什么是 Spring Aware？一篇文章快速上手并搞懂实现原理

逃离我推掉我的手 2024-04-29 03:15 34阅读 0赞

### 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

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`@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。

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`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：

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`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：

![image-20240214163346907][]

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

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`/* 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：

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`/* 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; }`

[image-20240214163346907]: https://img-blog.csdnimg.cn/img_convert/42b7d6bbab3959f2646e4f1e3ae86a40.webp?x-oss-process=image/format,png