VirtusLab · Gedochao · Nov 20, 2025
diff --git a/build.mill.scala b/build.mill.scala
@@ -524,6 +524,12 @@ trait Core extends ScalaCliCrossSbtModule
          |  def scala3NextPrefix = "${Scala.scala3NextPrefix}"
          |  def scala3LtsPrefix = "${Scala.scala3LtsPrefix}"
          |  def scala3Lts       = "${Scala.scala3Lts}"
+         |  
+         |  def scala38Versions = Seq(${Scala.scala38Versions
+          .sorted
+          .map(s => s"\"$s\"")
+          .mkString(", ")})
+         |  def scala38MinJavaVersion = ${Java.minimumScala38Java}
          |
          |  def workspaceDirName = "$workspaceDirName"
          |  def projectFileName = "$projectFileName"
@@ -1032,6 +1038,7 @@ trait CliIntegration extends SbtModule with ScalaCliPublishModule with HasTests
             .sorted
             .map(s => s"\"$s\"")
             .mkString(", ")})
+           |  def scala38MinJavaVersion        = ${Java.minimumScala38Java}
            |  def defaultScalafmtVersion       = "${Deps.scalafmtCli.dep.versionConstraint.asString}"
            |  def maxAmmoniteScala212Version   = "${Scala.maxAmmoniteScala212Version}"
            |  def maxAmmoniteScala213Version   = "${Scala.maxAmmoniteScala213Version}"

diff --git a/modules/core/src/main/scala/scala/build/CsUtils.scala b/modules/core/src/main/scala/scala/build/CsUtils.scala
@@ -2,4 +2,12 @@ package scala.build
 
 import coursier.version.Version
 
+import scala.build.internal.Constants
+
 extension (s: String) def coursierVersion: Version = Version(s)
+
+extension (csv: Version)
+  def isScala38OrNewer: Boolean =
+    Constants.scala38Versions
+      .map(_.coursierVersion)
+      .exists(_ <= csv)
diff --git a/modules/integration/src/test/scala/scala/cli/integration/RunTestDefinitions.scala b/modules/integration/src/test/scala/scala/cli/integration/RunTestDefinitions.scala
@@ -2430,23 +2430,32 @@ abstract class RunTestDefinitions
     }
   }
 
-  test(
-    s"run a simple hello world with the runner module on the classpath and Scala $actualScalaVersion"
-  ) {
-    val expectedMessage     = "Hello, world!"
-    val legacyRunnerWarning = "Defaulting to a legacy runner module version"
-    TestInputs(os.rel / "script.sc" -> s"""println("$expectedMessage")""")
-      .fromRoot { root =>
-        val res = os.proc(TestUtil.cli, "run", ".", "--runner", extraOptions)
-          .call(cwd = root, stderr = os.Pipe)
-        expect(res.out.trim() == expectedMessage)
-        val legacyWarningCheck = {
-          val check = res.err.trim().contains(legacyRunnerWarning)
-          if (actualScalaVersion.startsWith("2")) check else !check
-        }
-        expect(legacyWarningCheck)
-      }
+  for {
+    javaVersion <-
+      if isScala38OrNewer then
+        Constants.allJavaVersions.filter(_ >= Constants.scala38MinJavaVersion)
+      else Constants.allJavaVersions.filter(_ < 24)
   }
+    test(
+      s"run a simple hello world with the runner module on the classpath, Scala $actualScalaVersion and Java $javaVersion"
+    ) {
+      val expectedMessage     = "Hello, world!"
+      val legacyRunnerWarning = "Defaulting to a legacy runner module version"
+      TestInputs(os.rel / "script.sc" -> s"""println("$expectedMessage")""")
+        .fromRoot { root =>
+          val res =
+            os.proc(TestUtil.cli, "run", ".", "--runner", extraOptions, "--jvm", javaVersion)
+              .call(cwd = root, stderr = os.Pipe)
+          expect(res.out.trim() == expectedMessage)
+          val legacyWarningCheck = {
+            val check       = res.err.trim().contains(legacyRunnerWarning)
+            val shouldCheck =
+              javaVersion < Constants.scala38MinJavaVersion || actualScalaVersion.startsWith("2")
+            if shouldCheck then check else !check
+          }
+          expect(legacyWarningCheck)
+        }
+    }
 
