使用示例

用于拦截的Bean

public class Audience {

    private String name = "audience";

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void watch() {
        System.out.println("Watch movie");
    }
}

Advisor

@Aspect
public class AspectJAudience {
    /**
     * 定义一个公共的切点
     */
    @Pointcut("execution(* *.watch(..))")
    public void watch() {

    }

    /**
     * 目标方法执行之前调用
     */
    @Before("watch()")
    public void silenceCellPhone() {
        System.out.println("Silencing cell phone");
    }

    /**
     * 目标方法执行之前调用
     */
    @Before("watch()")
    public void takeSeat() {
        System.out.println("Taking seat");
    }

    /**
     * 目标方法执行完后调用
     */
    @AfterReturning("watch()")
    public void applause() {
        System.out.println("CLAP CLAP CLAP");
    }

    /**
     * 目标方法发生异常时调用
     */
    @AfterThrowing("watch()")
    public void demandRefund() {
        System.out.println("Demanding a refund");
    }

    /**
     * 环绕通知
     * @param p 通过它调用目标方法
     */
    @Around("watch()")
    public Object aroundWatch(ProceedingJoinPoint p) {
        Object o = null;
        try {
            System.out.println("Silencing cell phone");
            o = p.proceed();
            System.out.println("CLAP CLAP CLAP!!!");
        } catch (Throwable e) {
            System.out.println("Demanding a refund");
        }
        return o;
    }
}

配置文件

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
                            http://www.springframework.org/schema/beans/spring-beans.xsd
                            http://www.springframework.org/schema/aop
                            http://www.springframework.org/schema/aop/spring-aop.xsd">

    <aop:aspectj-autoproxy/>

    <bean id="audience" class="org.cellphone.uc.aop.Audience"/>
    <bean id="aspectJAudience" class="org.cellphone.uc.aop.AspectJAudience"/>
</beans>

测试

public class AspectJMain {

    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("spring/aop-test.xml");
        Audience audience = (Audience) context.getBean("audience");
        audience.watch();
    }
}

输出

Silencing cell phone
Silencing cell phone
Taking seat
Watch movie
CLAP CLAP CLAP!!!
CLAP CLAP CLAP

我们知道，Spring是否支持注解的AOP是由一个配置文件控制的，也就是aop:aspectj-autoproxy/，当在配置文件中声明了这句配置的时候，Spring就会支持注解的AOP，那么我们的分析就从这句注解开始。

Spring解析AOP注解

在AopNamespaceHandler中对应着这样一段函数

@Override
public void init() {
    // In 2.0 XSD as well as in 2.1 XSD.
    registerBeanDefinitionParser("config", new ConfigBeanDefinitionParser());
    registerBeanDefinitionParser("aspectj-autoproxy", new AspectJAutoProxyBeanDefinitionParser());
    registerBeanDefinitionDecorator("scoped-proxy", new ScopedProxyBeanDefinitionDecorator());

    // Only in 2.0 XSD: moved to context namespace as of 2.1
    registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser());
}

我们可以得知，在解析配置文件的时候，一旦遇到aspectj-autoproxy注解时就是使用解析器AspectJAutoProxyBeanDefinitionParser进行解析，那么我们来看一看AspectJAutoProxyBeanDefinitionParser的内部实现。

一、注册AspectJAutoProxyBeanDefinitionParser

所有解析器，因为是对BeanDefinitionParser接口的统一实现，入口都是从parse函数开始的，AspectJAutoProxyBeanDefinitionParser的parse函数如下：

@Override
@Nullable
public BeanDefinition parse(Element element, ParserContext parserContext) {
    // 注册AnnotationAwareAspectJAutoProxyCreator
    AopNamespaceUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(parserContext, element);
    // 对于注解中子类的处理
    extendBeanDefinition(element, parserContext);
    return null;
    
}

其中registerAspectJAnnotationAutoProxyCreatorIfNecessary函数是我们比较关心的，也是关键逻辑的实现。

/**
 * 注册AnnotationAwareAspectJAutoProxyCreator
 * @param parserContext
 * @param sourceElement
 */
public static void registerAspectJAnnotationAutoProxyCreatorIfNecessary(
ParserContext parserContext, Element sourceElement) {
    // 注册或升级定义beanName为		
    //org.springframework.aop.config.internalAutoProxyCreator的BeanDefinition
    BeanDefinition beanDefinition = 			
        AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(
            parserContext.getRegistry(), parserContext.extractSource(sourceElement));
    
    // 对于proxy-target-class以及expose-proxy属性的处理
    useClassProxyingIfNecessary(parserContext.getRegistry(), sourceElement);
    
    // 注册组件并通知，便于监听器做进一步处理
    registerComponentIfNecessary(beanDefinition, parserContext);
}

在registerAspectJAnnotationAutoProxyCreatorIfNecessary方法中主要完成了3件事情，基本上每行代码就是一个完整的逻辑。

1. 注册或者升级AnnotationAwareAspectJAutoProxyCreator

对于AOP的实现，基本上都是靠AnnotationAwareAspectJAutoProxyCreator去完成，它可以根据@Point注解定义的切点来自动代理相匹配的bean。但是为了配置简便，(这句话在说几把啥呢？) Spring使用了自定义配置来帮助我们自动注册AnnotationAwareAspectJAutoProxyCreator，其注册过程就是在这里实现的。

@Nullable
public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry,
        @Nullable Object source) {

    return registerOrEscalateApcAsRequired(AnnotationAwareAspectJAutoProxyCreator.class, registry, source);
}

