Event-driven architecture pattern in Spring Boot - Mini Project: Build & Apply

Event-driven architecture pattern with Spring Boot

📖 Scenario: You are building a simple Spring Boot application that uses event-driven architecture to notify when a new user registers.This pattern helps different parts of your app communicate by sending and listening to events, like sending a welcome message when a user signs up.

🎯 Goal: Create a Spring Boot app that defines a user registration event, publishes it when a user registers, and listens to that event to print a welcome message.

📋 What You'll Learn

Create a UserRegisteredEvent class extending ApplicationEvent

Create a UserService class that publishes UserRegisteredEvent

Create a UserRegisteredListener class that listens for UserRegisteredEvent

Trigger the event when a user registers

💡 Why This Matters

🌍 Real World

Event-driven architecture is used in modern applications to decouple components and improve scalability by reacting to events like user actions or system changes.

💼 Career

Understanding event-driven patterns is important for backend developers working with Spring Boot to build responsive and maintainable applications.

1

Create the UserRegisteredEvent class

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

Spring Boot

package com.example.demo.event;

// Your code here

Need a hint?

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

2

Create the UserService to publish events

Create a class called UserService annotated with @Service. Inject ApplicationEventPublisher with a private final field and constructor. Add a method registerUser(String username) that publishes a new UserRegisteredEvent with this as source and the username.

Spring Boot

package com.example.demo.service;

// Your code here

Need a hint?

Use constructor injection for ApplicationEventPublisher and call publishEvent inside registerUser.

3

Create the UserRegisteredListener to handle events

Create a class called UserRegisteredListener annotated with @Component. Add a method handleUserRegistered annotated with @EventListener that takes a UserRegisteredEvent parameter. Inside the method, print a welcome message using event.getUsername().

Spring Boot

package com.example.demo.listener;

// Your code here

Need a hint?

Use @EventListener on the method that takes UserRegisteredEvent and prints the welcome message.

4

Trigger the event in the main application

In the main Spring Boot application class, inject UserService with @Autowired. In the run method of CommandLineRunner, call registerUser("Alice") on the injected UserService to trigger the event.

Spring Boot

package com.example.demo;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import com.example.demo.service.UserService;

@SpringBootApplication
public class DemoApplication implements CommandLineRunner {

    @Autowired
    private UserService userService;

    @Override
    public void run(String... args) throws Exception {
        // Your code here
    }

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

Need a hint?

Use CommandLineRunner to run code after the app starts and call registerUser with "Alice".