登录仅适用于数据库中的最后一个用户 [英] Login works only for last user in the database

问题描述

由于某种原因，该登录仅适用于数据库中的最后一个用户.我有一个while循环，但我认为它使程序转到了最后一个用户.我尝试使用if语句，但是只有第一个用户可以登录.

For some reason the login only works for the last user in the database. I have a while loop, but I think it makes the program to go to the last user. I tried using if statement but then only the first user can log in.

  if (username!=null && password!=null) {
    pagename = "main";
    } else {
    username = request.getParameter("username");
    password = request.getParameter("password");

            while(results.next()) 
            { 
            if(results.getString(2).equals(password) && results.getString(1).equals(username)) 
            { 
            pagename="main";
            } 
            else 
            { 
            pagename="start";
            } 
    }
  }

这是怎么引起的，我该如何解决?

How is this caused and how can I solve it?

推荐答案

您要将整个数据库表复制到Java的内存中，并在while循环中对所有记录进行比较.当与记录匹配时，您不会中止while循环，因此它将继续循环遍历其余记录，因此pagename每次都会被开始"覆盖.

You are copying the entire DB table into Java's memory and doing the comparison in a while loop over all records. You are not aborting the while loop when there's a match with a record, so it continues looping over the remaining records and so the pagename get overridden with "start" everytime.

您需要添加break语句:

if (results.getString(2).equals(password) && results.getString(1).equals(username)) { 
    pagename="main";
    break;
}

或者更好的方法是让SQL执行它专为设计的工作，选择并精确返回所需的数据:

Or, better, let SQL do the job it is designed for, selecting and returning exactly the data you need:

preparedStatement = connection.prepareStatement("SELECT id FROM user WHERE username=? AND password=MD5(?)");
preparedStatement.setString(1, username);
preparedStatement.setString(2, password);
resultSet = preparedStatement.executeQuery();

if (resultSet.next()) {
    pagename = "main";
}
else {
    pagename = "start";
}

这更加有效和明智.

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