Spring进阶 - Spring AOP实现原理（一）AOP切面实现原理 - 编程导航

紧跟前文，前面我们学习了 AOP 基础，并进阶学习 IOC 初始化过程和 Bean 的生命周期等，而Spring AOP也是基于IOC的Bean加载来实现的。

本文介绍Spring AOP 原理解析，切面的实现过程。将切面类的所有切面方法根据使用的注解生成对应Advice，并将Advice连同切入点匹配器和切面类等信息一并封装到Advisor，为后续交给代理增强实现做准备的过程。

本文主要以阅读源码为主，与 IOC 容器初始化文章的源码阅读一样，本文也描述了源码阅读的流程，类似于地图，阅读源码迷路时看一下文字流程。

引言

我们应该从哪里入手看Spring AOP源码？和我们上文分析的IOC源码实现有什么关系呢？

前文聊到Spring AOP配置方式有 XML 和注解两种方式，XML核心配置如下：

可以看出AOP配置在Bean下，表明AOP是基于IOC容器加载来完成，这便是我们探索AOP的主要入口。

启动应用，初始化。

然后我们就能找到如下初始化的流程和aop对应的handler类，即parseCustomElement方法找到解析 aop:aspectj-autoproxy的handler(org.springframework.aop.config.AopNamespaceHandler)

其实你会发现，最重要的是知识点的关联关系，而不是知识点本身，单个知识点学一会谁学不会，前面的知识学好了，后续知识点不就是跑跑程序打个断点慢慢看的事了么？

入口找到，开始本文学习之旅。

上面的示例代码完整提供

初始化 Spring 项目，引入依赖 pom.xml ，最好是 java8 版本

▼
xml
复制代码
<dependencies>
  <dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-context</artifactId>
    <version>${spring.version}</version>
  </dependency>
  <dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-core</artifactId>
    <version>${spring.version}</version>
  </dependency>
  <dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-beans</artifactId>
    <version>${spring.version}</version>
  </dependency>
  <dependency>
    <groupId>org.aspectj</groupId>
    <artifactId>aspectjweaver</artifactId>
    <version>${aspectjweaver.version}</version>
  </dependency>
</dependencies>

User 实体类

▼
java
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public class User {

    /**
     * user's name.
     */
    private String name;

    /**
     * user's age.
     */
    private int age;

    /**
     * init.
     *
     * @param name name
     * @param age  age
     */
    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

dao 层代码

▼
java
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@Repository
public class UserDaoImpl {

    /**
     * init.
     */
    public UserDaoImpl() {
    }

    /**
     * mocked to find user list.
     *
     * @return user list
     */
    public List<User> findUserList() {
        return Collections.singletonList(new User("fency", 18));
    }
}

Service 类

▼
java
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@Service
public class UserServiceImpl {

    /**
     * user dao impl.
     */
    @Autowired
    private UserDaoImpl userDao;

    /**
     * init.
     */
    public UserServiceImpl() {
    }

    /**
     * find user list.
     *
     * @return user list
     */
    public List<User> findUserList() {
        return this.userDao.findUserList();
    }

    /**
     * set dao.
     *
     * @param userDao user dao
     */
    public void setUserDao(UserDaoImpl userDao) {
        this.userDao = userDao;
    }
}

切面类

▼
java
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package com.fency.springframework.aspect;

import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.reflect.MethodSignature;

import java.lang.reflect.Method;

public class LogAspect {

    /**
     * aspect for every methods under service package.
     */
    public Object businessService(ProceedingJoinPoint pjp) throws Throwable {
        // get attribute through annotation
        Method method = ((MethodSignature) pjp.getSignature()).getMethod();
        System.out.println("execute method: " + method.getName());

        // continue to process
        return pjp.proceed();
    }

}

XML 配置

aspect.xml

▼
xml
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<?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">

    <!-- 配置LogAspect切面 -->
    <bean id="logAspect" class="com.fency.springframework.aspect.LogAspect"/>
    
    <!-- 配置 AOP -->
    <aop:config>
        <!-- 定义切面 -->
        <aop:aspect id="logAspect" ref="logAspect">
            <!-- 定义切入点 -->
            <aop:pointcut id="userServiceMethods"
                          expression="execution(* com.fency.springframework.service.*.*(..))"/>

