Spring源码分析系列（一）--通过@EnableAspectJAutoProxy分析AOP加载过程

Spring源码分析系列（一）--通过@EnableAspectJAutoProxy分析AOP加载过程

  AOP是面向切面编程，是相对于OOP面向对象编程而言的。Spring的AOP的存在的目的是为了解耦。AOP可以让一组类共享相同的行为。在OOP中只能通过继承类和实现接口来实现，但是这样的缺点是会使代码的耦合度增加，且类继承只能为单继承，阻碍更多行为添加到一组类上，AOP的出现弥补了OOP的不足。

  使用@EnableAspectJAutoProxy注解开启Spring对AspectJ的支持。
  添加@EnableAspectJAutoProxy注解，表示开启AOP代理自动配置，如果使用@EnableAspectJAutoProxy注解，表示使用cglib进行代理对象的生成。设置@EnableAspectJAutoProxy(exposeProxy=true)表示通过AOP框架暴露该代理对象，aopContext能够访问。

  下面看一下@EnableAspectJAutoProxy这个类的源码

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(AspectJAutoProxyRegistrar.class)
public @interface EnableAspectJAutoProxy {

    /**
     * Indicate whether subclass-based (CGLIB) proxies are to be created as opposed
     * to standard Java interface-based proxies. The default is {@code false}.
     */
    // true--使用CGLIB基于类创建代理，false--使用java接口创建代理
    boolean proxyTargetClass() default false;

    /**
     * Indicate that the proxy should be exposed by the AOP framework as a {@code ThreadLocal}
     * for retrieval via the {@link org.springframework.aop.framework.AopContext} class.
     * Off by default, i.e. no guarantees that {@code AopContext} access will work.
     * @since 4.3.1
     */
    // 是否通过aop矿建暴露该代理对象，aopContext能够访问
    boolean exposeProxy() default false;

}

  通过上面的代码可以看出@EnableAspectJAutoProxy把AspectJAutoProxyRegistrar.class对象导入到了容器中。利用AspectJAutoProxyRegistrar给容器中注册了一个AnnotationAwareAspectJAutoProxyCreator。

AspectJAutoProxyRegistrar

AspectJAutoProxyRegistrar类中的registerBeanDefinitions方法主要实现了两个功能：

• 1.注册AnnotationAwareAspectJAutoProxyCreator
• 2.获取@EnableAspectJAutoProxy注解的属性信息

class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar {
    /**
     * Register, escalate, and configure the AspectJ auto proxy creator based on the value
     * of the @{@link EnableAspectJAutoProxy#proxyTargetClass()} attribute on the importing
     * {@code @Configuration} class.
     */
    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
        //注册AnnotationAwareAspectJAutoProxyCreator
        AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);
        //获取@EnableAspectJAutoProxy注解的属性信息
        AnnotationAttributes enableAspectJAutoProxy = AnnotationConfigUtils.attributesFor(importingClassMetadata,EnableAspectJAutoProxy.class);
        if (enableAspectJAutoProxy != null) {
            if (enableAspectJAutoProxy.getBoolean("proxyTargetClass")) {
                AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
            }
            if (enableAspectJAutoProxy.getBoolean("exposeProxy")) {
                AopConfigUtils.forceAutoProxyCreatorToExposeProxy(registry);
            }
        }
    }
}

1.注册AnnotationAwareAspectJAutoProxyCreator

@Nullable
public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry) {
    return registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry, null);
}

//注册AnnotationAwareAspectJAutoProxyCreator
@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");
    //判断是否包含目标bean
    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) {
                apcDefinition.setBeanClassName(cls.getName());
            }
        }
        return null;
    }

    RootBeanDefinition beanDefinition = new RootBeanDefinition(cls);
    beanDefinition.setSource(source);
    beanDefinition.getPropertyValues().add("order", Ordered.HIGHEST_PRECEDENCE);
    beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
    //将beanDefinition注册到registry中，name为AUTO_PROXY_CREATOR_BEAN_NAME
    //AUTO_PROXY_CREATOR_BEAN_NAME = org.springframework.aop.config.internalAutoProxyCreator
    registry.registerBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME, beanDefinition);
    return beanDefinition;
}