Example of Mockito ResultCaptor

ResultCaptor.java

    @SpyBean private IdGenerator idGenerator;
    @Autowired private SomeService service;

    @Test
    void createSubscription() {
        SomeRequest request = new SomeRequest("name");

        ResultCaptor<String> idCaptor = new ResultCaptor<>();
        doAnswer(idCaptor).when(idGenerator).generate();

        SomeResponse response = service.createResponse(request);

        assertThat(response.getId()).isEqualTo(idCaptor.getResult());
    }

    public static class ResultCaptor<T> implements Answer<T> {
        private T result = null;
        public T getResult() {
            return result;
        }

        @SuppressWarnings("unchecked")
        @Override
        public T answer(InvocationOnMock invocationOnMock) throws Throwable {
            result = (T) invocationOnMock.callRealMethod();
            return result;
        }
    }