Spring AOP是Spring的两大基石之一，不了解其基础概念的同学可以查看这两篇文章AOP基本概念和修饰者模式和JDK Proxy。
 如果从代码执行角度来看，Spring AOP的执行过程分为四大步骤：

• 步骤一：Spring框架生成Advisor实例，可以是@Aspect，@Async等注解生成的实例，也可以是程序员自定义的AbstractAdvisor子类的实例。
• 步骤二：Spring框架在目标实例初始化完成后，也就是使用BeanPostProcessor的postProcessAfterInitialization方法，根据Advisor实例中切入点Pointcut的定义，选择出适合该目标对象的Advisor实例。
• 步骤三：Spring框架根据Advisor实例生成代理对象。
• 步骤四：调用方法执行过程时，Spring框架执行Advisor实例的通知Advice逻辑。

 由于这四个步骤涉及的源码量较大，一篇文章无法直接完全讲解完，本篇文章只讲解第一步Advisor实例生成的源码分析。接下来的文章我们就依次讲解一下后续步骤中比较关键的逻辑。

Advisor类架构

  Spring中有大量的机制都是通过AOP实现的，比如说@Async的异步调用和@Transational。此外，用户也可以使用@Aspect注解定义切面或者直接继承AbstractPointcutAdvisor来提供切面逻辑。上述这些情况下，AOP都会生成对应的Advisor实例。

 我们先来看一下Advisor的相关类图。首先看一下org.aopalliance包下的类图。aopalliance是AOP组织下的公用包，用于AOP中方法增强和调用，相当于一个jsr标准，只有接口和异常，在AspectJ、Spring等AOP框架中使用。

 aopalliance定义了AOP的通知Advice和连接点Joinpoint接口，并且还有继承上述接口的MethodInterceptor和MethodInvocation。这两个类相信大家都很熟悉。

 然后我们来看一下Spring AOP中Advisor相关的类图。Advisor是Spring AOP独有的概念，比较重要的类有AbstractPointcutAdvisor和InstantiationModelAwarePointcutAdvisor。相关的讲解都在图中表明了，如果这张图中的概念和类同学们都熟识，那么对AOP的了解就已经很深入了。

获取所有Advisor实例

 AOP生成Advisor实例的函数入口是AbstractAdvisorAutoProxyCreator的findCandidateAdvisors函数。

// AbstractAdvisorAutoProxyCreator.java 找出当前所有的Advisor
protected List<Advisor> findCandidateAdvisors() {
    Assert.state(this.advisorRetrievalHelper != null, "No BeanFactoryAdvisorRetrievalHelper available");
    return this.advisorRetrievalHelper.findAdvisorBeans();
}

// AnnotationAwareAspectJAutoProxyCreator，是AbstractAdvisorAutoProxyCreator的子类
@Override
protected List<Advisor> findCandidateAdvisors() {
    // 调用父类的findCandidateAdvisor函数，一般找出普通的直接
    // 继承Advisor接口的实例，比如说`@Async`所需的`AsyncAnnotationAdvisor`
    List<Advisor> advisors = super.findCandidateAdvisors();
    // 为AspectJ的切面构造Advisor，也就是说处理@Aspect修饰的类，生成上文中说的`InstantiationModelAwarePointcutAdvisor`实例
    if (this.aspectJAdvisorsBuilder != null) {
        advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
    }
    return advisors;
}

 相关的ProxyCreator也有一个类体系，不过太过繁杂，而且重要性不大，我们就先略过，直接将具体的类。由上边代码可知AbstractAdvisorAutoProxyCreator 的findCandidateAdvisors 函数是直接获取Spring容器中的Advisor实例，比如说AsyncAnnotationAdvisor实例，或者说我们自定义的AbstractPointcutAdvisor的子类实例。AdvisorRetrievalHelper 的findAdvisorBeans 函数通过BeanFactory的getBean获取了所有类型为Advisor的实例。

 而AnnotationAwareAspectJAutoProxyCreator 看其类名就可知，是与AspectJ相关的创建器，用来获取@Aspect定义的Advisor实例，也就是InstantiationModelAwarePointcutAdvisor实例。

 接下去我们看一下BeanFactoryAspectJAdvisorsBuilder的buildAspectJAdvisors函数，它根据@Aspect修饰的切面实例生成对应的Advisor实例。

public List<Advisor> buildAspectJAdvisors() {
    List<String> aspectNames = this.aspectBeanNames;
    // 第一次初始化，synchronized加双次判断，和经典单例模式的写法一样。
    if (aspectNames == null) {
        synchronized (this) {
            aspectNames = this.aspectBeanNames;
            if (aspectNames == null) {
                // Spring源码并没有buildAspectJAdvisorsFirstly函数，为了方便理解添加。
                // 获取aspectNames，创建Advisor实例，并且存入aspectFactoryCache缓存
                return buildAspectJAdvisorsFirstly();
            }
        }
    }

