Aware接口的作用

简介

在Spring学习中出现了很多以Aware结尾的类或者接口, 这些类主要是为了辅助自定义组件来访问Spring容器底层的组件, bean属性等.
列如我们想要在程序代码中获取ApplicationContext, BeanFactory, bean的名字等

Aware接口定义:

package org.springframework.beans.factory;
public interface Aware {

}

这是一个空接口定义, 以下是一些常用实现了该接口的Aware组件类:

image.png

常用的Aware:

• BeanNameAware: 能够获取bean的名称
• BeanFactoryAware：获取BeanFactory实例
• ApplicationContextAware：获取ApplicationContext
• MessageSourceAware：获取MessageSource(资源国际化)
• ResourceLoaderAware：获取ResourceLoader(资源加载器)
• EnvironmentAware：获取Environment(运行环境)

使用案例

• 实现对应的xxxAware
• 将自定义的类注入到容器中

@Component
public class AwareTest implements BeanNameAware, ApplicationContextAware  {
    
    private ApplicationContext applicationContext;
    
    @Override
    public void setBeanName(String name) {
        System.out.println("Bean的名称: " + name);        
    }
    
    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        System.out.println("接收到了IOC容器的applicationContext: " + applicationContext);
        this.applicationContext = applicationContext;
    }
    
}

运行容器打印结果:

Bean的名称: awareTest
接收到了IOC容器的applicationContext: org.springframework.context.annotation.AnnotationConfigApplicationContext@2hf3bc2

总结:

把Spring底层的组件可以注入到自定义的bean中, 使用的是BeanPostProcessor原理, 在初始化之前调用set设置对应的值:

org.springframework.context.support.ApplicationContextAwareProcessor#invokeAwareInterfaces

    private void invokeAwareInterfaces(Object bean) {
          //如果bean实现了Aware接口, 调用对应的set进行装配
        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);
            }
        }

    }

由此可见Aware组件功能是依赖BeanPostProcessor后置处理器来处理的.