Spring源码解析:Spring Aware 原理解析

写在前面

Spring源码解析系列文章是我最近在学习了一段Spring，SpringBoot和SpringMVC等一系列的源码后整理的文章。我会随着我对Spring的理解来进行相应的文章的整合和归纳。

我决定从提出一个问题开始。然后围绕着解决这个问题的方式来剖析源码。当我们把一个问题搞懂了的时候。里面的源码我们也就清楚了。事实上看源码不是目标。解决一些日常的困惑才是目标。

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;
    
    @Override
    public 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