📢 Spring Application Events: A Complete Guide with Real-world Code Examples

🔍 What Are Spring Events?

Spring Events provide a way for different components of a Spring application to communicate with each other in a loosely coupled way using the Publisher-Listener model. It follows the Observer Design Pattern.

🧠 Why Use Spring Events?

✅ Loose coupling
✅ Asynchronous event handling (if needed)
✅ Clean separation of concerns
✅ Easy to extend & test

📦 Core Concepts

Component	Description
`ApplicationEvent`	The event object (can also be a POJO from Spring 4.2+)
`ApplicationEventPublisher`	Used to publish events
`@EventListener`	Used to listen and act on events

🧱 Use Case Example: User Registration System

Let’s build a real-world example:
When a user registers, send a welcome email and log an audit trail using Spring Events.

1️⃣ Create a Custom Event

// src/main/java/com/example/events/UserRegisteredEvent.java
package com.example.events;

public class UserRegisteredEvent {
    private final String email;
    private final String username;

    public UserRegisteredEvent(String email, String username) {
        this.email = email;
        this.username = username;
    }

    public String getEmail() { return email; }
    public String getUsername() { return username; }
}

2️⃣ Create an Event Publisher

// src/main/java/com/example/service/UserService.java
package com.example.service;

import com.example.events.UserRegisteredEvent;
import org.springframework.context.ApplicationEventPublisher;
import org.springframework.stereotype.Service;

@Service
public class UserService {

    private final ApplicationEventPublisher publisher;

    public UserService(ApplicationEventPublisher publisher) {
        this.publisher = publisher;
    }

    public void registerUser(String email, String username) {
        // Logic to save user to DB (skipped)
        System.out.println("✅ User registered: " + username);

        // Fire event
        publisher.publishEvent(new UserRegisteredEvent(email, username));
    }
}

3️⃣ Create Event Listeners

a. Email Notification Listener

// src/main/java/com/example/listeners/EmailNotificationListener.java
package com.example.listeners;

import com.example.events.UserRegisteredEvent;
import org.springframework.context.event.EventListener;
import org.springframework.stereotype.Component;

@Component
public class EmailNotificationListener {

    @EventListener
    public void handleUserRegistered(UserRegisteredEvent event) {
        System.out.println("📧 Sending welcome email to " + event.getEmail());
        // Logic to send email
    }
}

b. Audit Log Listener

// src/main/java/com/example/listeners/AuditLogListener.java
package com.example.listeners;

import com.example.events.UserRegisteredEvent;
import org.springframework.context.event.EventListener;
import org.springframework.stereotype.Component;

@Component
public class AuditLogListener {

    @EventListener
    public void logUserRegistration(UserRegisteredEvent event) {
        System.out.println("📝 Audit: New user - " + event.getUsername());
        // Write to audit log
    }
}

4️⃣ Make It Asynchronous (Optional)

If you want to asynchronously handle events:

a. Enable Async support

// src/main/java/com/example/DemoApplication.java
@SpringBootApplication
@EnableAsync
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

b. Annotate listener with `@Async`

@Async
@EventListener
public void handleUserRegistered(UserRegisteredEvent event) {
    // async logic
}

🧪 5️⃣ Triggering the Event

Let’s wire this into a controller:

// src/main/java/com/example/controller/UserController.java
package com.example.controller;

import com.example.service.UserService;
import org.springframework.web.bind.annotation.*;

@RestController
@RequestMapping("/api/users")
public class UserController {

    private final UserService userService;

    public UserController(UserService userService) {
        this.userService = userService;
    }

    @PostMapping("/register")
    public String register(@RequestParam String email, @RequestParam String username) {
        userService.registerUser(email, username);
        return "User registration successful!";
    }
}

📌 Summary

Step	Action
✅ 1	Create event class
✅ 2	Publish event from business logic
✅ 3	Create one or more listeners
✅ 4	(Optional) Use `@Async` for async handling

⚙️ Project Structure

src/main/java/com/example
├── controller
│   └── UserController.java
├── events
│   └── UserRegisteredEvent.java
├── listeners
│   ├── AuditLogListener.java
│   └── EmailNotificationListener.java
├── service
│   └── UserService.java
└── DemoApplication.java