JDBC驱动程序在空的ResultSet上引发“ ResultSet Closed”异常

JDBC驱动程序在空的ResultSet上引发“ ResultSet Closed”异常

是否为空，但执行以下 *** 作始终会 出错 ：

resultSet = statement.executeQuery(sql);string = resultSet.getString(1); // Epic fail. The cursor isn't set yet.

这不是错误。这是记录的行为。每个不错的JDBC教程都提到了它。您需要先使用设置ResultSet的游标，

next()

然后才能访问任何数据。

如果您实际上对所谓的唯一行是否 存在 感兴趣，则只需检查的结果

next()

。例如在一个虚构

UserDAO

类中：

public boolean exist(String username, String password) throws SQLException {    boolean exist = false;    try (        Connection connection = database.getConnection();        PreparedStatement statement = connection.prepareStatement("SELECt id FROM user WHERe username = ? AND password = MD5(?)");    ) {        statement.setString(1, username);        statement.setString(2, password);        try (ResultSet resultSet = statement.executeQuery()) { exist = resultSet.next();        }    }    return exist;}

如果你真的希望只有 零 或 一个 行，那么就这样做：

public User find(String username, String password) throws SQLException {    User user = null;    try (        Connection connection = database.getConnection();        PreparedStatement statement = connection.prepareStatement("SELECt id, username, email, birthdate FROM user WHERe username = ? AND password = MD5(?)");    ) {        statement.setString(1, username);        statement.setString(2, password);        try (resultSet = statement.executeQuery()) { if (resultSet.next()) {     user = new User(         resultSet.getLong("id"),         resultSet.getString("username"),         resultSet.getString("email"),         resultSet.getDate("birthdate"));  }        }    }    return user;}

然后只需在业务/域对象中相应地处理它，例如

User user = userDAO.find(username, password);if (user != null) {    // Login?}else {    // Show error?}

如果你真的希望只有 零 或 许多 行，那么就这样做：

public List<User> list() throws SQLException {    List<User> users = new ArrayList<User>();    try (        Connection connection = database.getConnection();        PreparedStatement statement = connection.prepareStatement("SELECt id, username, email, birthdate FROM user");        ResultSet resultSet = statement.executeQuery();    ) {        while (resultSet.next()) { users.add(new User(     resultSet.getLong("id"),     resultSet.getString("username"),     resultSet.getString("email"),     resultSet.getDate("birthdate")));        }    }    return users;}

然后只需在业务/域对象中相应地处理它，例如

List<User> users = userDAO.list();if (!users.isEmpty()) {    int count = users.size();    // ...}else {    // Help, no users?}