Event-driven architecture pattern in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to publish an event using Spring Boot's ApplicationEventPublisher.

Spring Boot

applicationEventPublisher.[1](new CustomEvent(this, "Hello"));

Drag options to blanks, or click blank then click option'

ApublishEvent

BsendEvent

CfireEvent

DraiseEvent

Attempts:

3 left

💡 Hint

Common Mistakes

Using methods like sendEvent or fireEvent which do not exist.

In Spring Boot, publishEvent is the method used to send events.

2fill in blank

medium

Complete the code to listen for events using @EventListener annotation.

Spring Boot

@EventListener
public void handleEvent([1] event) {
    System.out.println(event.getMessage());
}

Drag options to blanks, or click blank then click option'

AObject

BCustomEvent

CApplicationEvent

DEvent

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic types like Object or ApplicationEvent which may not match the event.

The method parameter should be the specific event class you want to listen to, here CustomEvent.

3fill in blank

hard

Fix the error in the event class by completing the constructor correctly.

Spring Boot

public class CustomEvent extends ApplicationEvent {
    private final String message;

    public CustomEvent(Object source, String message) {
        super([1]);
        this.message = message;
    }

    public String getMessage() {
        return message;
    }
}

Drag options to blanks, or click blank then click option'

Asource

Bmessage

Cthis

Devent

Attempts:

3 left

💡 Hint

Common Mistakes

Passing message or this instead of source to the superclass constructor.

The superclass constructor requires the source object, so pass source.

4fill in blank

hard

Fill both blanks to create a listener method that listens asynchronously and logs the event message.

Spring Boot

@Async
@EventListener
public void onEvent([1] event) {
    logger.[2]("Received: " + event.getMessage());
}

Drag options to blanks, or click blank then click option'

ACustomEvent

Binfo

Cdebug

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong event type or logging level like debug instead of info.

The listener method should accept CustomEvent and use logger.info to log the message.

5fill in blank

hard

Fill all three blanks to define a Spring Boot event publisher bean and publish an event inside it.

Spring Boot

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

@Component
public class EventPublisher {

    private final ApplicationEventPublisher [1];

    public EventPublisher(ApplicationEventPublisher [2]) {
        this.[3] = [2];
    }

    public void publish(String message) {
        [1].publishEvent(new CustomEvent(this, message));
    }
}

Drag options to blanks, or click blank then click option'

Apublisher

BeventPublisher

CapplicationEventPublisher

DeventPub

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for field and constructor parameter causing confusion.

The standard name used is applicationEventPublisher for clarity and consistency.

Practice

1. What is the main purpose of the event-driven architecture pattern in Spring Boot?

easy

A. To store data in a relational database

B. To allow different parts of the application to communicate by sending and receiving events

C. To create user interfaces with HTML and CSS

D. To handle HTTP requests directly without any middleware

Solution

1. Step 1: Understand event-driven architecture basics

   This pattern lets components send and listen to events to communicate asynchronously.

2. Step 2: Identify Spring Boot usage

   Spring Boot uses Java classes and annotations to implement this pattern for flexible communication.

3. Final Answer:

   To allow different parts of the application to communicate by sending and receiving events -> Option B

4. Quick Check:

   Event-driven communication = To allow different parts of the application to communicate by sending and receiving events [OK]

Hint: Event-driven means parts talk by sending/listening events [OK]

Common Mistakes:

• Confusing event-driven with database storage
• Thinking it handles UI design
• Assuming it processes HTTP requests directly

2. Which annotation is used in Spring Boot to mark a method as an event listener?

easy

A. @EventHandler

B. @Listener

C. @HandleEvent

D. @EventListener

Solution

1. Step 1: Recall Spring Boot event listener annotation

   Spring Boot uses @EventListener to mark methods that listen for events.

2. Step 2: Check other options

   @EventHandler, @Listener, and @HandleEvent are not valid Spring Boot annotations for event listening.

3. Final Answer:

   @EventListener -> Option D

4. Quick Check:

   @EventListener marks event methods [OK]

Hint: Remember: @EventListener listens to events in Spring Boot [OK]

Common Mistakes:

• Using @EventHandler which is not a Spring Boot annotation
• Confusing with @Listener or @HandleEvent which don't exist
• Misspelling the annotation name

3. Given this Spring Boot event publishing code snippet, what will be printed?

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

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

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

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

medium

A. Event received

B. No output

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand event publishing

   The Publisher publishes a MyEvent instance using ApplicationEventPublisher.

2. Step 2: Understand event listening

   The Listener class has a method annotated with @EventListener that listens for MyEvent and prints "Event received".

3. Final Answer:

   Event received -> Option A

4. Quick Check:

   Publishing triggers listener print = Event received [OK]

Hint: Publishing event triggers listener print statement [OK]

Common Mistakes:

• Thinking no output because listener is missing
• Assuming compilation error due to event class
• Expecting runtime exception without error

4. What is wrong with this Spring Boot event listener code?

@Component
public class MyListener {
    @EventListener
    public void handleEvent(String event) {
        System.out.println("Event: " + event);
    }
}

medium

A. EventListener methods cannot accept String as event parameter

B. Missing @Component annotation

C. Method must return a value

D. EventListener annotation is misspelled

Solution

1. Step 1: Check event parameter type

   Spring events can be any object, including String, but using String as event parameter is discouraged because it may cause ambiguity and unintended triggers.

2. Step 2: Understand best practice

   Using custom event classes extending ApplicationEvent or plain objects is preferred for clarity and to avoid conflicts.

3. Final Answer:

   EventListener methods cannot accept String as event parameter -> Option A

4. Quick Check:

   String event param is invalid for event listener [OK]

Hint: EventListener needs custom event class, not String [OK]

Common Mistakes:

• Thinking @Component is missing
• Believing method must return a value
• Assuming annotation is misspelled

5. You want to build a Spring Boot app where multiple components react to a user registration event asynchronously. Which approach best fits the event-driven architecture pattern?

hard

A. Send HTTP requests manually between components

B. Call all components directly from the registration method without events

C. Create a UserRegisteredEvent class, publish it with ApplicationEventPublisher, and have multiple @EventListener methods handle it

D. Use a database trigger to notify components

Solution

1. Step 1: Identify event-driven pattern usage

   Creating a custom event class and publishing it allows decoupled communication between components.

2. Step 2: Understand asynchronous handling

   Multiple @EventListener methods can react independently and asynchronously to the event.

3. Step 3: Evaluate other options

   Direct calls, database triggers, or manual HTTP requests do not follow Spring Boot event-driven best practices.

4. Final Answer:

   Create a UserRegisteredEvent class, publish it with ApplicationEventPublisher, and have multiple @EventListener methods handle it -> Option C

5. Quick Check:

   Custom event + publisher + listeners = Create a UserRegisteredEvent class, publish it with ApplicationEventPublisher, and have multiple @EventListener methods handle it [OK]

Hint: Use custom event class and @EventListener for async reactions [OK]

Common Mistakes:

• Calling components directly, losing decoupling
• Relying on database triggers for app events
• Manually sending HTTP requests between components