Haskforce Plugin interferes with Java Builds

First of all thanks for the great work on the Haskforce plugin. It is very useful while taking my first steps in the Haskell world.

However I ran into an issue with a Java project today, that uses 1.6 as target JVM. The default JVM installed on my system is 1.8. Got Intellij2016 with Haskforce 0.3-beta.32 plugin installed. When when I tried to rebuild my Java project today, I got the following error:

Error:Internal error: (java.lang.UnsupportedClassVersionError) com/intellij/execution/ExecutionException : Unsupported major.minor version 52.0
java.lang.UnsupportedClassVersionError: com/intellij/execution/ExecutionException : Unsupported major.minor version 52.0
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(ClassLoader.java:637)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:621)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
    at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
    at com.haskforce.jps.HaskForceBuilder.createModuleLevelBuilders(HaskForceBuilder.java:20)
    at org.jetbrains.jps.incremental.BuilderRegistry.<init>(BuilderRegistry.java:54)
    at org.jetbrains.jps.incremental.BuilderRegistry.<init>(BuilderRegistry.java:33)
    at org.jetbrains.jps.incremental.BuilderRegistry$Holder.<clinit>(BuilderRegistry.java:36)
    at org.jetbrains.jps.incremental.BuilderRegistry.getInstance(BuilderRegistry.java:43)
    at org.jetbrains.jps.cmdline.BuildRunner.runBuild(BuildRunner.java:132)
    at org.jetbrains.jps.cmdline.BuildSession.runBuild(BuildSession.java:294)
    at org.jetbrains.jps.cmdline.BuildSession.run(BuildSession.java:125)
    at org.jetbrains.jps.cmdline.BuildMain$MyMessageHandler$1.run(BuildMain.java:232)
    at org.jetbrains.jps.service.impl.SharedThreadPoolImpl$1.run(SharedThreadPoolImpl.java:44)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:439)
    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
    at java.util.concurrent.FutureTask.run(FutureTask.java:138)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
    at java.lang.Thread.run(Thread.java:695)

Since the stacktrace shows some Haskforce methods, I decided to disable the Haskforce plugin. After that rebuilding Java projects worked fine again.

Note that things are also working fine if I configure 1.8 as target JVM also. Myself and a colleague were able to reproduce the issue with a minimal Java project also.

[carymrobbins]

Note that the resolution to this was to simply not run the module builders if the running Java version was < 8. So using Java 8 features should generally be ok.

[carymrobbins]

@LeanderK - No, actually HaskForce is built with Java 8. However, the external builder modules (jps-shared and jps-plugin) target Java 6. So only those modules are restricted to Java 6 features.