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")
}
}
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.
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
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.
I was getting this problem in a project that combined .java & .scala files.
The solution for me was:
Remove all .java files
Edit the scala code as needed so it compiles without them.
Add the .java files back in.
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.
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
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.
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.
Related
Using Eclipse 2018.12, I've installed the Eclipse Kotlin plugin (0.8.20.20200316-1305), created a Kotlin project (using the "New..." options under "Kotlin") and wrote a "Hello World!" program:
Test.kt:
fun main() = System.out.println("Hello World!")
However, when I try to run it I get the message
Error: Could not find or load main class TestKt
Caused by: java.lang.ClassNotFoundException: TestKt
Upon further inspection it seems the class has never been compiled. There is no corresponding .class file anywhere. Under the project directory I see Eclipse configuration directory .settings and files .project and .classpath. Besides that, I have an empty bin directory and a src directory with Test.kt only.
I found two similar questions about this in SO:
In Unable to Run Kotlin Application in Eclipse, the accepted answer simply indicates working with a new version of Eclipse and Kotlin plugin, both older than what I have now. Besides, the problem may have been solved in that case due to a new installation rather than the version.
Kotlin - Error: Could not find or load main class _DefaultPackage is quite old and the accepted answer does not apply anymore. It was about not naming the main class properly, but in my case there is not even a byte code file to be found.
Running "Project -> Compile Kotlin classes" had no effect.
How can I get this simple example to run?
Update: I've updated to 2020-06 (not that it should matter since Eclipse Kotlin lists 2018-12 in its requirements) and replaced Zuly by AdoptOpenJDK HotSpotJDK 11. The error persists. The Eclipse log does not show any related messages.
Update 2: re-created the project in a brand-new workspace but the problem persists.
I'm very new to Scala and I tried to run Scala project under Eclipse. I used sbt to create one, then ran sbteclipse to prepare it for Eclipse and imported it successfully. However when I try to run it I get
Error: Unable to initialize main class Main
Caused by: java.lang.NoClassDefFoundError: scala/Function0
error. Scala, sbt and Java are installed, because when I try to run the same project via console, using sbt, it works. What am I missing?
Thanks for any help!
It looks like the classpath of your Eclipse project is incomplete: it's missing the Scala library. Can you double check in Project Settings that the scala library is present?
If all you want to do is try a simple program, an simpler solution is to create a New Scala Project using the Eclipse wizard.
I just set up a Scala project in Eclipse (Oxygen 4.7.3a, on Ubuntu 18.04) with the Scala-IDE plugin (4.7.0.v-2_12-201709291352-71a28d0) with a simple HelloWorld object. If I set "Properties -> Scala Compiler" to 2.12 everything works as expected: the project compiles and runs. But if I choose any other version (for instance, "Latest 2.10 bundle (dynamic)" I get prompted to do a full rebuild (as expected) but when I go to "Project -> Build" nothing happens. If I have errors in my code, the editor underlines them, but they won't show in "Problems". If I try to run it I get a class not found error.
I get the feeling that it is not actually compiling. Any ideas on what I need to configure differently?
I finally solved it by removing openjdk-11 and installing openjdk-8. After doing that and reinstalling Eclipse and Scala-IDE everything works fine.
It would have been helpful having some feedback from Eclipse regarding what was not working. I figured it out because I tried to build with SBT and the error lead to this SO question: Failed to initialize compiler: object java.lang.Object in compiler mirror not found
I tried the update-alternatives option (also changing the Java path in eclipse.ini) but then I got the "object java.lang.Object in compiler mirror not found" error in Eclipse problems. Since I didn't need openjdk-11 anyway I simply switched to openjdk-8.
I've created a play project with play 2.3.7
In the project directory, I ran activator and ran the eclipse command to generate eclipse project files.
When I go to eclipse (I'm using the Scala IDE from typesafe Build id: 4.0.0-vfinal-20150119-1023-Typesafe , there is an error in my Application.scala file:
object index is not a member of package views.html
Is there something amiss with my setup? The app runs fine when I execute run at the activation console prompt.
Thanks!
EDIT: Added code
package controllers
import play.api._
import play.api.mvc._
object Application extends Controller {
def index = Action {
Ok(views.html.index("Your new application is ready."))
}
}
The error is on the 'Ok..' line.
There is a file in views called index.scala.html, and the app runs file when I run it from activator..
Occasionally after adding a view in Play 2.4.x, IntelliJ IDEA sometimes gets confused and absolutely refuses to build. Even rebuild Project fails:
This still happens from time-to-time in IDEA 15. And when it does, the command line provides the quickest, most-reliable fix:
sbt clean; sbt compile
That's it! IDEA will now compile the project as expected.
Update:
In the rare case that sbt compile completed successfully on the command line, but IntelliJ IDEA 15 still gives the same "object x is not a member" error, then this has solved IDEA's confusion:
File Menu:
The other solutions did not work for me. Some would give me different errors, some would clear the Problems tab but leave me with a red squiggle under views.html.index and auto-complete would not work with the scala.html templates.
