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
@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
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
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
/*
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
/*
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。