How to use shouldDescribeNull method of org.mockito.internal.matchers.MatchersToStringTest class

You should instantiante testRef as follows:

Test testRef = new Test(taskJob) {

    public void updateAndReschedule(Long id, String message) {
        //do nothing
    }

};

No need for the spy.

Since you are using powermock features (@PrepareForTest, PowerMockito.whenNew etc.), you have to run your test with the PowerMockRunner.

@RunWith(PowerMockRunner.class)

Because ClassB#geResult is not private, you may also simplify your code and replace

when( mockedClassB, "getResult", any(String[].class) ).thenReturn(someValue); 

by

when(mockedClassB.getResult(any(String[].class))).thenReturn(someValue);

Usually when unit testing you want to mock all external dependencies of a class. That way the unit test can remain independent and focused on the class under test.

Nevertheless, if you want to mix Spring autowiring with Mockito mocks, an easy solution is to annotate with both @InjectMocks and @Autowired:

  @InjectMocks
  @Autowired
  private UploadServiceImpl uploadService;

The net effect of this is that first Spring will autowire the bean, then Mockito will immediately overwrite the mocked dependencies with the available mocks.

They both achieve the same result. Using an annotation (@Mock) is usually considered "cleaner", as you don't fill up your code with boilerplate assignments that all look the same.

Note that in order to use the @Mock annotation, your test class should be annotated with @RunWith(MockitoJUnitRunner.class) or contain a call to MockitoAnnotations.initMocks(this) in its @Before method.

Weird that your solution seems to work.
According to their documentation on Github it says.

Mocking of final classes and methods is an incubating, opt-in feature. It uses a combination of Java agent instrumentation and subclassing in order to enable mockability of these types. As this works differently to our current mechanism and this one has different limitations and as we want to gather experience and user feedback, this feature had to be explicitly activated to be available ; it can be done via the mockito extension mechanism by creating the file src/test/resources/mockito-extensions/org.mockito.plugins.MockMaker containing a single line:

mock-maker-inline

After you created this file, Mockito will automatically use this new engine and one can do :

 final class FinalClass {
   final String finalMethod() { return "something"; }
 }

 FinalClass concrete = new FinalClass(); 

 FinalClass mock = mock(FinalClass.class);
 given(mock.finalMethod()).willReturn("not anymore");

 assertThat(mock.finalMethod()).isNotEqualTo(concrete.finalMethod());

In subsequent milestones, the team will bring a programmatic way of using this feature. We will identify and provide support for all unmockable scenarios. Stay tuned and please let us know what you think of this feature!

My working structure now looks like this.

Use PowerMockito on top of Mockito.

Example code:

@RunWith(PowerMockRunner.class)
@PrepareForTest(DriverManager.class)
public class Mocker {

    @Test
    public void shouldVerifyParameters() throws Exception {

        //given
        PowerMockito.mockStatic(DriverManager.class);
        BDDMockito.given(DriverManager.getConnection(...)).willReturn(...);

        //when
        sut.execute(); // System Under Test (sut)

        //then
        PowerMockito.verifyStatic();
        DriverManager.getConnection(...);

    }

More information:

• Why doesn't Mockito mock static methods?

If I understand correctly, the problem is not really to test the observer, but to test the result of an asynchronous method call. To do that, create an observer which blocks until its update() method has been called. Something like the following:

public class BlockingGameObserver extends GameInstanceObserver {
    private CountDownLatch latch = new CountDownLatch(1);

    @Override
    public void update() {
        latch.countDown();
    }

    public void waitUntilUpdateIsCalled() throws InterruptedException {
        latch.await();
    }
}

And in your test:

private BlockingGameObserver observer;

@Before
public void setUp() throws IOException, Exception {
    observer = new BlockingGameObserver();
    GameInstance.getInstance().addObserver(observer); 
}

@Test
public void getRoomUserData_usingNullKey_shouldThrowUserDataDoesntExist() throws InterruptedException, UserDataDoesntExistException {
    serverService.createRoom("exampleRoom");   
    observer.waitUntilUpdateIsCalled();
    assertEquals("exampleRoom",
                 GameInstance.getInstance().getRoom("exampleRoom").getRoomId());
}

Your brackets are in the wrong place, try this:

doNothing().when(mockRegistrationPeristImpl).create(any(Registration.class));

Assuming that your code looks like this:

Account acct = ...
Mockito.when(accountDAO.updateAccount(acct)).thenThrow(new DataBaseException());

This will throw an exception if you execute above method only with Account acct, other objects will not throw anything. So if you are executing this method with different object, nothing will happen.

To check this, you can define a rule, that any execution of this method, with any instance of Account will throw an exception:

 Mockito.when(accountDAO.updateAccount(Mockito.any(Account.class))).thenThrow(new DataBaseException());