            <!-- 配置环绕通知 -->
            <aop:around method="businessService" pointcut-ref="userServiceMethods"/>
        </aop:aspect>
    </aop:config>

</beans>

daos.xml

▼
java
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<?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:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
                           http://www.springframework.org/schema/beans/spring-beans.xsd
                           http://www.springframework.org/schema/context
                           http://www.springframework.org/schema/context/spring-context.xsd">

    <!-- 启用注解扫描 -->
    <context:component-scan base-package="com.fency.springframework.dao"/>
    
    <!-- 配置UserDaoImpl bean -->
    <bean id="userDao" class="com.fency.springframework.dao.UserDaoImpl"/>

</beans>

services.xml

▼
java
复制代码
<?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:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
                           http://www.springframework.org/schema/beans/spring-beans.xsd
                           http://www.springframework.org/schema/context
                           http://www.springframework.org/schema/context/spring-context.xsd">

    <!-- 启用注解扫描 -->
    <context:component-scan base-package="com.fency.springframework.service"/>
    
    <!-- 配置UserServiceImpl bean，注入userDao依赖 -->
    <bean id="userService" class="com.fency.springframework.service.UserServiceImpl">
        <property name="userDao" ref="userDao"/>
    </bean>

</beans>

APP 启动类

▼
java
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public class App {

    /**
     * main interfaces.
     *
     * @param args args
     */
    public static void main(String[] args) {
        // create and configure beans
        ApplicationContext context =
                new FileSystemXmlApplicationContext("aspects.xml", "daos.xml", "services.xml");

        // retrieve configured instance

        UserServiceImpl service = context.getBean("userService", UserServiceImpl.class);

        // use configured instance
        List<User> userList = service.findUserList();

        // print info from beans
        userList.forEach(a -> System.out.println(a.getName() + "," + a.getAge()));
    }
}

文字描述阅读源码流程（XML为例）

为了避免阅读源码时迷路，提前给大家提供地图，这是接下来我们要做的事情，现在看不懂是正常的，迷路了别忘记回来看这里。

阶段一：标签解析

Spring 启动，IOC 容器从加载标签 XML 标签开始，XML 标签声明了多个自定义实例和 AOP 切面实例，IOC 容器加载所有的 XML 。
在 AopNamespaceHandler 中有一个 init()方法，我们发现每一个 aop 标签都有一个解析器，所以当你在 xml 使用 aop 这些标签，会被解析成 AOP 代理。
config 配置标签的解析，初始化 ConfigBeanDefinitionParser 类，执行内部 parse 方法。
aspect-autoproxy 标签解析，初始一个解析类，内部有 parse 方法，但是一直调用下去会发现 AOP的创建工作是交给 AnnotationAwareAspectJAutoProxyCreator 类完成

AnnotationAwareAspectJAutoProxyCreator 有两个重要的方法： **postProcessBeforeInstantiation** **（本文详细讲解）**和 postProcessAfterInstantiation（下一篇文章讲解）。到这里就挖到宝了。

阶段二：**postProcessBeforeInstantiation** 方法

方法内有两个重要的判断，判断是否给这个实例加 aop 的代理能力。
1. 是否是 AOP 基础类？是基础类就不加 aop 能力。
2. 是否跳过？跳过就不加 aop能力
如果实例是基础类，且要被跳过，那么它是 aop 切面类，把切面转成 Advisor 对象，内部会调用一个 getAdvisor() 的方法来转换。
在 getAdvisor() 方法又会调用 getPointcut() 方法获取表达式切点。最后转成出来一个 advisor 对象。
advisor 对象是 advice 的包装类，所以我们要获取 advice 信息，封装到 advisor 内部。

大费周章得到的 advisor 是什么？

advisor 约等于通知（Advice）和切入点（Pointcut），一个 advisor 是 AOP 设计思想的两个术语的具体实现。一个切面类有多个通知，就有多个切点表达式，那么就有多个 advisor.

