How to use Scala in IntelliJ IDEA (or: why is it so difficult to get a working IDE for Scala)?

Question

I recently gave up trying to use Scala in Eclipse (basic stuff like completion doesn't work). So now I'm trying IntelliJ. I'm not getting very far.

I've been able to edit programs (within syntax highlighting and completion... yay!). But I'm unable to run even the simplest "Hello World". This was the original error:

Scala signature Predef has wrong version
Expected 5.0
found: 4.1 in .... scala-library.jar

But that was yesterday with IDEA 9.0.1. See below...

UPDATE

Today I uninstalled IntelliJ 9.0.1, and installed 9.0.2 Early Availability, with the 4/14 stable version of the Scala plug-in.

Then I setup a project from scratch through the wizards:

• new project from scratch
• JDK is 1.6.u20
• accept the default (project) instead of global / module
• accept the download of Scala 2.8.0beta1 into project's lib folder

Created a new class:

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

For my efforts, I now have a brand-new error :)

Here it is:

Scalac internal error: class java.lang.ClassNotFoundException [java.net.URLClassLoader$1.run(URLClassLoader.java:202), java.security.AccessController.doPrivileged(Native Method), java.net.URLClassLoader.findClass(URLClassLoader.java:190), java.lang.ClassLoader.loadClass(ClassLoader.java:307), sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301), java.lang.ClassLoader.loadClass(ClassLoader.java:248), java.lang.Class.forName0(Native Method), java.lang.Class.forName(Class.java:169), org.jetbrains.plugins.scala.compiler.rt.ScalacRunner.main(ScalacRunner.java:72)]

FINAL UPDATE

I uninstalled 9.0.2 EA and reinstalled 9.0.1, but this time went with the 2.7.3 version of Scala rather than the default 2.7.6, because 2.7.3 is the one shown in the screen-shots at the IntelliJ website (I guess the screen-shots prove that they actually tested this version!). Now everything works!!!

Aggregation

Have you added scala compiler and library jars to project dependencies? – Rhaetic

Had a similar problem, but in my case it was because of a package declaration at the top of the file – Davila

Nowadays everything has changed, both Idea and Scala. Idea has the SBT integration now. All the answers are irrelevant now, and all this stuff is easier now. – Whence

Answer 1

I have encountered the same scalac error when trying to run a Scala project in Intellij Idea 9.0.2 and I've managed to find a solution by chance :). These are the steps I took in creating the project and running it.

I have created a Scala project in Intellij Idea 9.0.2 final (it was released today). I have installed the Scala plugin, restarted the IDE and created a new Scala project (with the name "TestScala") with scala-2.8.0.Beta1 as project library. Once the project is created and the scala libraries downloaded, I have created a Test.scala file with the following content:

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

After that, I created a launch configuration ("Edit Configurations"), choosing the "Application" template. I set as main class Test and choose the project name ("TestScala") in the "Use classpath and JDK of module" combo box. When I run the configuration I get the same error as you reported ("Scalac internal error: class java.lang.ClassNotFoundException") .

Now comes the freaky part :). I right click on the project, choose "Module Settings", have a look on all settings but I don't change anything . Click "apply" and "ok", try to run configuration again and it works :) .

I use Intellij Idea 9.0.2 the final release (build 95-66); Ubuntu 9.10 and JDK 1.6.0_18. I also have to mention that I had a JDK configured in Intellij, otherwise there is an extra step to configure it.

UPDATE:

When checking the setting of the module, one needs to click on the Module->Scala and Facets->Scala (expand it and click on Scala(ProjectName)) . Both of these settings are about the scala compiler and scala library location. I would guess these values are not properly set when the project is created but are saved once the user touches them and saves the settings.

Uncharted

Thanks! This is consistent with what I observed, but was not immediately obvious until I saw your answer! – Aggregation

OMG! Thanks a lot for this hint! Saved my much much much trouble! :) – Blisse

Genius. Thank you, and I will blame the IntelliJ developers at the next possible occasion. – Flashgun

Answer 2

To answer your question, it's difficult to get a working IDE for Scala for two reasons: (a) Scala is only just beginning to reach a wide audience and (b) due to (a), there is no business case for spending time on a Scala IDE.

Also, if you are old enough to cast your mind back and young enough to still remember, you would know that for the first five or more years of Java, we were stuck with okay-ish tools like JBuilder that did little more than compile your code when you said so - no error highlighting, no auto-importing, and the word refactoring didn't even exist. If you want to pioneer, you need to be prepared to cut some of the road yourself, or at least bush-bash.

I know it won't help you, but I have successfully used IDEA for Scala on Linux, Mac and Windows. I typically have the Scala SDK installed somewhere locally and point IDEA at that rather than using the 'download' option.

Presently, I am mostly using an EAP version of IDEA 9 on Mac OS X with Scala 2.8.0.Beta1-RC5 and it's working well (except that fsc doesn't seem to worked with mixed sources).

You could try your luck over at the IDEA Scala Plugin Discussion Forum, though I haven't had a great lot of responses to my own postings there.

Mcadams

Answer 3

Installing the plug-in is prerequisite one.

The next thing you should do is define a library (global or project-specific; I use global) that holds the Scala library and compiler JAR files (at a minimum, that's scala-compiler.jar and scala-library.jar). Adding source JARs and a documentation JAR or URLs is a good idea, too. Then make this library a dependency of any modules in your project that include Scala code.

Lastly, find the Scala facets in those modules and de-select both check-boxes there.

Hrvatska

Answer 4

I just did a fresh install and had exactly this same problem myself. It turned out that, because I had created the file in the root package, IDEA had added a package statement at the top with naming a package. I assume that this then got compiled as "package object Main" - valid syntax in 2.8? Anyway, I deleted the line that said package and it all worked fine.

Mcadams

Answer 5

I had the same problem yesterday while trying to set it up. Solution is pretty simple, you just have to set scala somewhere in project settings.

Beast

I did go to Settings to add the Scala Plugin. Is there something ELSE I need to do? Thanks – Aggregation

The only unobvious thing thats not on main tutorial is to add scala library to yours module dependency. This is in Module setting -> dependecies tab AFAIR. Works perfectly even with 2.8 rc. – Beast

Answer 6

You are mixing code compiled with two different Scala versions.

Kelle

That's impossible... it's only a 3-line "hello world" program! – Aggregation

no, really ... 3 lines of code cannot possibly have been "compiled with two different Scala versions". There is a possibility IntelliJ is finding files from prior experiments; so I'm going to try a full uninstall/reinstall now. – Aggregation

BTW what IDE do you use? Thanks – Aggregation

Me? I use IDEA. I find it works well and has for quite a while. As to the mixed-version hypothesis, it applies across all the code, so you have to execute with libraries compiled by the same version of the Scala compiler as that with which your own code was compiled. – Hrvatska

Answer 7

I use Netbeans to write scala programs. So far it works very well with my codes. You can try the plugin here: http://wiki.netbeans.org/Scala68v1.

Hobson

Answer 8

I was getting this error and also had to right click on the project and "Open Module Settings". However, it was more than just hitting apply. I had to make sure that my Content Root was correct for each project. For some reason, there were some incorrect Source and Test Folders.

My project uses maven as the main build tool and importing the project into Intellij is probably what created these incorrect settings.

Invocate

Hm, actually I wanted to suggest to build IDEA projects from maven build files, since this is how I successfully created more than 10 projects in the last 12 months. – Soyuz

Answer 9

I had similar problem, following this blog post instructions solved the problem for me

Divorce