In Spring boot application, adding listener via SpringApplication.addListeners() or SpringApplicationBuilder.listeners() allows us to listen to application events even before ApplicationContext is created. In this quick example, we will see how to do that.
Example
public class MyApplicationListener implements ApplicationListener<ApplicationEvent> {
@Override
public void onApplicationEvent(ApplicationEvent event) {
System.out.println("event: " + event);
}
}
The Main class
@SpringBootApplication
public class ExampleMain {
public static void main(String[] args) throws InterruptedException {
SpringApplication sa = new SpringApplication();
sa.addListeners(new MyApplicationListener());
sa.setSources(new HashSet<>(Arrays.asList(ExampleMain.class)));
ConfigurableApplicationContext context = sa.run(args);
MyBean bean = context.getBean(MyBean.class);
bean.doSomething();
}
}
Output
event: org.springframework.boot.context.event.ApplicationStartedEvent[source=org.springframework.boot.SpringApplication@402f32ff] event: org.springframework.boot.context.event.ApplicationEnvironmentPreparedEvent[source=org.springframework.boot.SpringApplication@402f32ff] event: org.springframework.boot.context.event.ApplicationPreparedEvent[source=org.springframework.boot.SpringApplication@402f32ff] 2017-09-07 11:36:24.881 INFO 9556 --- [ main] s.c.a.AnnotationConfigApplicationContext : Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@7e9a5fbe: startup date [Thu Sep 07 11:36:24 CDT 2017]; root of context hierarchy 2017-09-07 11:36:25.274 INFO 9556 --- [ main] o.s.j.e.a.AnnotationMBeanExporter : Registering beans for JMX exposure on startup event: org.springframework.context.event.ContextRefreshedEvent[source=org.springframework.context.annotation.AnnotationConfigApplicationContext@7e9a5fbe: startup date [Thu Sep 07 11:36:24 CDT 2017]; root of context hierarchy] event: org.springframework.boot.context.event.ApplicationReadyEvent[source=org.springframework.boot.SpringApplication@402f32ff] -- in doSomething() -- 2017-09-07 11:36:25.284 INFO 9556 --- [ Thread-2] s.c.a.AnnotationConfigApplicationContext : Closing org.springframework.context.annotation.AnnotationConfigApplicationContext@7e9a5fbe: startup date [Thu Sep 07 11:36:24 CDT 2017]; root of context hierarchy event: org.springframework.context.event.ContextClosedEvent[source=org.springframework.context.annotation.AnnotationConfigApplicationContext@7e9a5fbe: startup date [Thu Sep 07 11:36:24 CDT 2017]; root of context hierarchy] 2017-09-07 11:36:25.285 INFO 9556 --- [ Thread-2] o.s.j.e.a.AnnotationMBeanExporter : Unregistering JMX-exposed beans on shutdown
In above output, we can see some extra boot specific events are fired. Following is a quick description of those events:
ApplicationStartedEvent
Event published as early as conceivably possible as soon as a SpringApplication has been started - before the Environment or ApplicationContext is available, but after the ApplicationListeners have been registered.
ApplicationEnvironmentPreparedEvent
Event published when a SpringApplication is starting up and the Environment is first available for inspection and modification.
ApplicationPreparedEvent
Event published as when a SpringApplication is starting up and the ApplicationContext is fully prepared but not refreshed. The bean definitions will be loaded and the Environment is ready for use at this stage.
ApplicationReadyEvent
This event is published as late as conceivably possible to indicate that the application is ready to service requests.
ApplicationFailedEvent
This event is published, if SpringApplication fails to start.
Example Project
Dependencies and Technologies Used:
Spring Boot 1.5.6.RELEASE Corresponding Spring Version 4.3.10.RELEASE
spring-boot-starter : Core starter, including auto-configuration support, logging and YAML.
