Metoda pojedynczej klasy testu jednostkowego w systemie Android przy użyciu programu PowerMock

Muszę wykonać testy jednostkowe metod klasy Singleton, która wewnętrznie korzysta z RxJava Singles i używa struktury testowej PowerMock do mockowania statycznej klasy i metod. Próbowałem różnych metod, aby mockować metody Schedulers.io () i AndroidSchedulers.mainThread (), ale to nie działa. Otrzymuję błąd java.lang.NullPointerException w wierszu .subscribeOn (Schedulers.io ()) w metodzie UserApi.verifyUserData () .

Singleton Class UserApi (testowana klasa)

final public class UserApi {
    private CompositeDisposable compositeDisposable;
    private String userID; 
    //private final SchedulerProvider schedulerProvider;

    private UserApi(String userId) {
        super();
        this.userID = userId;
        //this.schedulerProvider = schedulerProvider;
    }

    public static UserApi getInstance() {
        return SingletonHolder.sINSTANCE;
    }

    private static final class SingletonHolder {
        private static final UserApi sINSTANCE;

        static {
            String uuid = UUID.randomUUID().toString();
            sINSTANCE = new UserApi(uuid);
        }
    }

    // Rest Api call

    public void verifyUserData(byte[] doc, byte[] img) {
        this.compositeDisposable = new CompositeDisposable();
        String docStr = Base64.encodeToString(doc, Base64.NO_WRAP);
        String imgStr = Base64.encodeToString(img, Base64.NO_WRAP);

        final Disposable apiDisposable = IdvManager.getInstance().getUserManager().verifyUserData(docStr, imgStr)
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Consumer<JsonObject>() {
                    @Override
                    public void accept(JsonObject verifyResponse) throws Exception {
                        pollResult();
                    }
                }, new Consumer<Throwable>() {
                    @Override
                    public void accept(Throwable error) throws Exception {
                      // handle error code...
                    }
                });
        this.compositeDisposable.add(apiDisposable);
    }

    private void pollResult() {
        // code here...
    }

}

Klasa i interfejs UserManager

public interface UserManager {

    Single<JsonObject> verifyUserData(String docStr, String imgStr);

}

final class UserManagerImpl implements UserManager {

    private final UserService userService;

    UserManagerImpl(final Retrofit retrofit) {
        super();
        this.userService = retrofit.create(UserService.class);
    }
    @Override
    public Single<JsonObject> verifyUserData(String docStr, String imgStr) {
     // Code here...
    }
}

Test jednostkowy

@RunWith(PowerMockRunner.class)
@PrepareForTest({IdvManager.class, Base64.class, Schedulers.class, AndroidSchedulers.class, UserApi.class})
public class UserApiTest {

    @Mock
    public UserManager userManager;
    @Mock
    private Handler handler;

    private IdvManager idvManager;
    private Schedulers schedulers;

    private UserApi spyUserApi;
    private TestScheduler testScheduler;
    private String userID;

    @Before
    public void setUp() {
        MockitoAnnotations.initMocks(this);
        testScheduler = new TestScheduler();
        handler = new Handler();
        PowerMockito.suppress(constructor(IdvManager.class));
        // mock static
        PowerMockito.mockStatic(IdvManager.class);
        PowerMockito.mockStatic(Schedulers.class);
        PowerMockito.mockStatic(AndroidSchedulers.class);
        PowerMockito.mockStatic(Base64.class);
        // Create mock for class
        idvManager = PowerMockito.mock(IdvManager.class);
        schedulers = PowerMockito.mock(Schedulers.class);
        PowerMockito.when(IdvManager.getInstance()).thenReturn(IdvManager);
        when(idvManager.getUserManager()).thenReturn(userManager);

        spyUserApi = PowerMockito.spy(UserApi.getInstance());
        // TestSchedulerProvider testSchedulerProvider = new TestSchedulerProvider(testScheduler);

        when(Base64.encodeToString((byte[]) any(), anyInt())).thenAnswer(new Answer<Object>() {
            @Override
            public Object answer(InvocationOnMock invocation) throws Throwable {
                return java.util.Base64.getEncoder().encodeToString((byte[]) invocation.getArguments()[0]);
            }
        });

        when(schedulers.io()).thenReturn(testScheduler);
        when(AndroidSchedulers.mainThread()).thenReturn(testScheduler);
        userID = UUID.randomUUID().toString();
    }

    @After
    public void clearMocks() {
        //Mockito.framework().clearInlineMocks();
    }

    @Test
    public void verifyUserData_callsPollResult_returnsResponse() {
        // Input
        String docStr = "iVBORw0KGgoAAAANSUhEUgAAAJ4AAACeCAYAAADDhbN7AA.....";
        // Output
        JsonObject verifyResponse = new JsonObject();
        verifyResponse.addProperty("status", "Response created");
        doReturn(Single.just(verifyResponse)).when(userManager).verifyUserData(docStr, docStr);
        // spy method call
        spyUserApi.verifyUserData(docFrontArr, docFrontArr);
        testScheduler.triggerActions();
        // assert
        verify(userManager).verifyUserData(docStr, docStr);

    }
}

Błąd

java.lang.NullPointerException
    at com.rahul.manager.UserApi.verifyUserData(UserApi.java:60)
    at com.rahul.manager.UserApiTest.verifyUserData_callsPollResult_returnsResponse(UserApiTest.java:171)
    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)

Nie jestem pewien, czy mogę przetestować metody klasy Singleton, szpiegując rzeczywiste wystąpienie klasy Singleton za pomocą PowerMock.