noClassDefFoundError using Scala Plugin for Eclips

问题:

I successfully implemented and ran several Scala tutorials in Eclipse using the Scala plugin. Then suddenly I tried to compile and run an example, and this error came up:

Exception in thread "main" java.lang.NoClassDefFoundError: hello/HelloWorld
Caused by: java.lang.ClassNotFoundException: hello.HelloWorld
 at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
 at java.security.AccessController.doPrivileged(Native Method)
 at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
 at java.lang.ClassLoader.loadClass(ClassLoader.java:315)
 at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:330)
 at java.lang.ClassLoader.loadClass(ClassLoader.java:250)
 at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:398)

After this point I could no longer run any Scala programs in Eclipse. I tried cleaning and rebuilding my project, closing and reopening my project, and closing and reopening Eclipse.

Eclipse version number 3.5.2 and Scala plugin 2.8.0

Here is the original code:

package hello

object HelloWorld {
 def main(args: Array[String]){
  println("hello world")
 }
}

回答1:

If you see this when you attempt to run as a Scala application then the most likely explanation is that your project didn't compile and no class files were generated. Please check whether or not that's the case: look in your project's output folder for hello/HelloWorld.class.

If your project didn't compile that could either be because there's an error which you've missed (and if this error isn't being reported in the Problems view that could be a bug, in which case please open a ticket on Trac) or because you've turned off automatic builds and not done a manual build of your project.

回答2:

I had the same problem. Project doesn't compile but there are no errors highlighted and AFAIK the code is OK. It seems to be a problem with the Run Configurations.

Solution 1: Delete the existing Run Configuration for your object and create a new one

Solution 2: Create a new object and cut / paste all your code into that file

回答3:

When running "clean" does not un-hose Eclipse, I next try saving my work, exiting Eclipse, and re-starting. That usually gets things going again, but not always. A few times I've had to update the Scala plugin with a more recent version (I'm using the latest nightly), to get things working again. I doubt that this worked because the new plugin happened to fix the bug, but rather expect that loading the new plugin gives the whole Eclipse-Scala system a "total reset" that gets it unhosed.

回答4:

I was getting this problem in a project that combined .java & .scala files. The solution for me was:

1. Remove all .java files
2. Edit the scala code as needed so it compiles without them.
3. Add the .java files back in.
4. Edit the scala code back.

The other solutions given here didn't work for me. I tried: clean project, restarting Eclipse, closing-&-opening the project, creating a new .scala file. No joy.

I'm using Eclipse 3.7 (latest stable), Scala IDE 2.0.0 and Scala 2.9 on Ubuntu Linux 11.10.

The symptoms in my case were:

• My project was working, but then it stopped compiling for no apparent reason. The IDE didn't show any compilation errors for .scala files, but there were no .class files in the output directory & I got a NoClassDefError if I tried to run anything.
• If I created a deliberate error in a .scala file, that did get picked up as a compilation error.
• The .java files were registering errors due to the missing scala classes.

I suppose there's probably a boot-strapping bug somewhere in the IDE plugin for .java/.scala mixes. I've done hybrid projects with this setup without problems, so it's only triggered in some situations. I don't know what the trigger is, but once triggered, there's no nice solution.

回答5:

I had moved my one and only class/object/application to a package, but had not added the package declaration.

sbt compiled and ran fine; eclipse would not

Adding the package declaration at the top of the file fixed it.

Scala 2.8.3 plugin; no compile error

回答6:

I encountered this error too but after doing the suggestions here (cleaning, deleting Run Configuration etc), I realized that I set the workspace wrongly that is why the class is not being found.

An indication that this is a problem is when the same error occurs when you try to compile a java project.

回答7:

I encountered this error (compilation worked in sbt but failed in eclipse) when I created a new package object called "common". Deleting the package object in eclipse caused the compile error to go away. There was nothing in it.

I was using sbt-eclipse to build the eclipse project. I'm using scala eclipse 3.0.0-vfinal-20130326-1146-Typesafe.