Spring Boot应用事件监听

1. Spring Boot特有的应用事件

除了Spring框架的事件，Spring Boot的 SpringApplication 也发送了一些自己的事件：

1. ApplicationStartingEvent :在任何处理(除了注册 listener 和 initializer )开始之前发送。
2. ApplicationEnvironmentPreparedEvent : 在 context 创建之前，而用到 context 中的 Environment 已经被识别时发送。
3. ApplicationContextInitializedEvent :  SpringApplication 正在启动， ApplicationContext 已准备好且 ApplicationContextInitializer 已被调用但是bean的定义还没有被加载时发送。
4. ApplicationPreparedEvent : 在 context 刷新之前，在bean的定义已经被加载之后调用。
5. ApplicationStartedEvent : 在任何应用和 command-line runner 调用之前，而 context 已经被刷新时发送。
6. ApplicationReadyEvent : 在任何应用和 command-line runner 被调用的时候发送，它意味着应用可以接受请求了。
7. ApplicationFailedEvent : 在启动时有异常的时候发送。

有些事件是在 ApplicationContext 创建之前触发的，所以我们不能用常规的注册成bean的事件监听方式：

@EventListener
ApplicationListener<Event>

像 ApplicationStartedEvent 和 ApplicationReadyEvent 是 ApplicationContext 创建之后触发的，可以用上述两种方式来监听事件。

2. 如何监听这些事件

我们可以通过下面的方式注册监听：

2.1.  SpringApplication.addListeners(...)

SpringApplication application = new SpringApplication(StartEventsApplication.class);
application.addListeners(
        (ApplicationListener<ApplicationStartingEvent>) event -> log.info("----------- 监听Spring Boot:" + event.getClass().getSimpleName()),
        (ApplicationListener<ApplicationEnvironmentPreparedEvent>) event -> log.info("----------- 监听Spring Boot:" + event.getClass().getSimpleName()),
        (ApplicationListener<ApplicationContextInitializedEvent>) event -> log.info("----------- 监听Spring Boot:" + event.getClass().getSimpleName()),
        (ApplicationListener<ApplicationPreparedEvent>) event -> log.info("----------- 监听Spring Boot:" + event.getClass().getSimpleName()),
        (ApplicationListener<ApplicationStartedEvent>) event -> log.info("----------- 监听Spring Boot:" + event.getClass().getSimpleName()),
        (ApplicationListener<ApplicationReadyEvent>) event -> log.info("----------- 监听Spring Boot:" + event.getClass().getSimpleName())
);
application.run(args);

2.2.  SpringApplicationBuilder.listeners(...)

src/main/resources/META-INF/spring.factories 下：

new SpringApplicationBuilder()
            .sources(StartEventsApplication.class)
            .listeners(
                    (ApplicationListener<ApplicationStartingEvent>) event -> log.info("----------- 监听Spring Boot:" + event.getClass().getSimpleName()),
                    (ApplicationListener<ApplicationEnvironmentPreparedEvent>) event -> log.info("----------- 监听Spring Boot:" + event.getClass().getSimpleName()),
                    (ApplicationListener<ApplicationContextInitializedEvent>) event -> log.info("----------- 监听Spring Boot:" + event.getClass().getSimpleName()),
                    (ApplicationListener<ApplicationPreparedEvent>) event -> log.info("----------- 监听Spring Boot:" + event.getClass().getSimpleName()),
                    (ApplicationListener<ApplicationStartedEvent>) event -> log.info("----------- 监听Spring Boot:" + event.getClass().getSimpleName()),
                    (ApplicationListener<ApplicationReadyEvent>) event -> log.info("----------- 监听Spring Boot:" + event.getClass().getSimpleName())
                    )
            .run(args);

2.3.  META-INF/spring.factories

org.springframework.context.ApplicationListener=top.wisely.startevents.listeners.ApplicationContextInitializedEventListener, \
                                                top.wisely.startevents.listeners.ApplicationEnvironmentPreparedEventListener, \
                                                top.wisely.startevents.listeners.ApplicationPreparedEventListener, \
                                                top.wisely.startevents.listeners.ApplicationReadyEventListener, \
                                                top.wisely.startevents.listeners.ApplicationStartedEventListener, \
                                                top.wisely.startevents.listeners.ApplicationStartingEventListener

监听器只需实现 ApplicationListener<要监听的接口类型> 接口，无需手动注册为bean:

public class ApplicationStartedEventListener implements ApplicationListener<ApplicationStartedEvent> {
    @Override
    public void onApplicationEvent(ApplicationStartedEvent event) {
        log.info("----------- 监听Spring Boot:" + event.getClass().getSimpleName());
    }
}