摘要：spring boot提供了一系列的监听器，方便我们开发人员使用和扩展。

本文咱们详细讲解一下spring boot中的监听器。

spring boot中支持的事件类型定在org.springframework.boot.context.event包中，目前支持的事件类型有如下6种：

1. ApplicationFailedEvent

2. ApplicationPreparedEvent

3. ApplicationReadyEvent

4. ApplicationStartedEvent

5. SpringApplicationEvent

6. ApplicationEnvironmentPreparedEvent

监听器的使用

第一：首先定义一个自己使用的监听器类并实现ApplicationListener接口。

第二：通过SpringApplication类中的addListeners方法将自定义的监听器注册进去。

ApplicationFailedEvent

ApplicationFailedEvent：该事件为spring boot启动失败时的操作

/**
* spring boot 启动的时候出现异常事件
* @author www.shareniu.com
*
*/
public class ShareniuApplicationFailedEventListener implements ApplicationListener<ApplicationFailedEvent> {
@Override
public void onApplicationEvent(ApplicationFailedEvent event) {
System.out.println("--------------:ShareniuApplicationFailedEventListener");
Throwable exception = event.getException();
System.out.println(exception);
}
}

可以通过ApplicationFailedEvent 获取Throwable实例对象获取异常信息并处理。

ApplicationPreparedEvent

ApplicationPreparedEvent：上下文准备事件。

上下文context已经准备完毕 ，可以通过ApplicationPreparedEvent获取到ConfigurableApplicationContext实例对象。ConfigurableApplicationContext类继承ApplicationContext类，但需要注意这个时候spring容器中的bean还没有被完全的加载，因此如果通过ConfigurableApplicationContext获取bean会报错的。比如报错：

Exception in thread "main" java.lang.IllegalStateException: org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext@69b0fd6f has not been refreshed yet

获取到上下文之后，可以将其注入到其他类中，毕竟ConfigurableApplicationContext为引用类型

public class ShareniuApplicationPreparedEventListener implements ApplicationListener<ApplicationPreparedEvent> {
@Override
public void onApplicationEvent(ApplicationPreparedEvent event) {
System.out.println("###############"+"ShareniuApplicationPreparedEventListener");
ConfigurableApplicationContext applicationContext = event.getApplicationContext();
//如果执行下面代码则报错
//ShareniuDemo shareniuDemo = applicationContext.getBean(ShareniuDemo.class);
//System.out.println(shareniuDemo);
}
}

ApplicationReadyEvent

ApplicationReadyEvent：上下文已经准备ok。

这个时候就可以通过ApplicationReadyEvent获取ConfigurableApplicationContext，然后通过ConfigurableApplicationContext 获取bean的信息。

public class ShareniuApplicationReadyEventListener implements ApplicationListener<ApplicationReadyEvent> {
@Override
public void onApplicationEvent(ApplicationReadyEvent event) {
System.out.println("--------------------:ShareniuApplicationReadyEventListener");
ConfigurableApplicationContext applicationContext = event.getApplicationContext();
//ShareniuDemo可以根基自身情况进行测试
ShareniuDemo shareniuDemo = applicationContext.getBean(ShareniuDemo.class);
}
}

ApplicationStartedEvent

ApplicationStartedEvent：spring boot 启动监听类。

可以在SpringApplication启动之前做一些手脚，比如修改SpringApplication实例对象中的属性值

public class ShareniuApplicationStartedEventListener  implements ApplicationListener<ApplicationStartedEvent>{
@Override
public void onApplicationEvent(ApplicationStartedEvent event) {
SpringApplication springApplication = event.getSpringApplication();
springApplication.setShowBanner(false);
System.out.println("##############################ShareniuApplicationStartedEventListener");
}
}

SpringApplicationEvent

SpringApplicationEvent：获取SpringApplication

public class ShareniuSpringApplicationEventListener implements ApplicationListener<SpringApplicationEvent> {
@Override
public void onApplicationEvent(SpringApplicationEvent event) {
System.out.println("-----------------------:ShareniuSpringApplicationEventListener");
SpringApplication springApplication = event.getSpringApplication();
System.out.println("###############"+springApplication);
}
}

ApplicationEnvironmentPreparedEvent

ApplicationEnvironmentPreparedEvent：环境事先准备，spring boot中的环境已经准备ok

可以通过ApplicationEnvironmentPreparedEvent获取到SpringApplication、ConfigurableEnvironment等等信息， 可以通过ConfigurableEnvironment实例对象来修改以及获取默认的环境信息。

public class ShasreniuApplicationEnvironmentPreparedEventListener  implements ApplicationListener<ApplicationEnvironmentPreparedEvent>{
@Override
public void onApplicationEvent(ApplicationEnvironmentPreparedEvent event) {
System.out.println("###############"+"ShasreniuApplicationEnvironmentPreparedEventListener");
SpringApplication springApplication = event.getSpringApplication();
ConfigurableEnvironment environment = event.getEnvironment();
long timestamp = event.getTimestamp();
Object source = event.getSource();
System.out.println("########################"+springApplication);
System.out.println("########################"+environment);
System.out.println("########################"+timestamp);
System.out.println("########################"+source);

MutablePropertySources propertySources = environment.getPropertySources();
if (propertySources!=null) {
Iterator<PropertySource<?>> iterator = propertySources.iterator();
while (iterator.hasNext()) {
PropertySource<?> propertySource = (PropertySource<?>) iterator.next();

System.out.println("##############:propertySource"+propertySource);
}
}
}
}

监听器注册

@RestController
@SpringBootApplication()
public class Application {
@RequestMapping("/")
String index() {
return "xxxxxxxxxxxxx";
}
public static void main(String[] args) {
SpringApplication springApplication = new SpringApplication(Application.class);
springApplication.addListeners(new ShareniuApplicationStartedEventListener());
springApplication.addListeners(new ShasreniuApplicationEnvironmentPreparedEventListener());
springApplication.addListeners(new ShareniuApplicationPreparedEventListener());
springApplication.addListeners(new ShareniuApplicationFailedEventListener());
springApplication.addListeners(new ShareniuApplicationReadyEventListener());
springApplication.addListeners(new ShareniuSpringApplicationEventListener());
springApplication.run(args);
}
}

建议

实际项目开发中，可以根据自身需要添加不同的监听器。随后的文章，咱们一起分析监听器的内部实现原理。

欢迎关注微信公众号，您的肯定是对我最大的支持