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

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

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

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);
    }
}

Hint

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

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