diff --git a/core/api/daemon/src/mill/api/daemon/ExecResult.scala b/core/api/daemon/src/mill/api/daemon/ExecResult.scala
index debc11c357cb..03bf312513ad 100644
--- a/core/api/daemon/src/mill/api/daemon/ExecResult.scala
+++ b/core/api/daemon/src/mill/api/daemon/ExecResult.scala
@@ -67,9 +67,10 @@ object ExecResult {
     def flatMap[V](f: T => ExecResult[V]): Failing[V]
 
     override def asFailing: Option[ExecResult.Failing[T]] = Some(this)
-    def throwException: Nothing = this match {
-      case f: ExecResult.Failure[?] => throw new Result.Exception(f.msg)
-      case f: ExecResult.Exception => throw f.throwable
+    def throwException: Nothing = throw exception
+    private[mill] def exception: Throwable = this match {
+      case f: ExecResult.Failure[?] => new Result.Exception(f.msg)
+      case f: ExecResult.Exception => f.throwable
     }
   }
 
diff --git a/libs/javalib/src/mill/javalib/JavaModule.scala b/libs/javalib/src/mill/javalib/JavaModule.scala
index 31824924cbb6..2bfa70fc6032 100644
--- a/libs/javalib/src/mill/javalib/JavaModule.scala
+++ b/libs/javalib/src/mill/javalib/JavaModule.scala
@@ -492,6 +492,25 @@ trait JavaModule
    */
   def unmanagedClasspath: T[Seq[PathRef]] = Task { Seq.empty[PathRef] }
 
+  /**
+   * Same class path as [[unmanagedClasspath]], but ensures every entry is a JAR file.
+   *
+   * If you'd like to add unmanaged class path entries, add them to [[unmanagedClasspath]].
+   * `unmanagedClasspathAsJars` will pick them up automatically, and convert directory entries
+   * to JAR files if needed.
+   */
+  private[mill] def unmanagedClasspathAsJars: T[Seq[PathRef]] =
+    Task {
+      unmanagedClasspath().zipWithIndex.flatMap {
+        case (ref, refIdx) if os.isDir(ref.path) =>
+          val jar = Task.dest / s"$refIdx.jar"
+          Jvm.createJar(jar, Seq(ref.path))
+          Seq(PathRef(jar))
+        case (ref, _) if os.exists(ref.path) => Seq(ref)
+        case (_, _) => Nil
+      }
+    }
+
   /**
    * The `coursier.Dependency` to use to refer to this module
    */
@@ -835,6 +854,24 @@ trait JavaModule
    */
   def compileResources: T[Seq[PathRef]] = Task.Sources { "compile-resources" }
 
+  /**
+   * Same class path as [[compileResources]], but ensures every entry is a JAR file.
+   *
+   * If you'd like to add compile resources entries, add them to [[compileResources]].
+   * `compileResourcesAsJars` will pick them up automatically, and convert directory entries
+   * to JAR files if needed.
+   */
+  private[mill] def compileResourcesAsJars: T[Seq[PathRef]] = Task {
+    compileResources().zipWithIndex.flatMap {
+      case (ref, refIdx) if os.isDir(ref.path) =>
+        val jar = Task.dest / s"$refIdx.jar"
+        Jvm.createJar(jar, Seq(ref.path))
+        Seq(PathRef(jar))
+      case (ref, _) if os.exists(ref.path) => Seq(ref)
+      case (_, _) => Nil // filtering out non-existing entries, to only keep actual JAR files
+    }
+  }
+
   /**
    * Folders containing source files that are generated rather than
    * handwritten; these files can be generated in this task itself,
@@ -1018,6 +1055,14 @@ trait JavaModule
    */
   def compileClasspath: T[Seq[PathRef]] = Task { compileClasspathTask(CompileFor.Regular)() }
 
+  /**
+   * All classfiles and resources from upstream modules and dependencies
+   * necessary to compile this module, all packaged as JAR files.
+   */
+  def compileClasspathAsJars: T[Seq[PathRef]] = Task {
+    resolvedMvnDeps() ++ transitiveJars() ++ localCompileClasspathAsJars()
+  }
+
   /**
    * All classfiles and resources from upstream modules and dependencies
    * necessary to compile this module.
@@ -1053,6 +1098,14 @@ trait JavaModule
     compileResources() ++ unmanagedClasspath()
   }
 
