从本机查询和转换中检索数据

Question

我有以下查询

@Override
public List<PlayerDetails> testPlayerQuerry() {
    return copyPlayersToDetails(em.createNativeQuery("select player_name from player p\n"
            + "join Player_Game pg on p.player_id = pg.fk_player_id\n"
            + "join Game g on pg.fk_game_id = g.game_id\n"
            + "where g.game_id = 2").getResultList());
}

据我所知，它返回字符串列表？由于我得到的错误

原因：java.lang.ClassCastException：java.lang.String 无法转换为entities.Player 在 utils.SessionFacade.copyPlayersToDetails(SessionFacade.java:105)

抛出错误的类可以很好地处理返回玩家字符串的查询，例如从玩家返回 *

private List<PlayerDetails> copyPlayersToDetails(List< Player> players) {
    List<PlayerDetails> list = new ArrayList<PlayerDetails>();
    Iterator i = players.iterator();
    while (i.hasNext()) {
        Player player = (Player) i.next();
        PlayerDetails details = new PlayerDetails(player.getPlayerId(),
                player.getPlayerName(), player.getPlayerRating());
        list.add(details);
    }
    return list;
}

我必须如何转换此方法或如何（如果可能）将存储在此字符串列表中的这些值放入 JSF 页面，以便检索查询结果？

Answer 1

您的copyPlayersToDetails() 方法需要Player 实体列表，而不是String 列表。这就是为什么你会在这里得到ClassCastException：

Player player = (Player) i.next();

因为i.next() 返回一个String。您的 Java 编译器没有报错的原因是 Query.getResultList()（不幸的是）返回了一个原始类型列表。

您应该将本机查询更改为：

@Override
public List<PlayerDetails> testPlayerQuerry() {
    return copyPlayersToDetails(em.createNativeQuery("select p.* from player p\n"
            + "join Player_Game pg on p.player_id = pg.fk_player_id\n"
            + "join Game g on pg.fk_game_id = g.game_id\n"
            + "where g.game_id = 2", Player.class).getResultList());
}

进一步批评

• 您应该（可能）使用绑定变量
• 您无需加入Game 表

改为这样写：

@Override
public List<PlayerDetails> testPlayerQuerry() {
    return copyPlayersToDetails(em.createNativeQuery("select p.* from player p\n"
          + "join Player_Game pg on p.player_id = pg.fk_player_id\n"
          + "where pg.fk_game_id = :1", Player.class).setParameter(1, 2).getResultList());
}