Running a JUnit 4 test from a Scala script

I'm using Scala now and then as a language for side projects. One of the nice things about Scala is that you can bootstrap with a single-file script, similar to Python or Ruby. Since I enjoy building things with test-driven development, it's not long after I write a script that I want to start writing tests for classes and functions. However, when I tried to do this in the straight-forward way, it doesn't quite work. Herein, the problem, and its solution.

As a first attempt, I expected this to work:

	class ArithmeticTest {
	@org.junit.Test def basicArithmetic() {
	org.junit.Assert.assertEquals(4, 2+2)
	}
	}
	import org.junit.internal.TextListener
	val core = new org.junit.runner.JUnitCore()
	core.addListener(new TextListener(new org.junit.internal.RealSystem()))
	core.run(classOf[ArithmeticTest])

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However, running this produces a failure in the infrastructure:

	.E
	Time: 0.004
	There was 1 failure:
	1) initializationError(Main$$anon$1$ArithmeticTest)
	java.lang.Exception: Test class should have exactly one public zero-argument constructor
	at org.junit.runners.BlockJUnit4ClassRunner.validateZeroArgConstructor(BlockJUnit4ClassRunner.java:155)
	at org.junit.runners.BlockJUnit4ClassRunner.validateConstructor(BlockJUnit4ClassRunner.java:133)
	at org.junit.runners.BlockJUnit4ClassRunner.collectInitializationErrors(BlockJUnit4ClassRunner.java:121)
	at org.junit.runners.ParentRunner.validate(ParentRunner.java:269)

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What's going on here is that when scala is run in single-file script mode, it implicitly wraps all of the declarations within the declaration of an anonymous singleton object (Main$$anon). When JUnit tries to reflectively create an ArithmeticTest object, it runs into the fact that, from the Java perspective, ArithmeticTest is a non-static inner class of Main$$anon. To get around this requires a bit of a dance, luckily made fairly short due to Scala's compact OO verbiage:

	class ArithmeticTest {
	@org.junit.Test def basicArithmetic() {
	org.junit.Assert.assertEquals(4, 2+2)
	}
	}
	def runTests(fac: () => Object) = {
	import org.junit.internal.TextListener
	val core = new org.junit.runner.JUnitCore()
	core.addListener(new TextListener(new org.junit.internal.RealSystem()))
	core.run(new org.junit.runner.Request() {
	import org.junit.runners.BlockJUnit4ClassRunner;
	override def getRunner = new BlockJUnit4ClassRunner(fac().getClass()) {
	override def validateZeroArgConstructor(errors: java.util.List[Throwable]) = {}
	override def createTest() = { fac() }
	}
	})
	}
	runTests(() => new ArithmeticTest)

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If I find myself doing this often, I imagine I'll be pulling this out into an importable library. Share and Enjoy.