How to mock bean with @PostConstruct method

Question

I need to mock some class that contains @PostConstruct method, but I get NullPointerException inside @PostConstruct method.

My code looks like the following:

@SpringJUnitConfig(MyTestClass.Config.class)
class MyTestClass {
    @Autowired
    MyClass myClass;

    @Autowired
    MyService service;

    @Test
    void testService() {
        assertNotNull(myClass);
    }

    @Configuration
    static class Config {
        @Bean
        MyService service(MyClass myClass) {
             return Mockito.spy(new MyService(myClass));
        }

        @Bean
        MyClass myClass() {
            return mock(MyClass.class);
        }

        @Bean
        ObjectMapper mapper() {
            return new ObjectMapper();
        }

    }
}

public class MyClass {
    private ObjectMapper mapper;

    @Autowired
    public MyClass(ObjectMapper mapper) {
          this.mapper = mapper;
    }
    
    @PostConstruct
    void init() {
        mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, true); // I get NPE bacause mapper is null.
    }
}

How can I avoid this exception? I can't use @MockBean, because I have to inject this mock into another bean.