Kod źródłowy do wpisu o niestandardowych walidatorach dla Java Bean Validation, znajdującego się na blogu https://blog.mloza.pl/java-bean-validation-niestandardowy-walidator-danych/

CustomValidatorController.java

@RestController
public class CustomValidatorController {
    @PostMapping("/zipCode")
    @ResponseBody
    public String zipCodeBody(
            @RequestBody @Valid RequestWithZipCode body // #1
    ) {
        return "OK! Everything is valid";
    }
}

CustomValidatorControllerTest.java

@WebMvcTest
public class CustomValidatorControllerTest {

    @Autowired
    private MockMvc mvc;

    @Autowired
    private ObjectMapper objectMapper;

    @Test
    public void shouldReturnErrorWhenInputIsInvalid() throws Exception {
        mvc.perform(post("/zipCode")
                .contentType(MediaType.APPLICATION_JSON)
                .content(invalidBody()))
                .andDo(mvcResult -> System.out.println(mvcResult.getResponse().getContentAsString()))
                .andExpect(status().isBadRequest());
    }

    @Test
    public void shouldReturnOkWhenInputIsValid() throws Exception {
        mvc.perform(post("/zipCode")
                .contentType(MediaType.APPLICATION_JSON)
                .content(validBody()))
                .andDo(mvcResult -> System.out.println(mvcResult.getResponse().getContentAsString()))
                .andExpect(status().is2xxSuccessful());
    }

    private String invalidBody() throws JsonProcessingException {
        return objectMapper.writeValueAsString(
                new RequestWithZipCode("1-21"));
    }

    private String validBody() throws JsonProcessingException {
        return objectMapper.writeValueAsString(
                new RequestWithZipCode("31-521"));
    }
}

RequestWithZipCode.java

public class RequestWithZipCode {

    @ZipCode // #1
    private String zipCode;

    // geter, seter, konstruktory...
    
}

ZipCode.java

@Target({FIELD})
@Retention(RUNTIME)
@Constraint(validatedBy = ZipCodeValidator.class). // #1
@Documented
public @interface ZipCode {
    String message() default "Podany kod pocztowy jest nieprawidłowy"; // #2

    Class<?>[] groups() default {}; // #3

    Class<? extends Payload>[] payload() default {}; // #4
}

ZipCodeValidator.java

public class ZipCodeValidator implements ConstraintValidator<ZipCode, String> {
    @Override
    public boolean isValid(String value, ConstraintValidatorContext context) {
        Pattern pattern = Pattern.compile("^[0-9]{2}-[0-9]{3}$");
        return pattern.matcher(value).matches();
    }
}