Mocking static classes with Powermock2 and Kotlin

Question

in Android I've been using the version 1.6.1 of Powermock and all this implementation worked really good for statics. It's not working now at all when I changed to 2.0.0-beta.5. Indeed, it didn't even work upgrading from my previous 1.6.1 to 1.7.1.

I have this implementation:

// Power Mockito
testImplementation "org.powermock:powermock-api-mockito2:2.0.0-beta.5"
testImplementation "org.powermock:powermock-module-junit4-rule-agent:2.0.0-beta.5"
testImplementation "org.powermock:powermock-module-junit4:2.0.0-beta.5"
//testImplementation 'org.powermock:powermock-module-junit4-rule:2.0.0-beta.5'

// Mockito
testImplementation "org.mockito:mockito-core:2.11.0"
testImplementation "com.nhaarman:mockito-kotlin-kt1.1:1.5.0"
androidTestImplementation("com.nhaarman:mockito-kotlin-kt1.1:1.5.0", {
    exclude group: 'org.mockito', module: 'mockito-core'
})
androidTestImplementation 'org.mockito:mockito-android:2.11.0'

and I'm trying to mock a static the same way I was doing with 1.6.1:

@RunWith(PowerMockRunner::class)
@PrepareForTest(SharedPreferencesHelper.Companion::class, ConfigUseCaseTests::class)
class ConfigUseCaseTests {

    lateinit var context: Context

    @Before
    fun setUp() {
        context = mock()
    }

    @Test
    fun getConfigs_fromJson() {
        PowerMockito.mockStatic(SharedPreferencesHelper.Companion::class.java)

        val instance = mock<SharedPreferencesHelper.Companion>()
        doReturn("foo")
                .whenever(instance)
                .loadString(isA(), anyString(), anyString(), anyString())
//        whenever(instance.loadString(isA(), anyString(), anyString(), anyString())).thenReturn("foo") // This shows the same error
        PowerMockito.whenNew(SharedPreferencesHelper.Companion::class.java)
                .withAnyArguments()
                .thenReturn(instance)

        val mockedFoo = instance.loadString(context, "", "", "") // This shows "foo"

        val mockedCompanion = SharedPreferencesHelper.loadString(context, "", "", "") // This is throwing NullPointerException

        Assert.assertEquals(mockedCompanion, "foo")
    }
}

My SharedPreferencesHelper looks like:

class SharedPreferencesHelper {
    companion object {

        @Suppress("NON_FINAL_MEMBER_IN_OBJECT")
        open fun loadString(context: Context, fileName: String, key: String, defaultValue: String?): String? {
            val sharedPreferences = getWithFileName(context, fileName)
            return if (!sharedPreferences.contains(key)) {
                defaultValue
            } else {
                sharedPreferences.getString(key, defaultValue)
            }
        }
    }
}

I've tried to play with the open but it didn't work.

Exception: (I don't understand it at all)

java.lang.NullPointerException
    at my.package.ConfigUseCaseTests.getConfigs_fromJson(ConfigUseCaseTests.kt:45)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.internal.runners.TestMethod.invoke(TestMethod.java:68)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.runTestMethod(PowerMockJUnit44RunnerDelegateImpl.java:326)
    at org.junit.internal.runners.MethodRoadie$2.run(MethodRoadie.java:89)
    at org.junit.internal.runners.MethodRoadie.runBeforesThenTestThenAfters(MethodRoadie.java:97)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.executeTest(PowerMockJUnit44RunnerDelegateImpl.java:310)
    at org.powermock.modules.junit4.internal.impl.PowerMockJUnit47RunnerDelegateImpl$PowerMockJUnit47MethodRunner.executeTestInSuper(PowerMockJUnit47RunnerDelegateImpl.java:131)

I can say, sometimes IT WORKS!! I'm adding the video because it looks amazing that it happens just sometimes: https://youtu.be/YZObVLcERBo (watch at the middle and the end)

Answer 1

The way companion objects are created on compilation is by creating a static field inside the surrounding class. It get's instantiated on the static scope (before the test is instantiated).

This is how it looks when decompiled in Java:

public final class SharedPreferencesHelper {
  public static final SharedPreferencesHelper.Companion Companion = new 
  SharedPreferencesHelper.Companion((DefaultConstructorMarker)null);
  // ...
}

For this to work you'll have to assign the given field with your mock instead of intercepting the creation of the Companion object. This doesn't even require to use PowerMock and can be done with reflexion: https://dzone.com/articles/how-to-change-private-static-final-fields