@Nullable
private static BeanDefinition registerOrEscalateApcAsRequired(Class<?> cls, BeanDefinitionRegistry registry,
        @Nullable Object source) {

    Assert.notNull(registry, "BeanDefinitionRegistry must not be null");

    // 如果已经存在了自动代理创建器且存在的自动代理创建器与现在的不一致，那么需要根据优先级来判断到底需要使用哪一种
    if (registry.containsBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME)) {
        BeanDefinition apcDefinition = registry.getBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME);
        if (!cls.getName().equals(apcDefinition.getBeanClassName())) {
            int currentPriority = findPriorityForClass(apcDefinition.getBeanClassName());
            int requiredPriority = findPriorityForClass(cls);
            if (currentPriority < requiredPriority) {
                // 改变bean最重要的就是改变bean所对应的className属性
                apcDefinition.setBeanClassName(cls.getName());
            }
        }
        // 如果已经存在自动代理创建器并且与将要创建的一致，那么无需再次创建
        return null;
    }

    RootBeanDefinition beanDefinition = new RootBeanDefinition(cls);
    beanDefinition.setSource(source);
    beanDefinition.getPropertyValues().add("order", Ordered.HIGHEST_PRECEDENCE);
    beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
    registry.registerBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME, beanDefinition);
    return beanDefinition;
}

以上代码实现了自动注册AnnotationAwareAspectJAutoProxyCreator类的功能，同时这里还涉及了—个优先级的问题，如果巳经存在了自动代理创建器，而且存在的自动代理创建器与现在的不一致，那么需要根据优先级来判断到底需要使用哪个。

创建AOP代理

上一节讲解了通过自定义配置完成了对AnnotationAwareAspectJAutoProxyCreator类型的自动注册，那么这个类到底做了什么工作来完成AOP的操作呢？首先我们看看AnnotationAwareAspectJAutoProxyCreator类的层次结构，如下图所示。

在类的层级中，我们看到AnnotationAwareAspectJAutoProxyCreator实现了BeanPostProcessor接口，而实现BeanPostProcessor后，当Spring加载这个Bean时会在初始化后调用其postProcessAfterInitialization方法，而我们对于AOP逻辑的分析也由此开始。

假设目前有一个bean，在创建它的时候走到了postProcessAfterInitialization过程：

/**
 * Create a proxy with the configured interceptors if the bean is
 * identified as one to proxy by the subclass.
 * @see #getAdvicesAndAdvisorsForBean
 */
@Override
public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
    if (bean != null) {
        // 根据给定的bean的class和name构建出key，格式：beanClassName_beanName
        Object cacheKey = getCacheKey(bean.getClass(), beanName);
        if (!this.earlyProxyReferences.contains(cacheKey)) {
            // 如果它适合被代理，则需要封装指定bean
            return wrapIfNecessary(bean, beanName, cacheKey);
        }
    }
    return bean;
}

/**
 * Wrap the given bean if necessary, i.e. if it is eligible for being proxied.
 * @param bean the raw bean instance
 * @param beanName the name of the bean
 * @param cacheKey the cache key for metadata access
 * @return a proxy wrapping the bean, or the raw bean instance as-is
 */
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
    // 如果已经处理过
    if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
        return bean;
    }
    // 无需增强
    if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
        return bean;
    }
    // 给定的bean类是否代表一个基础设施类，基础设施类不应代理，或者配置了指定bean不需要自动代理
    if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
        this.advisedBeans.put(cacheKey, Boolean.FALSE);
        return bean;
    }

    // Create proxy if we have advice.
    // 如果存在增强方法则创建代理
    Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
    // 如果获取到了增强则需要针对增强创建代理
    if (specificInterceptors != DO_NOT_PROXY) {
        this.advisedBeans.put(cacheKey, Boolean.TRUE);
        // 创建代理
        Object proxy = createProxy(
                bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
        this.proxyTypes.put(cacheKey, proxy.getClass());
        return proxy;
    }

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

函数中我们已经看到了代理创建的雏形。当然，真正开始之前还需要经过一些判断，比如是否已经处理过或者是否是需要跳过的bean，而真正创建代理的代码是从getAdvicesAndAdvisorsForBean开始的。

创建代理主要包含了两个步骤：

（1）获取增强方法或者增强器；

（2）根据获取的增强进行代理。

@Override
@Nullable
protected Object[] getAdvicesAndAdvisorsForBean(
        Class<?> beanClass, String beanName, @Nullable TargetSource targetSource) {

    List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
    if (advisors.isEmpty()) {
        return DO_NOT_PROXY;
    }
    return advisors.toArray();
}

/**
 * Find all eligible Advisors for auto-proxying this class.
 * @param beanClass the clazz to find advisors for
 * @param beanName the name of the currently proxied bean
 * @return the empty List, not {@code null},
 * if there are no pointcuts or interceptors
 * @see #findCandidateAdvisors
 * @see #sortAdvisors
 * @see #extendAdvisors
 */
protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
    List<Advisor> candidateAdvisors = findCandidateAdvisors();
    List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
    extendAdvisors(eligibleAdvisors);
    if (!eligibleAdvisors.isEmpty()) {
        eligibleAdvisors = sortAdvisors(eligibleAdvisors);
    }
    return eligibleAdvisors;
}

