Kotlin with Spring Boot Tutorial: REST APIs, JPA, Hibernate, and JWT Security with Real Backend Projects

This Kotlin with Spring Boot tutorial explains how to build enterprise-grade backend applications using Spring Boot and Kotlin. It covers REST controllers, JPA and Hibernate for database access, and JWT-based security. The chapter follows clean architecture principles and includes real-world backend projects such as user management, e-commerce, and blog APIs.

Kotlin with Spring Boot (Complete Tutorial)

Spring Boot with Kotlin

Spring Boot simplifies backend development by providing auto-configuration and production-ready features.

Project Setup (Gradle Kotlin DSL)


dependencies {
implementation("org.springframework.boot:spring-boot-starter-web")
implementation("org.springframework.boot:spring-boot-starter-data-jpa")
implementation("org.springframework.boot:spring-boot-starter-security")
implementation("io.jsonwebtoken:jjwt-api:0.11.5")
runtimeOnly("io.jsonwebtoken:jjwt-impl:0.11.5")
runtimeOnly("io.jsonwebtoken:jjwt-jackson:0.11.5")
}

Best Practices

  1. Use Kotlin DSL for Gradle
  2. Enable null safety
  3. Follow layered architecture

REST Controllers

REST controllers expose APIs to clients.

Example


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

@GetMapping
fun getUsers(): List<String> =
listOf("User1", "User2")

@PostMapping
fun createUser(@RequestBody name: String): String =
"User $name created"
}

Best Practices

  1. Keep controllers thin
  2. Use DTOs for requests and responses
  3. Handle exceptions globally

JPA and Hibernate

JPA handles database persistence.

Entity Example


@Entity
data class User(
@Id @GeneratedValue(strategy = GenerationType.IDENTITY)
val id: Long = 0,
val username: String,
val email: String
)

Repository Example


interface UserRepository : JpaRepository<User, Long>

Best Practices

  1. Use immutable entities where possible
  2. Avoid business logic in entities
  3. Use pagination for large datasets

Security Using JWT

JWT provides stateless authentication.

JWT Utility (Simplified)


@Component
class JwtUtil {

private val secret = "secretKey"

fun generateToken(username: String): String =
Jwts.builder()
.setSubject(username)
.signWith(Keys.hmacShaKeyFor(secret.toByteArray()))
.compact()
}

Security Configuration (Concept)

  1. Authenticate user
  2. Generate JWT
  3. Validate JWT for each request

Best Practices

  1. Store secrets securely
  2. Use HTTPS only
  3. Set token expiration

Backend Projects

User Management System

Features

  1. User registration and login
  2. JWT authentication
  3. Role-based access

Tech Stack

  1. Kotlin
  2. Spring Boot
  3. JPA
  4. JWT

E-Commerce Backend

Features

  1. Product catalog
  2. Cart and orders
  3. Secure checkout

Tech Stack

  1. Kotlin
  2. Spring Boot
  3. Hibernate
  4. REST APIs

Blog Application API

Features

  1. CRUD operations
  2. User authentication
  3. Comment system

Tech Stack

  1. Kotlin
  2. Spring Boot
  3. JPA
  4. JWT

Best Practices Summary

  1. Follow clean architecture
  2. Use DTOs and mappers
  3. Secure APIs properly
  4. Write unit and integration tests
  5. Handle errors consistently

Chapter Summary

This chapter covered Kotlin backend development using Spring Boot, including REST controllers, JPA and Hibernate, and JWT security. With real backend project ideas, it prepares developers for enterprise-level Kotlin backend roles.