Warn when .java & .scala sources are used in a mixed compilation with --server=false

modules/build/src/main/scala/scala/build/Build.scala

@@ -1199,6 +1199,24 @@ object Build {
       )
     }
 
+    if sources.hasJava && sources.hasScala && options.useBuildServer.contains(false) then {
+      val javaPaths = sources.paths
+        .filter(_._1.last.endsWith(".java"))
+        .map(_._1.toString) ++
+        sources.inMemory
+          .filter(_.generatedRelPath.last.endsWith(".java"))
+          .map(_.originalPath.fold(identity, _._2.toString))
+      val javaPathsList =
+        javaPaths.map(p => s"  $p").mkString(System.lineSeparator())
+      logger.message(
+        s"""$warnPrefix With ${Console.BOLD}--server=false${Console.RESET}, .java files are not compiled to .class files.
+           |scalac parses .java sources for type information (cross-compilation), but without the build server (Bloop/Zinc) nothing compiles them to bytecode.
+           |Affected .java files:
+           |$javaPathsList
+           |Remove --server=false or compile Java files separately to avoid runtime NoClassDefFoundError.""".stripMargin
+      )
+    }
+
     buildClient.clear()
     buildClient.setGeneratedSources(scope, generatedSources)
 

modules/build/src/test/scala/scala/build/tests/BuildTestsScalac.scala

@@ -1,3 +1,32 @@
 package scala.build.tests
 
-class BuildTestsScalac extends BuildTests(server = false)
+class BuildTestsScalac extends BuildTests(server = false) {
+
+  test("warn about Java files in mixed compilation with --server=false") {
+    val recordingLogger = new RecordingLogger()
+    val inputs          = TestInputs(
+      os.rel / "Side.java" ->
+        """public class Side {
+          |    public static String message = "Hello";
+          |}
+          |""".stripMargin,
+      os.rel / "Main.scala" ->
+        """@main def main() = println(Side.message)
+          |""".stripMargin
+    )
+    val options = defaultScala3Options.copy(useBuildServer = Some(false))
+    inputs.withBuild(options, buildThreads, bloopConfigOpt, logger = Some(recordingLogger)) {
+      (_, _, maybeBuild) =>
+        assert(maybeBuild.isRight)
+        val hasWarning = recordingLogger.messages.exists { msg =>
+          msg.contains(".java files are not compiled to .class files") &&
+          msg.contains("--server=false") &&
+          msg.contains("Affected .java files")
+        }
+        assert(
+          hasWarning,
+          s"Expected warning about Java files with --server=false in: ${recordingLogger.messages.mkString("\n")}"
+        )
+    }
+  }
+}

modules/build/src/test/scala/scala/build/tests/TestInputs.scala

@@ -8,7 +8,7 @@ import scala.build.compiler.{BloopCompilerMaker, SimpleScalaCompilerMaker}
 import scala.build.errors.BuildException
 import scala.build.input.{Inputs, ScalaCliInvokeData}
 import scala.build.options.{BuildOptions, Scope}
-import scala.build.{Build, BuildThreads, Builds}
+import scala.build.{Build, BuildThreads, Builds, Logger}
 import scala.util.Try
 import scala.util.control.NonFatal
 