对于指定bean的增强方法的获取一定是包含两个步骤的，获取所有的增强以及寻找所有增强中适用于bean的增强并应用， findCandidateAdvisors 与 findAdvisorsThatCanApply 便是做了这两件事情。当然，如果无法找到对应的增强器便返回DO_NOT_PROXY，其中DO_NOT_PROXY=null。

一、获取增强器

由于我们分析的是使用注解进行的AOP，所以对于findCandidateAdvisors的实现其实是由 AnnotationAwareAspectJAutoProxyCreator类完成的，我们继续跟踪 AnnotationAwareAspectJAutoProxyCreator 的 findCandidateAdvisors 方法。

@Override
protected List<Advisor> findCandidateAdvisors() {
    // Add all the Spring advisors found according to superclass rules.
    // 当使用注解方式配置AOP的时候并不是丢弃了对XML配置的支持，
    // 在这里调用父类方法加栽配置文件中的AOP声明
    List<Advisor> advisors = super.findCandidateAdvisors();
    // Build Advisors for all AspectJ aspects in the bean factory.
    if (this.aspectJAdvisorsBuilder != null) {
        advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
    }
    return advisors;
}

AnnotationAwareAspectJAutoProxyCreator 间接继承了AbstractAdvisorAutoProxyCreator， 在实现获取增强的方法中除了保留父类的获取配置文件中定义的增强外，同时添加了获取 Bean 的注解增强的功能，其实现正是由 this.aspectJAdvisorsBuilder.buildAspectJAdvisors() 来实现的。

在真正研究代码之前读者可以尝试着自己去想象一下解析思路，看看自己的实现与Spring是否有差别呢？或者我们一改以往的方式，先来了解函数提供的大概功能框架，读者可以在头脑中尝试实现这些功能点，看看是否有思路。

（1）尝试所有beanName，这一步骤中所有在beanFactory中注册的Bean都会被提取出来。

（2）遍历所有beanName，并找出声明AspectJ注解的类，进行进一步的处理。

（3）对标记为AspectJ注解的类进行增强器的提取。

（4）将提取结果加入缓存。

现在我们来看看函数实现，对Spring中所有的类进行分析，提取Advisor。

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

    if (aspectNames == null) {
        synchronized (this) {
            aspectNames = this.aspectBeanNames;
            if (aspectNames == null) {
                List<Advisor> advisors = new ArrayList<>();
                aspectNames = new ArrayList<>();
                // 获取所有的beanName
                String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                        this.beanFactory, Object.class, true, false);
                // 循环所有的beanName找出对应的增强方法
                for (String beanName : beanNames) {
                    // 不合法的bean则略过，由子类定义规则，默认返回true
                    if (!isEligibleBean(beanName)) {
                        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.
                    // 获取对应的bean的类型
                    Class<?> beanType = this.beanFactory.getType(beanName);
                    if (beanType == null) {
                        continue;
                    }
                    // 如果存在Aspect注解
                    if (this.advisorFactory.isAspect(beanType)) {
                        aspectNames.add(beanName);
                        AspectMetadata amd = new AspectMetadata(beanType, beanName);
                        if (amd.getAjType().getPerClause().getKind() == PerClauseKind.SINGLETON) {
                            MetadataAwareAspectInstanceFactory factory =
                                    new BeanFactoryAspectInstanceFactory(this.beanFactory, beanName);
                            // 解析标记Aspect注解中的增强方法
                            List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);
                            if (this.beanFactory.isSingleton(beanName)) {
                                this.advisorsCache.put(beanName, classAdvisors);
                            }
                            else {
                                this.aspectFactoryCache.put(beanName, factory);
                            }
                            advisors.addAll(classAdvisors);
                        }
                        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;
            }
        }
    }

    if (aspectNames.isEmpty()) {
        return Collections.emptyList();
    }
    // 记录在缓存中
    List<Advisor> advisors = new ArrayList<>();
    for (String aspectName : aspectNames) {
        List<Advisor> cachedAdvisors = this.advisorsCache.get(aspectName);
        if (cachedAdvisors != null) {
            advisors.addAll(cachedAdvisors);
        }
        else {
            MetadataAwareAspectInstanceFactory factory = this.aspectFactoryCache.get(aspectName);
            advisors.addAll(this.advisorFactory.getAdvisors(factory));
        }
    }
    return advisors;
}

至此，我们已经完成了Advisor的提取，在上面的步骤中最为重要也最为繁杂的就是增强器的获取。而这一功能委托给了getAdvisors方法去实现（this.advisorFactory.getAdvisors(factory)）

@Override
public List<Advisor> getAdvisors(MetadataAwareAspectInstanceFactory aspectInstanceFactory) {
    // 获取标记为AspectJ的类
    Class<?> aspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
    // 获取标记为AspectJ的name
    String aspectName = aspectInstanceFactory.getAspectMetadata().getAspectName();
    // 验证
    validate(aspectClass);

    // We need to wrap the MetadataAwareAspectInstanceFactory with a decorator
    // so that it will only instantiate once.
    MetadataAwareAspectInstanceFactory lazySingletonAspectInstanceFactory =
            new LazySingletonAspectInstanceFactoryDecorator(aspectInstanceFactory);

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

    // If it's a per target aspect, emit the dummy instantiating aspect.
    if (!advisors.isEmpty() && lazySingletonAspectInstanceFactory.getAspectMetadata().isLazilyInstantiated()) {
        // 如果寻找的增强器不为空而且又配置了增强延迟初始化，那么需要在首位加入同步实例化增强器
        Advisor instantiationAdvisor = new SyntheticInstantiationAdvisor(lazySingletonAspectInstanceFactory);
        advisors.add(0, instantiationAdvisor);
    }

