How does ApplicationContextAware work in Spring?

Question

In spring, if a bean implements ApplicationContextAware, then it is able to access the applicationContext. Therefore it is able to get other beans. e.g.

public class SpringContextUtil implements ApplicationContextAware {     private static ApplicationContext applicationContext;           public void setApplicationContext(ApplicationContext context) throws BeansException {       applicationContext = context;     }      public static ApplicationContext getApplicationContext() {       return applicationContext;     } } 

Then SpringContextUtil.getApplicationContext.getBean("name") can get the bean "name".

To do this, we should put this SpringContextUtil inside the applications.xml, e.g.

<bean class="com.util.SpringContextUtil" /> 

Here the bean SpringContextUtil doesn't include the property applicationContext. I guess when spring bean initialize, this property is set. But how is this done? How does the method setApplicationContext get called?

Answer 1

When spring instantiates beans, it looks for a couple of interfaces like ApplicationContextAware and InitializingBean. If they are found, the methods are invoked. E.g. (very simplified)

Class<?> beanClass = beanDefinition.getClass(); Object bean = beanClass.newInstance(); if (bean instanceof ApplicationContextAware) {     ((ApplicationContextAware) bean).setApplicationContext(ctx); } 

Note that in newer version it may be better to use annotations, rather than implementing spring-specific interfaces. Now you can simply use:

@Inject // or @Autowired private ApplicationContext ctx; 

Answer 2

Spring source code to explain how ApplicationContextAware work
when you use ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
In AbstractApplicationContext class,the refresh() method have the following code:

// Prepare the bean factory for use in this context. prepareBeanFactory(beanFactory); 

enter this method,beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this)); will add ApplicationContextAwareProcessor to AbstractrBeanFactory.

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)); ........... 

When spring initialize bean in AbstractAutowireCapableBeanFactory, in method initializeBean,call applyBeanPostProcessorsBeforeInitialization to implement the bean post process. the process include inject the applicationContext.

@Override     public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)             throws BeansException {         Object result = existingBean;         for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {             result = beanProcessor.postProcessBeforeInitialization(result, beanName);             if (result == null) {                 return result;             }         }         return result;     } 

when BeanPostProcessor implement Objectto execute the postProcessBeforeInitialization method,for example ApplicationContextAwareProcessor that added before.

private void invokeAwareInterfaces(Object bean) {         if (bean instanceof Aware) {             if (bean instanceof EnvironmentAware) {                 ((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());             }             if (bean instanceof EmbeddedValueResolverAware) {                 ((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(                         new EmbeddedValueResolver(this.applicationContext.getBeanFactory()));             }             if (bean instanceof ResourceLoaderAware) {                 ((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);             }             if (bean instanceof ApplicationEventPublisherAware) {                 ((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);             }             if (bean instanceof MessageSourceAware) {                 ((MessageSourceAware) bean).setMessageSource(this.applicationContext);             }             if (bean instanceof ApplicationContextAware) {                 ((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);             }         }     }