# Adding Kotlin to Java + Spring Boot Project

### Kotlin?

* `Kotlin` is a development language created by **JetBrains**, well-known for producing **IntelliJ IDEA**. It can be said to be a newly created language by a group of experts with deep knowledge in the **Java** language, aimed at maintaining enterprise-level applications with minimal effort and ease of maintenance. After several years of beta testing, version **1.0** was first released in 2016, and as of the time of writing, version **1.9.23** has been released.
    

### Java and Kotlin in the same project?

* `Kotlin` shares the same **JVM** ecosystem as **Java**. Just as **Java** source code is compiled into the intermediate language **.class** by `javac`, **Kotlin** source code is also compiled into **.class** by `kotlinc`. The two compiled files are executed identically without discrimination by the **JVM**. With the help of modern development tools such as `Gradle` and `IntelliJ IDEA`, we can conveniently write in both languages coexisting in the same project.
    

### First Steps in Adding Kotlin

* **IntelliJ IDEA**, true to the creators of **Kotlin**, provides a `Convert Java File to Kotlin` feature that allows you to automatically convert **.java** files to **.kt** with just one click. (Right-click on the **Java** file you want to convert in the project tree to see this feature.) It's not completely perfect, so depending on the size and complexity of the source code, some manual modifications might be necessary after the conversion. Running this feature for the first time not only converts the source file but also automatically updates the **build.gradle** file, making it incredibly convenient.
    

### Spring Boot 3 + Spring Data JPA + Querydsl

* For relatively simple projects, running the `Convert Java File to Kotlin` feature and a few manual tasks might be sufficient. Additionally, to incorporate **Kotlin** into one of the most popular technology stacks in backend development, **Spring Boot 3 + Spring Data JPA + Querydsl**, simply adding the following content to the `build.gradle` file in the project root will make everything work perfectly.
    

```kotlin
# /gradle/wrapper/gradle-wrapper.properties
...
kotlinVersion=2.0.0
springBootVersion=3.3.0
querydslVersion=5.1.0

# /build.gradle
import org.jetbrains.kotlin.gradle.tasks.KotlinCompile

buildscript {
    ext {
        kotlinVersion = '2.0.0'
        springBootVersion = '3.3.0'
        querydslVersion = '5.1.0'
    }
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:${springBootVersion}")
        classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:${kotlinVersion}")
        classpath("org.jetbrains.kotlin:kotlin-allopen:${kotlinVersion}")
        classpath("org.jetbrains.kotlin:kotlin-noarg:${kotlinVersion}")
    }
}

plugins {
    ...
    id 'org.jetbrains.kotlin.jvm' version "${kotlinVersion}"
    id 'org.jetbrains.kotlin.plugin.spring' version "${kotlinVersion}"
    id 'org.jetbrains.kotlin.plugin.jpa' version "${kotlinVersion}"
    id 'org.jetbrains.kotlin.kapt' version "${kotlinVersion}"
    id 'org.jetbrains.kotlin.plugin.lombok' version "${kotlinVersion}"
    id 'io.freefair.lombok' version '8.6'
    id 'idea'
}

dependencies {
    ...
    implementation 'org.jetbrains.kotlin:kotlin-reflect'
    implementation 'org.jetbrains.kotlin:kotlin-stdlib-jdk8'
    implementation 'com.fasterxml.jackson.module:jackson-module-kotlin'
    // Disable existing Java parts when applying Querydsl
    // annotationProcessor "com.querydsl:querydsl-apt:${querydslVersion}:jakarta"
    kapt "com.querydsl:querydsl-apt:${querydslVersion}:jakarta"
}

kapt {
    keepJavacAnnotationProcessors = true
}

tasks.withType(KotlinCompile).configureEach {
    kotlinOptions {
        freeCompilerArgs += '-Xjsr305=strict'
        jvmTarget = '21'
    }
}
```

### References

* [JetBrains - Mixing Java and Kotlin in one project](https://kotlinlang.org/docs/mixing-java-kotlin-intellij.html)
    
* [Using Kotlin in a Java project: 10 lessons learned](https://xebia.com/blog/using-kotlin-in-a-java-project-10-lessons-learned/)
    
* [Building web applications with Spring Boot and Kotlin](https://spring.io/guides/tutorials/spring-boot-kotlin/)
    
* [Spring Boot and Kotlin](https://www.baeldung.com/spring-boot-kotlin)
    
* [Reddit - Spring Boot with Kotlin?](https://www.reddit.com/r/SpringBoot/comments/16zsuod/spring_boot_with_kotlin/)