    // Find introduction fields.
    // 获取DeclareParents注解
    for (Field field : aspectClass.getDeclaredFields()) {
        Advisor advisor = getDeclareParentsAdvisor(field);
        if (advisor != null) {
            advisors.add(advisor);
        }
    }

    return advisors;
}

函数中首先完成了对增强器的获取，包括获取注解以及根据注解生成增强的步骤，然后考虑到在配置中可能会将增强配置成延迟初始化，那么需要在首位加入同步实例化增强器以保证增强使用之前的实例化，最后是对DeclareParents注解的获取，下面将详细介绍每个步骤。

1. 普通增强器的获取

普通增强器的获取逻辑通过getAdvisor方法实现，实现步骤包括对切点的注解的获取以及根据注解信息生成增强。

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

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

    // 切点信息的获取
    AspectJExpressionPointcut expressionPointcut = getPointcut(
            candidateAdviceMethod, aspectInstanceFactory.getAspectMetadata().getAspectClass());
    if (expressionPointcut == null) {
        return null;
    }

    // 根据切点信息生成增强器
    return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
            this, aspectInstanceFactory, declarationOrderInAspect, aspectName);
}

（1） 切点信息的获取。所谓获取切点信息就是指定注解的表达式信息的获取，如@Before("watch()")。

@Nullable
private AspectJExpressionPointcut getPointcut(Method candidateAdviceMethod, Class<?> candidateAspectClass) {
    // 获取方法上的注解
    AspectJAnnotation<?> aspectJAnnotation =
            AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
    if (aspectJAnnotation == null) {
        return null;
    }

    // 使用AspectJExpressionPointcut实例封装获取的信息
    AspectJExpressionPointcut ajexp =
            new AspectJExpressionPointcut(candidateAspectClass, new String[0], new Class<?>[0]);
    // 提取得到的注解中的表达式如：
    // @Pointcut("execution(* *.*watch*(..))")中的execution(* *.*watch*(..))
    ajexp.setExpression(aspectJAnnotation.getPointcutExpression());
    if (this.beanFactory != null) {
        ajexp.setBeanFactory(this.beanFactory);
    }
    return ajexp;
}

    /**
 * Find and return the first AspectJ annotation on the given method
 * (there <i>should</i> only be one anyway...).
 */
@SuppressWarnings("unchecked")
@Nullable
protected static AspectJAnnotation<?> findAspectJAnnotationOnMethod(Method method) {
    // 设置敏感的注解类
    Class<?>[] classesToLookFor = new Class<?>[] {
            Before.class, Around.class, After.class, AfterReturning.class, AfterThrowing.class, Pointcut.class};
    for (Class<?> c : classesToLookFor) {
        AspectJAnnotation<?> foundAnnotation = findAnnotation(method, (Class<Annotation>) c);
        if (foundAnnotation != null) {
            return foundAnnotation;
        }
    }
    return null;
}

// 获取指定方法上的注解并使用AspectJAnnotation封装
@Nullable
private static <A extends Annotation> AspectJAnnotation<A> findAnnotation(Method method, Class<A> toLookFor) {
    A result = AnnotationUtils.findAnnotation(method, toLookFor);
    if (result != null) {
        return new AspectJAnnotation<>(result);
    }
    else {
        return null;
    }
}

（2）根据切点信息生成增强。所有的增强都由Advisor的实现类InstantiationModelAwarePointcutAdvisorImpl统一封装的。

public InstantiationModelAwarePointcutAdvisorImpl(AspectJExpressionPointcut declaredPointcut,
        Method aspectJAdviceMethod, AspectJAdvisorFactory aspectJAdvisorFactory,
        MetadataAwareAspectInstanceFactory aspectInstanceFactory, int declarationOrder, String aspectName) {

    this.declaredPointcut = declaredPointcut;
    this.declaringClass = aspectJAdviceMethod.getDeclaringClass();
    this.methodName = aspectJAdviceMethod.getName();
    this.parameterTypes = aspectJAdviceMethod.getParameterTypes();
    this.aspectJAdviceMethod = aspectJAdviceMethod;
    this.aspectJAdvisorFactory = aspectJAdvisorFactory;
    this.aspectInstanceFactory = aspectInstanceFactory;
    this.declarationOrder = declarationOrder;
    this.aspectName = aspectName;

    if (aspectInstanceFactory.getAspectMetadata().isLazilyInstantiated()) {
        // Static part of the pointcut is a lazy type.
        Pointcut preInstantiationPointcut = Pointcuts.union(
                aspectInstanceFactory.getAspectMetadata().getPerClausePointcut(), this.declaredPointcut);

        // Make it dynamic: must mutate from pre-instantiation to post-instantiation state.
        // If it's not a dynamic pointcut, it may be optimized out
        // by the Spring AOP infrastructure after the first evaluation.
        this.pointcut = new PerTargetInstantiationModelPointcut(
                this.declaredPointcut, preInstantiationPointcut, aspectInstanceFactory);
        this.lazy = true;
    }
    else {
        // A singleton aspect.
        this.pointcut = this.declaredPointcut;
        this.lazy = false;
        this.instantiatedAdvice = instantiateAdvice(this.declaredPointcut);
    }
}

