Add support for passing java properties into scala-cli from .scala-jvmopts

Author: lwronski

build.sc

@@ -16,7 +16,8 @@ import $file.project.settings, settings.{
   localRepoResourcePath,
   platformExecutableJarExtension,
   workspaceDirName,
-  projectFileName
+  projectFileName,
+  jvmPropertiesFileName
 }
 import $file.project.deps, deps.customRepositories
 import $file.project.website
@@ -428,6 +429,7 @@ trait Core extends ScalaCliSbtModule with ScalaCliPublishModule with HasTests
          |
          |  def workspaceDirName = "$workspaceDirName"
          |  def projectFileName = "$projectFileName"
+         |  def jvmPropertiesFileName = "$jvmPropertiesFileName"
          |
          |  def defaultGraalVMJavaVersion = ${deps.graalVmJavaVersion}
          |  def defaultGraalVMVersion = "${deps.graalVmVersion}"

modules/cli/src/main/scala/scala/cli/ScalaCli.scala

@@ -163,16 +163,33 @@ object ScalaCli {
       s"Java >= 17 is required to run $fullRunnerName (found Java $javaMajorVersion)"
     )
 
-  private def loadJavaPropertiesFromResources() = {
+  def loadJavaProperties(cwd: os.Path) = {
+    // load java properties from scala-cli-properties resource file
     val prop = new java.util.Properties()
     val cl   = getClass.getResourceAsStream("/java-properties/scala-cli-properties")
     if cl != null then
       prop.load(cl)
       prop.stringPropertyNames().forEach(name => System.setProperty(name, prop.getProperty(name)))
+    // load java properties from .scala-jvmopts located in the current working directory and filter only java properties and warning if someone used other options
+    val jvmopts = cwd / Constants.jvmPropertiesFileName
+    if os.exists(jvmopts) && os.isFile(jvmopts) then
+      val jvmoptsContent        = os.read(jvmopts)
+      val jvmoptsLines          = jvmoptsContent.linesIterator.toSeq
+      val (javaOpts, otherOpts) = jvmoptsLines.partition(_.startsWith("-D"))
+      javaOpts.foreach { opt =>
+        opt.stripPrefix("-D").split("=", 2).match {
+          case Array(key, value) => System.setProperty(key, value)
+          case _                 => System.err.println(s"Warning: Invalid java property: $opt")
+        }
+      }
+      if otherOpts.nonEmpty then
+        System.err.println(
+          s"Warning: Only java properties are supported in .scala-jvmopts file. Other options are ignored: ${otherOpts.mkString(", ")} "
+        )
   }
 
   private def main0(args: Array[String]): Unit = {
-    loadJavaPropertiesFromResources() // load java properties to detect launcher kind
+    loadJavaProperties(cwd = os.pwd) // load java properties to detect launcher kind
     val remainingArgs = LauncherOptions.parser.stopAtFirstUnrecognized.parse(args.toVector) match {
       case Left(e) =>
         System.err.println(e.message)

modules/cli/src/test/scala/cli/tests/SetupScalaCLITests.scala

@@ -0,0 +1,35 @@
+package cli.tests
+
+import com.eed3si9n.expecty.Expecty.expect
+
+import java.util.Properties
+
+import scala.build.internal.Constants
+import scala.build.tests.TestInputs
+import scala.cli.ScalaCli
+import scala.util.Properties
+
+class SetupScalaCLITests extends munit.FunSuite {
+
+  test(s"should read java properties from file") {
+    val key   = "scala-cli"
+    val value = "true"
+    val inputs = TestInputs(
+      os.rel / Constants.jvmPropertiesFileName ->
+        s"""-Xmx2048m
+           |-Xms128m
+           |-Xss8m
+           |-D$key=$value
+           |""".stripMargin
+    )
+    inputs.fromRoot(root =>
+      // save current props to restore them after test
+      val currentProps = System.getProperties.clone().asInstanceOf[Properties]
+      ScalaCli.loadJavaProperties(root)
+      expect(sys.props.get(key).contains(value))
+
+      // restore original props
+      System.setProperties(currentProps)
+    )
+  }
+}

project/settings.sc

@@ -836,8 +836,9 @@ trait ScalaCliModule extends ScalaModule {
   }
 }
 
-def workspaceDirName = ".scala-build"
-def projectFileName  = "project.scala"
+def workspaceDirName      = ".scala-build"
+def projectFileName       = "project.scala"
+def jvmPropertiesFileName = ".scala-jvmopts"
 
 final case class License(licenseId: String, name: String, reference: String)
 object License {

website/docs/under-the-hood.md

@@ -46,6 +46,19 @@ In order set them, the `-D` command-line flags must be placed as the first optio
 scala-cli -Dfoo1=bar1 -Dfoo2=bar2 run ...
 ```
 
+The Scala CLI can also load Java properties from the `.scala-jvmopts` file present in the current working
+directory and import these Java properties into Scala CLI. Any java options in the `.scala-jvmopts` that are not
+recognizable as Java properties will be ignored.
+
+The example below shows that the Java properties `foo1` and `foo2` from the `.scala-jvmopts` file will be passed
+into the Scala CLI:
+```bash ignore
+$ cat .scala-jvmopts
+-Dfoo1=bar1 
+-Dfoo2=bar2
+$ scala-cli run ...
+```
+
 :::note
 - `scala-cli run . -Dfoo=bar` would pass the java property into your Scala app
 - `scala-cli -Dfoo=bar run .` would pass the java property into `scala-cli.`