这就是**postProcessBeforeInstantiation** 方法做的事情，第一步的判断确保不会给 aop 切面类实例加 aop 代理能力，避免无限递归。后面的步骤是把 aop 切面实例转成一个 advisor 对象。

总结流程： 解析标签 -> 解析时调用 postProcessBeforeInstantiation 方法 -> 得到一个 advisor 对象。

有了Adisor, 注入到合适的位置并交给代理（cglib和jdk)实现了。这里是在 postProcessAfterInstantiation 执行，也就是下一篇文章讲解的部分。

接下来按照这个流程，看源码。

aop配置标签的解析

我们找到了AopNamespaceHandler，其实就是注册BeanDefinition的解析器BeanDefinitionParser，将aop:xxxxxx配置标签交给指定的parser来处理。

看一下 AopNamespaceHandler 的源码，将XML的aop:xxx配置标签交给Parser来处理。

▼
java
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public class AopNamespaceHandler extends NamespaceHandlerSupport {

	/**
	 * Register the {@link BeanDefinitionParser BeanDefinitionParsers} for the
	 * '{@code config}', '{@code spring-configured}', '{@code aspectj-autoproxy}'
	 * and '{@code scoped-proxy}' tags.
	 */
	@Override
	public void init() {
		// In 2.0 XSD as well as in 2.5+ XSDs
        // 注册解析<aop:config> 配置
		registerBeanDefinitionParser("config", new ConfigBeanDefinitionParser());
        // 注册解析<aop:aspectj-autoproxy> 配置
		registerBeanDefinitionParser("aspectj-autoproxy", new AspectJAutoProxyBeanDefinitionParser());
		registerBeanDefinitionDecorator("scoped-proxy", new ScopedProxyBeanDefinitionDecorator());

		// Only in 2.0 XSD: moved to context namespace in 2.5+
		registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser());
	}

}

我们来对 config配置标签和aspectj-autoproxy标签解析。

config配置标签的解析

<aop:config/>由ConfigBeanDefinitionParser这个类处理（java8才有这个类），作为parser类最重要的就是parse方法，源码：

▼
java
复制代码
@Override
@Nullable
public BeanDefinition parse(Element element, ParserContext parserContext) {
    CompositeComponentDefinition compositeDef =
            new CompositeComponentDefinition(element.getTagName(), parserContext.extractSource(element));
    parserContext.pushContainingComponent(compositeDef);

    configureAutoProxyCreator(parserContext, element);

    List<Element> childElts = DomUtils.getChildElements(element);
    for (Element elt: childElts) {
        String localName = parserContext.getDelegate().getLocalName(elt);
        if (POINTCUT.equals(localName)) {
            parsePointcut(elt, parserContext);
        }
        else if (ADVISOR.equals(localName)) {
            parseAdvisor(elt, parserContext);
        }
        else if (ASPECT.equals(localName)) {
            parseAspect(elt, parserContext);
        }
    }

    parserContext.popAndRegisterContainingComponent();
    return null;
}

打个断点看看

parseAspect的方法如下, 处理方法不难，我这里就不展开了

▼
java
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private void parseAspect(Element aspectElement, ParserContext parserContext) {
    String aspectId = aspectElement.getAttribute(ID);
    String aspectName = aspectElement.getAttribute(REF);

    try {
        this.parseState.push(new AspectEntry(aspectId, aspectName));
        List<BeanDefinition> beanDefinitions = new ArrayList<>();
        List<BeanReference> beanReferences = new ArrayList<>();

        List<Element> declareParents = DomUtils.getChildElementsByTagName(aspectElement, DECLARE_PARENTS);
        for (int i = METHOD_INDEX; i < declareParents.size(); i++) {
            Element declareParentsElement = declareParents.get(i);
            beanDefinitions.add(parseDeclareParents(declareParentsElement, parserContext));
        }

        // We have to parse "advice" and all the advice kinds in one loop, to get the
        // ordering semantics right.
        NodeList nodeList = aspectElement.getChildNodes();
        boolean adviceFoundAlready = false;
        for (int i = 0; i < nodeList.getLength(); i++) {
            Node node = nodeList.item(i);
            if (isAdviceNode(node, parserContext)) {
                if (!adviceFoundAlready) {
                    adviceFoundAlready = true;
                    if (!StringUtils.hasText(aspectName)) {
                        parserContext.getReaderContext().error(
                                "<aspect> tag needs aspect bean reference via 'ref' attribute when declaring advices.",
                                aspectElement, this.parseState.snapshot());
                        return;
                    }
                    beanReferences.add(new RuntimeBeanReference(aspectName));
                }
                AbstractBeanDefinition advisorDefinition = parseAdvice(
                        aspectName, i, aspectElement, (Element) node, parserContext, beanDefinitions, beanReferences);
                beanDefinitions.add(advisorDefinition);
            }
        }