在封装过程中只是简单地将信息封装在类的实例中，所有的信息单纯地赋值，在实例初始化的过程中还完成了对于增强器的初始化。因为不同的增强所体现的逻辑是不同的，比如@Before("watch()")与@After("watch()")标签的不同就是增强器增强的位置不同，所以就需要不同的增强器来完成不同的逻辑，而根据注解中的信息初始化对应的增强器就是在instantiateAdvice函数中实现的。

private Advice instantiateAdvice(AspectJExpressionPointcut pointcut) {
    Advice advice = this.aspectJAdvisorFactory.getAdvice(this.aspectJAdviceMethod, pointcut,
            this.aspectInstanceFactory, this.declarationOrder, this.aspectName);
    return (advice != null ? advice : EMPTY_ADVICE);
}

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

    Class<?> candidateAspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
    validate(candidateAspectClass);

    AspectJAnnotation<?> aspectJAnnotation =
            AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
    if (aspectJAnnotation == null) {
        return null;
    }

    // If we get here, we know we have an AspectJ method.
    // Check that it's an AspectJ-annotated class
    if (!isAspect(candidateAspectClass)) {
        throw new AopConfigException("Advice must be declared inside an aspect type: " +
                "Offending method '" + candidateAdviceMethod + "' in class [" +
                candidateAspectClass.getName() + "]");
    }

    if (logger.isDebugEnabled()) {
        logger.debug("Found AspectJ method: " + candidateAdviceMethod);
    }

    AbstractAspectJAdvice springAdvice;
    // 根据不同的注解类型封装不同的增强器
    switch (aspectJAnnotation.getAnnotationType()) {
        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;
}

从函数中可以看到，Spring会根据不同的注解生成不同的增强器，例如AtBefore会对应AspectJMethodBeforeAdvice，而在AspectJMethodBeforeAdvice中完成了增强方法的逻辑。我们尝试分析下几个常用的增强器实现。 （重点）

• MethodBeforeAdviceInterceptor

public class MethodBeforeAdviceInterceptor implements MethodInterceptor, Serializable {

    private MethodBeforeAdvice advice;


    /**
     * Create a new MethodBeforeAdviceInterceptor for the given advice.
     * @param advice the MethodBeforeAdvice to wrap
     */
    public MethodBeforeAdviceInterceptor(MethodBeforeAdvice advice) {
        Assert.notNull(advice, "Advice must not be null");
        this.advice = advice;
    }

    @Override
    public Object invoke(MethodInvocation mi) throws Throwable {
        this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis());
        return mi.proceed();
    }

}

其中的MethodBeforeAdvice代表着前置增强的AspectJMethodBeforeAdvice，跟踪before方法：

@Override
public void before(Method method, Object[] args, @Nullable Object target) throws Throwable {
    invokeAdviceMethod(getJoinPointMatch(), null, null);
}

protected Object invokeAdviceMethod(
        @Nullable JoinPointMatch jpMatch, @Nullable Object returnValue, @Nullable Throwable ex)
        throws Throwable {

    return invokeAdviceMethodWithGivenArgs(argBinding(getJoinPoint(), jpMatch, returnValue, ex));
}

protected Object invokeAdviceMethodWithGivenArgs(Object[] args) throws Throwable {
    Object[] actualArgs = args;
    if (this.aspectJAdviceMethod.getParameterCount() == 0) {
        actualArgs = null;
    }
    try {
        ReflectionUtils.makeAccessible(this.aspectJAdviceMethod);
        // TODO AopUtils.invokeJoinpointUsingReflection
        // 激活增强方法
        return this.aspectJAdviceMethod.invoke(this.aspectInstanceFactory.getAspectInstance(), actualArgs);
    }
    catch (IllegalArgumentException ex) {
        throw new AopInvocationException("Mismatch on arguments to advice method [" +
                this.aspectJAdviceMethod + "]; pointcut expression [" +
                this.pointcut.getPointcutExpression() + "]", ex);
    }
    catch (InvocationTargetException ex) {
        throw ex.getTargetException();
    }
}

invokeAdviceMethodWithGivenArgs方法中的aspectJAdviceMethod正是对与前置增强的方法，在这里实现了调用。

• AspectJAfterAdvice

后置增强与前置增强有稍许不一致的地方。回顾之前讲过的前置增强，大致的结构是在拦截器链中放置MethodBeforeAdviceInterceptor，而在MethodBeforeAdviceInterceptor中又放置了AspectJMethodBeforeAdvice，并在调用invoke时首先串联调用。但是在后置增强的时候却不一样，没有提供中间的类，而是直接在拦截器中使用了中间的AspectJAfterAdvice。

public class AspectJAfterAdvice extends AbstractAspectJAdvice
        implements MethodInterceptor, AfterAdvice, Serializable {

    public AspectJAfterAdvice(
            Method aspectJBeforeAdviceMethod, AspectJExpressionPointcut pointcut, AspectInstanceFactory aif) {

        super(aspectJBeforeAdviceMethod, pointcut, aif);
    }


    @Override
    public Object invoke(MethodInvocation mi) throws Throwable {
        try {
            return mi.proceed();
        }
        finally {
            // 激活增强方法
            invokeAdviceMethod(getJoinPointMatch(), null, null);
        }
    }

    @Override
    public boolean isBeforeAdvice() {
        return false;
    }

    @Override
    public boolean isAfterAdvice() {
        return true;
    }

}

2. 增加同步实例化增强器

如果寻找的增强器不为空而且又配置了增强延迟初始化，那么就需要在首位加入同步实例化增强器。同步实例化增强器SyntheticInstantiationAdvisor如下：

protected static class SyntheticInstantiationAdvisor extends DefaultPointcutAdvisor {

    public SyntheticInstantiationAdvisor(final MetadataAwareAspectInstanceFactory aif) {
        super(aif.getAspectMetadata().getPerClausePointcut(), (MethodBeforeAdvice)
                // 简单初始化aspect
                (method, args, target) -> aif.getAspectInstance());
    }
}