@@ -94,7 +94,8 @@ final case class TestInputs(
     fromDirectory: Boolean = false,
     buildTests: Boolean = true,
     actionableDiagnostics: Boolean = false,
-    skipCreatingSources: Boolean = false
+    skipCreatingSources: Boolean = false,
+    logger: Option[Logger] = None
   )(f: (os.Path, Inputs, Either[BuildException, Builds]) => T): T =
     withCustomInputs(fromDirectory, None, skipCreatingSources) { (root, inputs) =>
       val compilerMaker = bloopConfigOpt match {
@@ -108,13 +109,14 @@ final case class TestInputs(
         case None =>
           SimpleScalaCompilerMaker("java", Nil)
       }
+      val log    = logger.getOrElse(TestLogger())
       val builds =
         Build.build(
           inputs,
           options,
           compilerMaker,
           None,
-          TestLogger(),
+          log,
           crossBuilds = false,
           buildTests = buildTests,
           partial = None,
@@ -131,7 +133,8 @@ final case class TestInputs(
     buildTests: Boolean = true,
     actionableDiagnostics: Boolean = false,
     scope: Scope = Scope.Main,
-    skipCreatingSources: Boolean = false
+    skipCreatingSources: Boolean = false,
+    logger: Option[Logger] = None
   )(f: (os.Path, Inputs, Either[BuildException, Build]) => T): T =
     withBuilds(
       options,
@@ -140,7 +143,8 @@ final case class TestInputs(
       fromDirectory,
       buildTests = buildTests,
       actionableDiagnostics = actionableDiagnostics,
-      skipCreatingSources = skipCreatingSources
+      skipCreatingSources = skipCreatingSources,
+      logger = logger
     ) {
       (p, i, builds) =>
         f(

modules/build/src/test/scala/scala/build/tests/TestLogger.scala

@@ -10,8 +10,43 @@ import java.io.PrintStream
 import scala.build.Logger
 import scala.build.errors.{BuildException, Diagnostic}
 import scala.build.internals.FeatureType
+import scala.collection.mutable.ListBuffer
 import scala.scalanative.build as sn
 
+/** Logger that records all message() and log() calls for test assertions. */
+final class RecordingLogger(delegate: Logger = TestLogger()) extends Logger {
+  val messages: ListBuffer[String] = ListBuffer.empty
+
+  override def error(message: String): Unit      = delegate.error(message)
+  override def message(message: => String): Unit = {
+    val msg = message
+    messages += msg
+    delegate.message(msg)
+  }
+  override def log(s: => String): Unit = {
+    val msg = s
+    messages += msg
+    delegate.log(msg)
+  }
+  override def log(s: => String, debug: => String): Unit         = delegate.log(s, debug)
+  override def debug(s: => String): Unit                         = delegate.debug(s)
+  override def log(diagnostics: Seq[Diagnostic]): Unit           = delegate.log(diagnostics)
+  override def log(ex: BuildException): Unit                     = delegate.log(ex)
+  override def debug(ex: BuildException): Unit                   = delegate.debug(ex)
+  override def exit(ex: BuildException): Nothing                 = delegate.exit(ex)
+  override def coursierLogger(message: String): CacheLogger      = delegate.coursierLogger(message)
+  override def bloopRifleLogger: BloopRifleLogger                = delegate.bloopRifleLogger
+  override def scalaJsLogger: ScalaJsLogger                      = delegate.scalaJsLogger
+  override def scalaNativeTestLogger: sn.Logger                  = delegate.scalaNativeTestLogger
+  override def scalaNativeCliInternalLoggerOptions: List[String] =
+    delegate.scalaNativeCliInternalLoggerOptions
+  override def compilerOutputStream: PrintStream = delegate.compilerOutputStream
+  override def verbosity: Int                    = delegate.verbosity
+  override def experimentalWarning(featureName: String, featureType: FeatureType): Unit =
+    delegate.experimentalWarning(featureName, featureType)
+  override def flushExperimentalWarnings: Unit = delegate.flushExperimentalWarnings
+}
+
 case class TestLogger(info: Boolean = true, debug: Boolean = false) extends Logger {
   override def log(diagnostics: Seq[Diagnostic]): Unit = {
     diagnostics.foreach { d =>

modules/integration/src/test/scala/scala/cli/integration/CompileTestDefinitions.scala

@@ -62,9 +62,7 @@ abstract class CompileTestDefinitions
         |""".stripMargin
   )
 
