Merge pull request #1669 from kretes/1648-warn-about-main-classes

Fix for https://github.com/sbt/sbt/issues/1648 - warn about multiple or no main classes

main/src/main/scala/sbt/Defaults.scala

@@ -264,9 +264,9 @@ object Defaults extends BuildCommon {
     definedSbtPlugins <<= discoverPlugins,
     discoveredSbtPlugins <<= discoverSbtPluginNames,
     inTask(run)(runnerTask :: Nil).head,
-    selectMainClass := mainClass.value orElse selectRunMain(discoveredMainClasses.value),
+    selectMainClass := pickMainClass(discoveredMainClasses.value) orElse askForMainClass(discoveredMainClasses.value),
     mainClass in run := (selectMainClass in run).value,
-    mainClass := selectPackageMain(discoveredMainClasses.value),
+    mainClass := pickMainClassOrWarn(discoveredMainClasses.value, streams.value.log),
     run <<= runTask(fullClasspath, mainClass in run, runner in run),
     runMain <<= runMainTask(fullClasspath, runner in run),
     copyResources <<= copyResourcesTask
@@ -662,10 +662,22 @@ object Defaults extends BuildCommon {
       new Package.Configuration(srcs, path, options)
     }
 
-  def selectRunMain(classes: Seq[String]): Option[String] =
+  @deprecated("use Defaults.askForMainClass", "0.13.7")
+  def selectRunMain(classes: Seq[String]): Option[String] = askForMainClass(classes)
+  @deprecated("use Defaults.pickMainClass", "0.13.7")
+  def selectPackageMain(classes: Seq[String]): Option[String] = pickMainClass(classes)
+  def askForMainClass(classes: Seq[String]): Option[String] =
     sbt.SelectMainClass(Some(SimpleReader readLine _), classes)
-  def selectPackageMain(classes: Seq[String]): Option[String] =
+  def pickMainClass(classes: Seq[String]): Option[String] =
     sbt.SelectMainClass(None, classes)
+  private def pickMainClassOrWarn(classes: Seq[String], logger: Logger): Option[String] = {
+    classes match {
+      case Nil                           => logger.warn("No main class detected")
+      case multiple if multiple.size > 1 => logger.warn("Multiple main classes detected.  Run 'show discoveredMainClasses' to see the list")
+      case _                             =>
+    }
+    pickMainClass(classes)
+  }
 
   def doClean(clean: Seq[File], preserve: Seq[File]): Unit =
     IO.withTemporaryDirectory { temp =>

notes/0.13.7.markdown

@@ -11,6 +11,7 @@
   [@tmandke]: https://github.com/tmandke
   [@topping]: https://github.com/topping
   [@WarsawScala]: https://github.com/WarsawScala
+  [@kretes]: https://github.com/kretes
   [1237]: https://github.com/sbt/sbt/issues/1237
   [1430]: https://github.com/sbt/sbt/issues/1430
   [1544]: https://github.com/sbt/sbt/issues/1544
@@ -33,6 +34,7 @@
   [1631]: https://github.com/sbt/sbt/pull/1631
   [1642]: https://github.com/sbt/sbt/pull/1642
   [1683]: https://github.com/sbt/sbt/pull/1683
+  [1648]: https://github.com/sbt/sbt/pull/1648
 
 ### Fixes with compatibility implications
 
@@ -49,6 +51,7 @@
 - Adds support for publishing to a Maven repository with `file` URLs. [#1618][1618] by [@jsuereth][@jsuereth]
 - Don't hardcode existing relations in `TextAnalysisFormat`. [#1572][1572] by [@Duhemm][@Duhemm]
 - Adds `developers` key. [#1590][1590] by [@jedesah][@jedesah]
+- Will warn when none or multiple main classes detected. [#1648][1648] by [@kretes][@kretes]
 
 ### Bug fixes