unable to build a project with JDK 8

[Rogach]

I'm trying to upgrade an old project that still uses Java 8, and I'm trying to migrate it from SBT to Mill. But trying to build even the simplest example project fails - see below.

Here's the build.mill I used:

//| mill-jvm-version: zulu:21
package build
import mill.*, scalalib.*

object foo extends ScalaModule {
  override def scalaVersion = "2.13.18"
  def jvmId = "adopt:8.0-292"
}

And here's the error:

mill foo.compile
build.mill-61] compile compiling 3 Scala sources to out/mill-build/compile.dest/classes ...
build.mill-61] done compiling
50] foo.compile
50] Exception in thread "main" java.lang.UnsupportedClassVersionError: mill/client/lock/Locks has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0
50] 	at java.lang.ClassLoader.defineClass1(Native Method)
50] 	at java.lang.ClassLoader.defineClass(ClassLoader.java:756)
50] 	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
50] 	at java.net.URLClassLoader.defineClass(URLClassLoader.java:468)
50] 	at java.net.URLClassLoader.access$100(URLClassLoader.java:74)
50] 	at java.net.URLClassLoader$1.run(URLClassLoader.java:369)
50] 	at java.net.URLClassLoader$1.run(URLClassLoader.java:363)
50] 	at java.security.AccessController.doPrivileged(Native Method)
50] 	at java.net.URLClassLoader.findClass(URLClassLoader.java:362)
50] 	at java.lang.ClassLoader.loadClass(ClassLoader.java:418)
50] 	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:352)
50] 	at java.lang.ClassLoader.loadClass(ClassLoader.java:351)
50] 	at mill.javalib.zinc.ZincWorkerMain$ZincWorkerTcpServer.<init>(ZincWorkerMain.scala:37)
50] 	at mill.javalib.zinc.ZincWorkerMain$.main$$anonfun$1(ZincWorkerMain.scala:19)
50] 	at mill.api.SystemStreamsUtils$.withTopLevelSystemStreamProxy(SystemStreamsUtils.scala:75)
50] 	at mill.javalib.zinc.ZincWorkerMain$.main(ZincWorkerMain.scala:31)
50] 	at mill.javalib.zinc.ZincWorkerMain.main(ZincWorkerMain.scala)
50/50, 1 failed] ============================== foo.compile ============================== 18s
50] [error] foo.compile
java.lang.Exception: server launch failed (timeout was 10000ms)
  mill.client.ServerLauncher.retryWithTimeout(ServerLauncher.java:188)
  mill.client.ServerLauncher.launchOrConnectToServer(ServerLauncher.java:132)
  mill.javalib.worker.JvmWorkerImpl.$init$$$anonfun$2(JvmWorkerImpl.scala:131)
  mill.util.RefCountedCache.get(RefCountedCache.scala:29)
  mill.javalib.worker.SubprocessZincApi.apply(SubprocessZincApi.scala:85)
  mill.javalib.worker.JvmWorkerImpl.apply(JvmWorkerImpl.scala:49)
  mill.scalalib.ScalaModule.compileTask$$anonfun$1(ScalaModule.scala:289)
  mill.api.Task$Anon.evaluate(Task.scala:436)
java.lang.RuntimeException: java.lang.IllegalStateException: Failed to launch 'mill.javalib.zinc.ZincWorkerMain' for:
  javaHome = Some(/home/user/.cache/coursier/arc/https/github.com/AdoptOpenJDK/openjdk8-binaries/releases/download/jdk8u292-b10/OpenJDK8U-jdk_x64_linux_hotspot_8u292b10.tar.gz/jdk8u292-b10)
  runtimeOptions =
  daemonDir = /home/user/proj/out/mill.javalib.JvmWorkerModule/internalWorker.dest/zinc-worker/834896655/daemon

Failure:
ProcessDied{server=LaunchedServer.OsProcess{process=3691339}, outputs=ServerLaunchOutputs{No daemon stdout

