how to build validation error code for i18n

Question

I'm writing REST-API using Spring MVC framework.
I'm using bean-validation like:

class Person {
    @NotNull
    String name;
    @NotNull
    String email;
    @Min(0)
    Integer age;
}

I'm validating Person using @Valid annotation into controllers:

@PostMapping
public Person create(@Valid @RequestBody Person person) {return ...;}

To make errors human-readable I use spring's top level error handler:

@ControllerAdvice
public class CustomExceptionHandler {

    @ExceptionHandler(MethodArgumentNotValidException.class)
    @ResponseBody String handle(MethodArgumentNotValidException ex) {
        List<String> errors = ex.getBindingResult().getFieldErrors().stream()
                .map(this::buildMessage)
                .collect(Collectors.toList());
        return errors.toString();
    }

    private String buildMessage(FieldError fe) {
        return fe.getField() + " " + fe.getDefaultMessage();
    }
}

So my errors looks like: [name may not be null, email may not be null]

Now I need to use language independent error code that will be parsed by different UIs to implement i18n.
Is there any way to build full error code? (that contains field name)

I see the following solutions:

Use custom message every time I used annotation (ugly):

class Person {
    @NotNull(message="app.error.person.name.not.null")
    String name;
    @NotNull(message="app.error.person.email.not.null")
    String email;
    @Min(0)(message="app.error.person.age.below.zero")
    Integer age;
}

Build correct code into my exception handler(don't know how):

private String buildMessage(FieldError fe) {
    return "app.error." +
            fe.getObjectName() + "." +
            fe.getField() + "." +
            fe.getDefaultMessage().replaceAll("\\s", "");//don't know how to connect to concrete annotation
}

so message will be like app.error.person.name.maynotbenull

Re-write all annotations and validators for them to build correct message by removing default ConstraintViolation and adding custom (overhead)

Priyaranjan Behera · Accepted Answer · 2017-03-21 06:52:01Z

2

No need to specify message inside annotation . It will be an overhead

@ControllerAdvice
public class CustomExceptionHandler {

  @Autowired
  MessageSource messageSource;

  @ExceptionHandler(MethodArgumentNotValidException.class)
  @ResponseBody String handle(MethodArgumentNotValidException ex) {
    List<String> errors = ex.getBindingResult().getFieldErrors().stream()
            .map(this::buildMessage)
            .collect(Collectors.toList());
    return errors.toString();
  }

  private String buildMessage(FieldError fe) {
    StringBuilder errorCode = new StringBuilder("");
    String localizedErrorMsg = "";
    errorCode.append("error").append(".");
    errorCode.append(fe.getObjectName()).append(".");
    errorCode.append(fe.getField()).append(".");
    errorCode.append(fe.getCode().toLowerCase());

        try {
            localizedErrorMsg = this.messageSource.getMessage(errorCode,(Object[]) null, LocaleContextHolder.getLocale());
        } catch (Exception ex) {
            localizedErrorMsg = fe.getDefaultMessage();
        }
    return localizedErrorMsg;
  }
}

And in the message file(i18n) use the following format

error.person.name.notnull = Name must not be null
error.person.email.notnull = Email must not be null
error.person.age.min= Minimum age should greater than 0.

Using this you don't have to write any message code in the annotation. Hopefully this will help.

edited Mar 21, 2017 at 6:52

answered Mar 20, 2017 at 14:57

Priyaranjan Behera

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5 Comments

Vova Yatsyk Over a year ago

I don't need to localize my message, but ok, I can just throw away try block.

Vova Yatsyk Over a year ago

I want to ask in which case fe.getCode() will return null?

Priyaranjan Behera Over a year ago

fe.getCode() will never return null. MethodArgumentNotValidException will be thrown because of that code only

Vova Yatsyk Over a year ago

but we have constructor for FieldError that set codes to null

Priyaranjan Behera Over a year ago

In FiledError 2 constructor are available. If annotation is present it will call the second one which will set the value of code

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