如果要使用SpringAOP的功能，必须要添加一个@EnableAspectJAutoProxy注解，有了这个注解才能支持@Aspect等相关的一系列AOP注解的功能，这个注解就相当于在传统的xml配置文件中添加 <aop:aspectj-autoproxy>一样。
在学习SpringAOP相关的知识之前，建议一定要先搞清楚IOC相关的知识，如果不清楚的可以先阅读Spring专栏中IOC系列的文章。
接下来就来看一下@EnableAspectJAutoProxy注解到底做了什么？
@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}.
*/
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
*/
boolean exposeProxy() default false;
}
看到@Import注解，直接跟进去。@Import注解不了解的可以看这篇文章，解析Spring中@Import的实现原理
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.
*/
// spring启动时，会调用到这个方法
@Override
public void registerBeanDefinitions(
AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
// 注册AnnotationAwareAspectJAutoProxyCreator
AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);
AnnotationAttributes enableAspectJAutoProxy =
AnnotationConfigUtils.attributesFor(importingClassMetadata, EnableAspectJAutoProxy.class);
if (enableAspectJAutoProxy != null) {
if (enableAspectJAutoProxy.getBoolean("proxyTargetClass")) {
AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
}
if (enableAspectJAutoProxy.getBoolean("exposeProxy")) {
AopConfigUtils.forceAutoProxyCreatorToExposeProxy(registry);
}
}
}
}
@Nullable
public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry) {
return registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry, null);
}
@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");
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;
}
// 把AnnotationAwareAspectJAutoProxyCreator封装成RootBeanDefinition
RootBeanDefinition beanDefinition = new RootBeanDefinition(cls);
beanDefinition.setSource(source);
beanDefinition.getPropertyValues().add("order", Ordered.HIGHEST_PRECEDENCE);
beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
// 把需要spring管理的bean放入beanDefinitionMap和beanDefinitionNames，分别是一个map和list容器。
// spring在实例化bean时，就会遍历容器中的bean然后依次处理。
registry.registerBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME, beanDefinition);
return beanDefinition;
}
可以看出@EnableAspectJAutoProxy注解最主要的作用实际上就是通过@Import注解把AnnotationAwareAspectJAutoProxyCreator这个对象注入到spring容器中。
再看一下AnnotationAwareAspectJAutoProxyCreator类的结构图
AnnotationAwareAspectJAutoProxyCreator类间接的实现了BeanPostProcessor接口，实现这个接口的目的就是为了能够通过前置处理器、后置处理器完成一些事情。
这部分代码来自IOC初始化bean对象的过程
protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
invokeAwareMethods(beanName, bean);
return null;
}, getAccessControlContext());
}
else {
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
//调动前置处理器
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
invokeInitMethods(beanName, wrappedBean, mbd);
}
catch (Throwable ex) {
throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
}
if (mbd == null || !mbd.isSynthetic()) {
//调动后置处理器
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
throws BeansException {
Object result = existingBean;
for (BeanPostProcessor processor : getBeanPostProcessors()) {
//从刚刚的类图中可以看出AnnotationAwareAspectJAutoProxyCreator类或者继承类中，肯定有重写这个方法
Object current = processor.postProcessAfterInitialization(result, beanName);
if (current == null) {
return result;
}
result = current;
}
return result;
}
分析一下这段流程，AnnotationAwareAspectJAutoProxyCreator因为间接继承了抽象类AbstractAutoProxyCreator，而这个抽象类又间接实现了BeanPostPorcessor接口，所以在每个bean对象最后初始化的时候，通过postProcessAfterInitialization方法完成了AOP相关的处理。
本文就没有继续接着postProcessAfterInitialization方法继续深入，因为还是打算另起一篇文章着重分析，可以先下一个结论，这个方法就是扫描并解析aop注解的入口，如果当前bean对象符合配置的增加规则，最终就会返回实现增加方法的代理类。