I have this working in Java:

@TestInstance(TestInstance.Lifecycle.PER_CLASS)
public class OtherTest {

    @RegisterExtension
    MyExtension myExtension = new MyExtension();

    @Test
    void myTest() {
        System.out.println("my test");
    }
}

However, the Kotlin version fails:

@TestInstance(TestInstance.Lifecycle.PER_CLASS)
open class SomethingTest {

    val something = Something()

    @RegisterExtension
    open var myExtension = MyExtension()

    @Test
    fun `test do something`(): Unit {
        assert(something.doSomething())

    }

//    companion object {
//        @JvmField
//        @RegisterExtension
//        val myExtension = MyExtension()
//    }
}

with the exception

org.junit.platform.commons.PreconditionViolationException: Failed to register extension via @RegisterExtension field [private org.example.MyExtension org.example.SomethingTest.myExtension]: field must not be private.
at app//org.junit.platform.commons.util.Preconditions.condition(Preconditions.java:296)

The open‘s and var are attempts to get around this error. I suspect Junit is wrapping and trying to set the variable. Using ‘javap’ there’s definite differences between the Java and Kotlin version as the variable is only exposed through the getter/setters:

public class org.example.SomethingTest {
  public org.example.SomethingTest();
  public final org.example.Something getSomething();
  public final org.example.MyExtension getMyExtension();
  public final void setMyExtension(org.example.MyExtension);
  public final void test do something();
}

The companion object (commented) is the workaround although I prefer not use static types.

Ideas?

My extension is simple enough:

class MyExtension : BeforeAllCallback, AfterAllCallback, BeforeEachCallback, AfterEachCallback {

    override fun beforeAll(context: ExtensionContext?) {
        println("beforeAll called")
    }

    override fun afterAll(context: ExtensionContext?) {
        println("afterAll called")
    }

    override fun beforeEach(context: ExtensionContext?) {
        println("beforeEach called")
    }

    override fun afterEach(context: ExtensionContext?) {
        println("afterEach called")
    }
}