Can you tell about your experience using the @Spy annotation in testing?

The @Spy annotation in Java is used in testing with Mockito to create a partial mock object (spy). Unlike @Mock, which creates a fully mocked object, @Spy allows you to use real methods of the object, but you can override the behavior of individual methods.

This is useful when you want to test a class but control the behavior of some of its methods, for example, to avoid calling external services or heavy operations.

Example of usage:

import static org.mockito.Mockito.*;

public class MyServiceTest {

    @Spy
    private MyService myService = new MyService();

    @Test
    public void testPartialMock() {
        // Override the behavior of a method
        doReturn("mocked result").when(myService).someMethod();

        // Calling another method will be real
        String result = myService.callAnotherMethod();

        // Check the result
        assertEquals("expected result", result);
    }
}

It is important to remember that for @Spy to work correctly, the object should be either created manually or initialized using MockitoAnnotations or MockitoExtension.