道兄，帮我看看这验证用户名字是否相同的问题

这是一个bean ,验证名字是否存在
public boolean queryName()
    {
     try
       {
        if(this.isValid())
        {
                 
                 String sql = "select * from '"+tableName+"' where '"+columName+"'= ?";
                 Mysql mysql = new Mysql(sql);
                 mysql.setString(1, "name");
                 rs = mysql.executeQuery();
              if(rs.next())
              {
                  String tempName = rs.getString("Username");
                  if(tempName==null && tempName.equals(""))  valid = false;
              }
           
            }
     }
     catch(Exception e)
     {
                System.out.println(e);
      }  
     return valid;
        }
    }
这是我写的jsp调用上面那个bean
<jsp:useBean id="IsParity" class="weixiu.IsParity" />
<%
   
   String username = null;  username = request.getParameter("username");
   IsParity.setName("username");
   IsParity.setTable("login");
   IsParity.setColum("Username");

   if(IsParity.queryName())
   {
	   out.println("Sorry!");
   }
   else
   {
	   out.println("go....");
   }

  %>
<p class="indent">