springboot启动流程（十一）aop切面处理过程
所有文章
https://www.cnblogs.com/lay2017/p/11478237.html
正文
spring的两大核心就是ioc和aop。在关于ioc依赖注入的文章中，我们了解了如何根据BeanDefinition创建Bean，然后在BeanPostProcessor中处理@Autowired和@Resource两个注解，自动注入Bean。
本文将讲解另外一块核心内容，aop切面。
AOP自动配置
首先，aop切面基于springboot的自动配置。如果不了解自动配置，请参考自动配置机制的文章。
为此，我们先找到aop自动配置的类AopAutoConfiguration
@Configuration @ConditionalOnClass({ EnableAspectJAutoProxy.class, Aspect.class, Advice.class, AnnotatedElement.class }) @ConditionalOnProperty(prefix = "spring.aop", name = "auto", havingValue = "true", matchIfMissing = true) public class AopAutoConfiguration { @Configuration @EnableAspectJAutoProxy(proxyTargetClass = false) @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false", matchIfMissing = false) public static class JdkDynamicAutoProxyConfiguration { } @Configuration @EnableAspectJAutoProxy(proxyTargetClass = true) @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true", matchIfMissing = true) public static class CglibAutoProxyConfiguration { } }
我们将以Cglib为主要的了解对象，可以看到CglibAutoProxyConfiguration上注解了一个@EnableAspectJAutoProxy，它意味着开启切面代理，我们打开该注解
@Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @Documented @Import(AspectJAutoProxyRegistrar.class) public @interface EnableAspectJAutoProxy { // }
我们看到，@Import注解导入了AspectJAutoProxyRegistrar。在自动配置的文章中，我们了解到ConfigurationClassParser将会处理@Import注解。这里我们不再关注如何处理@Import注解，直接打开AspectJAutoProxyRegistrar类看看
class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar { @Override public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) { // 注册自动切面代理的创建器 AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry); // 省略 } }
跟进registerAspectJAnnotationAutoProxyCreatorIfNecessary
public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry) { return registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry, null); }
继续跟进registerAspectJAnnotationAutoProxyCreatorIfNecessary
public static BeanDefinition registerAspectJAnnotationAutoProxyCreatorIfNecessary( BeanDefinitionRegistry registry, @Nullable Object source) { return registerOrEscalateApcAsRequired(AnnotationAwareAspectJAutoProxyCreator.class, registry, source); }
我们看到，调用registerOrEscalateApcAsRequired方法指定了一个类AnnotationAwareAspectJAutoProxyCreator，这个类将作为创建代理的类
跟进registerOrEscalateApcAsRequired方法
private static BeanDefinition registerOrEscalateApcAsRequired( Class<?> cls, BeanDefinitionRegistry registry, @Nullable Object source) { // // 构建BeanDefinition RootBeanDefinition beanDefinition = new RootBeanDefinition(cls); beanDefinition.setSource(source); beanDefinition.getPropertyValues().add("order", Ordered.HIGHEST_PRECEDENCE); beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE); // 注册到Bean容器 registry.registerBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME, beanDefinition); return beanDefinition; }
可以看到，最终是注册到了Bean容器中，作为BeanDefinition存在。我们可以认为aop的自动配置过程就是为了创建AnnotationAwareAspectJAutoProxyCreator这个类的BeanDefinition。
注册AnnotationAwareAspectJAutoProxyCreator
首先，我们先看看AnnotationAwareAspectJAutoProxyCreator的继承结构
可以看到，AnnotationAwareAspectJAutoProxyCreator最终是实现了BeanPostProcessor，那BeanPostProcessor是什么时候被创建为Bean的呢？
这里，我们得回到refresh容器过程的refresh方法中，跟进AbstractApplicationContext的refresh方法
public void refresh() throws BeansException, IllegalStateException { synchronized (this.startupShutdownMonitor) { // try { // // 注册BeanDefinition到容器 invokeBeanFactoryPostProcessors(beanFactory); // 注册后置处理器到容器 registerBeanPostProcessors(beanFactory); // } // } }
在注册BeanDefinition之后，就会把BeanPostProcessor的BeanDefinition转化为了Bean注册到容器中。registerBeanPostProcessors结束以后，AnnotationAwareAspectJAutoProxyCreator就以Bean的形式存在于BeanFactory中了。
触发AnnotationAwareAspectJAutoProxyCreator
接着，我们再看AnnotationAwareAspectJAutoProxyCreator被注册为Bean以后，是在什么位置被触发的。前面，ioc依赖注入的文章中我们提到过createBean方法，将会根据BeanDefinition创建Bean。
跟进AbstractAutowireCapableBeanFactory的createBean方法
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException { // try { // BeanPostProcessors 调用 Object bean = resolveBeforeInstantiation(beanName, mbdToUse); if (bean != null) { return bean; } } // try { // 创建实例对象 Object beanInstance = doCreateBean(beanName, mbdToUse, args); // return beanInstance; } // }
我们看到，在doCreateBean之前，先触发resolveBeforInstantiation方法，调用了AnnotationAwareAspectJAutoProxyCreator。
切面解析
跟进resolveBeforeInstantiation
