Event publishing with ApplicationEventPublisher in Spring Boot

Introduction

Event publishing lets parts of your app talk to each other without being tightly connected. It helps keep your code clean and organized.

When you want to notify other parts of your app about something that happened, like a user signing up.

When you want to separate concerns, so one part handles an action and another part reacts to it.

When you want to trigger background tasks after a main action, like sending an email after order placement.

When you want to build a flexible system where new features can listen to events without changing existing code.

Syntax

Spring Boot

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

@Component
public class MyEventPublisher {
    private final ApplicationEventPublisher eventPublisher;

    public MyEventPublisher(ApplicationEventPublisher eventPublisher) {
        this.eventPublisher = eventPublisher;
    }

    public void publish() {
        MyEvent event = new MyEvent(this);
        eventPublisher.publishEvent(event);
    }
}

The event class extends ApplicationEvent and usually has a constructor with a source object.

The publisher class uses ApplicationEventPublisher to send events.

Examples

This event carries extra data (username) about the event.

Spring Boot

public class UserRegisteredEvent extends ApplicationEvent {
    private final String username;

    public UserRegisteredEvent(Object source, String username) {
        super(source);
        this.username = username;
    }

    public String getUsername() {
        return username;
    }
}

Publisher sends the event with extra data.

Spring Boot

@Component
public class UserEventPublisher {
    private final ApplicationEventPublisher eventPublisher;

    public UserEventPublisher(ApplicationEventPublisher eventPublisher) {
        this.eventPublisher = eventPublisher;
    }

    public void publishUserRegistered(String username) {
        UserRegisteredEvent event = new UserRegisteredEvent(this, username);
        eventPublisher.publishEvent(event);
    }
}

Sample Program

This Spring Boot app publishes a user registered event and listens to it. When you run it, it prints messages before and after publishing the event.

Spring Boot

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ApplicationEvent;
import org.springframework.context.ApplicationEventPublisher;
import org.springframework.context.event.EventListener;
import org.springframework.stereotype.Component;

@SpringBootApplication
public class EventDemoApplication {
    public static void main(String[] args) {
        var context = SpringApplication.run(EventDemoApplication.class, args);
        UserEventPublisher publisher = context.getBean(UserEventPublisher.class);
        System.out.println("Publishing user registered event...");
        publisher.publishUserRegistered("alice");
    }
}

class UserRegisteredEvent extends ApplicationEvent {
    private final String username;

    public UserRegisteredEvent(Object source, String username) {
        super(source);
        this.username = username;
    }

    public String getUsername() {
        return username;
    }
}

@Component
class UserEventPublisher {
    private final ApplicationEventPublisher eventPublisher;

    public UserEventPublisher(ApplicationEventPublisher eventPublisher) {
        this.eventPublisher = eventPublisher;
    }

    public void publishUserRegistered(String username) {
        UserRegisteredEvent event = new UserRegisteredEvent(this, username);
        eventPublisher.publishEvent(event);
    }
}

@Component
class UserEventListener {
    @EventListener
    public void handleUserRegistered(UserRegisteredEvent event) {
        System.out.println("User registered event received for user: " + event.getUsername());
    }
}

Important Notes

Publishing an event is fast and usually runs in the same thread by default.

Listeners can be synchronous or asynchronous if configured.

Common mistake: forgetting to annotate listener methods with @EventListener.

Summary

Use ApplicationEventPublisher to send events inside your app.

Create event classes by extending ApplicationEvent.

Listeners react to events using @EventListener annotation.

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