3. 获取DeclareParents注解

DeclareParents主要用于引介增强的注解形式的实现，而其实现方式与普通增强很类似，只不过使用DeclareParentsAdvisor对功能进行封装。

private Advisor getDeclareParentsAdvisor(Field introductionField) {
    DeclareParents declareParents = introductionField.getAnnotation(DeclareParents.class);
    if (declareParents == null) {
        // Not an introduction field
        return null;
    }

    if (DeclareParents.class == declareParents.defaultImpl()) {
        throw new IllegalStateException("'defaultImpl' attribute must be set on DeclareParents");
    }

    return new DeclareParentsAdvisor(
            introductionField.getType(), declareParents.value(), declareParents.defaultImpl());
}

二、寻找匹配的增强器

前面的函数中已经完成了所有增强器的解析，但是对于所有增强器来讲，并不一定都适用于当前的bean，还要挑取出适合的增强器，也就是满足我们配置的通配符的增强器。具体实现在findAdvisorsThatCanApply中。

protected List<Advisor> findAdvisorsThatCanApply(
        List<Advisor> candidateAdvisors, Class<?> beanClass, String beanName) {

    ProxyCreationContext.setCurrentProxiedBeanName(beanName);
    try {
        // 过滤已经得到的advisors
        return AopUtils.findAdvisorsThatCanApply(candidateAdvisors, beanClass);
    }
    finally {
        ProxyCreationContext.setCurrentProxiedBeanName(null);
    }
}

继续看findAdvisorsThatCanApply：

public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
    if (candidateAdvisors.isEmpty()) {
        return candidateAdvisors;
    }
    List<Advisor> eligibleAdvisors = new ArrayList<>();
    // 首先处理引介增强
    for (Advisor candidate : candidateAdvisors) {
        if (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {
            eligibleAdvisors.add(candidate);
        }
    }
    boolean hasIntroductions = !eligibleAdvisors.isEmpty();
    for (Advisor candidate : candidateAdvisors) {
        // 引介增强已经处理
        if (candidate instanceof IntroductionAdvisor) {
            // already processed
            continue;
        }
        // 对于普通bean的处理
        if (canApply(candidate, clazz, hasIntroductions)) {
            eligibleAdvisors.add(candidate);
        }
    }
    return eligibleAdvisors;
}

findAdvisorsThatCanApply函数的主要功能是寻找增强器中适用于当前class的增强器。引介增强与普通的增强的处理是不一样的，所以分开处理。而对于真正的匹配在canApply中实现

public static boolean canApply(Advisor advisor, Class<?> targetClass, boolean hasIntroductions) {
    if (advisor instanceof IntroductionAdvisor) {
        return ((IntroductionAdvisor) advisor).getClassFilter().matches(targetClass);
    }
    else if (advisor instanceof PointcutAdvisor) {
        PointcutAdvisor pca = (PointcutAdvisor) advisor;
        return canApply(pca.getPointcut(), targetClass, hasIntroductions);
    }
    else {
        // It doesn't have a pointcut so we assume it applies.
        return true;
    }
}

public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) {
    Assert.notNull(pc, "Pointcut must not be null");
    if (!pc.getClassFilter().matches(targetClass)) {
        return false;
    }

    MethodMatcher methodMatcher = pc.getMethodMatcher();
    if (methodMatcher == MethodMatcher.TRUE) {
        // No need to iterate the methods if we're matching any method anyway...
        return true;
    }

    IntroductionAwareMethodMatcher introductionAwareMethodMatcher = null;
    if (methodMatcher instanceof IntroductionAwareMethodMatcher) {
        introductionAwareMethodMatcher = (IntroductionAwareMethodMatcher) methodMatcher;
    }

    Set<Class<?>> classes = new LinkedHashSet<>();
    if (!Proxy.isProxyClass(targetClass)) {
        classes.add(ClassUtils.getUserClass(targetClass));
    }
    classes.addAll(ClassUtils.getAllInterfacesForClassAsSet(targetClass));

    for (Class<?> clazz : classes) {
        Method[] methods = ReflectionUtils.getAllDeclaredMethods(clazz);
        for (Method method : methods) {
            if (introductionAwareMethodMatcher != null ?
                    introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions) :
                    methodMatcher.matches(method, targetClass)) {
                return true;
            }
        }
    }

    return false;
}

三、创建代理

在获取了所有对应bean的增强器后，便可以进行代理的创建了。

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

    if (this.beanFactory instanceof ConfigurableListableBeanFactory) {
        AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);
    }

    ProxyFactory proxyFactory = new ProxyFactory();
    // 获取当前类中相关属性
    proxyFactory.copyFrom(this);

    if (!proxyFactory.isProxyTargetClass()) {
        // 决定对于给定的bean是否应该使用targetClass而不是他的接口代理，
        // 检査 proxyTargetClass 设置以及 preserveTargetClass 属性
        if (shouldProxyTargetClass(beanClass, beanName)) {
            proxyFactory.setProxyTargetClass(true);
        }
        else {
            evaluateProxyInterfaces(beanClass, proxyFactory);
        }
    }

    Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
    // 加入增强器
    proxyFactory.addAdvisors(advisors);
    // 设置要代理的类
    proxyFactory.setTargetSource(targetSource);
    // 定制代理
    customizeProxyFactory(proxyFactory);
    // 用来控制代理工厂被配置之后，是否还允许修改通知。
    // 缺省值是false (即在代理被配置之后，不允许修改代理的配置)。
    proxyFactory.setFrozen(this.freezeProxy);
    if (advisorsPreFiltered()) {
        proxyFactory.setPreFiltered(true);
    }

    return proxyFactory.getProxy(getProxyClassLoader());
}

对于代理类的创建及处理，Spring委托给了 ProxyFactory去处理，而在此函数中主要是对 ProxyFactory的初始化操作，进而对真正的创建代理做准备，这些初始化操作包括如下内容。

（1）获取当前类中的属性。

（2）添加代理接口。

（3）封装 Advisor 并加人到 ProxyFactory 中。

（4）设置要代理的类。

（5）当然在Spring中还为子类提供了定制的函数customizeProxyFactory，子类可以在此函数中进行对ProxyFactory的进一步封装。

（6）进行获取代理操作。

其中，封装Advisor并加人到ProxyFactory中以及创建代理是两个相对繁琐的过程，可以通过ProxyFactory提供的addAdvisor方法直接将增强器置人代理创建工厂中，但是将拦截器封装为增强器还是需要一定的逻辑的。

protected Advisor[] buildAdvisors(@Nullable String beanName, @Nullable Object[] specificInterceptors) {
    // Handle prototypes correctly...
    // 解析注册的所有interceptorName
    Advisor[] commonInterceptors = resolveInterceptorNames();

    List<Object> allInterceptors = new ArrayList<>();
    if (specificInterceptors != null) {
        // 加入拦截器
        allInterceptors.addAll(Arrays.asList(specificInterceptors));
        if (commonInterceptors.length > 0) {
            if (this.applyCommonInterceptorsFirst) {
                allInterceptors.addAll(0, Arrays.asList(commonInterceptors));
            }
            else {
                allInterceptors.addAll(Arrays.asList(commonInterceptors));
            }
        }
    }
    if (logger.isDebugEnabled()) {
        int nrOfCommonInterceptors = commonInterceptors.length;
        int nrOfSpecificInterceptors = (specificInterceptors != null ? specificInterceptors.length : 0);
        logger.debug("Creating implicit proxy for bean '" + beanName + "' with " + nrOfCommonInterceptors +
                " common interceptors and " + nrOfSpecificInterceptors + " specific interceptors");
    }

    Advisor[] advisors = new Advisor[allInterceptors.size()];
    for (int i = 0; i < allInterceptors.size(); i++) {
        // 拦截器进行封装转化为Advisor
        advisors[i] = this.advisorAdapterRegistry.wrap(allInterceptors.get(i));
    }
    return advisors;
}

@Override
public Advisor wrap(Object adviceObject) throws UnknownAdviceTypeException {
    // 如果要封装的对象本身就是Advisor类型的那么无需在做过多处理
    if (adviceObject instanceof Advisor) {
        return (Advisor) adviceObject;
    }
    // 因为此封装方法只对Advisor与Advice两种类型的数据有效，如果不是将不能封装
    if (!(adviceObject instanceof Advice)) {
        throw new UnknownAdviceTypeException(adviceObject);
    }
    Advice advice = (Advice) adviceObject;
    if (advice instanceof MethodInterceptor) {
        // So well-known it doesn't even need an adapter.
        // 如果是MethodInterceptor类型则使用DefaultPointcutAdvisor封装
        return new DefaultPointcutAdvisor(advice);
    }
    // 如果存在Advisor的适配器那么也同样需要进行封装
    for (AdvisorAdapter adapter : this.adapters) {
        // Check that it is supported.
        if (adapter.supportsAdvice(advice)) {
            return new DefaultPointcutAdvisor(advice);
        }
    }
    throw new UnknownAdviceTypeException(advice);
}

由于Spring中涉及过多的拦截器、增强器、增强方法等方式来对逻辑进行增强，所以非常有必要统一封装成Advisor来进行代理的创建，完成了增强的封装过程，那么解析最重要的一步就是代理的创建与获取了。

public Object getProxy(@Nullable ClassLoader classLoader) {
    return createAopProxy().getProxy(classLoader);
}

1. 创建代理

protected final synchronized AopProxy createAopProxy() {
    if (!this.active) {
        activate();
    }
    // 创建代理
    return getAopProxyFactory().createAopProxy(this);
}

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() || Proxy.isProxyClass(targetClass)) {
            return new JdkDynamicAopProxy(config);
        }
        return new ObjenesisCglibAopProxy(config);
    }
    else {
        return new JdkDynamicAopProxy(config);
    }
}

到此已经完成了代理的创建，不管我们之前是否有阅读过Spring的源代码，但是都或多或少听过对于Spring的代理中JDKProxy的实现和CglibProxy的实现。Spring是如何选取的呢？网上的介绍到处都是，现在我们就从源代码的角度分析，看看到底Spring是如何选择代理方式的。

从if的判断条件中可以看到3个方面影响这Spring的判断。

• optimize：用来控制通过CGLIB创建的代理是否使用激进的优化策略，除非完全了解AOP代理如何处理优化，否则不推荐用户使用这个设置。目前这个属性仅用于CGLIB 代理，对于JDK动态代理（缺省代理）无效。
• proxyTargetClass：这个属性为true时，目标类本身被代理而不是目标类的接口。如果这个属性值被设为true，CGLIB代理将被创建，设置方式：<aop:aspectj-autoproxy proxy-target-class="true"/>。

• hasNoUserSuppliedProxylnterfaces：是否存在代理接口

下面是对JDK与Cglib方式的总结。

• 如果目标对象实现了接口，默认情况下会采用JDK的动态代理实现AOP。
• 如果目标对象实现了接口，可以强制使用CGLIB实现AOP。

• 如果目标对象没有实现了接口，必须采用CGLIB库，Spring会自动在JDK动态代理 和CGLIB之间转换。

如何强制使用CGLIB实现AOP?