-  test(
-    "java files with no using directives should not produce warnings about using directives in multiple files"
-  ) {
+  {
     val inputs = TestInputs(
       os.rel / "Bar.java" ->
         """public class Bar {}
@@ -73,12 +71,23 @@ abstract class CompileTestDefinitions
         """public class Foo {}
           |""".stripMargin
     )
-
-    inputs.fromRoot { root =>
-      val warningMessage = "Using directives detected in multiple files"
-      val output         = os.proc(TestUtil.cli, "compile", extraOptions, ".")
-        .call(cwd = root, stderr = os.Pipe).err.trim()
-      expect(!output.contains(warningMessage))
+    test(
+      "java files with no using directives should not produce warnings about using directives in multiple files"
+    ) {
+      inputs.fromRoot { root =>
+        val warningMessage = "Using directives detected in multiple files"
+        val output         = os.proc(TestUtil.cli, "compile", extraOptions, ".")
+          .call(cwd = root, stderr = os.Pipe).err.trim()
+        expect(!output.contains(warningMessage))
+      }
+    }
+    test("Pure Java with --server=false: no warning about .java files not being compiled") {
+      inputs.fromRoot { root =>
+        val warningMessage = ".java files are not compiled to .class files"
+        val output         = os.proc(TestUtil.cli, "compile", "--server=false", extraOptions, ".")
+          .call(cwd = root, stderr = os.Pipe).err.text()
+        expect(!output.contains(warningMessage))
+      }
     }
   }
 
@@ -140,7 +149,7 @@ abstract class CompileTestDefinitions
   }
 
   test(
-    "having target + using directives in files should not produce warnings about using directives in multiple files"
+    "having target + using directives in files: no using-directives or .java-not-compiled warnings"
   ) {
     val inputs = TestInputs(
       os.rel / "Bar.java" ->
@@ -160,14 +169,14 @@ abstract class CompileTestDefinitions
       val output         = os.proc(TestUtil.cli, "--power", "compile", extraOptions, ".")
         .call(cwd = root).err.trim()
       expect(!output.contains(warningMessage))
+      expect(!output.contains(".java files are not compiled to .class files"))
     }
   }
 
-  test(
-    "warn about directives in multiple files"
-  ) {
-    val inputs = TestInputs(
-      os.rel / "Bar.java" ->
+  {
+    val javaSourceFile = "Bar.java"
+    val inputs         = TestInputs(
+      os.rel / javaSourceFile ->
         """//> using jvm 17
           |public class Bar {}
           |""".stripMargin,
@@ -176,12 +185,24 @@ abstract class CompileTestDefinitions
           |class Foo {}
           |""".stripMargin
     )
+    test("warn about directives in multiple files") {
+      inputs.fromRoot { root =>
+        val warningMessage = "Using directives detected in multiple files"
+        val output         = os.proc(TestUtil.cli, "--power", "compile", extraOptions, ".")
+          .call(cwd = root, stderr = os.Pipe).err.trim()
+        expect(output.contains(warningMessage))
+      }
+    }
 
-    inputs.fromRoot { root =>
-      val warningMessage = "Using directives detected in multiple files"
-      val output         = os.proc(TestUtil.cli, "--power", "compile", extraOptions, ".")
-        .call(cwd = root, stderr = os.Pipe).err.trim()
-      expect(output.contains(warningMessage))
+    test("mixed .java/.scala: with --server=false warn about .java not compiled") {
+      inputs.fromRoot { root =>
+        val warningMessage = ".java files are not compiled to .class files"
+        val output         =
+          os.proc(TestUtil.cli, "--power", "compile", extraOptions, ".", "--server=false")
+            .call(cwd = root, stderr = os.Pipe).err.trim()
+        expect(output.contains(warningMessage))
+        expect(output.contains(javaSourceFile))
+      }
     }
   }
 
@@ -699,7 +720,9 @@ abstract class CompileTestDefinitions
       }
   }
 
-  test("pass java options to scalac when server=false") {
+  test(
+    "pass java options to scalac when server=false (Scala-only, no .java-not-compiled warning)"
+  ) {
     val inputs = TestInputs(
       os.rel / "Main.scala" ->
         """object Main extends App {
@@ -721,6 +744,7 @@ abstract class CompileTestDefinitions
       val out = res.out.text()
       expect(out.contains("Error occurred during initialization of VM"))
       expect(out.contains("Too small maximum heap"))
+      expect(!out.contains(".java files are not compiled to .class files"))
     }
   }