文件上传在web应用中是非常常见的，现在我就介绍下基于servlet的文件上传，基于Struts2的文件上传可以看：
页面端代码：
<%@ page language="java" import="java.util.*" pageEncoding="GBK"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>注册</title>
</head>
<body>
<form name="form1" onsubmit="return on_submit()"
action="RegisterServlet" method="post" enctype="multipart/form-data">
<input type="text" name="uname1" id="password" />
<input type="text" name="uname2" id="uname2"/>
<input type="password" name="password1" id="password" />
<input type="password" name="password2" id="password"/>
<input type="radio" value="男" checked="checked" name="sex"/>男
<input type="radio" value="女" name="sex"/>女
<input type="text" name="email" value="" class="box" id="login" />
<br/><br/>
<input type="file" name="file1" id="file"/>
<input type="submit" name="submit" value="完成注册" />
</form>
</body>
</html>
这里要注意的一点就是存在文件上传的form表单必须封装为enctype="multipart/form-data"；这里我们直接与后台进行交互，不进行Ajax交互，需要使用ajax可以看：
http://www.cnblogs.com/shenliang123/category/372520.html
下面我们继续来看servlet的代码实现：
package com.xidian.bbs.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import java.net.InetAddress;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.Statement;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.jsp.JspFactory;
import javax.servlet.jsp.PageContext;
import com.jspsmart.upload.*;
import com.xidian.bbs.bean.Bean;
import com.xidian.bbs.bean.RegisterBean;
import com.xidian.bbs.util.DBAccess;
import com.xidian.bbs.util.IpTimeStamp;
@SuppressWarnings("serial")
public class RegisterServlet extends HttpServlet{
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
response.setCharacterEncoding("GBK");
request.setCharacterEncoding("GBK");
SmartUpload smart=new SmartUpload();
try{
//PageContext是jsp的内置对象，在servlet不能直接使用，需要做一些处理
JspFactory _jspxFactory = null;
PageContext pageContext = null;
_jspxFactory = JspFactory.getDefaultFactory();
pageContext = _jspxFactory.getPageContext(this,request,response,"",true,8192,true);
smart.initialize(pageContext);//初始化上传操作
smart.upload();
IpTimeStamp its=new IpTimeStamp(InetAddress.getLocalHost().getHostAddress());//request.getRemoteAddr()获得用户的ip地址
//System.out.println("获取的ip为"+InetAddress.getLocalHost().getHostAddress());
//如果要实现文件的批量上传，则只需用for循环，将getFile(0)中的0改为i即可
String ext=smart.getFiles().getFile(0).getFileExt();//此为得到文件的扩展名,getFile(0)为得到唯一的一个上传文件
String fileName=its.getIpTimeRand()+"."+ext;
//System.out.println("获取 的文件名为"+fileName);
//this.getServletContext().getRealPath("/")为得到tomcat的跟目录，放于upload文件夹中，java.io.File.separator是一种安全操作
//String realPath="";
//this.getServletContext().getRealPath("/")+
smart.getFiles().getFile(0).saveAs("/headupload"+java.io.File.separator+fileName);
String realPath="headupload/"+fileName+"";
//
//由于前面的form表单已经进行了封装，这里就不能简单的用request.getparameter()来获取表单参数
String uname1 = smart.getRequest().getParameter("uname1");//昵称
String upass1 = smart.getRequest().getParameter("password1");
String sex = smart.getRequest().getParameter("sex");
String uname2 = smart.getRequest().getParameter("uname2");//用户名
String email = smart.getRequest().getParameter("email");
PrintWriter out = response.getWriter();
//以下是持久层操作，省略。。。。。。。。。。
}
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doGet(request,response);
}
}
上面使用到的ip+时间戳的类IpTimeStamp对文件进行重命名：http://www.cnblogs.com/shenliang123/archive/2012/04/19/2456645.html