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) { Object bean = null; if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) { if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) { // if (targetType != null) { bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName); // } } // } return bean; }
继续跟进applyBeanPostProcessorBeforeInstantiation方法
protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) { for (BeanPostProcessor bp : getBeanPostProcessors()) { if (bp instanceof InstantiationAwareBeanPostProcessor) { InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp; // 触发postProcesBeforeInstantiation Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName); if (result != null) { return result; } } } return null; }
继续跟进AbstractAutoProxyCreator的postProcessBeforeInstantiation方法
public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) { Object cacheKey = getCacheKey(beanClass, beanName); if (!StringUtils.hasLength(beanName) || !this.targetSourcedBeans.contains(beanName)) { // if (isInfrastructureClass(beanClass) || shouldSkip(beanClass, beanName)) { this.advisedBeans.put(cacheKey, Boolean.FALSE); return null; } } // return null; }
isInfrastructureCLass将返回false，跟进AspectJAwareAdvisorAutoProxyCreator的shouldSkip
protected boolean shouldSkip(Class<?> beanClass, String beanName) { // 返回所有的通知，如@Before @After @AfterThrowing @Round List<Advisor> candidateAdvisors = findCandidateAdvisors(); for (Advisor advisor : candidateAdvisors) { if (advisor instanceof AspectJPointcutAdvisor && ((AspectJPointcutAdvisor) advisor).getAspectName().equals(beanName)) { return true; } } return super.shouldSkip(beanClass, beanName); }
这里的findCandidateAdvisors方法，将会从beanFactory中获得注解了@Aspect的类元数据，然后获取其中定义的Advisor。
到这一步，我们就已经获得了完成了切面部分的解析工作。
代理增强
回到AbstractAutowireCapableBeanFactory的createBean方法
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException { // try { // BeanPostProcessors 调用 Object bean = resolveBeforeInstantiation(beanName, mbdToUse); if (bean != null) { return bean; } } // try { // 创建实例对象 Object beanInstance = doCreateBean(beanName, mbdToUse, args); // return beanInstance; } // }
获取了Advisor，那么再看看Advisor被增强到Bean上的过程，跟进doCreateBean
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args) throws BeanCreationException { // if (instanceWrapper == null) { // 创建实例对象 instanceWrapper = createBeanInstance(beanName, mbd, args); } // Object exposedObject = bean; try { // 自动注入 populateBean(beanName, mbd, instanceWrapper); // 初始化Bean，创建代理的入口 exposedObject = initializeBean(beanName, exposedObject, mbd); } // return exposedObject; }
我们找到initializeBean，这个初始化Bean的入口，将从这里开始关注代理增强部分，跟进initializeBean方法
protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) { // if (mbd == null || !mbd.isSynthetic()) { wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName); } return wrappedBean; }
继续跟进applyBeanPostProcessorsAfterInitialization方法
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName) throws BeansException { Object result = existingBean; for (BeanPostProcessor processor : getBeanPostProcessors()) { Object current = processor.postProcessAfterInitialization(result, beanName); if (current == null) { return result; } result = current; } return result; }
这里将会调用AbstractAutoProxyCreator的postProcessAfterInitialization方法，跟进方法
public 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; }
继续跟进wrapIfNecessary
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) { // // 获取适用于当前Bean的Advisors Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null); 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; }
getAdvicesAndAdvisorsForBean方法将会获取到可以增强到该Bean的Advisor，然后createProxy将会创建代理类，并一路返回，如果是单例，则注册到缓存中
跟进createProxy看看创建代理
protected Object createProxy(Class<?> beanClass, @Nullable String beanName, @Nullable Object[] specificInterceptors, TargetSource targetSource) { // // 获取可用的advisor Advisor[] advisors = buildAdvisors(beanName, specificInterceptors); proxyFactory.addAdvisors(advisors); // // 创建并返回代理对象 return proxyFactory.getProxy(getProxyClassLoader()); }
跟进getProxy
public Object getProxy(@Nullable ClassLoader classLoader) { return createAopProxy().getProxy(classLoader); }
createAopProxy将会根据条件返回Cglib实现或者jdk动态代理的实现，然后调用它们的getProxy方法去获取代理对象
总结
本文省略了不少细节内容，大体逻辑是从：aop自动配置 -> 解析@Aspect切面对象 -> 代理增强，这么一个逻辑行文的。
自动配置的核心就是为了导入一个AnnotationAwareAspectJAutoProxyCreator，该类实现了BeanPostProcessor。所以在创建Bean实例对象之前会触发解析@Aspect切面对象，获取Advisor。在生成Bean实例对象之后，会再触发该类对Bean实例对象做代理增强，增强的Advisor来自之前的解析结果。代理增强的实现有cglib和jdk动态代理两种。
最后，增强过的代理Bean如果是单例，将跟以前一样添加到缓存对象中。
