什么是 Spring Aware？一篇文章快速上手并搞懂实现原理

什么是 Aware 接口

源码对Aware接口的注释是这样的：

A marker superinterface indicating that a bean is eligible to be notified by the Spring container of a particular framework object through a callback-style method. The actual method signature is determined by individual subinterfaces but should typically consist of just one void-returning method that accepts a single argument.

Aware 接口是「标记接口」，实现了 Aware 接口的 Bean，能够被 Spring 通过回调的方式，获取到一些与 Spring 容器相关的信息，比如 ApplicationContext，BeanFactory 等等。

说白了，想要让一个类获取 Spring 容器相关的信息，或者说持有 Spring 信息比如 ApplicationContext，这时候就得实现 Aware 接口了。

Aware 快速开始

这里以BeanFactoryAware为例：

java 复制代码

 @Component
 public class TestAware implements BeanFactoryAware {
 
     private BeanFactory beanFactory;
 
     @Override
     public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
         this.beanFactory = beanFactory;
         System.out.println("Aware 接口调用 获取到 BeanFactory");
     }
 
     public Object get(String name){
         return beanFactory.getBean(name);
     }
 }

实现对应的 Aware 接口，就可以拿到对应的实例对象。

Aware 的实现原理

对于 Aware 接口，它本质是 A marker superinterface，是一个标记接口，熟悉 BeanPostProcessor 的同学们就有话要说了。

下面是 BeanPostProcessor 的 Javadoc：

Typically, post-processors that populate beans via marker interfaces or the like will implement postProcessBeforeInitialization

关注这几个词，populate beans via marker interfaces，是 通过标记接口填充bean 的意思，这不就是 Aware 接口吗。但根据 Aware 接口提供的信息不同，实现方式也会有所不同。

开始读源码

在 initializeBean 方法中，首先执行 invokeAwareMethods 方法。

java 复制代码

     protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
 
         // 1. Aware
         invokeAwareMethods(beanName, bean);
 
         // 2. BeanPostProcessorsBeforeInitialization
         Object wrappedBean = bean;
         if (mbd == null || !mbd.isSynthetic()) {
             wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
         }
 
         // 3. InitMethods
             invokeInitMethods(beanName, wrappedBean, mbd);
         // 4. BeanPostProcessorsAfterInitialization
         if (mbd == null || !mbd.isSynthetic()) {
             wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
         }
 
         return wrappedBean;
     }

invokeAwareMethods 会处理下面这几个 Aware 接口：

java 复制代码

 private void invokeAwareMethods(String beanName, Object bean) {
    if (bean instanceof Aware) {
       if (bean instanceof BeanNameAware) {
          ((BeanNameAware) bean).setBeanName(beanName);
       }
       if (bean instanceof BeanClassLoaderAware) {
          ClassLoader bcl = getBeanClassLoader();
          if (bcl != null) {
             ((BeanClassLoaderAware) bean).setBeanClassLoader(bcl);
          }
       }
       if (bean instanceof BeanFactoryAware) {
           // 回调 setBeanFactory 
          ((BeanFactoryAware) bean).setBeanFactory(AbstractAutowireCapableBeanFactory.this);
       }
    }
 }

而在 applyBeanPostProcessorsBeforeInitialization 方法中，有这么几个 BeanPostProcessors：

ApplicationContextAwareProcessor 长这样，这个类提供的是 embeddedValueResolver 和 ApplicationContext 相关信息：

java 复制代码

 /*
 BeanPostProcessor implementation that supplies the ApplicationContext, Environment, or StringValueResolver for the ApplicationContext to beans that implement the EnvironmentAware, EmbeddedValueResolverAware, ResourceLoaderAware, ApplicationEventPublisherAware, MessageSourceAware, and/or ApplicationContextAware interfaces.
 */ 
 class ApplicationContextAwareProcessor implements BeanPostProcessor {
 
    private final ConfigurableApplicationContext applicationContext;
     
    @Override
    @Nullable
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
       // 不需要回调的情况
       if (!(... || bean instanceof ApplicationStartupAware)) {
          return bean;
       }
       // 回调
       invokeAwareInterfaces(bean);
        
       return bean;
    }
     
    // 回调方法 
    private void invokeAwareInterfaces(Object bean) {
         // ... 
       if (bean instanceof ApplicationContextAware) {
          ((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
       }
    }
 
 }

而 WebApplicationContextServletContextAwareProcessor 提供的是 WebApplicationContextServletContext 相关信息，它的 Before 处理是这样的：

java 复制代码

 /*
 Can be used when registering the processor can occur before the ServletContext or ServletConfig have been initialized.
 */
 @Override
 public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
    if (getServletContext() != null && bean instanceof ServletContextAware) {
       ((ServletContextAware) bean).setServletContext(getServletContext());
    }
    if (getServletConfig() != null && bean instanceof ServletConfigAware) {
       ((ServletConfigAware) bean).setServletConfig(getServletConfig());
    }
    return bean;
 }

至此，Aware 接口就基本分析完毕了。最后留有一个疑问是：为什么 invokeAwareMethods 方法处理的 Aware 接口和其它 Aware 接口的处理方式不同？我找到一个 issue 可能和这有关：Support Aware interfaces for ImportBeanDefinitionRegistrar $SPR-9568$ ，感兴趣的可以尝试看一下，也欢迎在评论区留下你的答案。