Unwanted mock with mockito

Julien Berthoud :

I'm doing unit tests with jUnit 5 and Mockito. In one test, I had to mock a method. Everything works fine. In another test later I'm invoking the same method but this time I don't want Mockito to do anything. Nevertheless, Mockito returns an unasked null value, which makes my test fail. I thought it was due to the first test, so I added Mockito.reset(). But it didn't change anything.

Do you understand what's happening behind the scene?

I ran the debug mode to have more info about the object created by Mockito. Among other infos I can read

invocationForStubbing: ecritureComptable.toString();

But I don't know where and when this method is called. Any help appreciated.

EDIT.... Thank you guys. I edited my post to make it clearer and also because I have now a better idea of what possibly happened:

1/ Before each test, I create a mock of the object DaoProxy and I use the mode RETURNS_DEEP_STUBS, which have mockito mock objects nested in DaoProxy

  @BeforeAll
    private static void injectMockDao() {
        DaoProxy daoProxyMock = mock(DaoProxy.class, Mockito.RETURNS_DEEP_STUBS);
        AbstractBusinessManager.configure(null, daoProxyMock, null);
    }

2/ For a specific test method, I use :

when(getDaoProxy().getComptabiliteDao().getEcritureComptableByRef(Mockito.anyString())).thenReturn(ecritureBDD);

and reset it after use, hopping that by the next call of the all chain, Mockito won't do anything (but it didn't work) :

reset(getDaoProxy().getComptabiliteDao().getEcritureComptableByRef(Mockito.anyString()));

3/ In another test later, i make a call to

getDaoProxy().getComptabiliteDao().getEcritureComptableByRef()

and Mockito - although unasked - returns a null object.

The input of @Gavin makes me assume this is because of the RETURNS_DEEP_STUBS-Mocking of DaoProxy. Mockito mocks the nested object, but since it has no info on what it should return, it returns the default object value : null. This explains why in this case the reset didn't help.

Gavin :

If your object is marked for mocking with @Mock or you have used the mock method to create it then Mockito will return the default value for the type, which for objects is null.

In the failing test you could try to either provide a mocked value in the usual way, or inject a "real" instance of the object being mocked, I believe it is possible to have Mockito to provide a mocked response on a "real" instance of an object.