如何从MySQL检测空的ResultSet？

只需检查是否

ResultSet#next()

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

您可以如下使用

if (userDAO.exist(username, password)) {    // Proceed with login?} else {    // Show error?}

另外，您也可以让它返回一个完全错误的消息，

User

null

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

与

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