java – JDBC驱动程序在空的ResultSet上抛出“ResultSet Closed

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

这不是一个bug.这是documented behaviour.每个decent JDBC tutorial都提到它.您需要使用next()设置ResultSet的游标才能访问任何数据.

如果你真的感兴趣,不管是否存在唯一的行,那么只需检查next()的结果.例如在虚构的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;
}

如果你真的期望只有零行或一行,那么只需要做一些事情：

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,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?
}

如果你真的期望只有零行或多行,那么只需要做一些事情：

public List<User> list() throws SQLException {
    List<User> users = new ArrayList<User>();

    try (
        Connection connection = database.getConnection();
        PreparedStatement statement = connection.prepareStatement("SELECT id,birthdate FROM user");
        ResultSet resultSet = statement.executeQuery();
    ) {
        while (resultSet.next()) {
            users.add(new User(
                resultSet.getLong("id"),resultSet.getDate("birthdate")));
        }
    }

    return users;
}

然后在业务/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?
}