how to mock methods in Singleton

NonLazySingleton.java

// sample code written to showcase how to use Mockito to mock certain methods to test logic in Singleton classes

public class MySingletonClass {
    // not lazy initiated object.
    private static final MySingeltonClass INSTANCE = new MySingletonClass();
    
    // can be used by unit test by Mocking it. Mockito invokes this default constructor.
    @VisibleForTesting
    MySingletonClass {
        
    }
    
    // method used by main code
    public static final MySingletonClass getInstance() {
        return INSTANCE;
    }
    
    // a method you want to mock
    public String getRecentMessage(UUID userId) {
        // some wierd logic just to showcase some logic
        List<UserMessage> messages = getUserMessagesFromDB(userId, 1);
        if (messages == null || messages.size() == 0) {
            UserMessage message = new UserMessage(userId, "initial message");
            saveMessageToDB(message);
            return message;
        } else {
            return messages.get(0);
        }
    }
    
    @VisibleForTesting
    void saveMessageToDB(UserMessage message) {
        UserMessageDao.getInstance().saveMessage(message);
    }
    
    @VisibleForTesting
    List<UserMessage> getUserMessagesFromDB(UUID userId, int recent) {
        // accesssing databse here..
        return UserMessageDao.getInstance().getUserMessages(userId, recent)
    }
}

// Unit test class in same package as main class
public class MySingletonClassTest {
    
    @Mock
    // invokes default constructor. So during unit testing, there are 2 instances created, one for MySingletonClass (due to static initialization)
    // and 2nd one for proxy class generated which derives from MySingletonClass.
    // If that is a problem, you can change Singleton to be lazy initialization. I have not put lazy initialization code to focus
    // on mockito usage for singleton.
    private MySingletonClass singletonInstance;
    
    @Before
    public void setup() {
        MockitoAnnotations.initMocks(this);
    }
    
    @Test
    public void testWhenNoMessageExists() {
        when(singletonInstance.getUserMessagesFromDB(any(UUID.class), anyInt()).thenReturn(null);
        UUID userId = UUID.randomUUID();
        UserMessage message = singletonInstance.getRecentMessage(userId);
        assertTrue(message != null);
        assertEquals("initial message", message.getMessage());
    }
}