Setting Up Your First Kotlin Spring Boot Project

Creating your first Kotlin Spring Boot project doesn't have to be complicated. This guide will walk you through the process step by step, ensuring you have a solid foundation for building modern web applications.

Key Takeaways

• How to set up a new Kotlin Spring Boot project using Spring Initializr
• Understanding the basic project structure
• Essential dependencies for web development
• Running and testing your first application

Prerequisites

• JDK 17 or later installed
• An IDE (IntelliJ IDEA recommended)
• Basic Kotlin knowledge

Creating the Project

The easiest way to start is using Spring Initializr. You can either:

1. Visit start.spring.io
2. Use your IDE's Spring Initializr integration (recommended)

Let's create a simple web application:

// build.gradle.kts
plugins {
    id("org.springframework.boot") version "3.2.2"
    id("io.spring.dependency-management") version "1.1.4"
    kotlin("jvm") version "1.9.22"
    kotlin("plugin.spring") version "1.9.22"
}

group = "com.example"
version = "0.0.1-SNAPSHOT"

java {
    sourceCompatibility = JavaVersion.VERSION_17
}

dependencies {
    implementation("org.springframework.boot:spring-boot-starter-web")
    implementation("com.fasterxml.jackson.module:jackson-module-kotlin")
    implementation("org.jetbrains.kotlin:kotlin-reflect")
    testImplementation("org.springframework.boot:spring-boot-starter-test")
}

Project Structure

After creation, your project will have this basic structure:

src
├── main
│   ├── kotlin
│   │   └── com
│   │       └── example
│   │           └── demo
│   │               ├── DemoApplication.kt
│   │               └── HelloController.kt
│   └── resources
│       └── application.properties
└── test
    └── kotlin
        └── com
            └── example
                └── demo
                    └── DemoApplicationTests.kt

Your First Controller

Let's create a simple controller to verify everything works:

// HelloController.kt
package com.example.demo

import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.RestController

@RestController
class HelloController {
    @GetMapping("/")
    fun hello() = mapOf(
        "message" to "Welcome to Kotlin Spring Boot!",
        "timestamp" to System.currentTimeMillis()
    )
}

Running the Application

Your main application file will look like this:

// DemoApplication.kt
package com.example.demo

import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication

@SpringBootApplication
class DemoApplication

fun main(args: Array<String>) {
    runApplication<DemoApplication>(*args)
}

Run the application and visit http://localhost:8080 to see your JSON response.

Testing

Spring Boot provides excellent testing support. Here's a basic test for our controller:

// DemoApplicationTests.kt
package com.example.demo

import org.junit.jupiter.api.Test
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc
import org.springframework.boot.test.context.SpringBootTest
import org.springframework.test.web.servlet.MockMvc
import org.springframework.test.web.servlet.get

@SpringBootTest
@AutoConfigureMockMvc
class DemoApplicationTests {

    @Autowired
    private lateinit var mockMvc: MockMvc

    @Test
    fun `should return welcome message`() {
        mockMvc.get("/")
            .andExpect {
                status { isOk() }
                jsonPath("$.message") { exists() }
                jsonPath("$.timestamp") { exists() }
            }
    }
}

Common Issues and Solutions

1. Port already in use
3. Java version mismatch
   • Ensure your JAVA_HOME points to JDK 17 or later
   • Verify Gradle's Java compatibility settings

Solution: Change the port in application.properties:

server.port=8081

Next Steps

• Add more dependencies based on your needs (JPA, Security, etc.)
• Configure application properties
• Set up database connections
• Implement more complex endpoints

Remember: Spring Boot with Kotlin provides a great developer experience, especially when you follow the conventions. Start simple and gradually add complexity as needed.