Event publishing with ApplicationEventPublisher in Spring Boot - Mini Project: Build & Apply

Event publishing with ApplicationEventPublisher

📖 Scenario: You are building a simple Spring Boot application that needs to notify other parts of the app when a user registers. This notification will be done using Spring's event publishing system.

🎯 Goal: Create a Spring Boot application that publishes a custom event called UserRegisteredEvent using ApplicationEventPublisher. The event should carry the username of the registered user.

📋 What You'll Learn

Create a custom event class UserRegisteredEvent that extends ApplicationEvent and holds a username.

Create a Spring service class UserService that has a method registerUser(String username).

Inject ApplicationEventPublisher into UserService and publish UserRegisteredEvent inside registerUser.

Add a simple event listener component that listens for UserRegisteredEvent and prints the username.

💡 Why This Matters

🌍 Real World

Event publishing is useful in real applications to decouple components. For example, when a user registers, you might want to send a welcome email, update analytics, or log the event without tightly coupling these actions.

💼 Career

Understanding Spring's event system is important for building scalable and maintainable backend applications. Many enterprise applications use event-driven designs to improve modularity and responsiveness.

1

Create the custom event class

Create a class called UserRegisteredEvent that extends ApplicationEvent. Add a private final String username field, a constructor that takes Object source and String username, and a getter method getUsername().

Spring Boot

// Your code here

Need a hint?

Remember to extend ApplicationEvent and call super(source) in the constructor.

2

Add ApplicationEventPublisher to UserService

Create a Spring service class called UserService. Inject ApplicationEventPublisher using constructor injection and store it in a private final field called eventPublisher.

Spring Boot

import org.springframework.context.ApplicationEventPublisher;
import org.springframework.stereotype.Service;

// Your code here

Need a hint?

Use @Service annotation and constructor injection for ApplicationEventPublisher.

3

Publish UserRegisteredEvent in registerUser method

Inside UserService, add a public method registerUser(String username). Inside this method, create a new UserRegisteredEvent with this as source and the username. Then publish the event using eventPublisher.publishEvent().

Spring Boot

    // Your code here

Need a hint?

Create the event object and call publishEvent on eventPublisher.

4

Add event listener to handle UserRegisteredEvent

Create a Spring component class called UserRegisteredListener. Add a method handleUserRegisteredEvent(UserRegisteredEvent event) annotated with @EventListener. Inside the method, print the username using System.out.println("User registered: " + event.getUsername()).

Spring Boot

import org.springframework.context.event.EventListener;
import org.springframework.stereotype.Component;

// Your code here

Need a hint?

Use @Component and @EventListener annotations to create the listener.