Configuring JVM Versions

By default, Mill uses the same JVM that it itself is running on to compile/test/run Java/Scala/Kotlin modules, typically whatever java command you have available globally or what you have configured in your Build Header Config. This page goes into more detail about downloading and using a custom Java home on a per-module basis.

Setting the JVM version of a JavaModule

Configuring custom JVMs is done by setting the def jvmId of any JavaModule, ScalaModule, or KotlinModule

The jvmId string has the form:

"{name}:{version}"

To see what Jvms are available for download look at the index for your os and architecture here.

build.mill (download, browse)

import mill._, javalib._
import mill.define.ModuleRef

object foo extends JavaModule {
  def jvmId = "temurin:18.0.2"

  object test extends JavaTests with TestModule.Junit4
}

> ./mill foo.run
Foo running on Java 18.0.2

> ./mill foo.test
Testing with JVM version: 18.0.2
Test foo.FooTest.testSimple finished...

Selecting a custom JVM via jvmId means that JVM is used for compiling, testing, and running that module via Mill. Note that .assembly is not affected, as JVM assembly jars do not bundle a JVM and have to be run using a JVM installed on the target host machine.

Selecting JVM Index Versions

By default, Mill comes bundled with a version of the JVM index that was published when each version of Mill is released. This ensures that the JVM versions you pick are stable, but means that the latest JVM versions may not be available. You can pass in the JVM index version explicitly via def jvmIndexVersion below, choosing a published index version from the Maven Central:

JVM Indices on Maven Central

Or alternatively pass in "latest.release" to pick the latest JVM index available, although that might mean the JVM version is no longer stable and might change over time as new releases are published:

import scalalib._

object bar extends ScalaModule {
  def scalaVersion = "2.13.12"
  def jvmId = "temurin:23.0.1"
  def jvmIndexVersion = "latest.release"
}

> ./mill bar.run
Bar running on Java 23.0.1

Explicit JVM Download URLs

You can also pass in the JVM download URL explicitly. Note that if you do so, you need to ensure yourself that you are downloading the appropriate JVM distribution for your operating system and CPU architecture. In the example below we switch between Mac/ARM and Linux/X64, but you may have additional cases if you need to support Windows or other OS/CPU combinations

import kotlinlib._

object qux extends KotlinModule {
  def kotlinVersion = "2.0.20"
  def jvmId =
    if (sys.props("os.name") == "Mac OS X") {
      "https://github.com/adoptium/temurin22-binaries/releases/download/jdk-22.0.2%2B9/OpenJDK22U-jdk_aarch64_mac_hotspot_22.0.2_9.tar.gz"
    } else {
      "https://github.com/adoptium/temurin22-binaries/releases/download/jdk-22.0.2%2B9/OpenJDK22U-jdk_x64_linux_hotspot_22.0.2_9.tar.gz"
    }

}

> ./mill qux.run
Qux running on Java 22.0.2

Locally-Installed JVMs

Lastly, you can point Mill at any JVM distribution installed locally on disk via:

object baz extends JavaModule {
  def javaHome = Some(PathRef(os.Path("/my/java/home"), quick = true))
}