java - 用于 OneToMany 的 Spring RowMapper？

Question

我正在尝试为两个对象调整两个 RowMappers，这两个对象之间具有 @OneToMany 关系。

假设，我有两种方法:

public Account findAccount(long id) {
    SQL = "SELECT * FROM accounts WHERE id = ?";
    Account account = template.queryForObject(SQL, new Object[] { id }, MAP_ACCOUNT);
    return account;
}


public Card findCard(String number) {
    SQL = "SELECT * FROM cards WHERE number = ?";
    Card card = template.queryForObject(SQL, new Object[] { number }, MAP_CARD);
    return card;
}

和两个行映射器:

   private final RowMapper<Card> MAP_CARD = new RowMapper<Card>() {

    public Card mapRow(ResultSet rs, int rowNum) throws SQLException {
        Account account = findAccount(rs.getLong("account_id"));
        Card card = new DefaultCard(rs.getString("number"), account);
        return card;
    }

};

private final RowMapper<Account> MAP_ACCOUNT = new RowMapper<Account>() {

    public Account mapRow(ResultSet rs, int rowNum) throws SQLException {
        SQL = "SELECT * FROM cards where account_id = " + rs.getLong("id");
        List<Card> cards = template.query(SQL, MAP_CARD);
        Account account = new DefaultAccount(rs.getLong("id"), rs.getString("username"), cards);
        return account;
    }
};

运行 findAccount 或 findCard 方法将引发异常，指出“连接过多!”。这是因为它们之间的行映射器存在循环依赖性。我知道我以错误的方式完成了此操作，我想知道如何正确重写行映射器。非常感谢。

Answer 1

首先，您的 java 对象构造函数是“递归”紧密耦合的。 Account 和 Card 构造函数期望彼此作为参数。没有卡也可以有账户，对吗？因此删除 Account 构造函数的卡片列表。

现在进入查询，加载帐户卡时有两种情况:

1 - 从账户加载卡牌:您已经拥有该账户，无需为每张卡查询账户。因此，您可以拥有一个通过参数接收帐户的 MAP_CARD_FROM_ACCOUNT 行映射器。

2- 加载单张卡:在这种情况下，您只需要卡及其帐户，因此对于 MAP_CARD 映射器，您可以进行查询以返回卡和帐户信息:SELECT * FROM 卡 C, 帐户 a WHERE c.account_id=a.id and number = ?

下面是映射器代码的示例:

 private final RowMapper<Card> MAP_CARD_FROM_ACCOUNT = new RowMapper<Card>() {

    public void setAccount(Account account){
       this.account = account;
    }

    public Card mapRow(ResultSet rs, int rowNum) throws SQLException {
        Card card = new DefaultCard(rs.getString("number"), account);
        return card;
    }

};

 private final RowMapper<Card> MAP_CARD = new RowMapper<Card>() {

    public Card mapRow(ResultSet rs, int rowNum) throws SQLException {
        Account account = new Account(rs.getLong("a.account_id"), rs.getString("a.username");
        Card card = new DefaultCard(rs.getString("c.number"), account);
        return card;
    }

};

private final RowMapper<Account> MAP_ACCOUNT = new RowMapper<Account>() {

    public Account mapRow(ResultSet rs, int rowNum) throws SQLException {
        SQL = "SELECT * FROM cards where account_id = " + rs.getLong("id");
        Account account = new DefaultAccount(rs.getLong("id"), rs.getString("username"));
        MAP_CARD_FROM_ACCOUNT.setAccount(account)
        List<Card> cards = template.query(SQL, MAP_CARD_FROM_ACCOUNT);
        account.setCards(cards);
        return account;
    }
};