    if (aspectNames.isEmpty()) {
        return Collections.emptyList();
    }
    List<Advisor> advisors = new ArrayList<>();
    // 遍历aspectNames,依次获取对应的Advisor实例，或者是MetadataAwareAspectInstanceFactory生成的Advisor实例
    for (String aspectName : aspectNames) {
        List<Advisor> cachedAdvisors = this.advisorsCache.get(aspectName);
        // cache可以取到实例，该Advisor是单例的
        if (cachedAdvisors != null) {
            advisors.addAll(cachedAdvisors);
        }
        else {
            // 取得Advisor对应的工厂类实例，再次生成Advisor实例，该Advisor是多实例的。
            MetadataAwareAspectInstanceFactory factory = this.aspectFactoryCache.get(aspectName);
            advisors.addAll(this.advisorFactory.getAdvisors(factory));
        }
    }
    return advisors;
}

 buildAspectJAdvisors函数执行时分为两种情况，第一个未初始化时，也就是aspectNames为null时，执行buildAspectJAdvisorsFirstly进行第一次初始化，在这一过程中生成切面名称列表aspectBeanNames和要返回的Advisor 列表，并且将生成的Advisor实例放置到advisorsCache中。

 第二种情况则是已经初始化后再次调用，遍历aspectNames，从advisorsCache 取出对应的Advisor实例，或者从advisorsCache取出Advisor对应的工厂类对象，再次生成Advisor实例。

public List<Advisor> buildAspectJAdvisorsFirstly() {
    List<Advisor> advisors = new ArrayList<>();
    List<String> aspectNames = new ArrayList<>();
    // 调用BeanFactoryUtils获取所有bean的名称
    String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
            this.beanFactory, Object.class, true, false);
    for (String beanName : beanNames) {
        if (!isEligibleBean(beanName)) {
            continue;
        }
        // 获取对应名称的bean实例
        Class<?> beanType = this.beanFactory.getType(beanName);
        if (beanType == null) {
            continue;
        }
        /**
         * AbstractAspectJAdvisorFactory类的isAspect函数来判断是否为切面实例
         * 判断条件为是否被@Aspect修饰或者是由AspectJ编程而来。
         */
        if (this.advisorFactory.isAspect(beanType)) {
            aspectNames.add(beanName);
            AspectMetadata amd = new AspectMetadata(beanType, beanName);
            // 切面的属性为单例模式
            if (amd.getAjType().getPerClause().getKind() == PerClauseKind.SINGLETON) {
                MetadataAwareAspectInstanceFactory factory =
                        new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);
                // 获取一个切面中所有定义的Advisor实例。一个切面可以定义多个Advisor。
                List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);
                // 单例模式，只需要将生成的Advisor添加到缓存
                if (this.beanFactory.isSingleton(beanName)) {
                    this.advisorsCache.put(beanName, classAdvisors);
                }
                // 多实例模式，需要保存工厂类，便于下一次再次生成Advisor实例。
                else {
                    this.aspectFactoryCache.put(beanName, factory);
                }
                advisors.addAll(classAdvisors);
            }
            else {
                MetadataAwareAspectInstanceFactory factory =
                        new PrototypeAspectInstanceFactory(this.beanFactory, beanName);
                this.aspectFactoryCache.put(beanName, factory);
                advisors.addAll(this.advisorFactory.getAdvisors(factory));
            }
        }
    }
    this.aspectBeanNames = aspectNames;
    return advisors;
}

 buildAspectJAdvisorsFirstly函数的逻辑如下：

• 首先使用BeanFactoryUtils获取了BeanFactory中所有的BeanName，然后进而使用BeanFactory获取所有的Bean实例。
• 遍历Bean实例，通过ReflectiveAspectJAdvisorFactory的isAspect函数判断该实例是否为切面实例，也就是被@Aspect注解修饰的实例。
• 如果是，则使用ReflectiveAspectJAdvisorFactory，根据切面实例的定义来生成对应的多个Advisor实例，并且将其加入到advisorsCache中。

生成InstantiationModelAwarePointcutAdvisorImpl实例

 ReflectiveAspectJAdvisorFactory 的getAdvisors 函数会获取@Aspect修饰的实例中所有没有被@Pointcut修饰的方法，然后调用getAdvisor函数，并且将这些方法作为参数。

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

    validate(aspectInstanceFactory.getAspectMetadata().getAspectClass());
    // 获得该方法上的切入点条件表达式
    AspectJExpressionPointcut expressionPointcut = getPointcut(
            candidateAdviceMethod, aspectInstanceFactory.getAspectMetadata().getAspectClass());
    if (expressionPointcut == null) {
        return null;
    }
    // 生成Advisor实例
    return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
            this, aspectInstanceFactory, declarationOrderInAspect, aspectName);
}

private AspectJExpressionPointcut getPointcut(Method candidateAdviceMethod, Class<?> candidateAspectClass) {
    // 获得该函数上@Pointcut, @Around, @Before, @After, @AfterReturning, @AfterThrowing注解的信息
    AspectJAnnotation<?> aspectJAnnotation =
            AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
    // 没有上述注解，则直接返回
    if (aspectJAnnotation == null) {
        return null;
    }