No daemon stderr

}}

  mill.client.ServerLauncher.lambda$launchOrConnectToServer$0(ServerLauncher.java:164)
  mill.client.ServerLauncher.retryWithTimeout(ServerLauncher.java:178)
  mill.client.ServerLauncher.launchOrConnectToServer(ServerLauncher.java:132)
  mill.javalib.worker.JvmWorkerImpl.$init$$$anonfun$2(JvmWorkerImpl.scala:131)
  mill.util.RefCountedCache.get(RefCountedCache.scala:29)
  mill.javalib.worker.SubprocessZincApi.apply(SubprocessZincApi.scala:85)
  mill.javalib.worker.JvmWorkerImpl.apply(JvmWorkerImpl.scala:49)
  mill.scalalib.ScalaModule.compileTask$$anonfun$1(ScalaModule.scala:289)
  mill.api.Task$Anon.evaluate(Task.scala:436)
java.lang.IllegalStateException: Failed to launch 'mill.javalib.zinc.ZincWorkerMain' for:
  javaHome = Some(/home/user/.cache/coursier/arc/https/github.com/AdoptOpenJDK/openjdk8-binaries/releases/download/jdk8u292-b10/OpenJDK8U-jdk_x64_linux_hotspot_8u292b10.tar.gz/jdk8u292-b10)
  runtimeOptions =
  daemonDir = /home/user/proj/out/mill.javalib.JvmWorkerModule/internalWorker.dest/zinc-worker/834896655/daemon

Failure:
ProcessDied{server=LaunchedServer.OsProcess{process=3691339}, outputs=ServerLaunchOutputs{No daemon stdout

No daemon stderr

}}

  mill.javalib.worker.JvmWorkerImpl.$anonfun$4(JvmWorkerImpl.scala:127)
  mill.client.ServerLauncher.lambda$launchOrConnectToServer$0(ServerLauncher.java:160)
  mill.client.ServerLauncher.retryWithTimeout(ServerLauncher.java:178)
  mill.client.ServerLauncher.launchOrConnectToServer(ServerLauncher.java:132)
  mill.javalib.worker.JvmWorkerImpl.$init$$$anonfun$2(JvmWorkerImpl.scala:131)
  mill.util.RefCountedCache.get(RefCountedCache.scala:29)
  mill.javalib.worker.SubprocessZincApi.apply(SubprocessZincApi.scala:85)
  mill.javalib.worker.JvmWorkerImpl.apply(JvmWorkerImpl.scala:49)
  mill.scalalib.ScalaModule.compileTask$$anonfun$1(ScalaModule.scala:289)
  mill.api.Task$Anon.evaluate(Task.scala:436)

[Rogach]

Oh, turns out Mill doesn't support Java 8 at all. Found a line in the docs right after submitting the issue:

Note that Mill requires a minimum of Java 17 for the build tool itself to run. However, it supports user-defined modules down to Java 11.

https://mill-build.org/mill/cli/build-header.html#_mill_jvm_version

That's unfortunate. Sorry for bothering.

[lefou]

Since 1.0, Mill requires Java 11 at runtime.
Since Mill 1.1.0, it's Java 17.

It should be still possible, to target older Java versions. Either use the respective options like the -source, -target and --release for javac and equivalent options for scalac. Or you can select a specific JVM to use when building specific modules with jvmId task, which only works down to Java 11.

[Rogach]

Perhaps I can try using older Mill (pre 1.0.0) to start the migration?

The plan is to upgrade to newer Java version as well, but to do that I first wanted to have a solid build process, that should help when working on the migration. For example having LSP support would be really nice.

[lefou]

Perhaps I can try using older Mill (pre 1.0.0) to start the migration?

If it is important to you to use an older JVM for building, that yes. You might want to go with Mill 0.12.17. Some API has changed in Mill 1.x, so you should pay attention to the various deprecation messages, to minimize the migration work required when you switch to 1.x.

The plan is to upgrade to newer Java version as well, but to do that I first wanted to have a solid build process, that should help when working on the migration. For example having LSP support would be really nice.

Of course, the best supported is always the latest stable version, so feature-wise and re the editing experience, using Mill 1.x is recommended. You can use the --release 8 option to produce Java 8 bytecode (add it to javacOptions and scalacOptions). And I think, you should be able to configure a RunModule to run your built app with Java 8. If you need to run the unit tests with Java 8, you can't use the built-in support of Mill 1.x though.

[Rogach]

This project uses JavaFX, which was unbundled from JDK starting from Java 11, so simply using --release 8 probably won't cut it.

Thanks for the pointers!