Using JUnit @Rule with ScalaTest (e.g. TemporaryFolder)

Question

I would like to be able to use JUnit rules such as TemporaryFolder or other TestRules we have already developed in-house. What is the best method to accomplish that? I'm aware of JUnitSuite but it doesn't seem to pick up the @Rule annotation. I would like to use a different ScalaTest suite anyway.

So my questions are:

• Are JUnit rules supported by a ScalaTest suit?
• If not, is there a library out there which would make using Junit TestRules possible?
• If not, how to use JUnit TestRules in Scala tests?
• Or is there a more appropriate Scala-specific approach for acomplishing what TemporaryFolder, or, e.g., Stefan Birkner's System Rules provide?

---

Here's what I tried with JUnitSuite:

class MyTest extends JUnitSuite {
  //@Rule
  //val temporaryFolder = new TemporaryFolder() // throws java.lang.Exception: The @Rule 'temporaryFolder' must be public.

  @Rule
  def temporaryFolder = new TemporaryFolder()

  @Test
  def test: Unit = {
    assert(temporaryFolder.newFile() !== null) // java.lang.IllegalStateException: the temporary folder has not yet been created
  }
}

Answer 1

You could solve the problem by creating a member field of type TemporaryFolder and returning this field value by the @Rule function.

class MyTest extends JUnitSuite {

  val _temporaryFolder = new TemporaryFolder

  @Rule
  def temporaryFolder = _temporaryFolder

  @Test
  def test: Unit = {
    assert(temporaryFolder.newFile() !== null)
  }
}

Answer 2

Here is what I came up based on ScalaTest's documentation on fixtures. Still, I would like to know if there is a better solution.

1. Loan-fixture method

   class LoanFixtureTest extends FunSuite {
     def withRule[T <: TestRule](rule: T)(testCode: T => Any): Unit = {
       rule(
         new Statement() {
           override def evaluate(): Unit = testCode(rule)
         },
         Description.createSuiteDescription("JUnit rule wrapper")
       ).evaluate()
     }
   
     test("my test") {
       withRule(new TemporaryFolder()) { temporaryFolder =>
         assert(temporaryFolder.newFile() !== null)
       }
     }
   }
   

   • Pros: allows applying the rule only to tests where it is needed
   • Cons: not very elegant usage; clumsy when multiple TestRules are required

2. Using stackable mixins with withFixture(test: NoArgTest) override

   trait TemporaryFolderFixture1 extends SuiteMixin {
     this: Suite =>
     val temporaryFolder = new TemporaryFolder
   
     abstract override def withFixture(test: NoArgTest) = {
       var outcome: Outcome = null
       val statementBody = () => outcome = super.withFixture(test)
       temporaryFolder(
         new Statement() {
           override def evaluate(): Unit = statementBody()
         },
         Description.createSuiteDescription("JUnit rule wrapper")
       ).evaluate()
       outcome
     }
   }
   
   class StackableTraitFixtureTest extends FunSuite with TemporaryFolderFixture1 {
     test("my test") {
       assert(temporaryFolder.newFile() !== null)
     }
   }
   

   • Pros: very simple usage, conveniently allows mixing multiple rules in
   • Cons: requires having a mixin for every rule; rules need to be invoked even for tests that don't need them; rule cannot be used e.g. in BeforeAfterEach#beforeEach()

3. Overriding withFixture(test: OneArgTest)

   trait TemporaryFolderFixture2 {
     thisFixture: org.scalatest.fixture.FunSuite =>
     type FixtureParam = TemporaryFolder
   
     override protected def withFixture(test: OneArgTest): Outcome = {
       val temporaryFolder = new TemporaryFolder()
       var outcome: Outcome = null
       temporaryFolder(
         new Statement() {
           override def evaluate(): Unit = {
             outcome = withFixture(test.toNoArgTest(temporaryFolder))
           }
         },
         Description.createSuiteDescription("JUnit rule wrapper")
       ).evaluate()
       outcome
     }
   }
   
   class OneArgWithFixtureTest extends org.scalatest.fixture.FunSuite with TemporaryFolderFixture2 {
     test("my test") { temporaryFolder =>
       assert(temporaryFolder.newFile() !== null)
     }
   }
   

   • Cons: allows only one TestRule, making in generic to work with any rule instead of just TestRule would require an extra effort

Which one do you like the best?

Answer 3

This worked for me. Based on answer. So annotation will be applied to to the (synthetic) getter method

import org.junit._
import scala.annotation.meta.getter

class MyTest extends JUnitSuite {

  @(Rule @getter)
  val tempFolder = new TemporaryFolder

}

Just make sure to use junit version >4.11.