通过java服务器/客户端将记录添加到MySQL数据库

Question

我正在制作一个基本的java服务器（稍后将提供客户端），它将访问我的MySQL数据库。它只需检查输入的用户名是否存在。如果确实如此，则创建记录。在玩了一堆不同的东西试图让它工作后，我遇到了（rs.next），当我使用它时，我得到了一个错误：找不到它的符号。 这是我的代码

    public void run()
    {
        final String queryCheck = "SELECT * from account WHERE username = ?";

        try
        {

            ObjectInputStream FromClient = new ObjectInputStream(
                    client.getInputStream());
            ObjectOutputStream ToClient = new ObjectOutputStream(
                    client.getOutputStream());
            InetAddress inetAddress = client.getInetAddress();
            String host = "jdbc:mysql://localhost/midterm";
            String user = "scott";
            String pass = "tiger";
            String database = "midterm";
            Connection conn = DriverManager.getConnection(host,user,pass);
            PreparedStatement stmt = conn.prepareStatement(queryCheck);
            System.out.print("Connected to database\n");

            System.out.println("Connect to Client " + clientID + " at " + inetAddress.getHostName() + " " + inetAddress.getHostAddress());
            // create data input and out streams

            while(true)
            {
                Object objReceived = FromClient.readObject();
                Object objReceived2 = FromClient.readObject();
                String username = objReceived.toString();
                String userpass = objReceived2.toString();
                System.out.print("\nInput received: " + "\nUsername: " + username + " Password: " + userpass);

                try
                {

                    stmt.setString(1, username);
                    //stmt.setString(2, userpass);
                    ResultSet rs = stmt.executeQuery();
                    boolean recordAdded = false;

                    if (!rs.next())
                    {
                        stmt.addBatch();
                        stmt.executeBatch();
                        recordAdded = true;
                    }
                    if (recordAdded)
                    {
                        String txt = ("Account Created!!");
                        ToClient.writeObject(txt);
                        System.out.println("Account has been Created!");
                        recordAdded = false;

                    }else
                    {
                        String txt = ("Username already exists!");
                        ToClient.writeObject(txt);
                        System.out.println("\nAccount already exists!");
                    }
                }
                catch (Exception ex)
                {
                }
            //conn.close();
            }
        } catch (Exception e) {
        System.err.println("Got an exception! ");
        System.err.println(e.getMessage());
        }

        }

坦率地说，我不确定我是否走上了正确的道路。我在这里也看到过类似的文章，但是他们的代码可以使用next（），而我的代码不能。任何反馈都将不胜感激

Answer 1

这里

while (!rs.next) 

你说的是，而rs没有下一个元素

若要在ResultSet上迭代，必须删除!，这样您将有：

while (rs.next)  // you say, while there are more elements, loop...
{
  stmt.addBatch();
  stmt.executeBatch();
  recordAdded = true;
}

Answer 2

            while (!rs.next)  // I have also tried stmt.next
            {
                stmt.addBatch();
                stmt.executeBatch();
                recordAdded = true;
            }

将上述代码替换为：

            if(rs.next())
            {
                stmt.addBatch();
                stmt.executeBatch();
                recordAdded = true;
            }