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

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

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

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

Hint

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

Practice

1. What is the primary role of ApplicationEventPublisher in a Spring Boot application?

easy

A. To manage HTTP requests and responses

B. To listen and handle events automatically

C. To configure database connections

D. To send or publish events within the application

Solution

1. Step 1: Understand the purpose of ApplicationEventPublisher

   This interface is designed to publish events to listeners within the Spring application context.

2. Step 2: Differentiate from listeners and other components

   Listeners handle events, but ApplicationEventPublisher only sends or publishes them.

3. Final Answer:

   To send or publish events within the application -> Option D

4. Quick Check:

   Event sending = ApplicationEventPublisher [OK]

Hint: Publisher sends events; listeners receive them [OK]

Common Mistakes:

• Confusing publisher with listener
• Thinking it manages HTTP or database
• Assuming it handles event processing

2. Which of the following is the correct way to inject ApplicationEventPublisher in a Spring Boot component?

easy

A. @Autowired private ApplicationEventPublisher publisher;

B. private ApplicationEventPublisher publisher = new ApplicationEventPublisher();

C. @Inject ApplicationEventPublisher publisher = null;

D. ApplicationEventPublisher publisher = ApplicationEventPublisher.getInstance();

Solution

1. Step 1: Recall Spring dependency injection syntax

   Spring uses @Autowired to inject beans automatically into components.

2. Step 2: Check each option for correctness

   @Autowired private ApplicationEventPublisher publisher; uses @Autowired correctly. private ApplicationEventPublisher publisher = new ApplicationEventPublisher(); tries to instantiate interface (not allowed). @Inject ApplicationEventPublisher publisher = null; mixes @Inject with assignment (not typical in Spring). ApplicationEventPublisher publisher = ApplicationEventPublisher.getInstance(); uses a non-existent getInstance method.

3. Final Answer:

   @Autowired private ApplicationEventPublisher publisher; -> Option A

4. Quick Check:

   @Autowired injects beans properly [OK]

Hint: Use @Autowired for Spring bean injection [OK]

Common Mistakes:

• Trying to instantiate ApplicationEventPublisher directly
• Mixing injection annotations incorrectly
• Using static getInstance() which doesn't exist

3. Given the following code snippet, what will be printed when publishEvent() is called?

public class MyEvent extends ApplicationEvent {
    public MyEvent(Object source) { super(source); }
}

@Component
public class MyPublisher {
    @Autowired
    private ApplicationEventPublisher publisher;

    public void publishEvent() {
        publisher.publishEvent(new MyEvent(this));
    }
}

@Component
public class MyListener {
    @EventListener
    public void handleMyEvent(MyEvent event) {
        System.out.println("Event received");
    }
}

medium

A. No output

B. Compilation error

C. Event received

D. Runtime exception

Solution

1. Step 1: Understand event publishing and listening

   The publisher sends a MyEvent instance. The listener listens for MyEvent and prints "Event received".

2. Step 2: Confirm no errors in code

   Code uses correct annotations and event class extending ApplicationEvent, so no errors or exceptions.

3. Final Answer:

   Event received -> Option C

4. Quick Check:

   Listener prints message on event [OK]

Hint: Listener prints message when event is published [OK]

Common Mistakes:

• Forgetting @EventListener annotation
• Not extending ApplicationEvent in event class
• Expecting output without publishing event

4. Identify the error in the following Spring Boot event publishing code:

@Component
public class EventSender {
    private ApplicationEventPublisher publisher;

    public void send() {
        publisher.publishEvent(new CustomEvent(this));
    }
}

public class CustomEvent extends ApplicationEvent {
    public CustomEvent(Object source) { super(source); }
}

medium

A. send method should be static

B. ApplicationEventPublisher is not injected, so publisher is null

C. publishEvent method does not exist

D. CustomEvent does not extend ApplicationEvent

Solution

1. Step 1: Check ApplicationEventPublisher usage

   The publisher field is declared but never injected or assigned, so it remains null.

2. Step 2: Consequence of null publisher

   Calling publishEvent on null causes NullPointerException at runtime.

3. Final Answer:

   ApplicationEventPublisher is not injected, so publisher is null -> Option B

4. Quick Check:

   Missing injection causes null publisher [OK]

Hint: Always inject ApplicationEventPublisher before use [OK]

Common Mistakes:

• Forgetting @Autowired on publisher
• Assuming interface can be instantiated
• Thinking method must be static

5. You want to publish a custom event UserCreatedEvent and ensure multiple listeners react differently. Which approach correctly implements this in Spring Boot?

hard

A. Create UserCreatedEvent extending ApplicationEvent, inject ApplicationEventPublisher, publish event, and annotate multiple methods with @EventListener for UserCreatedEvent

B. Create UserCreatedEvent as a simple POJO, call listeners manually without publishing events

C. Use ApplicationEventPublisher to publish String messages instead of event objects

D. Create UserCreatedEvent but do not extend ApplicationEvent; rely on @EventListener to catch any object

Solution

1. Step 1: Define event class properly

   Custom events should extend ApplicationEvent for Spring to recognize them.

2. Step 2: Publish event and create multiple listeners

   Inject ApplicationEventPublisher to publish event. Use @EventListener on multiple methods to react differently.

3. Step 3: Evaluate other options

   Create UserCreatedEvent as a simple POJO, call listeners manually without publishing events skips event system, losing decoupling. Use ApplicationEventPublisher to publish String messages instead of event objects misuses publisher with Strings. Create UserCreatedEvent but do not extend ApplicationEvent; rely on @EventListener to catch any object breaks event contract, listeners may not work.

4. Final Answer:

   Create UserCreatedEvent extending ApplicationEvent, inject ApplicationEventPublisher, publish event, and annotate multiple methods with @EventListener for UserCreatedEvent -> Option A

5. Quick Check:

   Proper event class + publisher + listeners = correct [OK]

Hint: Extend ApplicationEvent and use @EventListener for multiple reactions [OK]

Common Mistakes:

• Not extending ApplicationEvent for custom events
• Calling listeners manually instead of publishing
• Publishing wrong object types like String