如何在 Kotlin 中使用 MethodSource 编写 Nested ParametrizedTest？

Question

I'm learning how to write test in Kotlin with JUnit 5. I like using feature like @Nested , @ParametrizedTest and @MethodSource when I was writing Java.But when I switch to Kotlin, I encounter an issue:

I figured out how to use

• @Nested by referring this JUnit test in nested Kotlin class not found when running gradle test
• @ParametrizedTest and @MethodSource by referring JUnit 5 for Kotlin Developers - section 4.2

But when I put this together, I got

Companion object is not allowed here.

inside the inner class.

---

Test to reproduce the error

internal class SolutionTest {
    @Nested
    inner class NestedTest {
        @ParameterizedTest
        @MethodSource("testCases")
        fun given_input_should_return_expected(input: Int, expected: Int) {
            // assert
        }

        // error in below line
        companion object {
            @JvmStatic
            fun testCases(): List<Arguments> {
                return emptyList()
            }
        }
    }
}

Is it possible to solve this error? So I can use @Nested , @ParametrizedTest and @MethodSource together?

Answer 1

You can specify the method by using fully qualified name of the outer class @MethodSource("com.example.SolutionTest#testCases")

Here is an example nested test.

package com.example

// imports

internal class SolutionTest {

  @Test
  fun outerTest() {
    // ...
  }

  @Nested
  inner class NestedTest {

    @ParameterizedTest
    @MethodSource("com.example.SolutionTest#testCases")
    fun given_input_should_return_expected(input: Int, expected: Int) {
      Assertions.assertEquals(input + 2, expected)
    }
  }

  companion object {
    @JvmStatic
    fun testCases(): List<Arguments> {
      return listOf(
          Arguments.of(1, 3),
          Arguments.of(2, 4),
      )
    }
  }
}

Answer 2

If you don't mind having @TestInstance(PER_CLASS) , you can then define your MethodSource method just like a normal Kotlin method, without needing a companion object:

internal class SolutionTest {
    @Nested
    @TestInstance(PER_CLASS)
    inner class NestedTest {
        @ParameterizedTest
        @MethodSource("testCases")
        fun given_input_should_return_expected(input: Int, expected: Int) {
            // assert
        }

        fun testCases(): List<Arguments> {
            return emptyList()
        }
    }
}