        AspectComponentDefinition aspectComponentDefinition = createAspectComponentDefinition(
                aspectElement, aspectId, beanDefinitions, beanReferences, parserContext);
        parserContext.pushContainingComponent(aspectComponentDefinition);

        List<Element> pointcuts = DomUtils.getChildElementsByTagName(aspectElement, POINTCUT);
        for (Element pointcutElement : pointcuts) {
            parsePointcut(pointcutElement, parserContext);
        }

        parserContext.popAndRegisterContainingComponent();
    }
    finally {
        this.parseState.pop();
    }
}

aspectj-autoproxy配置标签的解析

这个部分仅仅是看调用关系，比较简单。

<aop:aspectj-autoproxy/>则由AspectJAutoProxyBeanDefinitionParser这个类处理的，我们看下parse 方法

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java
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@Override
@Nullable
public BeanDefinition parse(Element element, ParserContext parserContext) {
    // 注册AspectJAnnotationAutoProxyCreator
    AopNamespaceUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(parserContext, element);
    // 拓展BeanDefinition
    extendBeanDefinition(element, parserContext);
    return null;
}

AopNamespaceUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary方法对应如下

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java
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public static void registerAspectJAnnotationAutoProxyCreatorIfNecessary(
        ParserContext parserContext, Element sourceElement) {

    BeanDefinition beanDefinition = AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(
            parserContext.getRegistry(), parserContext.extractSource(sourceElement));
    useClassProxyingIfNecessary(parserContext.getRegistry(), sourceElement);
    registerComponentIfNecessary(beanDefinition, parserContext);
}

AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary对应如下

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

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

到这里，我们发现AOP的创建工作是交给AnnotationAwareAspectJAutoProxyCreator来完成的。

注解切面代理创建类(AnnotationAwareAspectJAutoProxyCreator)

AnnotationAwareAspectJAutoProxyCreator是如何工作的呢？这时候我们就要看AnnotationAwareAspectJAutoProxyCreator类结构关系了。

如下是类结构关系

它实现了两类接口：

BeanFactoryAware属于Bean级生命周期接口方法
InstantiationAwareBeanPostProcessor 和 BeanPostProcessor 这两个接口实现，一般称它们的实现类为“后处理器”，是容器级生命周期接口方法；

Bean级生命周期与容器级生命周期前文讲解过，没啥特殊的：

Bean 级：Bean 实例化 -> 销毁的全过程
容器级：容器准备 -> Bean级生命周期 -> 容器关闭全过程。简化步骤就是如此。

结合前文Spring Bean生命周期的流程，我们就可以定位到核心的初始化方法肯定在postProcessBeforeInstantiation和postProcessAfterInitialization中。

postProcessBeforeInstantiation

如下是上述类结构中postProcessBeforeInstantiation的方法，读者在自己看代码的时候建议打个断点看，可以方便理解

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java
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@Override
public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) {
    Object cacheKey = getCacheKey(beanClass, beanName);

    if (!StringUtils.hasLength(beanName) || !this.targetSourcedBeans.contains(beanName)) {
        // 如果已经在缓存中，则忽略
        if (this.advisedBeans.containsKey(cacheKey)) {
            return null;
        }
        // 是否是aop基础类？是否跳过？
        if (isInfrastructureClass(beanClass) || shouldSkip(beanClass, beanName)) {
            this.advisedBeans.put(cacheKey, Boolean.FALSE);
            return null;
        }
    }

    // Create proxy here if we have a custom TargetSource.
    // Suppresses unnecessary default instantiation of the target bean:
    // The TargetSource will handle target instances in a custom fashion.
    TargetSource targetSource = getCustomTargetSource(beanClass, beanName);
    if (targetSource != null) {
        if (StringUtils.hasLength(beanName)) {
            this.targetSourcedBeans.add(beanName);
        }
        Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(beanClass, beanName, targetSource);
        Object proxy = createProxy(beanClass, beanName, specificInterceptors, targetSource);
        this.proxyTypes.put(cacheKey, proxy.getClass());
        return proxy;
    }

    return null;
}

判断是否是aop基础类

postProcessBeforeInstantiation 方法内有一个步骤是 **isInfrastructureClass(beanClass)，判断是否是aop基础类，如果是基础类那么不能被AOP代理，**源码：