+  /**
+   * The *input* classfiles/resources from this module, used during compilation,
+   * excluding upstream modules and third-party dependencies, packaged as JAR files.
+   */
+  def localCompileClasspathAsJars: T[Seq[PathRef]] = Task {
+    compileResourcesAsJars() ++ unmanagedClasspathAsJars()
+  }
+
   /**
    * Resolved dependencies
    */
@@ -1127,6 +1180,13 @@ trait JavaModule
       localClasspath()
   }
 
+  override def runClasspathAsJars: T[Seq[PathRef]] = Task {
+    super[RunModule].runClasspathAsJars() ++ // Remove '[RunModule]' when we can break bin-compat
+      resolvedRunMvnDeps().toSeq ++
+      transitiveJars() ++
+      Seq(jar())
+  }
+
   /**
    * A jar containing only this module's resources and compiled classfiles,
    * without those from upstream modules and dependencies
diff --git a/libs/javalib/src/mill/javalib/RunModule.scala b/libs/javalib/src/mill/javalib/RunModule.scala
index a2773afc3836..c2c77144f65e 100644
--- a/libs/javalib/src/mill/javalib/RunModule.scala
+++ b/libs/javalib/src/mill/javalib/RunModule.scala
@@ -59,9 +59,20 @@ trait RunModule extends WithJvmWorkerModule with RunModuleApi {
   /**
    * All classfiles and resources including upstream modules and dependencies
    * necessary to run this module's code.
+   *
+   * The returned class path can include directories. See `runClasspathAsJars`
+   * if you'd like the class path to contain only JAR files.
    */
   def runClasspath: T[Seq[PathRef]] = Task { Seq.empty[PathRef] }
 
