Master Spring Boot Event Listeners: 4 Ways to Implement Custom Events

Last week a friend was asked about Spring Boot event listeners in an interview, so we dive into the topic.

Spring Boot provides four ways to register event listeners:

Manually add a listener to ApplicationContext.

Load the listener into the Spring container.

Configure the listener in application.properties.

Use the @EventListener annotation.

Custom events are created by extending ApplicationEvent and defining a constructor; custom listeners implement ApplicationListener<T> and override onApplicationEvent.

Method 1

Create a listener class that implements ApplicationListener<MyEvent> and register it manually via ConfigurableApplicationContext.

public class MyListener1 implements ApplicationListener<MyEvent> {
    Logger logger = Logger.getLogger(MyListener1.class);
    public void onApplicationEvent(MyEvent event) {
        logger.info(String.format("%s listened to source: %s.", MyListener1.class.getName(), event.getSource()));
    }
}

In the Spring Boot main class, obtain the context and add the listener:

@SpringBootApplication
public class ListenerApplication {
    public static void main(String[] args) {
        ConfigurableApplicationContext context = SpringApplication.run(ListenerApplication.class, args);
        // register listener
        context.addApplicationListener(new MyListener1());
    }
}

Method 2

Annotate the listener class with @Component so that Spring registers it automatically.

@Component
public class MyListener2 implements ApplicationListener<MyEvent> {
    Logger logger = Logger.getLogger(MyListener2.class);
    public void onApplicationEvent(MyEvent event) {
        logger.info(String.format("%s listened to source: %s.", MyListener2.class.getName(), event.getSource()));
    }
}

Method 3

Define the listener class and configure its fully‑qualified name in application.properties using context.listener.classes.

public class MyListener3 implements ApplicationListener<MyEvent> {
    Logger logger = Logger.getLogger(MyListener3.class);
    public void onApplicationEvent(MyEvent event) {
        logger.info(String.format("%s listened to source: %s.", MyListener3.class.getName(), event.getSource()));
    }
}

context.listener.classes=com.listener.MyListener3

Method 4

Use the @EventListener annotation on a method; the class does not need to implement ApplicationListener.

@Component
public class MyListener4 {
    Logger logger = Logger.getLogger(MyListener4.class);
    @EventListener
    public void listener(MyEvent event) {
        logger.info(String.format("%s listened to source: %s.", MyListener4.class.getName(), event.getSource()));
    }
}

Custom event definition:

@SuppressWarnings("serial")
public class MyEvent extends ApplicationEvent {
    public MyEvent(Object source) {
        super(source);
    }
}

Test by publishing the event in the main class:

@SpringBootApplication
public class ListenerApplication {
    public static void main(String[] args) {
        ConfigurableApplicationContext context = SpringApplication.run(ListenerApplication.class, args);
        // register listener
        context.addApplicationListener(new MyListener1());
        // publish event
        context.publishEvent(new MyEvent("Test event."));
    }
}

Running the application prints ordered log messages from each listener, demonstrating that the four registration methods work and are invoked in order.

Source code: https://github.com/ingorewho/springboot-develope/tree/master/springboot-listener