1．客户端上传文件
客户端通过一个Jsp页面，上传文件到服务器，该Jsp页面必须含有File类表单，并且表单必须设置enctype="multipart/form-data"。提交表单时通过内置对象request，request.getInputStream();方法获得一个输入流。
在上传文件时，会有附加信息，如下所示：
-----------------------------7d71042a40328
Content-Disposition: form-data; name="fileforload"; filename="C:\Documents and Settings\ZJ\桌面\book.txt"
Content-Type: text/plain
//此处为文件内容
-----------------------------7d71042a40328
Content-Disposition: form-data; name="submit"
 
commit
-----------------------------7d71042a40328--
附加信息大小为297字节（不确定这个值，测试得到），可通过request.getContentLength()>297来判断是否上传了文件还是提交空字符串。
2．测试
fileupload.jsp负责提交文件，accept.jsp负责实现上传功能。
fileupload.jsp
<html>
<head>
  <meta http-equiv="Content-Type" content="text/html; charset=GB18030">
  <title>This page for FileUpload</title>
</head>
<body>
  <p>Choose the file for uploading:
  <form action="accept.jsp" method=post enctype="multipart/form-data">
    <input type=file name=fileforload size=30>
    <br>
    <input type=submit value=commit name=submit>
  </form>
</body>
</html>
 
accept.jsp
<html>
<head>
<%@ page language="java" import="java.io.*" %>
<meta http-equiv="Content-Type" content="text/html; charset=GB18030">
<title>This page for response</title>
</head>
<body>
    <%
    try {
       if (request.getContentLength() > 297) {
           InputStream in = request.getInputStream();
           File f = new File("d:/temp", "test.txt");
           FileOutputStream o = new FileOutputStream(f);
           byte b[] = new byte[1024];
           int n;
           while ((n = in.read(b)) != -1) {
               o.write(b, 0, n);
           }
           o.close();
           in.close();
           out.print("File upload success!");
           } else {
              out.print("No file!");
           }
       } catch (IOException e) {
           out.print("upload error.");
           e.printStackTrace();
       }
    %>
    </body>
</html>
 
服务器端得到的上传文件I like.txt，取名为test.txt
-----------------------------7d75b1540328
Content-Disposition: form-data; name="fileforload"; filename="C:\Documents and Settings\ZJ\桌面\I like.txt"
Content-Type: text/plain
 
我喜欢驾驭着代码在风驰电掣中创造完美;
我喜欢操纵着代码在随心所欲中体验生活;
我喜欢用心情代码编制我小小的与众不同;
每一段新的代码在我手中延生对我来说就象观看刹那花开的感动;
我不需要焦点.因为我就是焦点!
 
-----------------------------7d75b1540328
Content-Disposition: form-data; name="submit"
 
commit
-----------------------------7d75b1540328--
3．去除附加信息
按照HTTP协议，文件表单提交的信息中，前4行和后5行是表单本身的信息，中间部分才是上传的文件的内容。下例对上传的文件进行处理，获取文件名，并去除附加信息。
4．测试
fileupload.jsp不变，accept.jsp修改如下：
<html>
<head>
<%@ page language="java" import="java.io.*" %>
<meta http-equiv="Content-Type" content="text/html; charset=GB18030">
<title>The real file</title>
</head>
<body>
<%try{
    //use sessionid to create a temp file.
    String tempFileName=(String)session.getId();
    //create the temp file.
    File temp=new File("d:/temp",tempFileName);
    FileOutputStream o=new FileOutputStream(temp);
    if(request.getContentLength()>297){
      //write the upload content to the temp file.
      InputStream in=request.getInputStream();
      byte b[]=new byte[1024];
      int n;
      while((n=in.read(b))!=-1){
         o.write(b,0,n);
      }
      o.close();
      in.close();
      //read the temp file.
      RandomAccessFile random=new RandomAccessFile(temp,"r");
      //read Line2 to find the name of the upload file.
      int second=1;
      String secondLine=null;
      while(second<=2){
         secondLine=random.readLine();
         second++;
      }
      //get the last location of the dir char.'\\'.
      int position=secondLine.lastIndexOf('\\');
      //get the name of the upload file.
      String fileName=secondLine.substring(position+1,secondLine.length()-1);
      //relocate to the head of file.
      random.seek(0);
      //get the location of the char.'Enter' in Line4.
      long forthEndPosition=0;
      int forth=1;
      while((n=random.readByte())!=-1&&(forth<=4)){
         if(n=='\n'){
             forthEndPosition=random.getFilePointer();
             forth++;
         }
      }
      File realFile=new File("d:/temp",fileName);
      RandomAccessFile random2=new RandomAccessFile(realFile,"rw");
      //locate the end position of the content.Count backwards 6 lines.
      random.seek(random.length());
      long endPosition=random.getFilePointer();
      long mark=endPosition;
      int j=1;
      while((mark>=0)&&(j<=6)){
         mark--;
         random.seek(mark);
         n=random.readByte();
         if(n=='\n'){
             endPosition=random.getFilePointer();
             j++;
         }
      }
      //locate to the begin of content.Count for 4 lines's end position.
      random.seek(forthEndPosition);
      long startPoint=random.getFilePointer();
      //read the real content and write it to the realFile.
      while(startPoint<endPosition-1){
         n=random.readByte();
         random2.write(n);
         startPoint=random.getFilePointer();
      }
      random2.close();
      random.close();
      //delete the temp file.
      temp.delete();
      out.print("File upload success!");
    }
    else{
       out.print("No file!");
    }
}
catch(IOException e){
    out.print("upload error.");
    e.printStackTrace();
}
%>
</body>
</html>
（注：如果文件名是中文，会出现乱码。）
5．文件下载
Jsp内置对象response调用方法getOutputStream（）可以获取一个指向客户的输出流，服务器将文件写入这个流，然后可下载此文件。
6．测试
    download.jsp显示下载选项，LoadFile.java（Servlet）负责下载文件。
download.jsp
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=GB18030">
    <title>download page</title>
</head>
<body>
    <a href=loadFile>Download:test.zip</a>
</body>
</html>
 
LoadFile.java
package com.zj.sample;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStream;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
public class LoadFile extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)
           throws IOException, ServletException {
       OutputStream o = response.getOutputStream();
       byte b[] = new byte[1024];
       // the file to download.
       File fileLoad = new File("d:/temp", "test.rar");
       // the dialogbox of download file.
       response.setHeader("Content-disposition", "attachment;filename="
              + "test.rar");
       // set the MIME type.
       response.setContentType("application/x-tar");
       // get the file length.
       long fileLength = fileLoad.length();
       String length = String.valueOf(fileLength);
       response.setHeader("Content_Length", length);
       // download the file.
       FileInputStream in = new FileInputStream(fileLoad);
       int n = 0;
       while ((n = in.read(b)) != -1) {
           o.write(b, 0, n);
       }
    }
 
    public void doPost(HttpServletRequest request, HttpServletResponse response)  throws IOException, ServletException {
       doGet(request, response);
    }
}
 
web.xml（注册servlet）
<servlet>
    <servlet-name>LoadFileServlet</servlet-name>
    <servlet-class>com.zj.sample.LoadFile</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>LoadFileServlet</servlet-name>
    <url-pattern>/loadFile</url-pattern>
</servlet-mapping>