茅塞顿开：Spring Aware原理解析

问题

为什么被Spring管理的类。只要实现了Aware接口。Spring就会自动把对应的对象注入进来。这个bean如果想要使用直接使用就可以了。如下: Spring会自动把ApplicationContext注入到AwareTest这个bean里面。

public class AwareTest implements ApplicationContextAware {public ApplicationContext applicationContext;@Overridepublic void setApplicationContext(ApplicationContext applicationContext) throws BeansException {this.applicationContext = applicationContext;}public void checkAware(){if(applicationContext != null){System.out.println("ApplicationContext inject success");}}
}

源码解析：

1. 我们都知道在AbstractApplicationContext里面真正的启动ApplicationContext的函数是refresh()方法。具体的方法如下

public void refresh() throws BeansException, IllegalStateException {synchronized (this.startupShutdownMonitor) {// Prepare this context for refreshing.prepareRefresh();// Tell the subclass to refresh the internal bean factory.ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();// Prepare the bean factory for use in this context.prepareBeanFactory(beanFactory);try {// Allows post-processing of the bean factory in context subclasses.postProcessBeanFactory(beanFactory);// Invoke factory processors registered as beans in the context.invokeBeanFactoryPostProcessors(beanFactory);// Register bean processors that intercept bean creation.registerBeanPostProcessors(beanFactory);// Initialize message source for this context.initMessageSource();// Initialize event multicaster for this context.initApplicationEventMulticaster();// Initialize other special beans in specific context subclasses.onRefresh();// Check for listener beans and register them.registerListeners();// Instantiate all remaining (non-lazy-init) singletons.finishBeanFactoryInitialization(beanFactory);// Last step: publish corresponding event.finishRefresh();}catch (BeansException ex) {if (logger.isWarnEnabled()) {logger.warn("Exception encountered during context initialization - " +"cancelling refresh attempt: " + ex);}// Destroy already created singletons to avoid dangling resources.destroyBeans();// Reset 'active' flag.cancelRefresh(ex);// Propagate exception to caller.throw ex;}finally {// Reset common introspection caches in Spring's core, since we// might not ever need metadata for singleton beans anymore...resetCommonCaches();}}}

2. 可以看到有一个prepareBeanFactory的方法，我们再往里面看一下这个方法里面究竟有什么。可以发现给BeanFactory添加了一个BeanPostProcessor。名字是ApplicationContextAwareProcessor 

protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {// Tell the internal bean factory to use the context's class loader etc.beanFactory.setBeanClassLoader(getClassLoader());beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));// Configure the bean factory with context callbacks.beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));beanFactory.ignoreDependencyInterface(EnvironmentAware.class);... 多余的代码不贴出来了}

3. BeanPostProcessor这个接口我们都很熟悉了。我们看一下这个里面PostProcessBeforeInitialization()函数。传入bean和beanName。最后是调用了invokeAwareInterfaces(bean)这个函数。如果bean 是aware类的子类。那么就调用bean的set方法将ApplicationContext塞到bean里面。然后业务代码在需要使用ApplicationContext的时候就可以直接使用了。因为已经被set进来了。
 

public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {AccessControlContext acc = null;if (System.getSecurityManager() != null &&(bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)) {acc = this.applicationContext.getBeanFactory().getAccessControlContext();}if (acc != null) {AccessController.doPrivileged((PrivilegedAction