+  /**
+   * All classfiles and resources including upstream modules and dependencies
+   * necessary to run this module's code.
+   *
+   * Unlike `runClasspath`, all class path entries here are JAR files.
+   */
+  def runClasspathAsJars: T[Seq[PathRef]] = Task { Seq.empty[PathRef] }
+
   /**
    * The elements of the run classpath which are local to this module.
    * This is typically the output of a compilation step and bundles runtime resources.
diff --git a/libs/javalib/test/resources/classpath/app/compile-resources/empty b/libs/javalib/test/resources/classpath/app/compile-resources/empty
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/libs/javalib/test/resources/classpath/app/src/app/MyApp.java b/libs/javalib/test/resources/classpath/app/src/app/MyApp.java
new file mode 100644
index 000000000000..55e595e7fa78
--- /dev/null
+++ b/libs/javalib/test/resources/classpath/app/src/app/MyApp.java
@@ -0,0 +1,12 @@
+package app;
+
+import lib.Thing;
+
+public class MyApp {
+  public static void main(String[] args) throws java.net.URISyntaxException {
+    String appUri = MyApp.class.getProtectionDomain().getCodeSource().getLocation().toURI().toASCIIString();
+    String libUri = Thing.class.getProtectionDomain().getCodeSource().getLocation().toURI().toASCIIString();
+    System.out.println("App URI: " + appUri);
+    System.out.println("Lib URI: " + libUri);
+  }
+}
diff --git a/libs/javalib/test/resources/classpath/appAsJars/compile-resources/empty b/libs/javalib/test/resources/classpath/appAsJars/compile-resources/empty
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/libs/javalib/test/resources/classpath/build.mill b/libs/javalib/test/resources/classpath/build.mill
new file mode 100644
index 000000000000..cf6edea9f021
--- /dev/null
+++ b/libs/javalib/test/resources/classpath/build.mill
@@ -0,0 +1,2 @@
+import mill.*
+import mill.javalib.*
diff --git a/libs/javalib/test/resources/classpath/lib/src/lib/Thing.java b/libs/javalib/test/resources/classpath/lib/src/lib/Thing.java
new file mode 100644
index 000000000000..8e68af82229b
--- /dev/null
+++ b/libs/javalib/test/resources/classpath/lib/src/lib/Thing.java
@@ -0,0 +1,7 @@
+package lib;
+
+public class Thing {
+  public static String message() {
+    return "Foo";
+  }
+}
diff --git a/libs/javalib/test/src/mill/javalib/ClasspathTests.scala b/libs/javalib/test/src/mill/javalib/ClasspathTests.scala
new file mode 100644
index 000000000000..95edf1a2b3b5
--- /dev/null
+++ b/libs/javalib/test/src/mill/javalib/ClasspathTests.scala
@@ -0,0 +1,113 @@
+package mill.javalib
+
+import mill.api.{Discover, PathRef, Task}
+import mill.testkit.{TestRootModule, UnitTester}
+import mill.util.TokenReaders.*
+
+import utest.*
+
+import java.net.URI
+import java.nio.file.Paths
+import java.io.ByteArrayOutputStream
+import java.io.PrintStream
+
+object ClasspathTests extends TestSuite {
+
+  object TestCase extends TestRootModule {
+    object lib extends JavaModule
+
+    object app extends JavaModule {
+      def moduleDeps = Seq(lib)
+      def mainClass = Some("app.MyApp")
+    }
+
+    object appAsJars extends JavaModule {
+      def moduleDeps = Seq(app)
+      def mainClass = app.mainClass
+      def compileClasspath = compileClasspathAsJars
+      def runClasspath = runClasspathAsJars
+    }
+
+    lazy val millDiscover = Discover[this.type]
+  }
+
+  val tests: Tests = Tests {
+    test("test") {
+      val sources = os.Path(sys.env("MILL_TEST_RESOURCE_DIR")) / "classpath"
+
+      UnitTester(TestCase, sourceRoot = sources).scoped { eval =>
+        def classPathTaskValue(task: Task[Seq[PathRef]]) =
+          eval(task)
+            .left.map(f => throw f.exception)
+            .merge
+            .value
+            .map(_.path.subRelativeTo(TestCase.moduleDir))
+
+        val libCompileCp = classPathTaskValue(TestCase.lib.compileClasspath)
+        val expectedLibCompileCp = Seq(
+          os.sub / "lib/compile-resources"
+        )
+        assert(expectedLibCompileCp == libCompileCp)
+
+        val appCompileCp = classPathTaskValue(TestCase.app.compileClasspath)
+        val expectedAppCompileCp = Seq(
+          os.sub / "lib/compile-resources",
+          os.sub / "out/lib/compile.dest/classes",
+          os.sub / "app/compile-resources"
+        )
+        assert(expectedAppCompileCp == appCompileCp)
+
+        val appAsJarsCompileCp = classPathTaskValue(TestCase.appAsJars.compileClasspath)
+        val expectedAppAsJarsCompileCp = Seq(
+          os.sub / "out/lib/jar.dest/out.jar",
+          os.sub / "out/app/jar.dest/out.jar",
+          os.sub / "out/appAsJars/compileResourcesAsJars.dest/0.jar"
+        )
+        assert(expectedAppAsJarsCompileCp == appAsJarsCompileCp)
+      }
+
+      def locationFromOutput(kind: String, out: String): os.SubPath = {
+        val uriStr =
+          out.linesIterator.find(_.startsWith(s"$kind URI: ")).get.stripPrefix(s"$kind URI: ")
+        val path = os.Path(Paths.get(new URI(uriStr)))
+        path.subRelativeTo(TestCase.moduleDir)
+      }
+
+      val mainBaos = new ByteArrayOutputStream
+      val jarBaos = new ByteArrayOutputStream
+
+      UnitTester(
+        TestCase,
+        sourceRoot = sources,
+        outStream = new PrintStream(mainBaos, true)
+      ).scoped { eval =>
+        eval(TestCase.app.run()).left.map(f => throw f.exception)
+      }
+      UnitTester(
+        TestCase,
+        sourceRoot = sources,
+        outStream = new PrintStream(jarBaos, true)
+      ).scoped { eval =>
+        eval(TestCase.appAsJars.run()).left.map(f => throw f.exception)
+      }
+
+      val mainOut = new String(mainBaos.toByteArray)
+      val jarOut = new String(jarBaos.toByteArray)
+
+      val appDir = locationFromOutput("App", mainOut)
+      val appJar = locationFromOutput("App", jarOut)
+      val libDir = locationFromOutput("Lib", mainOut)
+      val libJar = locationFromOutput("Lib", jarOut)
+
+      val expectedAppDir = os.sub / "out/app/compile.dest/classes"
+      val expectedAppJar = os.sub / "out/app/jar.dest/out.jar"
+      val expectedLibDir = os.sub / "out/lib/compile.dest/classes"
+      val expectedLibJar = os.sub / "out/lib/jar.dest/out.jar"
+
+      assert(expectedAppDir == appDir)
+      assert(expectedAppJar == appJar)
+      assert(expectedLibDir == libDir)
+      assert(expectedLibJar == libJar)
+    }
+  }
+}