spring ApplicationContextAware的使用和执行时机

实际项目开发中，会写类似如下的工具类，方便做脚本执行或者一些判断，提供一些api方法等(可阅读：https://blog.csdn.net/qq_26437925/article/details/114213391 ，ApplicationContextAware就大派用场了)，如下

@Component
public class SpringApplicationUtil implements ApplicationContextAware {

    private static ApplicationContext applicationContext;

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        SpringApplicationUtil.applicationContext = applicationContext;
    }

    public static Object getBean(String beanName){
        return applicationContext.getBean(beanName);
    }

    public static List<String> getAllBeanNames(){
        String[] arr = applicationContext.getBeanDefinitionNames();
        if(arr == null || arr.length == 0){
            return new ArrayList<>();
        }
        return Arrays.stream(arr).collect(Collectors.toList());
    }

}

如下测试代码, 想拿到某个bean, 直接从这个util拿到

@Test
	public void test2() throws Exception {
		ApplicationContext ctx =
				new AnnotationConfigApplicationContext(LocalConfig.class, LocalConfig2.class);

		// 直接从工具类获取
		UserService userService = (UserService)SpringApplicationUtil.getBean("userService");
		Assert.assertNotNull(userService);
		userService.add(new User());

		((AnnotationConfigApplicationContext) ctx).close();
	}

本文来探究一下它是如何工作的。

直接debug可以看到执行链路，进行具体分析

可以发现在这个bean的初始化阶段，会执行所有的BeanPostProcessor扩展的postProcessBeforeInitialization方法，会发现执行到了ApplicationContextAwareProcessor

ApplicationContextAwareProcessor怎么来的？

org.springframework.context.support.AbstractApplicationContext的refresh方法中的prepareBeanFactory(beanFactory); 执行添加的，平台内置的一个beanPostProcessor

所以回到org.springframework.context.support.ApplicationContextAwareProcessor#postProcessBeforeInitialization 方法中，可以看到会执行Aware接口

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(this.embeddedValueResolver);
		}
		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);
		}
	}
}

执行ApplicationContextAware的setApplicationContext方法完成设置, 如下

其实就是AnnotationConfigApplicationContext自身，即Spring容器管理者自身。

再比如BeanAware接口, 一个Bean想知道自己的beanName，那么自己实现下BeanNameAware

public class X implements BeanNameAware {

    private String xName;

    @Autowired
    private Y y;


    public X() {
        System.out.println("X()");
    }

    public void sy(){
        System.out.println("y:" + y);
    }

    @Override
    public void setBeanName(String name) {
        this.xName = name;
    }

    public void sayMyName(){
        System.out.println("xName:" + xName);
    }