Constructor injection (preferred) in Spring Boot - Mini Project: Build & Apply

Constructor Injection in Spring Boot

📖 Scenario: You are building a simple Spring Boot application that manages messages. You want to use constructor injection to provide a service to a controller. Constructor injection is the preferred way to inject dependencies in Spring Boot because it makes your code easier to test and more reliable.

🎯 Goal: Create a Spring Boot controller that uses constructor injection to get a message service. The controller will return a greeting message from the service.

📋 What You'll Learn

Create a service class called MessageService with a method getMessage() that returns a string.

Create a controller class called MessageController that uses constructor injection to get the MessageService.

The controller should have a method greet() that returns the message from the service.

Use Spring annotations @Service and @RestController appropriately.

💡 Why This Matters

🌍 Real World

Constructor injection is widely used in Spring Boot applications to manage dependencies cleanly and make components easier to test and maintain.

💼 Career

Understanding constructor injection is essential for Java developers working with Spring Boot, as it is the preferred way to inject dependencies in professional projects.

1

Create the MessageService class

Create a class called MessageService annotated with @Service. Inside it, create a method called getMessage() that returns the string "Hello from MessageService!".

Spring Boot

@Service
public class MessageService {
    // Your code here
}

Need a hint?

Use @Service above the class. Define getMessage() to return the exact string.

2

Create the MessageController class with constructor

Create a class called MessageController annotated with @RestController. Add a private final field of type MessageService called messageService. Create a public constructor that takes a MessageService parameter called messageService and assigns it to the field.

Spring Boot

@RestController
public class MessageController {
    private final MessageService messageService;

    // Your code here
}

Need a hint?

Use @RestController above the class. Declare the field as private final. Create a constructor that assigns the parameter to the field.

3

Add the greet() method to MessageController

Inside MessageController, add a public method called greet() that returns a String. This method should return the result of calling getMessage() on the messageService field.

Spring Boot

@Service
public class MessageService {
    public String getMessage() {
        return "Hello from MessageService!";
    }
}

@RestController
public class MessageController {
    private final MessageService messageService;

    public MessageController(MessageService messageService) {
        this.messageService = messageService;
    }

    // Add greet() method here
}

Need a hint?

Define greet() to call getMessage() on the injected service and return its result.

4

Add @GetMapping to greet() method

Add the annotation @GetMapping("/greet") above the greet() method in MessageController so it handles HTTP GET requests at the path /greet.

Spring Boot

@Service
public class MessageService {
    public String getMessage() {
        return "Hello from MessageService!";
    }
}

@RestController
public class MessageController {
    private final MessageService messageService;

    public MessageController(MessageService messageService) {
        this.messageService = messageService;
    }

    // Add @GetMapping annotation here
    public String greet() {
        return messageService.getMessage();
    }
}

Need a hint?

Use @GetMapping("/greet") just above the greet() method.