Problem is with the this line where you are mocking the call to objectmapper.

Mockito.when((objMapper).readValue(“”,ConfigDetail.class)).thenReturn(configDetail);

Correct syntax is

Mockito.when(objMapper.readValue(“”,ConfigDetail.class)).thenReturn(configDetail);

Notice the bracket position. When using Spy or Verify, the bracket position is diff. then when using when-then syntax.

You can try something like this.

Mockito.when(mapper.convertValue(anyMap(), eq(Target.class))).then(invocationOnMock -> {
  if (invocationOnMock.getArguments()[0] == givenObjectOne) {
    return target1;
  } else if (invocationOnMock.getArguments()[0] == givenObjectTwo) {
    return target2;
  }
  return null;
});

More details about Answer from Mockito can be found here. https://javadoc.io/static/org.mockito/mockito-core/3.5.2/org/mockito/stubbing/Answer.html

I don't see the reason behind writing a test which mocks Object Mapper to throw some exception and then test whether the exception was thrown or not. Instead the test should be for the custom logic which is developed by you. If you have a method like this -

public void publicMethod(messageDto dto)  {
    try{
        privateMethod(dto, param, "other string");
    } catch(JsonProcessingException e){
       // do stuff
       dto.setXXX("XXX"); // The test should be testing this
    }
}

So, the test should be testing whether the dto is set correctly.

@Test
public void shouldFailJsonParsing() throws JsonProcessingException {
        // Given
        final Dto dto = new Dto();

        // When
        when(mapper.writeValueAsString(any()))
                .thenThrow(JsonProcessingException.class);

        // Invoke the method to be tested
        classToBeTestedActor.publicMethod(dto);

        // Test whether expected mock methods are invoked
        verify(mapper, times(1)).writeValueAsString(any());
        verifyNoMoreInteractions(mapper);

        // Assert results
        Assert.assertEquals("XXX", dto.getXXX());
    }

I get your point and I kind of agree with that. I just want to understand why, even if I am forcing the mapper to throw that exception, I am unable to assert it

I think the reason for this is that the exception is catched by you. That is the reason, that exception isn't propagated to your test.

public void publicMethod(messageDto dto) throws JsonProcessingException {
        try{
            privateMethod(dto, param, "other string");
        } catch(JsonProcessingException e){
           // do stuff
        }
    }

Try changing this to -

public void publicMethod(messageDto dto) throws JsonProcessingException {
        privateMethod(dto, param, "other string");
        
    }

I faced the same NPE with the objectmapper. This issue is resolved after adding , my test class has class level annotation with

@RunWith(MockitoJUnitRunner.class) 

and the fix

 @Before
    public void setupBefore() {
        MockitoAnnotations.initMocks(this);

    }