在ScalaTest中如何使用JUnit的@Rule（例如TemporaryFolder）

Question

我希望能够在测试中使用JUnit规则，例如TemporaryFolder或其他已经在内部开发的TestRule。 如何最好地实现这一目标？我知道有JUnitSuite，但它似乎无法识别@Rule注释。 无论如何，我想使用不同的ScalaTest套件。

因此，我的问题是：

• ScalaTest套件是否支持JUnit规则？
• 如果不支持，是否存在库可以使用JUnit的TestRule？
• 如果没有，如何在Scala测试中使用JUnit的TestRule？
• 还是有更适合的Scala特定方法来实现TemporaryFolder，或者例如Stefan Birkner的System Rules所提供的功能？

---

以下是我尝试使用JUnitSuite的内容：

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
  }
}

- Mifeet

Answer 1

你可以通过创建一个类型为TemporaryFolder的成员变量，并通过@Rule函数返回该成员变量值来解决问题。

class MyTest extends JUnitSuite {

  val _temporaryFolder = new TemporaryFolder

  @Rule
  def temporaryFolder = _temporaryFolder

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

- Till Rohrmann

---

啊，谢谢，那个可行 :). 没有创建多个 TemporaryFolder 实例。最终，我们采用了可堆叠的 mixins 解决方案，这样我们就不会被 JUnitSuite 锁定。但无论如何，很高兴知道 @Rule 可以在 scalatest 中得到支持。 - Mifeet

很高兴我能帮到你 :-) - Till Rohrmann

Answer 2

我基于ScalaTest的fixture文档得出以下方案，但我想知道是否有更好的解决方案。

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

你最喜欢哪一个？

- Mifeet

Answer 3

这对我有帮助。基于答案。所以注释将应用于（合成的）getter方法。

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

class MyTest extends JUnitSuite {

  @(Rule @getter)
  val tempFolder = new TemporaryFolder

}

请确保使用junit版本>4.11。

- zella