Manually call Spring Annotation Validation

If you have everything correctly configured, you can do this:

import javax.validation.Validator;

@Autowired
Validator validator;

Then you can use it to validate you object:

var errors = validator.validate(obj);

This link gives pretty good examples of using validations in Spring apps. https://reflectoring.io/bean-validation-with-spring-boot/

I have found an example to run the validation programmitically in this article.

class MyValidatingService {

  void validatePerson(Person person) {
    ValidatorFactory factory = Validation.buildDefaultValidatorFactory();
    Validator validator = factory.getValidator();
    Set<ConstraintViolation<Person>> violations = validator.validate(person);
    if (!violations.isEmpty()) {
      throw new ConstraintViolationException(violations);
    }
  } 
}

It throws 500 status, so it is recommended to handle it with custom exception handler.

@ControllerAdvice(annotations = RestController.class)
public class CustomGlobalExceptionHandler extends ResponseEntityExceptionHandler {
    @ExceptionHandler(ConstraintViolationException.class)
    public ResponseEntity<CustomErrorResponse> constraintViolationException(HttpServletResponse response, Exception ex) throws IOException {
        CustomErrorResponse errorResponse = new CustomErrorResponse();
        errorResponse.setTimestamp(LocalDateTime.now());
        errorResponse.setStatus(HttpStatus.BAD_REQUEST.value());
        errorResponse.setError(HttpStatus.BAD_REQUEST.getReasonPhrase());
        errorResponse.setMessage(ex.getMessage());
        return new ResponseEntity<>(errorResponse, HttpStatus.BAD_REQUEST);
    }
}

Second example is from https://www.mkyong.com/spring-boot/spring-rest-error-handling-example/

Update: Using validation is persistence layer is not recommended: https://twitter.com/odrotbohm/status/1055015506326052865

Spring provides LocalValidatorFactoryBean, which implements the Spring SmartValidator interface as well as the Java Bean Validation Validator interface.

// org.springframework.validation.SmartValidator - implemented by LocalValidatorFactoryBean
@Autowired
SmartValidator validator;

public String saveAccount(@ModelAttribute Account account, BindingResult result) {
    // ... custom logic
    validator.validate(account, result, Account.Step1.class);
    if (result.hasErrors()) {
        // ... on binding or validation errors
    } else {
        // ... on no errors
    }
    return "";
}

Here is a code sample from JSR 303 spec

Validator validator = Validation.buildDefaultValidatorFactory().getValidator();

Driver driver = new Driver();
driver.setAge(16);
Car porsche = new Car();
driver.setCar(porsche);


Set<ConstraintViolation<Driver>> violations = validator.validate( driver );

So yes, you can just get a validator instance from the validator factory and run the validation yourself, then check to see if there are violations or not. You can see in the javadoc for Validator that it will also accept an array of groups to validate against.

Obviously this uses JSR-303 validation directly instead of going through Spring validation, but I believe spring validation annotations will use JSR-303 if it's found in the classpath