NoClassDefFoundError with sbt and scala.swing

Question

I'm new to JVM land so I apologize if this is a common problem. I'm using Scala (2.12) with sbt 0.13.13 on OSX.

I'm working on a tiny app that depends on the GUI library scala.swing (2.10.x). I ran into a runtime issue almost immediately with example code (http://otfried.org/scala/index_28.html).

Specifically, when invoking sbt run I get a stacktrace leading with:

[error] (run-main-0) java.lang.NoClassDefFoundError: scala/Proxy$class
java.lang.NoClassDefFoundError: scala/Proxy$class
    at scala.swing.Window.<init>(Window.scala:25)
    at scala.swing.Frame.<init>(RichWindow.scala:75)
    at scala.swing.MainFrame.<init>(MainFrame.scala:19)

(Proxy appears to be a class/trait in the scala stdlib)

Reading on SO and elsewhere suggests this kind of exception is typically emitted when code present at compile time cannot be located subsequently at runtime. Indeed, the code compiles just fine, it is only when running the code that the problem occurs.

All suggestions I've found are to reconcile your classpath to resolve these issues. However, if the sbt console is to believed, my compile-time and run-time classpaths are identical:

> show compile:fullClasspath
[info] * Attributed(/Users/chris/Projects/thing2/target/scala-2.12/classes)
[info] * Attributed(/Users/chris/.ivy2/cache/org.scala-lang/scala-library/jars/scala-library-2.12.1.jar)
[info] * Attributed(/Users/chris/.ivy2/cache/org.scala-lang/scala-swing/jars/scala-swing-2.10.6.jar)
[success] Total time: 0 s, completed Dec 24, 2016 7:01:15 PM
> show runtime:fullClasspath
[info] * Attributed(/Users/chris/Projects/thing2/target/scala-2.12/classes)
[info] * Attributed(/Users/chris/.ivy2/cache/org.scala-lang/scala-library/jars/scala-library-2.12.1.jar)
[info] * Attributed(/Users/chris/.ivy2/cache/org.scala-lang/scala-swing/jars/scala-swing-2.10.6.jar)
[success] Total time: 0 s, completed Dec 24, 2016 7:01:19 PM

So, I find myself at a bit of a forensic impasse. Any suggestions on where to look next would be much appreciated. For clarity, this has only happened with scala.swing thus far. I have a couple other small projects in Scala that haven't had any issues. What's perplexing is the "missing" class seems to be scala standard lib material.

Answer 1

NoClassDefFoundError points to a problem where you mix libraries that were compiled for different major Scala versions. If you use Scala 2.12, you must also use the Swing module with a matching version. Before Scala 2.11, Swing has been published with an artifact like this:

"org.scala-lang" % "scala-swing" % scalaVersion.value

It was then moved to the org.scala-lang.modules group. Your build file should contain something like this:

scalaVersion := "2.12.1"

libraryDependencies += "org.scala-lang.modules" %% "scala-swing" % "2.0.0-M2"

(it seems the latest Scala 2.11 compatible version "1.0.2" has not been published for Scala 2.12, and so you need to jump straight to "2.0.0-M2" which should be mostly source compatible).