[求助]servlet编译时遇到的问题.

一个论坛程序是用jsp＋servletjavabean模式的
程序里面的所有的servlet类都出现了啊警告不知什么原因例如
一个LoginServlet就出现警告提示为：
add default serial version ID
add generated serial version ID
rename in file

源码如下：
package com.wxpn.tutorial.servlet;
import java.io.IOException;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.wxpn.tutorial.db.ConnectionPool;
import com.wxpn.tutorial.db.DB;

/**
* Servlet implementation class for Servlet: LoginServlet
*/
public class LoginServlet extends javax.servlet.http.HttpServlet implements
javax.servlet.Servlet {

public LoginServlet() {
super();
}

protected void doGet(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
}

protected void doPost(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
String result = "";

// 获取提交的用户名：
String strUserName = request.getParameter("txtUserName");
// 获取提交的密码：
String strPassword = request.getParameter("txtPassword");

// 创建数据库连接对象：
ConnectionPool connPool = DB.getConnPool();
Connection conn = connPool.getConnection();
Statement stmt = null;
ResultSet rs = null;

try {
// sql语句：
String sql = "select * from userinfo where username='"
+ strUserName + "' and userpwd='" + strPassword + "'";
// 创建数据记录集对象：
stmt = conn.createStatement();
// 执行sql语句，返回一个记录集到rs：
rs = stmt.executeQuery(sql);
// 如果记录集为非空，表明有相匹配的用户名和密码，登录成功：
if (rs.next()) {
// 登录成功时,把strUserName设成session变量UserName
request.getSession(true).setAttribute("userName", strUserName);

response.sendRedirect("index.jsp");
} else {
// 否则（记录集为空），表明失败：
request.setAttribute("error", "登录失败");
try {
request.getRequestDispatcher("/error.jsp").forward(request,
response);
} catch (Throwable t) {
getServletContext().log(t.getMessage());
}
}
} catch (SQLException sqlExc) {
sqlExc.printStackTrace();
} catch (Exception e) {
e.printStackTrace();
} finally {
// 关闭连接，释放数据库资源：
try {
if (rs != null) {
rs.close();
}
if (stmt != null) {
stmt.close();
}
connPool.freeConnection(conn);
} catch (SQLException sqlExc) {
sqlExc.printStackTrace();
}
}
}
}

这个问题该怎么解决啊？？本人刚开始学java问题很多，还请大家多多帮助我啊～