JAX-RS ExceptionMapper to handle Bean Validation's ConstraintViolationException

ConstraintViolationExceptionMapper.java

import com.google.common.collect.Iterables;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;
import javax.validation.ConstraintViolation;
import javax.validation.ConstraintViolationException;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;
import javax.ws.rs.ext.ExceptionMapper;
import javax.ws.rs.ext.Provider;

@Provider
public class ConstraintViolationExceptionMapper implements ExceptionMapper<ConstraintViolationException> {
    @Override
    public Response toResponse(ConstraintViolationException exception) {
        Set<ConstraintViolation<?>> constraintViolations = exception.getConstraintViolations();
        List<String> errorMessages = constraintViolations.stream()
                .map(constraintViolation -> {
                    String name = Iterables.getLast(constraintViolation.getPropertyPath()).getName();
                    return name + " " + constraintViolation.getMessage();
                })
                .collect(Collectors.toList());
        return Response
                .status(Response.Status.BAD_REQUEST)
                .type(MediaType.APPLICATION_JSON_TYPE)
                .entity(errorMessages)
                .build();
    }
}

The ConstraintViolationExceptionMapper extracts the last node of the path to get the field name, then prepends it to the error message so that the @NotNull annotation doesn't need to specify the field name in the error message:

eg.

public class User {
    @NotNull
    private String name;
    
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

instead of

public class User {
    @NotNull(message = "name {javax.validation.constraints.NotNull.message}")
    private String name;
    
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

The ConstraintViolationExceptionMapper combines all the error messages from all ConstraintViolation(s) into a List, which is then returned as a JSON array, together with a HTTP Status Code 400 (Bad Request).

This example uses Google Guava's Iterables#getLast(Iterable<T>) to get the last node of the path for brevity, it is not required otherwise.