Spring扩展点之ApplicationContextAware

ApplicationContextAware

作用

Spring中管理的对象有两类，一类是用户自定义的对象，一类是Spring容器对象，比如applicationContext、beanFactory等。用户自定义的对象，在注入时可以直接使用@Autowared等方法，但是如果需要使用容器对象，该怎么注入呢？

Spring提供了一个扩展方法，即ApplicationContextAware接口，实现容器对象的注入。

这个接口里面只有一个方法，即setApplicationContext

publicinterfaceApplicationContextAwareextendsAware{voidsetApplicationContext(ApplicationContextvar1)throwsBeansException;}

使用方法

创建即ApplicationContextAware接口的实现类，实现类中定义局部变量context，实现setApplicationContext方法，将容器对象接收到context中。

publicclassApplicationContextUtilimplementsApplicationContextAware{privatestaticApplicationContextcontext;publicvoidsetApplicationContext(ApplicationContextcontext)throwsBeansException{this.context=context;}publicstaticApplicationContextgetContext(){returncontext;}}

context变量不是必须的，如果后续业务中需要使用context，就保存下来，如果只在bean初始化时用一次就不需要。

执行时机

作为第一个BeanPostProcessor执行。

源码解读

首先看一下bean初始化时各个扩展点的执行顺序。 在bean初始化方法AbstractAutowireCapableBeanFactory#initializeBean()中，可以看出执行顺序是：

先调用bean的构造函数

再调用set方法

再调用beanPostProcessorsBeforeInitialization

调用init-method

调用beanPostProcessorsAfterInitialization

protectedObjectinitializeBean(StringbeanName,Objectbean,@NullableRootBeanDefinitionmbd){if(System.getSecurityManager()!=null){AccessController.doPrivileged((PrivilegedAction<Object>)()->{invokeAwareMethods(beanName,bean);returnnull;},getAccessControlContext());}else{invokeAwareMethods(beanName,bean);}ObjectwrappedBean=bean;if(mbd==null||!mbd.isSynthetic()){wrappedBean=applyBeanPostProcessorsBeforeInitialization(wrappedBean,beanName);}try{invokeInitMethods(beanName,wrappedBean,mbd);}catch(Throwableex){thrownewBeanCreationException((mbd!=null?mbd.getResourceDescription():null),beanName,"Invocationofinitmethodfailed",ex);}if(mbd==null||!mbd.isSynthetic()){wrappedBean=applyBeanPostProcessorsAfterInitialization(wrappedBean,beanName);}returnwrappedBean;}

其中，ApplicationContextAware是在beanPostProcessorsBeforeInitialization阶段。怎么知道的呢？

在AbstractApplicationContext#refresh()方法中，进行各个扩展点实现的设置。

publicvoidrefresh()throwsBeansException,IllegalStateException{synchronized(this.startupShutdownMonitor){//Preparethiscontextforrefreshing.prepareRefresh();//Tellthesubclasstorefreshtheinternalbeanfactory.ConfigurableListableBeanFactorybeanFactory=obtainFreshBeanFactory();//Preparethebeanfactoryforuseinthiscontext.prepareBeanFactory(beanFactory);try{//Allowspost-processingofthebeanfactoryincontextsubclasses.postProcessBeanFactory(beanFactory);//Invokefactoryprocessorsregisteredasbeansinthecontext.invokeBeanFactoryPostProcessors(beanFactory);//Registerbeanprocessorsthatinterceptbeancreation.registerBeanPostProcessors(beanFactory);//Initializemessagesourceforthiscontext.initMessageSource();//Initializeeventmulticasterforthiscontext.initApplicationEventMulticaster();//Initializeotherspecialbeansinspecificcontextsubclasses.onRefresh();//Checkforlistenerbeansandregisterthem.registerListeners();//Instantiateallremaining(non-lazy-init)singletons.finishBeanFactoryInitialization(beanFactory);//Laststep:publishcorrespondingevent.finishRefresh();}catch(BeansExceptionex){......省略......}finally{......省略......}}}

其中，prepareBeanFactory()方法中，可以看到，将ApplicationContextAwareProcessor加到BeanPostProcessor中了。

protectedvoidprepareBeanFactory(ConfigurableListableBeanFactorybeanFactory){//Telltheinternalbeanfactorytousethecontext'sclassloaderetc.beanFactory.setBeanClassLoader(getClassLoader());beanFactory.setBeanExpressionResolver(newStandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));beanFactory.addPropertyEditorRegistrar(newResourceEditorRegistrar(this,getEnvironment()));//Configurethebeanfactorywithcontextcallbacks.beanFactory.addBeanPostProcessor(newApplicationContextAwareProcessor(this));......省略......//Registerearlypost-processorfordetectinginnerbeansasApplicationListeners.beanFactory.addBeanPostProcessor(newApplicationListenerDetector(this));......省略......}

再看一下ApplicationContextAwareProcessor，它实现了BeanPostProcessor接口，覆写了postProcessBeforeInitialization()方法，在方法里面，如果发现bean是ApplicationContextAware类型，就调用setApplicationContext()方法。

classApplicationContextAwareProcessorimplementsBeanPostProcessor{privatefinalConfigurableApplicationContextapplicationContext;privatefinalStringValueResolverembeddedValueResolver;/***CreateanewApplicationContextAwareProcessorforthegivencontext.*/publicApplicationContextAwareProcessor(ConfigurableApplicationContextapplicationContext){this.applicationContext=applicationContext;this.embeddedValueResolver=newEmbeddedValueResolver(applicationContext.getBeanFactory());}@Override@NullablepublicObjectpostProcessBeforeInitialization(Objectbean,StringbeanName)throwsBeansException{if(!(beaninstanceofEnvironmentAware||beaninstanceofEmbeddedValueResolverAware||beaninstanceofResourceLoaderAware||beaninstanceofApplicationEventPublisherAware||beaninstanceofMessageSourceAware||beaninstanceofApplicationContextAware)){returnbean;}AccessControlContextacc=null;if(System.getSecurityManager()!=null){......省略......}else{invokeAwareInterfaces(bean);}returnbean;}privatevoidinvokeAwareInterfaces(Objectbean){if(beaninstanceofEnvironmentAware){((EnvironmentAware)bean).setEnvironment(this.applicationContext.getEnvironment());}if(beaninstanceofEmbeddedValueResolverAware){((EmbeddedValueResolverAware)bean).setEmbeddedValueResolver(this.embeddedValueResolver);}if(beaninstanceofResourceLoaderAware){((ResourceLoaderAware)bean).setResourceLoader(this.applicationContext);}if(beaninstanceofApplicationEventPublisherAware){((ApplicationEventPublisherAware)bean).setApplicationEventPublisher(this.applicationContext);}if(beaninstanceofMessageSourceAware){((MessageSourceAware)bean).setMessageSource(this.applicationContext);}if(beaninstanceofApplicationContextAware){((ApplicationContextAware)bean).setApplicationContext(this.applicationContext);}}}