ApplicationContextAware,ServletContextAware等Aware接口的理解

Aware接口理解

Aware的意思是感知
ApplicationContextAware可以获得 ApplicationContext上下文资源
ServletContextAware可以获得ServletContext上下文资源
…
实现xxAware接口的子类,会覆盖对应的setXX()方法，从而可以获得Spring容器中的xx资源

ApplicationContextAware实例

ApplicationContextAware接口，可以在Spring初始化实例 Bean的时候，可以通过这个接口将当前的Spring上下文传入。

注意：一定要让继承ApplicationContextAware接口的bean被Spring上下文管理
在application.xml文件中定义对应的bean标签，或者使用@Component标注。

@Component
public class Test implements ApplicationContextAware,ServletContextAware {
    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        System.out.println(applicationContext);
    }

    @Override
    public void setServletContext(ServletContext servletContext) {
        System.out.println(servletContext);
    }
}

我们看到实现ApplicationContextAware只覆盖了一个setApplicationContext方法
而这个方法在初始化时会注入ApplicationContext供子类Test设置

SpringBean初始化流程

debug代码我们发现ApplicationContextAwareProcessor的invokeAwareInterfaces上下文被注入，注入逻辑很简单，如果是ApplicationContextAware的实例，就set进去

        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);
            }
        }

调用堆栈

setApplicationContext:21, Test (com.microptech.eurekaregistry)
invokeAwareInterfaces:120, ApplicationContextAwareProcessor (org.springframework.context.support)
postProcessBeforeInitialization:96, ApplicationContextAwareProcessor (org.springframework.context.support)
applyBeanPostProcessorsBeforeInitialization:419, AbstractAutowireCapableBeanFactory (org.springframework.beans.factory.support)
initializeBean:1737, AbstractAutowireCapableBeanFactory (org.springframework.beans.factory.support)
doCreateBean:576, AbstractAutowireCapableBeanFactory (org.springframework.beans.factory.support)
createBean:498, AbstractAutowireCapableBeanFactory (org.springframework.beans.factory.support)
lambda$doGetBean$0:320, AbstractBeanFactory (org.springframework.beans.factory.support)
getObject:-1, 23880693 (org.springframework.beans.factory.support.AbstractBeanFactory$$Lambda$139)
getSingleton:222, DefaultSingletonBeanRegistry (org.springframework.beans.factory.support)
doGetBean:318, AbstractBeanFactory (org.springframework.beans.factory.support)
getBean:199, AbstractBeanFactory (org.springframework.beans.factory.support)
preInstantiateSingletons:846, DefaultListableBeanFactory (org.springframework.beans.factory.support)
finishBeanFactoryInitialization:863, AbstractApplicationContext (org.springframework.context.support)
refresh:546, AbstractApplicationContext (org.springframework.context.support)
refresh:142, ServletWebServerApplicationContext (org.springframework.boot.web.servlet.context)
refresh:775, SpringApplication (org.springframework.boot)
refreshContext:397, SpringApplication (org.springframework.boot)
run:316, SpringApplication (org.springframework.boot)
run:1260, SpringApplication (org.springframework.boot)
run:1248, SpringApplication (org.springframework.boot)
main:12, EurekaRegistryApplication (com.microptech.eurekaregistry)

Spring实例WebMvcConfigurationSupport

以WebMVC比较核心的ServletContext类为例

WebMvcConfigurationSupport
通过继承ApplicationContextAware和ServletContextAware 接口
可以通过ApplicationContext，ServletContext实现获取spring配置文件，设置异常处理器等功能

public class WebMvcConfigurationSupport implements ApplicationContextAware, ServletContextAware {
	@Override
	public void setApplicationContext(@Nullable ApplicationContext applicationContext) {
		this.applicationContext = applicationContext;
	}
	@Override
	public void setServletContext(@Nullable ServletContext servletContext) {
		this.servletContext = servletContext;
	}
}