简单理解，除了【业务目标对象】，其它【AOP框架自身组件】和【切面】都属于基础类，不应该被AOP代理。

▼
java
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@Override
protected boolean isInfrastructureClass(Class<?> beanClass) {
    // Previously we setProxyTargetClass(true) in the constructor, but that has too
    // broad an impact. Instead we now override isInfrastructureClass to avoid proxying
    // aspects. I'm not entirely happy with that as there is no good reason not
    // to advise aspects, except that it causes advice invocation to go through a
    // proxy, and if the aspect implements e.g the Ordered interface it will be
    // proxied by that interface and fail at runtime as the advice method is not
    // defined on the interface. We could potentially relax the restriction about
    // not advising aspects in the future.
    // 父类判断它是aop基础类 or 使用@Aspect注解
    return (super.isInfrastructureClass(beanClass) ||
            (this.aspectJAdvisorFactory != null && this.aspectJAdvisorFactory.isAspect(beanClass)));
}

父类判断它是否是aop基础类的方法 super.isInfrastructureClass(beanClass), 本质上就是判断该类是否实现了Advice, Pointcut, Advisor或者AopInfrastructureBean接口。

▼
java
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protected boolean isInfrastructureClass(Class<?> beanClass) {
    // 该类是否实现了Advice, Pointcut, Advisor或者AopInfrastructureBean接口
    boolean retVal = Advice.class.isAssignableFrom(beanClass) ||
            Pointcut.class.isAssignableFrom(beanClass) ||
            Advisor.class.isAssignableFrom(beanClass) ||
            AopInfrastructureBean.class.isAssignableFrom(beanClass);
    if (retVal && logger.isTraceEnabled()) {
        logger.trace("Did not attempt to auto-proxy infrastructure class [" + beanClass.getName() + "]");
    }
    return retVal;
}

是否应该跳过shouldSkip

postProcessBeforeInstantiation 方法内有一个步骤是 **shouldSkip(beanClass, beanName)，**通过断点辅助，candidateAdvisors与xml配置的通知是对应的。 java 8 才能跑出来这个效果，java11 源码改了。

这一步的意义是：防止切面自己被自己通知，从而避免无限递归调用和潜在的循环依赖问题, 并且把切面转成 advisor 也是从这里开始。

当循环到 UserServiceImpl 时，这是一个需要 AOP 切面的Bean, 那么我们debug 可以看出要给 Bean 添加的候选通知。

▼
java
复制代码
@Override
protected boolean shouldSkip(Class<?> beanClass, String beanName) {
    // TODO: Consider optimization by caching the list of the aspect names
    List<Advisor> candidateAdvisors = findCandidateAdvisors();
    for (Advisor advisor : candidateAdvisors) {
        if (advisor instanceof AspectJPointcutAdvisor &&
                ((AspectJPointcutAdvisor) advisor).getAspectName().equals(beanName)) {
            return true;
        }
    }
    return super.shouldSkip(beanClass, beanName);
}

切面方法转成Advisor

这里不做剖析，如果感兴趣想深入研究切面方法怎么转成 Advisor, 这里提供java8源码。

findCandidateAdvisors方法如下：

▼
java
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@Override
protected List<Advisor> findCandidateAdvisors() {
    // 在父类中找到所有的advisor：基于xml配置的<aop:before/>生成的
    List<Advisor> advisors = super.findCandidateAdvisors();
    // 为bean Factory中AspectJ切面构建advistor：通过AspectJ注解的方式生成Advisor类
    if (this.aspectJAdvisorsBuilder != null) {
        advisors.addAll(this.aspectJAdvisorsBuilder.buildAspectJAdvisors());
    }
    return advisors;
}

