Question

我有一个实现动作监听器的注册对话框。如果用户输入名称并且它已经存在，我想在控制台上打印一条消息。如果用户名不存在，MySQL应将其添加到数据库中。不幸的是，这段代码不起作用：

private void regButtonActionPerformed(java.awt.event.ActionEvent evt) {                                          
    if(!userBox.getText().equals(""))
    {
        try
        {
            Class.forName("com.mysql.jdbc.Driver");
            Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/genx", "root", "Warlock1989");
            Statement stmt = con.createStatement();
            String query = "SELECT name FROM accounts";
            ResultSet rs = stmt.executeQuery(query);
            while(rs.next())
            {
                String uname = rs.getString("name");
                if(!uname.equals(userBox.getText()))
                {
                    PreparedStatement pstmt = con.prepareStatement("INSERT INTO accounts(name) VALUES(?)");
                    pstmt.setString(1, userBox.getText());
                    pstmt.executeUpdate();
                    System.out.println("Username " + userBox.getText() + " has been registered.");
                }
                else
                {
                    System.out.println("Username " + userBox.getText() + " already exists.");
                }
            }
        }
        catch(Exception ex)
        {
            ex.printStackTrace();
        }
    }

Answer 1

您当前的方法从数据库加载所有记录，并尝试查找将导致内存异常的用户（如果数据库包含大量记录）。所以， 不要从数据库中获取所有记录而只是使用where name=?运行查询来检查用户是否已存在于数据库中，如下所示：

PreparedStatement pstmt1 = null;
PreparedStatement pstmt2 = null;
ResultSet rs = null;
try {

    String userInput = userBox.getText();

    String query = "SELECT name FROM accounts where name=?";
    pstmt1 = con.preparedStatement(query);
    pstmt1.setString(1, userInput);

    rs = pstmt1.executeQuery();

    if(rs.next()) {
            pstmt2 = con.prepareStatement("INSERT INTO accounts(name) VALUES(?)");
            pstmt2.setString(1, userInput);
            pstmt2.executeUpdate();
            System.out.println("Username " + userInput + " has been registered.");
    }  else {
            System.out.println("Username " + userInput + " already exists.");
    }

} catch(SQLException sqlexe) {
    //add logging
} finally {
    if(pstmt1 != null)
       pstmt1.close();
    if(pstmt2 != null)
       pstmt2.close();
     if(rs != null)
       rs.close();
}

您当前的代码未发布resultsset＆amp; preparedstatement个对象，因此请确保释放finally块中的资源。

检查MySQL Java中是否存在用户名

1 个答案: