CODEWITHSUNDEEP
javascala
Tim
Tim

Reputation: 99428

Does `scala` not need -jar, while `java` does, when executing a .jar file?

I was wondering why scala doesn't need -jar, while java does, when executing a .jar file?

For example:

scala target/some.jar

java -jar target/someother.jar

I look at the scala shell script, it invokes java as:

execCommand \
  "${JAVACMD:=java}" \
  $JAVA_OPTS \
  "${java_args[@]}" \
  "${classpath_args[@]}" \
  -Dscala.home="$SCALA_HOME" \
  $OVERRIDE_USEJAVACP \
  $WINDOWS_OPT \
   scala.tools.nsc.MainGenericRunner  "$@"

Is it correct that scala passes target/some.jar as part "$@" to java, and therefore the java command doesn't prefix target/some.jar with -jar?

Thanks.

Upvotes: 0

Views: 83

Answers (1)

Mario Galic
Mario Galic

Reputation: 48420

Analysing the source code of MainGenericRunner we see scala runner tries to guess what the user intended

  def runTarget(): Option[Throwable] = howToRun match {
    case AsObject =>
      ObjectRunner.runAndCatch(settings.classpathURLs, thingToRun, command.arguments)
    case AsScript if isE =>
      ScriptRunner(settings).runScriptText(combinedCode, thingToRun +: command.arguments)
    case AsScript =>
      ScriptRunner(settings).runScript(thingToRun, command.arguments)
    case AsJar    =>
      JarRunner.runJar(settings, thingToRun, command.arguments)
    case Error =>
      None
    case _  =>
      ...
  }

where howToRun potentially calls guessHowToRun

  private def guessHowToRun(target: String): GenericRunnerCommand.HowToRun = {
    if (!ok) Error
    else if (io.Jar.isJarOrZip(target)) AsJar
    else if (ScalaClassLoader.classExists(settings.classpathURLs, target)) AsObject
    else {
      val f = io.File(target)
      if (!f.hasExtension("class", "jar", "zip") && f.canRead) AsScript
      else {
        Console.err.println("No such file or class on classpath: " + target)
        Error
      }
    }
  }

We can be explicit by providing -howtorun flag:

scala -howtorun:jar some.jar

Scala apps can also be run via java -jar, for example, if we specify the main class in build.sbt

Compile / mainClass := Some("example.MyMain")

and execute sbt assembly, then the produced fat jar under target/ can be executed like so

java -jar myapp-assembly.jar

Upvotes: 5

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