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String的列表？由于我得到的错误

引起：java.lang.ClassCastException：java.lang.String无法强制转换为entities.Player 在utils.SessionFacade.copyPlayersToDetails（SessionFacade.java:105）

抛出错误的类，它可以正常运行查询返回玩家字符串，例如来自玩家的返回*

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;
}

我如何转换此方法或如何（如果可能）我可以将存储在此String列表中的值存入JSF页面，以便我可以检索查询结果？

Answer 1

您的copyPlayersToDetails()方法需要Player个实体列表，而不是String列表。这就是你在这里获得ClassCastException的原因：

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

因为i.next()返回String。您的Java编译器没有抱怨的原因是Query.getResultList()（不幸的是）返回原始类型列表。

您应该将原始查询更改为：

@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

你应该（可能）使用绑定变量
您不需要加入Game表格

改为写下：

@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());
}

从原生查询和转换中检索数据

1 个答案:

进一步批评