How to set up Kotlin's byte code version in Gradle project to Java 8?

Question

In Kotlin project, what is a proper Gradle script to make sure my classes will be compiled to byte code ver 52 (Java 8)?

For some reason my classes are compiled as ver 50 (Java 6) even though I set up source and target compatibility. At least this is what Idea shows me when I open file from directory build/classes/... after building the project.

My current set up looks like this.

buildscript {
    ext {
        kotlinVersion = '1.0.5-2'
        springBootVersion = '1.4.2.RELEASE'
    }
    repositories { mavenCentral() }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:${springBootVersion}")
        classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:${kotlinVersion}")
    }
}

apply plugin: 'kotlin'
apply plugin: 'org.springframework.boot'

tasks.withType(JavaCompile) {
    sourceCompatibility = JavaVersion.VERSION_1_8
    targetCompatibility = JavaVersion.VERSION_1_8
}

// I also tried this and it hasn't helped
//sourceCompatibility = 1.8
//targetCompatibility = 1.8

repositories { mavenCentral() }

dependencies {
    compile("org.jetbrains.kotlin:kotlin-stdlib:${kotlinVersion}")
    compile('org.springframework.cloud:spring-cloud-starter-stream-rabbit')
}

dependencyManagement { imports { mavenBom "org.springframework.cloud:spring-cloud-dependencies:Camden.SR2" } }

score 108 · Accepted Answer · edited Jun 04 '17 at 17:32

108

As Mark pointed out on Debop's answer, you have to configure both compileKotlin and compileTestKotlin. You can do it without duplication this way:

tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all {
  kotlinOptions {
    jvmTarget = "1.8"
  }
}

For a pure Kotlin project, I don't think the options sourceCompatibility and targetCompatibility do anything, so you may be able to remove them.

Ref: https://kotlinlang.org/docs/reference/using-gradle.html#compiler-options

edited Jun 04 '17 at 17:32

mkobit

43,979
12
156
150

answered Jun 01 '17 at 02:50

seanf

6,504
3
42
52

5

I would do this: `org.jetbrains.kotlin.gradle.tasks.KotlinCompile::class.java` – programaths Jan 06 '19 at 12:25
1

FYI, don't use `all` use `configureEach` this is the prefered way for task avoidance and will speed up your configuration phase on larger projects – Chris.Jenkins Jul 27 '21 at 18:29
1

Update for Kotlin 1.8, this is now the preferred syntax (kotlinOptions is [deprecated](https://kotlinlang.org/docs/whatsnew18.html#exposing-kotlin-compiler-options-as-gradle-lazy-properties)): `tasks.withType(KotlinCompile).configureEach { compilerOptions.jvmTarget.set(JvmTarget.JVM_1_8) }` – winne2 Mar 29 '23 at 08:33

score 77 · Answer 2 · answered Sep 01 '17 at 07:09

77

In case someone uses gradle with kotlin-dsl instead of groovy:

import org.jetbrains.kotlin.gradle.tasks.KotlinCompile

// compile bytecode to java 8 (default is java 6)
tasks.withType<KotlinCompile> {
    kotlinOptions.jvmTarget = "1.8"
}

answered Sep 01 '17 at 07:09

guenhter

11,255
3
35
66

2

The same solution in one line: `tasks.withType { kotlinOptions.jvmTarget = "1.8" }` – sylwano Feb 04 '21 at 16:36

score 40 · Answer 3 · answered Feb 16 '17 at 14:08

40

Kotlin 1.1 in Gradle. in console you have error about inline (your installed compiler is 1.0.x) If run gradle task in IntelliJ IDEA, your code compiled by kotlin 1.1 compiler

compileKotlin {
    sourceCompatibility = JavaVersion.VERSION_1_8
    targetCompatibility = JavaVersion.VERSION_1_8

    kotlinOptions {
        jvmTarget = "1.8"
        apiVersion = "1.1"
        languageVersion = "1.1"
    }
}

answered Feb 16 '17 at 14:08

Debop

1,035
10
9

5

Looks like you may need to duplicate this in a compileTestKotlin block to also apply this config to test code. If I'm wrong about this, though, I'd like to know about it and why. – Mark Apr 07 '17 at 16:54
You are correct. That's because this is configuring only the `compileKotlin` task. It's better to configure all tasks with type `KotlinCompile` to ensure that any existing, or added, tasks that compile Kotlin are configured to use the desired Java version. – Andrew A. Tasso Sep 18 '21 at 01:11