（1）添加 CGLIB 库，Spring_HOME/cglib/*.jar。

（2）在 Spring 配置文件中加人<aop:aspectj-autoproxy proxy-target-class="true"/>。

JDK动态代理和CGLIB字节码生成的区别？

• JDK动态代理只能对实现了接口的类生成代理，而不能针对类。
• CGLIB是针对类实现代理，主要是对指定的类生成一个子类，覆盖其中的方法，因为是继承，所以该类或方法最好不要声明成final。

Spring中的JDK动态代理

我们再次来回顾一下使用JDK代理的方式 （ AOP“原理”笔记 ） ，在整个创建过程中，对于InvocationHandler的创建是最为核心的，在自定义的InvocationHandler中需要重写3个函数。

• 构造函数，将代理的对象传入。
• invoke方法，此方法中实现了 AOP增强的所有逻辑。

• getProxy方法，此方法千篇一律，但是必不可少。

那么，我们看看Spring中的JDK代理实现是不是也是这么做的呢？继续之前的跟踪，到 达 JdkDynamicAopProxy 的 getProxy。

@Override
public Object getProxy(@Nullable 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);
}

通过之前的示例我们知道，JDKProxy的使用关键是创建自定义的InvocationHandler，而 InvocationHandler中包含了需要覆盖的函数getProxy，而当前的方法正是完成了这个操作。再次确认一下JdkDynamicAopProxy也确实实现了InvocationHandler接口，那么我们就可以推断出，在 JdkDynamicAopProxy 中一定会有个 invoke 函数，并且 JdkDynamicAopProxy 会把 AOP 的核心逻辑写在其中。查看代码，果然有这样个函数：

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

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

    try {
        // equals方法的处理
        if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
            // The target does not implement the equals(Object) method itself.
            return equals(args[0]);
        }
        // hash方法的处理
        else if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
            // The target does not implement the hashCode() method itself.
            return hashCode();
        }
        else if (method.getDeclaringClass() == DecoratingProxy.class) {
            // There is only getDecoratedClass() declared -> dispatch to proxy config.
            return AopProxyUtils.ultimateTargetClass(this.advised);
        }
        /**
         * Class类的isAssignableFrom(Class cls)方法：
         * 如果调用这个方法的class或接口与参数cls表示的类或接口相同，
         * 或者是参数cls表示的类或接口的父类，则返回true。
         * 形象的：自身类.class.isAssignableFrom(自身或子类.class) 返回true
         */
        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;
        // 有时候目标对象内部的自我调用将无法实施切面中的增强则需要荣国此属性暴露代理
        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.
        target = targetSource.getTarget();
        Class<?> targetClass = (target != null ? target.getClass() : null);

        // Get the interception chain for this method.
        // 获取当前方法的拦截器链
        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.
        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.
            // 如果没有发现任何拦截器那么直接调用切点方法
            Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
            retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
        }
        else {
            // We need to create a method invocation...
            // 将拦截器封装在ReflectiveMethodInvocation，
            // 以便于使用其proceed进行链接表用拦截器
            invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
            // Proceed to the joinpoint through the interceptor chain.
            // 执行拦截器链
            retVal = invocation.proceed();
        }

        // Massage return value if necessary.
        Class<?> returnType = method.getReturnType();
        // 返回结果
        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);
        }
    }
}

上面的函数中最主要的工作就是创建了一个拦截器链，并使用ReflectiveMethodInvocation类进行了链的封装，而在ReflectiveMethodInvocation类的proceed方法中实现了拦截器的逐一调用，那么我们继续来探究，在proceed方法中是怎么实现前置增强在目标方法前调用后置增强在目标方法后调用的逻辑呢？

@Override
@Nullable
public Object proceed() throws Throwable {
    //    We start with an index of -1 and increment early.
    // 执行完所有增强后执行切点方法
    if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
        return invokeJoinpoint();
    }

    // 获取下一个要执行的拦截器
    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.
        // 动态匹配
        InterceptorAndDynamicMethodMatcher dm =
                (InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
        Class<?> targetClass = (this.targetClass != null ? this.targetClass : this.method.getDeclaringClass());
        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.
            // 不匹配则不执行拦截器
            return proceed();
        }
    }
    else {
        // It's an interceptor, so we just invoke it: The pointcut will have
        // been evaluated statically before this object was constructed.
        /**
         * 普通拦截器，直接调用拦截器
         */
        // 将this作为参数传递以保证当前实例中调用链的执行
        return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
    }
}

在proceed方法中，或许代码逻辑并没有我们想象得那么复杂，ReflectiveMethodlnvocation中的主要职责是维护了链接调用的计数器，记录着当前调用链接的位置，以便链可以有序地进行下去，那么在这个方法中并没有我们之前设想的维护各种增强的顺序，而是将此工作委托给了各个增强器，使各个增强器在内部进行逻辑实现。