Build a Kotlin Spring Boot Web Service in Minutes

Why Kotlin for Backend Development

Kotlin, originally popular for mobile development, is increasingly used by backend developers because it runs on the JVM, offers seamless Java interoperability, and adds functional programming benefits while remaining easy to learn.

Spring Boot and Kotlin

Spring Boot simplifies the creation of Spring applications by providing convention‑over‑configuration, embedded servers, and starter packages. Spring Initializr supports both Java and Kotlin, allowing developers to generate a Maven‑based Kotlin Spring Boot project with the required dependencies.

Project Setup

Using Spring Initializr, generate a Maven project with the Spring Web dependency and download the archive. After extracting, open the project in IntelliJ IDEA. The pom.xml defines the parent Spring Boot version (2.2.2.RELEASE) and includes Kotlin dependencies.

<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>
  </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>
    </dependency>
  </dependencies>
  <build>
    <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>

Kotlin Application Code

The main application class and a simple controller are defined as follows:

@SpringBootApplication
class KotlinSpringbootApplication

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

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

Running the application and accessing http://localhost:8080/index returns the greeting, demonstrating how a functional Kotlin Spring Boot web service can be created with just a few lines of code.

Book Reference

The tutorial is extracted from the book Kotlin in Common Middleware , which covers Kotlin integration with Spring Boot, Redis, JPA, QueryDSL, MongoDB, Spring Security, RocketMQ, Elasticsearch, Swagger, and more, providing practical examples for building microservices.