@EnableAspectJAutoProxy注解的原理(SpringAOP的原理）

本文主要是介绍@EnableAspectJAutoProxy注解的原理(SpringAOP的原理），希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

spring使用aspectj提供的库进行切入点解析和匹配，解释与aspectj 5相同的注释。但是，AOP运行时仍然是纯SpringAOP，并且不依赖于AspectJ编译器或weaver。

springAOP不管是用jdk动态代理还是cglib动态代理，跟aspectj的最大区别是SpringAOp用的是动态代理运行期织入，而aspectj是使用的静态代理在编译期织入。

先看@EnableAspectJAutoProxy注解，通过Import引入AspectJAutoProxyRegistrar，AspectJAutoProxyRegistrar实现了ImportBeanDefinitionRegistrar接口。ImportBeanDefinitionRegistrar接口的作用就是把BeanDefinitionRegistry传递给我们，我们可以通过这个注册器完成beanDefinition的注册。

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(AspectJAutoProxyRegistrar.class)
public @interface EnableAspectJAutoProxy {boolean proxyTargetClass() default false;boolean exposeProxy() default false;}

接着再看AspectJAutoProxyRegistrar都做了些什么

class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar {@Overridepublic void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {//注册AnnotationAwareAspectJAutoProxyCreatorAopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);AnnotationAttributes enableAspectJAutoProxy =AnnotationConfigUtils.attributesFor(importingClassMetadata, EnableAspectJAutoProxy.class);//判断配置类AppConfig是否加了@EnableAspectJAutoProxy注解if (enableAspectJAutoProxy != null) {//默认是false指用jdk动态代理,值为true则开启cglib代理if (enableAspectJAutoProxy.getBoolean("proxyTargetClass")) {AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);}if (enableAspectJAutoProxy.getBoolean("exposeProxy")) {AopConfigUtils.forceAutoProxyCreatorToExposeProxy(registry);}}}}

首先注册了一个AnnotationAwareAspectJAutoProxyCreator，先看这个类的继承结构图

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

public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry, @Nullable Object source) {return registerOrEscalateApcAsRequired(AnnotationAwareAspectJAutoProxyCreator.class, registry, source);}

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;}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;}

很关键的一点就是，它实现了BeanPostProcessor接口，那么就能猜测出，大致就是在bean实例化完成后，put到singletomObjects(狭义上的spring容器，一个currentHashMap，存储beanName和bean对象)之前做一些判断，然后将代理对象或者实际对象放入singletonObject中。

接着再看AnnotationAwareAspectJAutoProxyCreator中BeanPostProcessor接口的实现方法，发现没有。在AbstractAutoProxyCreator中找到了

@Overridepublic Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) {Object cacheKey = getCacheKey(beanClass, beanName);if (!StringUtils.hasLength(beanName) || !this.targetSourcedBeans.contains(beanName)) {if (this.advisedBeans.containsKey(cacheKey)) {return null;}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;}

@Overridepublic Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {if (bean != null) {Object cacheKey = getCacheKey(bean.getClass(), beanName);if (this.earlyProxyReferences.remove(cacheKey) != bean) {return wrapIfNecessary(bean, beanName, cacheKey);}}return bean;}

在fater方法中调用wrapIfNecessary，这个方法会判断满足切点表达式的方法上的通知是否不为空，不为空就返回代理对象，反之返回实际对象

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {return bean;}if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {return bean;}if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {this.advisedBeans.put(cacheKey, Boolean.FALSE);return bean;}// Create proxy if we have advice.Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);if (specificInterceptors != DO_NOT_PROXY) {this.advisedBeans.put(cacheKey, Boolean.TRUE);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;}

可以测试一下，现在把通知注释掉

@Configuration
@ComponentScan("com.spring")
@EnableAspectJAutoProxy(proxyTargetClass = true)
public class AppConfig {}

@Aspect
@Component
public class TestAspectj {@Pointcut("execution(* com.spring.dao.*.*(..))")public void point(){//该方法就是一个标识方法，为pointcut提供一个依附的地方}//	@Before("point()")
//	public void before(){
//		System.out.println("Before");
//	}
//	@After("point()")
//	public void after(){
//		System.out.println("After");
//	}
}

public class Test {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();context.register(AppConfig.class);context.refresh();IndexDao indexDao = context.getBean(IndexDao.class);indexDao.query();}
}

没有通知的时候，返回的是实际IndexDao类型