How Kotlin Simplifies Spring Boot Backend Development: A Step‑by‑Step Guide

Kotlin’s Rise in Backend Development

With Kotlin becoming popular on the mobile side, it is also gaining attention among backend developers. Kotlin runs on the JVM, offers seamless interoperability with Java, easy onboarding, and functional programming advantages. Spring Initializr supports both Java and Kotlin.

Kotlin is developed by JetBrains, the same company behind IntelliJ IDEA, which provides strong Kotlin support, including Java‑to‑Kotlin code conversion and mixed‑language projects. Existing Java codebases can gradually adopt Kotlin for new modules.

The author, Yuan Kang, has been using Kotlin with Spring Boot for backend development and decided to write a book to share his practical experience with Java developers.

Kotlin Spring Boot Microservice Practice

The book “Kotlin Spring Boot Microservice Practice” demonstrates how to integrate Kotlin with common middleware such as Redis, JPA, QueryDSL, MongoDB, Spring Security, RocketMQ, Elasticsearch, and Swagger.

Spring Boot Overview

Spring Boot, created by the Pivotal team, simplifies the initial setup and development of Spring applications. Since its first release in April 2014, it has evolved through many versions, moving from Spring 4 to Spring 5 and adding support for reactive programming.

Year

Spring Boot version

Spring version

2014

1.0.x

4.0.x.RELEASE

2014‑2015

1.1.x

4.0.x.RELEASE

2015

1.2.x

4.1.x.RELEASE

2015‑2016

1.3.x

4.2.x.RELEASE

2016‑2017

1.4.x

4.3.x.RELEASE

2017‑2018

1.5.x

4.3.x.RELEASE

2018‑2019

2.0.x

5.0.x.RELEASE

2018‑2020

2.1.x

5.1.x.RELEASE

2019‑2020

2.2.x

5.2.x.RELEASE

Spring Boot follows the “convention over configuration” principle, eliminating the need for XML files and reducing boilerplate with starter dependencies. It provides auto‑configuration, production‑ready features such as metrics, health checks, and easy deployment via embedded containers like Tomcat, Jetty, or Undertow.

Creating a Kotlin Spring Boot Project

Use Spring Initializr to generate a Maven‑based Kotlin project, selecting the “Spring Web” dependency. Download and unzip the project.

Open the project in IntelliJ IDEA. The generated pom.xml defines the parent Spring Boot starter and sets Kotlin version 1.3.61 and Spring Boot version 2.2.2.RELEASE.

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.2.2.RELEASE</version>
    <relativePath/>
  </parent>
  <groupId>io.kang.example</groupId>
  <artifactId>kolinspringboot</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <properties>
    <java.version>1.8</java.version>
    <kotlin.version>1.3.61</kotlin.version>
  </properties>
  <dependencies>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter</artifactId>
    </dependency>
    <dependency>
      <groupId>org.jetbrains.kotlin</groupId>
      <artifactId>kotlin-reflect</artifactId>
    </dependency>
    <dependency>
      <groupId>org.jetbrains.kotlin</groupId>
      <artifactId>kotlin-stdlib-jdk8</artifactId>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-test</artifactId>
      <scope>test</scope>
      <exclusions>
        <exclusion>
          <groupId>org.junit.vintage</groupId>
          <artifactId>junit-vintage-engine</artifactId>
        </exclusion>
      </exclusions>
    </dependency>
  </dependencies>
  <build>
    <sourceDirectory>${project.basedir}/src/main/kotlin</sourceDirectory>
    <testSourceDirectory>${project.basedir}/src/test/kotlin</testSourceDirectory>
    <plugins>
      <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
      </plugin>
      <plugin>
        <groupId>org.jetbrains.kotlin</groupId>
        <artifactId>kotlin-maven-plugin</artifactId>
        <configuration>
          <args>
            <arg>-Xjsr305=strict</arg>
          </args>
          <compilerPlugins>
            <plugin>spring</plugin>
          </compilerPlugins>
        </configuration>
        <dependencies>
          <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-maven-allopen</artifactId>
            <version>${kotlin.version}</version>
          </dependency>
        </dependencies>
      </plugin>
    </plugins>
  </build>
</project>

Implement a simple web application with a main class and a controller.

KotlinSpringbootApplication.kt:

@SpringBootApplication
class KotlinSpringbootApplication {
    // Main function to launch the application
    fun main(args: Array<String>) {
        runApplication<KotlinSpringbootApplication>(*args)
    }
}

IndexController.kt:

@RestController
class IndexController {
    // Define the index endpoint
    @GetMapping("/index")
    fun index(): String {
        return "Hello, Kotlin for Spring Boot!!"
    }
}

Running the application and accessing http://localhost:8080/index returns “Hello, Kotlin for Spring Boot!!”, demonstrating how a functional backend service can be built with just a few lines of Kotlin code.