MySQL + JAVA异常:开始结果集之前 [英] MySQL + JAVA Exception: Before start of result set

问题描述

try
  {
   PreparedStatement  s = (PreparedStatement) conn.prepareStatement("SELECT voters.Check,count(*) FROM voting.voters where FirstName="+first+"and LastName="+last+" and SSN="+voter_ID);
   //java.sql.Statement k = conn.createStatement();

         rs=s.executeQuery();
               //s.executeQuery("SELECT voters.Check,count(*) FROM voting.voters where FirstName="+first+"and LastName="+last+" and SSN="+voter_ID);

         System.out.println(rs.first());
         c=rs.getInt(1);
         d=rs.getInt(2);

         System.out.println(c);
         System.out.println(d);

          if(c==1 && d==1)
          {
           s.executeUpdate("update cand set total=total+1 where ssn="+can_ID);
           System.out.println("Succeful vote");
           System.out.println("after vote");
           s.executeUpdate("update voters set voters.Check=1 where ssn="+voter_ID);
                 toclient=1;



             PreparedStatement  qw = (PreparedStatement) conn.prepareStatement("select FirstName from cand where ssn="+can_ID);

                 // rs=k.executeQuery("select FirstName from cand where ssn="+can_ID);
             rs1 = qw.executeQuery();//Error Here Plz help me
                  String name1= (String) rs1.getString(1);

                  System.out.println(name1);
                  s.executeUpdate("update voters set VTO="+name1+"where ssn="+voter_ID);
           System.out.println(rs.getString(1));

          }
          else
          {
           if(c != -1)
            toclient =2;
           if( d ==0)
            toclient =3;
           if( d>1)
            toclient =4;

          }
           System.out.println("out-----------");
           rs.close();

           s.close();

  }

   catch (SQLException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }

错误信息:

java.sql.SQLException: Before start of result set
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1072)
 at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:986)
 at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:981)

推荐答案

在您的代码段中，您创建了PreparedStatements，但未正确使用它们.准备好的语句旨在用作一种语句模板"，该语句在执行之前已绑定到值.引用javadoc:

In your code snippet you create PreparedStatements but you do not use them correctly. Prepared statements are meant to be used as a kind of 'statement template' which is bound to values before it executes. To quote the javadoc:

   PreparedStatement pstmt = con.prepareStatement(
                                 "UPDATE EMPLOYEES SET SALARY = ? WHERE ID = ?");
   pstmt.setBigDecimal(1, 153833.00)
   pstmt.setInt(2, 110592)

与您当前使用PreparedStatement相比，这有两个大优点:

This has two big advantages over your current usage of PreparedStatement:

• 一个PreparedStatement可以用于多次执行
• 防止可能的 SQL注入攻击

• one PreparedStatement can be used for multiple executes
• it prevents a possible SQL injection attack

这里的第二个是大问题，例如，如果您的变量first和last是在用户界面中收集的，并且未重新格式化，则可能会冒着一部分SQL输入这些值的风险，然后终止在您的陈述中！使用绑定参数，它们将仅用作值，而不是SQL语句的一部分.

The second one here is the biggie, if for instance your variables first and last are collected in a user interface and not reformatted, you run the risk of parts of SQL being input for those values, which then end up in your statements! Using bound parameters they will just be used as values, not part of the SQL statement.

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