在当前的bean Factory中通过AspectJ注解的方式生成Advisor类，buildAspectJAdvisors方法如下

这方法很长，本质上做的事情：用DCL双重锁的单例实现方式，拿到切面类里的切面方法，将其转换成advisor（并放入缓存中）。

▼
java
复制代码
/**
    * Look for AspectJ-annotated aspect beans in the current bean factory,
    * and return to a list of Spring AOP Advisors representing them.
    * <p>Creates a Spring Advisor for each AspectJ advice method.
    * @return the list of {@link org.springframework.aop.Advisor} beans
    * @see #isEligibleBean
    */
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<>();
                String[] beanNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                        this.beanFactory, Object.class, true, false);
                for (String beanName : beanNames) {
                    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.
                    Class<?> beanType = this.beanFactory.getType(beanName, false);
                    if (beanType == null) {
                        continue;
                    }
                    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);
                            List<Advisor> classAdvisors = this.advisorFactory.getAdvisors(factory);
                            // 单例加到advisorsCache, 非单例加到aspectFactoryCache
                            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);
                            // advisorFactory工厂获取advisors
                            advisors.addAll(this.advisorFactory.getAdvisors(factory));
                        }
                    }
                }
                this.aspectBeanNames = aspectNames;
                return advisors;
            }
        }
    }

    if (aspectNames.isEmpty()) {
        return Collections.emptyList();
    }
    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;
}

转换的成advisor的方法是：this.advisorFactory.getAdvisors，看getAdvisors源码

▼
java
复制代码
@Override
public List<Advisor> getAdvisors(MetadataAwareAspectInstanceFactory aspectInstanceFactory) {
    Class<?> aspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
    String aspectName = aspectInstanceFactory.getAspectMetadata().getAspectName();
    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)) {
        // Prior to Spring Framework 5.2.7, advisors.size() was supplied as the declarationOrderInAspect
        // to getAdvisor(...) to represent the "current position" in the declared methods list.
        // However, since Java 7 the "current position" is not valid since the JDK no longer
        // returns declared methods in the order in which they are declared in the source code.
        // Thus, we now hard code the declarationOrderInAspect to 0 for all advice methods
        // discovered via reflection in order to support reliable advice ordering across JVM launches.
        // Specifically, a value of 0 aligns with the default value used in
        // AspectJPrecedenceComparator.getAspectDeclarationOrder(Advisor).
        Advisor advisor = getAdvisor(method, lazySingletonAspectInstanceFactory, 0, 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()) {
        Advisor instantiationAdvisor = new SyntheticInstantiationAdvisor(lazySingletonAspectInstanceFactory);
        advisors.add(0, instantiationAdvisor);
    }

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

    return advisors;
}

getAdvisor方法如下

▼
java
复制代码
@Override
@Nullable
public Advisor getAdvisor(Method candidateAdviceMethod, MetadataAwareAspectInstanceFactory aspectInstanceFactory,
        int declarationOrderInAspect, String aspectName) {

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

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

    // 封装成advisor
    return new InstantiationModelAwarePointcutAdvisorImpl(expressionPointcut, candidateAdviceMethod,
            this, aspectInstanceFactory, declarationOrderInAspect, aspectName);
}

最后这里有一个重点 getPointcut() 方法，这是获取表达式的切点，详细看看

获取表达式的切点

切面类包含了切点，切面类转成 advisor 了，那么切点自然也是要转的。

获取表达式的切点的方法getPointcut如下：

▼
java
复制代码
@Nullable
private AspectJExpressionPointcut getPointcut(Method candidateAdviceMethod, Class<?> candidateAspectClass) {
    AspectJAnnotation<?> aspectJAnnotation =
            AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);
    if (aspectJAnnotation == null) {
        return null;
    }

    AspectJExpressionPointcut ajexp =
            new AspectJExpressionPointcut(candidateAspectClass, new String[0], new Class<?>[0]);
    ajexp.setExpression(aspectJAnnotation.getPointcutExpression());
    if (this.beanFactory != null) {
        ajexp.setBeanFactory(this.beanFactory);
    }
    return ajexp;
}

AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod的方法如下

▼
java
复制代码
private static final Class<?>[] ASPECTJ_ANNOTATION_CLASSES = new Class<?>[] {
        Pointcut.class, Around.class, Before.class, After.class, AfterReturning.class, AfterThrowing.class};