    AspectJExpressionPointcut ajexp =
            new AspectJExpressionPointcut(candidateAspectClass, new String[0], new Class<?>[0]);
    // 获得注解信息中的切入点判断表达式        
    ajexp.setExpression(aspectJAnnotation.getPointcutExpression());
    if (this.beanFactory != null) {
        ajexp.setBeanFactory(this.beanFactory);
    }
    return ajexp;
}

 getAdvisor函数就是根据作为参数传入的切面实例的方法上的注解来生成Advisor实例，也就是InstantiationModelAwarePointcutAdvisorImpl对象。依据方法上的切入点表达式生成AspectJExpressionPointcut 。
 我们都知道PointcutAdvisor实例中必然有一个Pointcut和Advice实例。修饰在方法上的注解包括：@Pointcut, @Around, @Before, @After, @AfterReturning和@AfterThrowing，所以InstantiationModelAwarePointcutAdvisorImpl会依据不同的不同的注解生成不同的Advice通知。

public InstantiationModelAwarePointcutAdvisorImpl(AspectJExpressionPointcut declaredPointcut,
        Method aspectJAdviceMethod, AspectJAdvisorFactory aspectJAdvisorFactory,
        MetadataAwareAspectInstanceFactory aspectInstanceFactory, int declarationOrder, String aspectName) {
    // .... 省略成员变量的直接赋值

    // 单例模式时
    this.pointcut = this.declaredPointcut;
    this.lazy = false;
    // 按照注解解析 Advice
    this.instantiatedAdvice = instantiateAdvice(this.declaredPointcut);
}

 InstantiationModelAwarePointcutAdvisorImpl的构造函数中会生成对应的Pointcut和Advice。instantiateAdvice函数调用了ReflectiveAspectJAdvisorFactory的getAdvice函数。

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

    Class<?> candidateAspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
    validate(candidateAspectClass);
    // 获取 Advice 注解
    AspectJAnnotation<?> aspectJAnnotation =
            AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
    if (aspectJAnnotation == null) {
        return null;
    }

    // 检查是否为AspectJ注解
    if (!isAspect(candidateAspectClass)) {
        throw new AopConfigException("Advice must be declared inside an aspect type: " +
                "Offending method '" + candidateAdviceMethod + "' in class [" +
                candidateAspectClass.getName() + "]");
    }
    
    AbstractAspectJAdvice springAdvice;
    // 按照注解类型生成相应的 Advice 实现类
    switch (aspectJAnnotation.getAnnotationType()) {
        case AtPointcut:
            if (logger.isDebugEnabled()) {
                logger.debug("Processing pointcut '" + candidateAdviceMethod.getName() + "'");
            }
            return null;
        case AtAround: // @Before 生成 AspectJMethodBeforeAdvice
            springAdvice = new AspectJAroundAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            break;
        case AtBefore: // @After 生成 AspectJAfterAdvice
            springAdvice = new AspectJMethodBeforeAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            break;
        case AtAfter: // @AfterReturning 生成 AspectJAfterAdvice
            springAdvice = new AspectJAfterAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            break;
        case AtAfterReturning: // @AfterThrowing 生成 AspectJAfterThrowingAdvice
            springAdvice = new AspectJAfterReturningAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            AfterReturning afterReturningAnnotation = (AfterReturning) aspectJAnnotation.getAnnotation();
            if (StringUtils.hasText(afterReturningAnnotation.returning())) {
                springAdvice.setReturningName(afterReturningAnnotation.returning());
            }
            break;
        case AtAfterThrowing: // @Around 生成 AspectJAroundAdvice
            springAdvice = new AspectJAfterThrowingAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            AfterThrowing afterThrowingAnnotation = (AfterThrowing) aspectJAnnotation.getAnnotation();
            if (StringUtils.hasText(afterThrowingAnnotation.throwing())) {
                springAdvice.setThrowingName(afterThrowingAnnotation.throwing());
            }
            break;
        default:
            throw new UnsupportedOperationException(
                    "Unsupported advice type on method: " + candidateAdviceMethod);
    }
 // 配置Advice
    springAdvice.setAspectName(aspectName);
    springAdvice.setDeclarationOrder(declarationOrder);
    // 获取方法的参数列表方法
    String[] argNames = this.parameterNameDiscoverer.getParameterNames(candidateAdviceMethod);
    if (argNames != null) {
        // 设置参数名称
        springAdvice.setArgumentNamesFromStringArray(argNames);
    }
    springAdvice.calculateArgumentBindings();

    return springAdvice;
}

 至此，Spring AOP就获取了容器中所有的Advisor实例，下一步在每个实例初始化完成后，根据这些Advisor的Pointcut切入点进行筛选，获取合适的Advisor实例，并生成代理实例。

后记

 Spring AOP后续文章很快就会更新，请大家继续关注。