For testing purposes we’ll use the following Player class, which has a few default constraints on name and score:

package com.farenda.javax.validation;

import javax.validation.constraints.Max;
import javax.validation.constraints.Min;
import javax.validation.constraints.Size;

public class Player {

    // name have to be 3 chars:
    @Size(min = 3, max = 3)
    private String name;

    // possible score in game:
    @Min(0) @Max(100)
    private int score;

    public Player(String name, int score) {
        this.name = name;
        this.score = score;
    }

    // just for logs
    @Override
    public String toString() {
        return "Player{name='" + name + '\'' + ", score=" + score + '}';
    }
}

The first thing we have to do is to create a Bean Validator. To do that we’ll instantiate a ValidatorFactory that will provide us a Validator before all tests:

package com.farenda.javax.validation;

import org.junit.AfterClass;
import org.junit.BeforeClass;

import javax.validation.Validator;
import javax.validation.ValidatorFactory;

public class PlayerValidationTest {
    private static ValidatorFactory validatorFactory;
    private static Validator validator;

    @BeforeClass
    public static void createValidator() {
        validatorFactory = Validation.buildDefaultValidatorFactory();
        validator = validatorFactory.getValidator();
    }

    //...

Both ValidatorFactory and Validator are thread-safe, so can be reused in many tests. You could even create an abstract test class for Bean Validation tests.
Also let’s dispose the ValidatorFactory after all tests to free any resources possibly allocated by the factory:

@AfterClass
public static void close() {
    validatorFactory.close();
}

Here’s the complete test for easy copy-past:

package com.farenda.javax.validation;

import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;

import javax.validation.ConstraintViolation;
import javax.validation.Validation;
import javax.validation.Validator;
import javax.validation.ValidatorFactory;
import java.util.Set;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertTrue;

public class PlayerValidationTest {

    private static ValidatorFactory validatorFactory;
    private static Validator validator;

    @BeforeClass
    public static void createValidator() {
        validatorFactory = Validation.buildDefaultValidatorFactory();
        validator = validatorFactory.getValidator();
    }

    @AfterClass
    public static void close() {
        validatorFactory.close();
    }

    @Test
    public void shouldHaveNoViolations() {
        //given:
        Player player = new Player("ABC", 44);

        //when:
        Set<ConstraintViolation<Player>> violations
                = validator.validate(player);

        //then:
        assertTrue(violations.isEmpty());
    }

    @Test
    public void shouldDetectInvalidName() {
        //given too short name:
        Player player = new Player("a", 44);

        //when:
        Set<ConstraintViolation<Player>> violations
                = validator.validate(player);

        //then:
        assertEquals(violations.size(), 1);

        ConstraintViolation<Player> violation
                = violations.iterator().next();
        assertEquals("size must be between 3 and 3",
                     violation.getMessage());
        assertEquals("name", violation.getPropertyPath().toString());
        assertEquals("a", violation.getInvalidValue());
    }
}

My impression is that many developers thing that testing validation is part of integration or even system testing and usually don’t do that. But here you can see that this really can be done on (almost) unit level and Bean Validation Unit Testing can done by developers together with other unit tests. Which is supercool! :-)

• How to setup Bean Validation in Java project
• JUnit Tutorial