/**
    * 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) {
    for (Class<?> clazz : ASPECTJ_ANNOTATION_CLASSES) {
        AspectJAnnotation<?> foundAnnotation = findAnnotation(method, (Class<Annotation>) clazz);
        if (foundAnnotation != null) {
            return foundAnnotation;
        }
    }
    return null;
}

findAnnotation方法如下

▼
java
复制代码
@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;
    }
}

AnnotationUtils.findAnnotation 获取注解方法如下

▼
java
复制代码
/**
    * Find a single {@link Annotation} of {@code annotationType} on the supplied
    * {@link Method}, traversing its super methods (i.e. from superclasses and
    * interfaces) if the annotation is not <em>directly present</em> on the given
    * method itself.
    * <p>Correctly handles bridge {@link Method Methods} generated by the compiler.
    * <p>Meta-annotations will be searched if the annotation is not
    * <em>directly present</em> on the method.
    * <p>Annotations on methods are not inherited by default, so we need to handle
    * this explicitly.
    * @param method the method to look for annotations on
    * @param annotationType the annotation type to look for
    * @return the first matching annotation, or {@code null} if not found
    * @see #getAnnotation(Method, Class)
    */
@Nullable
public static <A extends Annotation> A findAnnotation(Method method, @Nullable Class<A> annotationType) {
    if (annotationType == null) {
        return null;
    }

    // Shortcut: directly present on the element, with no merging needed?
    if (AnnotationFilter.PLAIN.matches(annotationType) ||
            AnnotationsScanner.hasPlainJavaAnnotationsOnly(method)) {
        return method.getDeclaredAnnotation(annotationType);
    }

    // Exhaustive retrieval of merged annotations...
    return MergedAnnotations.from(method, SearchStrategy.TYPE_HIERARCHY, RepeatableContainers.none())
            .get(annotationType).withNonMergedAttributes()
            .synthesize(MergedAnnotation::isPresent).orElse(null);
}

封装成Advisor

注：Advisor 是 advice的包装器，包含了advice及其它信息

由InstantiationModelAwarePointcutAdvisorImpl构造完成

▼
java
复制代码
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);
    }
}

通过pointcut获取advice

▼
java
复制代码
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);
}

交给aspectJAdvisorFactory获取

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

    // 获取切面类
    Class<?> candidateAspectClass = aspectInstanceFactory.getAspectMetadata().getAspectClass();
    validate(candidateAspectClass);

    // 获取切面注解
    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);
    }

    // 切面注解转换成advice
    AbstractAspectJAdvice springAdvice;

    switch (aspectJAnnotation.getAnnotationType()) {
        case AtPointcut: // AtPointcut忽略
            if (logger.isDebugEnabled()) {
                logger.debug("Processing pointcut '" + candidateAdviceMethod.getName() + "'");
            }
            return null;
        case AtAround:
            springAdvice = new AspectJAroundAdvice(
                    candidateAdviceMethod, expressionPointcut, aspectInstanceFactory);
            break;
        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;
        default:
            throw new UnsupportedOperationException(
                    "Unsupported advice type on method: " + candidateAdviceMethod);
    }

    // 最后将其它切面信息配置到advice
    springAdvice.setAspectName(aspectName);
    springAdvice.setDeclarationOrder(declarationOrder);
    String[] argNames = this.parameterNameDiscoverer.getParameterNames(candidateAdviceMethod);
    if (argNames != null) {
        springAdvice.setArgumentNamesFromStringArray(argNames);
    }
    springAdvice.calculateArgumentBindings();

    return springAdvice;
}

小结

回头看，主要是处理使用了@Aspect注解的切面类，然后将切面类的所有切面方法根据使用的注解生成对应Advice，并将Advice连同切入点匹配器和切面类等信息一并封装到Advisor的过程。

advisor 有了，再加上源码的流程，其实到这里切面就已经实现了。

回顾一下你在写切面编程的时候，你需要定义 @Aspect, @Around , 切点表达式（Pointcut）对吧，那就对应上源码了，@Aspect声明的了切面类实例，作用是源码中判断是否属于 aop 基础类，@Around , 切点表达式对应为 advisor, 切面实现了。

下一篇文章学 advisor 的用法。

Spring进阶 - Spring AOP实现原理（一）AOP切面实现原理

引言