   for (parallelInstancesCount <- Seq(2, 5, 10))
     test(

diff --git a/modules/integration/src/test/scala/scala/cli/integration/TestTestDefinitions.scala b/modules/integration/src/test/scala/scala/cli/integration/TestTestDefinitions.scala
@@ -4,6 +4,7 @@ import com.eed3si9n.expecty.Expecty.expect
 
 import scala.annotation.tailrec
 import scala.cli.integration.Constants.munitVersion
+import scala.cli.integration.TestUtil.StringOps
 
 abstract class TestTestDefinitions extends ScalaCliSuite with TestScalaVersionArgs {
   this: TestScalaVersion =>
@@ -1007,4 +1008,37 @@ abstract class TestTestDefinitions extends ScalaCliSuite with TestScalaVersionAr
       }
     }
   }
+
+  for {
+    javaVersion <-
+      if isScala38OrNewer then
+        Constants.allJavaVersions.filter(_ >= Constants.scala38MinJavaVersion)
+      else Constants.allJavaVersions.filter(_ < 24)
+    expectedMessage = "Hello, world!"
+    expectedWarning = s"Defaulting to a legacy test-runner module version"
+  }
+    test(s"run a simple test with Java $javaVersion") {
+      TestInputs(os.rel / "example.test.scala" ->
+        s"""//> using dep com.novocode:junit-interface:0.11
+           |import org.junit.Test
+           |
+           |class MyTests {
+           |  @Test
+           |  def foo(): Unit = {
+           |    assert(2 + 2 == 4)
+           |    println("$expectedMessage")
+           |  }
+           |}
+           |""".stripMargin).fromRoot { root =>
+        val res =
+          os.proc(TestUtil.cli, "test", ".", extraOptions, "--jvm", javaVersion)
+            .call(cwd = root, stderr = os.Pipe)
+        val out = res.out.trim()
+        expect(out.contains(expectedMessage))
+        if actualScalaVersion.startsWith("2") || javaVersion < Constants.scala38MinJavaVersion then
+          val err = res.err.trim()
+          expect(err.contains(expectedWarning))
+          expect(err.countOccurrences(expectedWarning) == 1)
+      }
+    }
 }
diff --git a/modules/options/src/main/scala/scala/build/Artifacts.scala b/modules/options/src/main/scala/scala/build/Artifacts.scala
@@ -128,6 +128,7 @@ object Artifacts {
     extraCompileOnlyJars: Seq[os.Path],
     extraSourceJars: Seq[os.Path],
     fetchSources: Boolean,
+    jvmVersion: Int,
     addJvmRunner: Option[Boolean],
     addJvmTestRunner: Boolean,
     addJmhDependencies: Option[String],
@@ -146,11 +147,13 @@ object Artifacts {
       scalaVersion = scalaParams.scalaVersion
     } yield scalaVersion).getOrElse(defaultScalaVersion)
 
+    val shouldUseLegacyJava8Runners  = jvmVersion < Constants.scala38MinJavaVersion
     val shouldUseLegacyScala3Runners =
       scalaVersion.startsWith("3") &&
       scalaVersion.coursierVersion < s"$scala3LtsPrefix.0".coursierVersion
     val shouldUseLegacyScala2Runners = scalaVersion.startsWith("2")
-    val shouldUseLegacyRunners       = shouldUseLegacyScala2Runners || shouldUseLegacyScala3Runners
+    val shouldUseLegacyScalaRunners  = shouldUseLegacyScala3Runners || shouldUseLegacyScala2Runners
+    val shouldUseLegacyRunners       = shouldUseLegacyScalaRunners || shouldUseLegacyJava8Runners
 
