Scala project won't compile in Eclipse; “Could not find the main class.”

问题

I have installed Eclipse 3.5.2 and today's Scala plugin from /update-current (that's Scala 2.8 final.) I can compile and run Scala projects consisting of a single singleton object that implements main().

But, if a project contains more classes, I receive the "Could not find the main class" error.

I have tried searching for the solution and I discovered:

Eclipse is correctly looking for the Main$ class, not the Main class
* under Debug Configurations, my main class is correctly identified as mypackage.Main
* my plugin is up to date and recommended for my version of Eclipse
* cleaning, restarting etc. doesn't help.

The same project will compile with scalac.

Thanks for any ideas on how to solve this.

EDIT: MatthieuF suggested I should post the code.

This snippet produces an error. It's not the most idiomatic code, but I wrote it that way to test my environment. I tried it as a single file and as separate files. It DOES work with scalac.

import swing._

class HelloFrame extends Frame {
        title = "First program"
        contents = new Label("Hello, world!")
}

object Hello {
  val frame = new HelloFrame    
  def main(args : Array[String]) : Unit = {
        frame.visible = true
   }
}

BUT, if I nest the definition of HelloFrame within Hello, it works. This snippet runs perfectly:

import swing._

object Hello {

    class HelloFrame extends Frame {
        title = "First program"
        contents = new Label("Hello, world!")
    }

    val frame = new HelloFrame

    def main(args : Array[String]) : Unit = {
        frame.visible = true
    }
}

回答1:

For me, the problem was that there was a build error (see Problems tab) which was preventing compilation; oops! The reason you see the error is that the run macro proceeds despite the failed compilation step, and attempts to run class files it expects to be there; they don't exist because there was a build error preventing compilation, so it says it can't find Main (not compiled).

Problem goes away when build can complete successfully, i.e. errors are fixed.

I guess, theoretically, there may be more complicated reasons your build is not completing successfully that are not listed in Problems.

回答2:

One possibility is that you are trying to launch using ctrl-F11, but from a different class.

The Scala Eclipse plugin does not obey the defaults for Java launching. In Preferences->Run/Debug->Launching, there are some options Launch Operation->Always Launch the previously selected application, etc. This currently does not work in the Scala eclipse plugin. To launch a specified main, you need to launch it from the editor for the class.

There has been a bug raised for this. http://scala-ide.assembla.com/spaces/scala-ide/tickets/1000023-scala-launch--does-not-follow-jdt-behaviour

EDIT: This is now (mostly) fixed.

回答3:

For me it was Eclipse specific problem. I noticed that .class file wasn't built at all. So bin directory doesn't have compiled classes. When I manually compiled *.scala file using *.sbt and copied it to bin directory it was working as expected. I tried different tips and tricks and it wasn't worked until I reinstalled Scala plugin in Eclipse .

回答4:

I'd solve similar problem by executig "Project->Clean.." with next automatically building.

回答5:

I had the same error message with a Java application made by myself.

The problem was that I deleted (though inside Eclipse) a jar that belonged to the Java build path, without deleting it from the Java build path (project's Properties window). When I did it the class could compile and run again.

回答6:

Make sure that the .class files exist, usually below the bin directory.

In particular, if you have errors in unrelated files in the same project then the compilation may fail, and no .class files will be produced.

回答7:

There can be the case of projects, containing errors, added to the build path of the application which prevents the completion of successful compilation. Make sure you remove any such project from the build path before running the application.

Removing these projects solved the problem for me.

回答8:

Do you have a proper build tool setup? Like sbt have you installed it?

You can check its version by $sbt --version

If it is not setup you can download from here http://www.scala-sbt.org/download.html

You might have to restart your eclipse after installation.

回答9:

Just copy your XXX.scala file code. Remove the package and create a new Scala Class. Paste your XXX.scala code. (If you are using maven, do a maven clean and build.) Run configuration again. This works for me.

回答10:

I have faced this issue. I have just deleted the package name, created scala class, Written the same code, Set Build to "Build Automatically". Finally, It works perfectly fine.

回答11:

Check scala-ide.log

For me the issue was that there were errors on:

AppData\Local\Temp\sbt_10d322fb\xsbt\ClassName.scala:16: error: not found: value enteringPhase enteringPhase(currentRun.flattenPhase.next) { s fullName separator }

回答12:

If you are using Intellij, mark directory as source root

来源：https://stackoverflow.com/questions/3953468/scala-project-wont-compile-in-eclipse-could-not-find-the-main-class

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eclipse

scala

eclipse-plugin

scala-ide