求大神帮忙解释这段代码！！谢谢！！

package web;

import javax.servlet.http.*;
import javax.servlet.*;

import java.util.*;
import java.io.*;

import io.*;
/**
* Description:
* <br/>网站: <a href="http://www.crazyit.org">疯狂Java联盟</a> 
* <br/>Copyright (C), 2001-2010, Yeeku.H.Lee
* <br/>This program is protected by copyright laws.
* <br/>Program Name:
* <br/>Date:
* @author  Yeeku.H.Lee [email protected]
* @version  1.0
*/
public class ReplyListServlet extends HttpServlet 
{
    public void service(HttpServletRequest request,
        HttpServletResponse response)
        throws ServletException,IOException
    {
        //设置解码方式
        request.setCharacterEncoding("UTF-8");
        ServletContext application = getServletContext();//连接上下文
        //获取文件名
        String file = request.getParameter("file");
        System.out.println(file);
        //如果file参数不存在
        if (file == null || file.trim().equals(""))
        {
            //重定向到list.xml
            response.sendRedirect("list.xml");
        }
        else
        {
            //以file参数指定的XML文档作为响应
            request.getRequestDispatcher("/WEB-INF/xml/" + file.trim())
                .forward(request, response);
        }
    }
}
我们刚学过xml，没有学java web，但是我们做一个java web项目，看不懂这段代码，求解释，谢谢