@XmlAccessorType(XmlAccessType.PUBLIC_MEMBER)
@XmlRootElement(name = "square")
public class Square {

   @Size(min = 4, max = 4)
   private int numberOfSides;

   public void setNumberOfSides(int numSides) {
       this.numberOfSides = numSides;
   }

   public int getNumberOfSides() {
      return this.numberOfSides;
   }
}

@RunWith(JUnit4.class)
public class BeanValidationTest {

   @Test
   public void Should_not_unmarshal_invalid_numberOfSides_value() throws JAXBException    {
     String xml = "<square>" +
                 "  <numberOfSides>3</numberOfSides>" +
                 "</square>";

     JAXBContext context = org.eclipse.persistence.jaxb.JAXBContext.newInstance(Square.class);

     Unmarshaller unmarshaller = context.createUnmarshaller();

     Square result = (Square) unmarshaller.unmarshal(new StringReader(xml));

     assertThat(result.getNumberOfSides(), equalTo(4));
   }
}

java.lang.AssertionError:
Expected: <4>
 but: was <3>