Enforcing Java version for Scala project in sbt?

My scala application will only run with Java 7 as it depends on libraries that only appeared in that version of the JDK.

How do I enforce that in sbt, so that the correct error message is shown immediately to the user if she is using the wrong version of Java when starting sbt to run/compile the application?

NOTE: There is NO Java™ source code to compile here. I only have Scala source code. The Scala code requires an import java.nio.file.Path that's available from Java 7.

Using javacOptions ++= Seq("-source", "1.7", "-target", "1.7") does not work if you have no Java sources.

But you can set the target JVM for the Scala compiler in build.sbt or Build.scala:

scalacOptions += "-target:jvm-1.7"

As a result it prints on a JDK 6:

$ sbt clean run
[info] Set current project to default-cd5534 (in build file:/tmp/so/)
[success] Total time: 0 s, completed 27.10.2013 14:31:43
[info] Updating {file:/tmp/so/}default-cd5534...
[info] Resolving org.fusesource.jansi#jansi;1.4 ...
[info] Done updating.
[info] Compiling 1 Scala source to /tmp/so/target/scala-2.10/classes...
[info] Running Main 
[error] (run-main) java.lang.UnsupportedClassVersionError: Main : Unsupported major.minor version 51.0
java.lang.UnsupportedClassVersionError: Main : Unsupported major.minor version 51.0
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:634)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
        at java.net.URLClassLoader.access$000(URLClassLoader.java:73)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:212)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:314)
[trace] Stack trace suppressed: run last compile:run for the full output.
java.lang.RuntimeException: Nonzero exit code: 1
        at scala.sys.package$.error(package.scala:27)
[trace] Stack trace suppressed: run last compile:run for the full output.
[error] (compile:run) Nonzero exit code: 1
[error] Total time: 4 s, completed 27.10.2013 14:31:47

Note: Maybe it works only for the latest SBT/Scalac version.

For anybody in the future, this is also a good way to do it. It halts execution immediately if it cannot find the right Java version:

initialize := {
  val _ = initialize.value // run the previous initialization
  val required = "1.8"
  val current  = sys.props("java.specification.version")
  assert(current == required, s"Unsupported JDK: java.specification.version $current != $required")
}

You put this in your build.sbt.

Enforcing Java version for Scala project in sbt?

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