAdvice = new AspectJAroundAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            break;
        case AtPointcut:
            if (logger.isDebugEnabled()) {
                logger.debug("Processing pointcut '" + candidateAdviceMethod.getName() + "'");
            }
            return null;
        default:
            throw new UnsupportedOperationException(
                    "Unsupported advice type on method: " + candidateAdviceMethod);
    }

    // Now to configure the advice...
    springAdvice.setAspectName(aspectName);
    springAdvice.setDeclarationOrder(declarationOrder);
    String[] argNames = this.parameterNameDiscoverer.getParameterNames(candidateAdviceMethod);
    if (argNames != null) {
        springAdvice.setArgumentNamesFromStringArray(argNames);
    }
    springAdvice.calculateArgumentBindings();
    return springAdvice;
}

#1 aspectInstanceFactory#getAspectMetadata获取切面信息，再获取对应的aspectJAnnotation
#2 根据切面类型，构建不同的通知实现类
@After对应AspectJAfterAdvice
@Around对应AspectJAroundAdvice
@Before对应AspectJMethodBeforeAdvice
@AfterReturning对应AspectJAfterReturningAdvice
@AfterThrowing对应AspectJAfterThrowingAdvice
除了AspectJAroundAdvice，这些通知实现类都实现了MethodInterceptor（方法拦截器接口）。

findAdvisorsThatCanApply 过滤单元切面

这里会遍历所有的单元切面，检查bean的class和class中是否有方法可以匹配对应的单元切面，如果没有则过滤该单元切面。代码比较繁琐，不展开了。

extendAdvisors(eligibleAdvisors)扩充单元切面

AspectJAwareAdvisorAutoProxyCreator.extendAdvisors -> AspectJProxyUtils.makeAdvisorChainAspectJCapableIfNecessary

spring会在advisors列表开始位置添加ExposeInvocationInterceptor。
ExposeInvocationInterceptor是一个特殊的MethodInterceptor，它将当前方法拦截器调用链放置到线程上下文中，以便有需要时使用。

创建代理对象

我们知道，spring通过动态代理类实现aop，有jdk动态代理和cglib两种方法。
如果要使用jdk动态代理，被代理类必须实现一个接口。
这里只关注jdk动态代理如何实现aop。

AbstractAutoProxyCreator#wrapIfNecessary方法#2步骤 -> AbstractAutoProxyCreator.createProxy(第三个参数就是Advices)

protected Object createProxy(
        Class<?> beanClass, String beanName, Object[] specificInterceptors, TargetSource targetSource) {

    ...
    
    Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);    // #1
    for (Advisor advisor : advisors) {
        proxyFactory.addAdvisor(advisor);    // #2
    }

    proxyFactory.setTargetSource(targetSource);
    customizeProxyFactory(proxyFactory);

    proxyFactory.setFrozen(this.freezeProxy);
    if (advisorsPreFiltered()) {
        proxyFactory.setPreFiltered(true);
    }

    return proxyFactory.getProxy(getProxyClassLoader());    // #3
}            

#1 类型检查及转化，如将Advices转换成Advisor
#2 将Advisor添加到proxyFactory中
#3 构造代理对象

proxyFactory.getProxy -> ProxyCreatorSupport.createAopProxy

protected final synchronized AopProxy createAopProxy() {
    if (!this.active) {
        activate();
    }
    return getAopProxyFactory().createAopProxy(this);    // #1
}

#1 构造代理对象，createAopProxy方法参数是this，将proxyFactory将自身作为参数，

DefaultAopProxyFactory#createAopProxy

public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
    if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) {    // #1
        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() || Proxy.isProxyClass(targetClass)) {
            return new JdkDynamicAopProxy(config);
        }
        return new ObjenesisCglibAopProxy(config);
    }
    else {    // #2
        return new JdkDynamicAopProxy(config);
    }
}

#1 根据用户配置和目标bean是否有实现接口，判断是否需要使用ObjenesisCglibAopProxy
#2 使用JDK动态代理

来看看JdkDynamicAopProxy#getProxy

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, true);
    findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
    return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);    // #1
}

#1 这里构造了jdk的动态代理对象，注意newProxyInstance方法第三个参数是this，即JdkDynamicAopProxy，JdkDynamicAopProxy实现了InvocationHandler。

Spring AOP源码解析内容过长，关于多个通知链式调用的解析留到下一篇文章解析

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