Validator receiving null objects ¿why?

Question

first of all thanks for everyone who tries to help me with this: I'm trying to validate a list with at least one element, if I check using

Set<ConstraintViolation<Object>> constraintViolations = VALIDATOR.validate(anObj);

The constraint works fine, but when I try to persist, the object with one element pass again and the list is null...

This is the part of the class with the list:

public class Team implements Serializable {
//...
@AtLeastOneNotNull
@ManyToMany
@JoinTable(
    name="teams_players"
    , joinColumns={
        @JoinColumn(name="id_team")
        }
    , inverseJoinColumns={
        @JoinColumn(name="id_player")
        }
    )   
private List<Player> players;
//...
}

The validator:

public class AtLeastOneNotNullValidator implements ConstraintValidator<AtLeastOneNotNull, List<?>> {

@Override
public void initialize(AtLeastOneNotNull constraint) {

}

@Override
public boolean isValid(List<?> aCollection, ConstraintValidatorContext aConstraintValidatorContext) {
    if(aCollection == null || aCollection.isEmpty())
        return false;
    else
        return true;
}
}

Annotation:

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Constraint(validatedBy = AtLeastOneNotNullValidator.class)
public @interface AtLeastOneNotNull {

String message() default "{[ERROR] Collection must have at least one element}";

Class<?>[] groups() default {};

Class<? extends Payload>[] payload() default {};

}

Test class:

public class TeamDAOTest {

private static Validator VALIDATOR;

TeamTestHelper teamTestHelper = null;
TeamDAO teamDaoTest = null;

@Before
public void initTestClass() {
    teamDaoTest = new TeamDAO();
    teamTestHelper = new TeamTestHelper();
    ValidatorFactory factory = Validation.buildDefaultValidatorFactory();
    VALIDATOR = factory.getValidator();
}

@After
public void endTestClass() {

}

@Test
public void savingRightTeam() {
    Team testTeam = teamTestHelper.getTeamOK();

 //     Set<ConstraintViolation<Team>> constraintViolations = VALIDATOR.validate(testTeam);
 //     
 //     assertEquals(0, constraintViolations.size());
 //             
    assertEquals("Inserting right Team", true, teamDaoTest.updateEntity(testTeam));
}

@Test
public void savingWrongTeam() {
    Team testTeam = teamTestHelper.getTeamKO_WithoutPlayers();

//      Set<ConstraintViolation<Team>> constraintViolations = VALIDATOR.validate(testTeam);
//      
//      assertEquals(1, constraintViolations.size());

    assertEquals("Inserting empty Team", false, teamDaoTest.updateEntity(testTeam));
}

}

And finally, this is what happens when I run this

First of all, you can see the Team created with the list with one element. Note: The helper creates a player and persist it first, then adds to a team and return it, and this is the Team that we can see.

In a second step, when it's trying to persist the element and gets through the validator, the list is empty... ¿why?

And finally, of course, with a null list, the merge method throws an exception:

Any idea of what could be happening? I don't know why this happens, as you can see, I commented the other lines, the test pass ok, but not when I try to update, it seems as another object, or as the object instantiates again

Thanks again to everyone

---

EDIT:

The getPlayer() and addPlayer() methods in Team object:

public List<Player> getPlayers() {
    if(this.players == null)
        this.players = new ArrayList<Player>();

    return this.players;
}

public boolean addPlayer(Player aPlayer){
    if(!getPlayers().contains(aPlayer)){
        this.players.add(aPlayer);
        return true;
    }
    else return false;

}

---

Edit 2: DAO, as you can see, by now I don't need more specific methods in TeamDAO

public class TeamDAO extends AbstractDAOLayer<Team> {

}

UpdateEntity method in AbstractDAOLayer

public boolean updateEntity(T anObject){
    try{
        this.beginTransaction();
        this.entityManager.merge(anObject);
        this.finishtTransaction();
        return true;
    }
    catch(Exception e){
        return false;
    }
}

Answer 1

Please check this. The problem has nothing to do with BV but rather with jpa merge semantic with many-to-many relationship.