Spring钩子接口Aware

2020-04-08 16:09:52来源：博客园阅读 ()

Spring钩子接口Aware

前言

我们在编写代码的时候，有的时候想要使用Spring的底层组件，类似于 ApplicationContext, BeanFactory等等

那我们实现Spring提供的钩子方法xxxAware。在创建对象的时候，会调用接口规定的方法注入相关的组件。

Aware接口

 1 /**
 2  * A marker superinterface indicating that a bean is eligible to be notified by the
 3  * Spring container of a particular framework object through a callback-style method.
 4  * The actual method signature is determined by individual subinterfaces but should
 5  * typically consist of just one void-returning method that accepts a single argument.
 6  *
 7  * <p>Note that merely implementing {@link Aware} provides no default functionality.
 8  * Rather, processing must be done explicitly, for example in a
 9  * {@link org.springframework.beans.factory.config.BeanPostProcessor}.
10  * Refer to {@link org.springframework.context.support.ApplicationContextAwareProcessor}
11  * for an example of processing specific {@code *Aware} interface callbacks.
12  *
13  * @author Chris Beams
14  * @author Juergen Hoeller
15  * @since 3.1
16  */
17 public interface Aware {}

一个标记的超级接口，表明这个bean会被spring容器注意到，通过一个回调的风格。这个我也称之为钩子方法。

我们可以看到如下图片，实现了Aware的接口的接口特别多，他们都是Spring自带的组件，我们通过Aware可以很方便的使用他们。

在此列举几个比较重要的接口，都是我经常用到的。

Aware子接口	描述
BeanNameAware	获取容器中 Bean 的名称
BeanFactoryAware	获取当前 BeanFactory ，这样可以调用容器的服务
ApplicationContextAware	注入IOC容器的，可以使用容器绝大部分功能
MessageSourceAware	获取 Message Source 相关文本信息
EmbeddedValueResolverAware	值解析器，比如{} #{}等等
EnvironmentAware	环境解析器，可以拿properties的时候挺好用的

ApplicationContextAware接口

 1 public interface ApplicationContextAware extends Aware {
 2 
 3     /**
 4      * Set the ApplicationContext that this object runs in.
 5      * Normally this call will be used to initialize the object.
 6      * <p>Invoked after population of normal bean properties but before an init callback such
 7      * as {@link org.springframework.beans.factory.InitializingBean#afterPropertiesSet()}
 8      * or a custom init-method. Invoked after {@link ResourceLoaderAware#setResourceLoader},
 9      * {@link ApplicationEventPublisherAware#setApplicationEventPublisher} and
10      * {@link MessageSourceAware}, if applicable.
11      * @param applicationContext the ApplicationContext object to be used by this object
12      * @throws ApplicationContextException in case of context initialization errors
13      * @throws BeansException if thrown by application context methods
14      * @see org.springframework.beans.factory.BeanInitializationException
15      */
16     void setApplicationContext(ApplicationContext applicationContext) throws BeansException;
17 
18 }

这个可以获取Spring的IOC容器。拿到这个IOC容器，你可以拥有Spring的绝大多数功能。

我们以ApplicationContextAware为例。直接继承XxxAware接口，就可以拿到他的相应字段了。

 1 @Component
 2 @Slf4j
 3 public class Person implements ApplicationContextAware, BeanFactoryAware,
 4         BeanNameAware, EnvironmentAware, EmbeddedValueResolverAware {
 5 
 6     @Override
 7     public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
 8         log.info("applicationContext = {}", applicationContext.getBean("person"));
 9     }
10 
11     @Override
12     public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
13         log.info("beanFactory = {}", beanFactory.containsBean("person"));
14     }
15 
16     @Override
17     public void setBeanName(String name) {
18         log.info("name = {}", name);
19     }
20 
21     @Override
22     public void setEmbeddedValueResolver(StringValueResolver resolver) {
23         // 字符解析器
24         log.info("val = {}", resolver.resolveStringValue("#{5 * 10}"));
25     }
26 
27     @Override
28     public void setEnvironment(Environment environment) {
29         // 这个可以获取环境变量的值
30         log.info("app = {}", environment.getProperty("app.secret"));
31     }
32 }

结果：

1 2020-04-04 21:35:39.474 [main] [] INFO  - [com.gdufe.osc.controller.Person java:37] [name = person]
2 2020-04-04 21:35:39.475 [main] [] INFO  - [com.gdufe.osc.controller.Person java:32] [beanFactory = true]
3 2020-04-04 21:35:39.475 [main] [] INFO  - [com.gdufe.osc.controller.Person java:48] [app = wenbochang888]
4 2020-04-04 21:35:39.491 [main] [] INFO  - [com.gdufe.osc.controller.Person java:42] [val = 50]
5 2020-04-04 21:35:39.493 [main] [] INFO  - [com.gdufe.osc.controller.Person java:27] [applicationContext = com.gdufe.osc.controller.Person@24386839]

Aware接口底层实现原理

我们看下如下的代码

 1 class ApplicationContextAwareProcessor implements BeanPostProcessor {
 2 
 3     @Override
 4     @Nullable
 5     public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {
 6         
 7         .....
 8         invokeAwareInterfaces(bean);
 9         .....
10 
11         return bean;
12     }
13 
14     private void invokeAwareInterfaces(Object bean) {
15         if (bean instanceof Aware) {
16             if (bean instanceof EnvironmentAware) {
17                 ((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());
18             }
19             if (bean instanceof EmbeddedValueResolverAware) {
20                 ((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);
21             }
22             if (bean instanceof ResourceLoaderAware) {
23                 ((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);
24             }
25             if (bean instanceof ApplicationEventPublisherAware) {
26                 ((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);
27             }
28             if (bean instanceof MessageSourceAware) {
29                 ((MessageSourceAware) bean).setMessageSource(this.applicationContext);
30             }
31             if (bean instanceof ApplicationContextAware) {
32                 ((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
33             }
34         }
35     }
36 }