What finally worked was to open the project's properties, go to Java Build Path > Source, and add both of the following directories:
target/scala-2.11/src_managed/main
target/scala-2.11/twirl/main
If you only do target/scala-2.11/twirl/main then you'll miss out on the class files generated from the conf directory.
In Scala IDE 4.0.0 thinks there's errors in an out-of-the-box Play Framework 2.3.7 program you can find the solution (in brief: adding target/scala-2.11/twirl/main folder to the compilation path), give it a try.
I had the same problem. I added target/scala-2.x/classes and target/scala-2.x/classes_managed to my Java build path and Eclipse stopped complaining.
Adding target/scala-2.11/twirl/main which is having views.html package to source fixed for me.
I had the same issue running Play 2.4.0-RC1 using default SBT layout (disablePlugins(PlayLayoutPlugin)) and solved it by adding to build.sbt:
sourceDirectories in (Compile, TwirlKeys.compileTemplates) :=
(unmanagedSourceDirectories in Compile).value
#brent-foust 's answer worked for me but only initially. Every time I rebuilt the project from within IDEA I would then get "not found: routes" errors from within target\scala-2.11\twirl\main\views\html\main.template.scala until I performed Brent's workaround again.
I eventually discovered the solution to that was changing a line in the .iml file from
<excludeFolder url="file://$MODULE_DIR$/target/scala-2.11/src_managed/main" />
to
<sourceFolder url="file://$MODULE_DIR$/target/scala-2.11/src_managed/main" isTestSource="false" />
I don't know what the long term implications of doing this are but it has fixed this problem. Some of the other similar problems mentioned might also be fixed by applying the same change to some of the other folders listed in the .iml.
I tried all solutions without any positiv result.
So I went to Preferences > Build, Execution, Deployment > Build Tools > sbt and checked Use sbt shell for imports and builds.
This let the compile button in intelliJ compile with the sbt shell.
I think this is better anyway, since a build server or something similar will compile the same way and not like intelliJ.
For me when importing the project into intellij making sure I "checked" the "auto import" checkbox did the trick.
1) Add the following line to your sbt.build file:
EclipseKeys.preTasks := Seq(compile in Compile)
2) Add the follwing line to your plugins.sbt file under the project folder:
addSbtPlugin("com.typesafe.sbteclipse" % "sbteclipse-plugin" % "5.1.0")
3) Run the "eclipse" command from within sbt
as explained in the documentation of the play framework:
Setting up your preferred IDE
Basically we need a way to put the compiled classes on the path for this to work.
I did the following to fix it.
Since the projects compiles to the target directory.
I went to the Project Properties -> Java Build Path and added a few folders that look like this,
target/scala-2.12/routes/main
target/scala-2.12/twirl
target/scala-2.12/twirl/main
Now i dont want you to assume you will have these exact folders in your case too. That depends on your project setup.
But you should add the folders inside the target/scala-2.x folder.
So, I downloaded Scala and configured paths, I can run Scala console from terminal, Scala plugin is installed and "hello world" is running...
The problem is that, when I write a "hello world" program:
object First {
def main(args: Array[String]): Unit = {
println(12)
}
}
it says: Cannot resolve symbol println
As I said, I can run this program and it prints out 12... Also, if I create a List or an array it "cannot resolve symbol" but everything runs with no problem at all...
In most cases I've found, there was problem with Java set up, but that's not the case here...
Within File->Project Structure, make sure that there is a scala library in Project Settings\Libraries
Or, make sure that you have added scala-compiler.jar, scala-library.jar to your project.
If it is still acting strange, try File->Invalidate Cache/Restart
I had a similar issue with IntelliJ for an SBT project I started, with a correctly installed Scala 2.11 library, etc. Invalidate Cache fixed it so that IntelliJ could find the symbols.
Ensure, that you have scala-library.jar and scala-compiler in your project libraries.
Then try invalidating cache (File->Invalidate Caches/Restart->Invalidate and Restart).
If it's still not working, try reloading all your maven projects. You can either reimport them manually or go to Maven Projects->Reimport All Maven Projects (blue arrows).
I had similar issue and the last thing worked for me.
I hope it helps :)
I have the same issue when I use the idea 15, and fixed it in these 2 steps:
File -> Project Structure -> Libraries -> + -> Scala SDK -> your version -> OK
Maven Projects -> choose your scala project -> Lifecycle -> clean -> compile
Done
Don't know if this will help, but it worked with my environment. Navigate to:
File > Project Structure > Modules
Then, when I tried to apply a minor change, I got a message about how the Content Roots was being shared between two different Modules (a conflict). After removing the conflicting Content Root(s) from one of the Modules, IntelliJ started resolving symbols correctly.
You will see Content Roots on the right-hand side of the dialog box under module "Sources" tab.
I have no idea if the conflict in "Content Root" was what kept IntelliJ from resolving symbols, but fixing this error cleared everything up without having to change anything else.
if your version IntelliJ IDEA 2016.3.4
Worksheet configuration error:: Can't find Scala module to run
project structure > Modules > + > new modules > scala > scala
I had this problem - it turned out to be caused by me upgrading Java on my Mac, so that my JDK's path (/Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home) was no longer valid. I went into my Project Structure settings and updated the path to /Library/Java/Home, then the project could see the proper Java libraries.