The Java EE 6 Tutorial, Volume I

Creating Criteria Queries in RequestBean's Business Methods

Many of the business methods in RequestBean define Criteria queries. One business method, getPlayersByPosition, returns a list of players that play a particular position on a team.

public List<PlayerDetails> getPlayersByPosition(String position) {"getPlayersByPosition");
    List<Player> players = null;
    try {
        CriteriaQuery<Player> cq = cb.createQuery(Player.class);
        if (cq != null) {
            Root<Player> player = cq.from(Player.class);

            // set the where clause
            cq.where(cb.equal(player.get(Player_.position), position));
            TypedQuery<Player> q = em.createQuery(cq);
            players = q.getResultList();

        return copyPlayersToDetails(players);
    } catch (Exception ex) {
        throw new EJBException(ex);

A query is object is created by calling the CriteriaBuilder object's createQuery method, with the type set to Player because the query will return a list of players.

The query root is the base entity from which the query will navigate to find the entity's attributes and related entities, and is created by calling the from method of the query object. This sets the from clause of the query.

The where clause, set by calling the where method on the query object, restricts the results of the query according to the conditions of an expression. The CriteriaBuilder.equal method compares the two expressions. In getPlayersByPosition, the position attribute of the Player_ Metamodel class, accessed by calling the get method of the query root, is compared to the position parameter passed to getPlayersByPosition.

The select clause of the query is set by calling the select method of the query object. The query will return Player entities, so the query root object is passed as a parameter to select.

The query object is prepared for execution by calling EntityManager.createQuery, which returns a TypedQuery<T> object with the type of the query, in this case Player. This typed query object is used to execute the query, which occurs when the getResultList method is called, and a List<Player> collection is returned.