Igor · Answer 4 · 2019-08-04T13:27:36.193

17

import org.jetbrains.kotlin.gradle.tasks.KotlinCompile

// JVM target applied to all Kotlin tasks across all subprojects

// Kotlin DSL
tasks.withType<KotlinCompile> {
    kotlinOptions.jvmTarget = JavaVersion.VERSION_1_8.toString()
}

//Groovy
tasks.withType(KotlinCompile) {
    kotlinOptions.jvmTarget = JavaVersion.VERSION_1_8
}

edited Aug 04 '19 at 13:27

answered May 29 '19 at 11:13

Igor

2,039
23
27

score 11 · Answer 5 · answered Dec 01 '19 at 13:15

In your build.gradle.kts:

    tasks {
        val java = "11"

        compileKotlin { 
           kotlinOptions { jvmTarget = java }
           sourceCompatibility = java
        }
    }

Or like this:

tasks {
    withType<KotlinCompile> { kotlinOptions { jvmTarget = "11" } }
}

score 7 · Answer 6 · answered Dec 13 '16 at 07:53

Kotlin 1.0 always produces Java 6 class files. Kotlin 1.1 will support generating Java 8 class files by passing -jvm-target 1.8 to the compiler. See

https://blog.jetbrains.com/kotlin/2016/07/first-glimpse-of-kotlin-1-1-coroutines-type-aliases-and-more/

for a discussion of Java 7/8 support.

hennr · Answer 7 · 2022-09-29T14:28:14.807

Since gradle 6.7 there is so called java toolchain support in gradle which I find easiest to use.

It allows you to configure the JDK to use for this project and will even download the required version if needed.

All you need is a compatible JRE to start gradle and this block in your gradle build file:

kotlin dsl

kotlin {
    jvmToolchain {
        // for gradle kotlin plugin < 1.7:
        (this as JavaToolchainSpec).languageVersion.set(JavaLanguageVersion.of(17))
        // for gradle kotlin plugin >= 1.7:
        languageVersion.set(JavaLanguageVersion.of(17))
    }
}

groovy dsl

kotlin {
    jvmToolchain {
        languageVersion.set(JavaLanguageVersion.of(17))
    }
}

Note: This will update the java compiler settings as well, so you don't need anything else in your build.gradle.kts

Read more about this in the docs:

https://kotlinlang.org/docs/gradle.html#set-custom-jdk-home

https://docs.gradle.org/current/userguide/toolchains.html

As per: https://kotlinlang.org/docs/gradle-configure-project.html#gradle-java-toolchains-support, there is even shorter syntax: `kotlin { jvmToolchain(17) }` — nouveu, Jun 02 '23 at 07:46

score 1 · Answer 8 · answered Apr 26 '17 at 01:48

1

For anyone like me who use maven to build mixed code of kotlin and java in intellij , you need to set

<kotlin.compiler.jvmTarget>1.8</kotlin.compiler.jvmTarget>

because maven build doesn't respect jvmTarget set in project setting.

answered Apr 26 '17 at 01:48

happy15

79
1
4

In which file should this be set? – Aleksandar Stefanović Jun 08 '17 at 11:26
In you Pom.xml, under the properties tag. – happy15 Jun 09 '17 at 15:22

score 1 · Answer 9 · answered Nov 02 '18 at 07:39

1

thx for seanf's anwser ,but if your DSL kotlin is 1.3.0:

tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile::class.java).all {
    kotlinOptions {
        jvmTarget = "1.8"
    }
}

answered Nov 02 '18 at 07:39

leonardosccd

1,503
11
12

`kotlin-dsl` was already answered with better syntax at https://stackoverflow.com/a/45994990/627727 – mkobit Feb 18 '19 at 21:01

score 1 · Answer 10 · answered Oct 20 '20 at 22:08

If you're getting an error like this: Cannot inline bytecode built with JVM target 1.8 into bytecode that is being built with JVM target 1.6. Please specify proper '-jvm-target' option.

Then, you can simply add the following in build.gradle (in Groovy) in order to tell kotlin to build using 1.8.

compileKotlin {
    kotlinOptions {
        jvmTarget = "1.8"
    }
}

score 1 · Answer 11 · answered Dec 09 '22 at 10:29

I tried all of the above and I was still getting warnings for compileTestKotlin.
This is what cleared all of them in the root build.gradle:

tasks.withType(org.jetbrains.kotlin.gradle.tasks.AbstractKotlinCompile).all {
  kotlinOptions {
    jvmTarget = 17
  }
}

How to set up Kotlin's byte code version in Gradle project to Java 8?

11 Answers11

Linked

Related