How to use Junit 5's TempDir with Kotlin?

Question

I want to convert the following (working) java test to Kotlin:

package my.project;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.io.TempDir;

import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;

import static java.util.Collections.singletonList;
import static org.junit.jupiter.api.Assertions.assertEquals;

public class MyTempFileTest {

    @TempDir
    public File tempFolder;

    @Test
    public void testTempFolder() {
        Assertions.assertNotNull(tempFolder);
    }

    @Test
    public void testTempFolderParam(@TempDir File tempFolder) {
        Assertions.assertNotNull(tempFolder);
    }
 }

With IntelliJ's built in converter it becomes:

package my.project

import org.junit.jupiter.api.Assertions
import org.junit.jupiter.api.Test
import org.junit.jupiter.api.io.TempDir
import java.io.File

class MyTempFileTest {
    @TempDir
    var tempFolder: File? = null
    
    @Test
    fun testTempFolder() {
        Assertions.assertNotNull(tempFolder)
    }

    @Test
    fun testTempFolderParam(@TempDir tempFolder: File?) {
        Assertions.assertNotNull(tempFolder)
    }
}

But this fails to initialize:

org.junit.jupiter.api.extension.ExtensionConfigurationException: @TempDir field [private java.io.File my.project.MyTempFileTest.tempFolder] must not be private.

Putting public in front of var however, makes no difference. I get the same error message and IntelliJ even suggests to remove the apparently 'redundant' public again.

Answer 1

Slightly more idiomatic solution from the Kotlin point of view that makes sure the type of the file property is non-nullable:

@field:TempDir
lateinit var tempFolder: File