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());
}

据我所知是哪个 return 字符串列表？由于我得到的错误

Caused by: java.lang.ClassCastException: java.lang.String cannot be cast to entities.Player at utils.SessionFacade.copyPlayersToDetails(SessionFacade.java:105)

抛出错误的 class 它可以很好地处理 returns player String 例如 return * from players

的查询

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那些值获取到 JSF 页面中以便检索查询结果？

Answer 1

您的 copyPlayersToDetails() 方法需要一个 Player 实体列表，而不是 String 列表。这就是你在这里得到 ClassCastException 的原因：

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

因为i.next()returns一个String。 Java 编译器没有报错的原因是 Query.getResultList()（不幸的是）returns 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
您不需要加入 Game table

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());
}

从本机查询和转换中检索数据

Retrieving data from native query and conversion

java

jpa

jpql

nativequery

进一步批评