Using JUnit #Rule with ScalaTest (e.g. TemporaryFolder) - scala

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.
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
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()
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.