Dependency Injection in Spring Boot

Dependency Injection (DI) is a fundamental concept in Spring Boot that helps achieve loose coupling and increased testability. This guide demonstrates how to use DI in Spring Boot with practical examples in Java and Kotlin.

🌟 Why Use Dependency Injection?

Dependency Injection allows developers to manage and inject dependencies into classes without manually instantiating them. This leads to:

• Better code modularity
• Simplified testing
• Easier maintenance

🌟 Prerequisites

📋 Ensure you have the following:

• ☕ Java Development Kit (JDK) 17+
• 📦 Maven or Gradle installed
• 🔤 A Java IDE (e.g., IntelliJ IDEA, Eclipse)

🛠️ Step 1: Add Dependencies

Include the necessary Spring Boot dependencies:

• Maven:

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter</artifactId>
</dependency>

• Gradle:

implementation 'org.springframework.boot:spring-boot-starter'

📋 Step 2: Create a Service

Define a simple service to demonstrate DI.

• Java
• Kotlin

package com.example.demo.service;

import org.springframework.stereotype.Service;

@Service
public class UserService {

    public String getUserById(String id) {
        return "User with ID: " + id;
    }

    public String getAllUsers() {
        return "Fetching all users.";
    }
}

package com.example.demo.service

import org.springframework.stereotype.Service

@Service
class UserService {

    fun getUserById(id: String): String {
        return "User with ID: $id"
    }

    fun getAllUsers(): String {
        return "Fetching all users."
    }
}

📖 Step 3: Inject Dependencies

Use annotations to inject the service into other components.

• Java
• Kotlin

package com.example.demo.controller;

import com.example.demo.service.UserService;
import lombok.RequiredArgsConstructor;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping("/api/users")
@RequiredArgsConstructor
public class UserController {

    private final UserService userService;

    @GetMapping("/{id}")
    public String getUserById(@PathVariable String id) {
        return userService.getUserById(id);
    }

    @GetMapping
    public String getAllUsers() {
        return userService.getAllUsers();
    }
}

package com.example.demo.controller

import com.example.demo.service.UserService
import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.PathVariable
import org.springframework.web.bind.annotation.RequestMapping
import org.springframework.web.bind.annotation.RestController

@RestController
@RequestMapping("/api/users")
class UserController(
    private val userService: UserService
) {

    @GetMapping("/{id}")
    fun getUserById(@PathVariable id: String): String = userService.getUserById(id)

    @GetMapping
    fun getAllUsers(): String = userService.getAllUsers()
}

▶️ Running the Application

Run the application using the following commands:

• Spring Boot (Java/Kotlin):

  ./mvnw spring-boot:run
  

Access the API at http://localhost:8080/api/users.

🧪 Test the API

You can test the API using the following cURL commands:

• Fetch all users:

curl -X GET http://localhost:8080/api/users

• Fetch a user by ID:

curl -X GET http://localhost:8080/api/users/1

This guide demonstrates how to implement Dependency Injection in Spring Boot with Java and Kotlin, showcasing service injection and REST API integration.