     val jvmTestRunnerDependencies =
       if addJvmTestRunner then {
@@ -160,12 +163,25 @@ object Artifacts {
           else runnerScala2LegacyVersion
         val testRunnerVersion0 =
           if shouldUseLegacyRunners then {
+            if shouldUseLegacyScalaRunners then
+              logger.message(
+                s"$warnPrefix Scala $scalaVersion is no longer supported by the test-runner module."
+              )
+            if shouldUseLegacyJava8Runners then
+              logger.message(
+                s"$warnPrefix Java $jvmVersion is no longer supported by the test-runner module."
+              )
             logger.message(
-              s"""$warnPrefix Scala $scalaVersion is no longer supported by the test-runner module.
-                 |$warnPrefix Defaulting to a legacy test-runner module version: $runnerLegacyVersion.
-                 |$warnPrefix To use the latest test-runner, upgrade Scala to at least $scala3LtsPrefix."""
-                .stripMargin
+              s"$warnPrefix Defaulting to a legacy test-runner module version: $runnerLegacyVersion."
             )
+            if shouldUseLegacyScalaRunners then
+              logger.message(
+                s"$warnPrefix To use the latest test-runner, upgrade Scala to at least $scala3LtsPrefix."
+              )
+            if shouldUseLegacyJava8Runners then
+              logger.message(
+                s"$warnPrefix To use the latest test-runner, upgrade Java to at least ${Constants.defaultJavaVersion}."
+              )
             runnerLegacyVersion
           }
           else testRunnerVersion
@@ -481,6 +497,25 @@ object Artifacts {
                   if shouldUseLegacyScala3Runners
                   then runnerScala30LegacyVersion
                   else runnerScala2LegacyVersion
+                if shouldUseLegacyScalaRunners then
+                  logger.message(
+                    s"$warnPrefix Scala $scalaVersion is no longer supported by the runner module."
+                  )
+                if shouldUseLegacyJava8Runners then
+                  logger.message(
+                    s"$warnPrefix Java $jvmVersion is no longer supported by the runner module."
+                  )
+                logger.message(
+                  s"$warnPrefix Defaulting to a legacy runner module version: $runnerLegacyVersion."
+                )
+                if shouldUseLegacyScalaRunners then
+                  logger.message(
+                    s"$warnPrefix To use the latest runner, upgrade Scala to at least $scala3LtsPrefix."
+                  )
+                if shouldUseLegacyJava8Runners then
+                  logger.message(
+                    s"$warnPrefix To use the latest runner, upgrade Java to at least ${Constants.defaultJavaVersion}."
+                  )
                 logger.message(
                   s"""$warnPrefix Scala $scalaVersion is no longer supported by the runner module.
                      |$warnPrefix Defaulting to a legacy runner module version: $runnerLegacyVersion.

diff --git a/modules/options/src/main/scala/scala/build/options/BuildOptions.scala b/modules/options/src/main/scala/scala/build/options/BuildOptions.scala
@@ -464,7 +464,7 @@ final case class BuildOptions(
     }
     val extraRepositories: Seq[Repository] = value(finalRepositories)
     val maybeArtifacts                     = Artifacts(
-      scalaArtifactsParamsOpt,
+      scalaArtifactsParamsOpt = scalaArtifactsParamsOpt,
       javacPluginDependencies = value(javacPluginDependencies),
       extraJavacPlugins = javaOptions.javacPlugins.map(_.value),
       defaultDependencies = value(defaultDependencies),
@@ -474,6 +474,7 @@ final case class BuildOptions(
       extraCompileOnlyJars = allExtraCompileOnlyJars,
       extraSourceJars = allExtraSourceJars,
       fetchSources = classPathOptions.fetchSources.getOrElse(false),
+      jvmVersion = javaHome().value.version,
       addJvmRunner = addRunnerDependency0,
       addJvmTestRunner = isTests && addJvmTestRunner,
       addJmhDependencies = jmhOptions.finalJmhVersion,

diff --git a/project/deps/package.mill.scala b/project/deps/package.mill.scala
@@ -90,6 +90,7 @@ object Scala {
 object Java {
   def minimumJavaLauncherJava  = 11
   def minimumBloopJava: Int    = 17
+  def minimumScala38Java: Int  = 17
   def minimumInternalJava: Int = 16
   def defaultJava: Int         = minimumBloopJava
   def cliKeyJavaVersions       = Seq(