实现文件上传：

在Web页面中添加上传输入项是

在Servlet中读取上传文件的数据，并保存到本地硬盘中。

文件上传的相关API：

1. Fileltem 接口：

1String getName()方法

String getFieldName()方法
void write(File file)方法
String getString()方法
public String getString（）
public String getString（java.lang.String encoding)

String getContentType()方法
boolean isInMemory()方法

void delete()方法

InputStream getlnputStream()方法

long getSize()方法

创建项目，导入JAR包：

创建上传页面：

```cpp
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>文件上传</title>
</head>
<body>
    <form action="UploadServlet" method="post"
enctype="multipart/form-data">
        <table width="600px">
            <tr>
                <td>上传者</td>
                <td><input type="text" name="name" /></td>
            </tr>
            <tr>
                <td>上传文件</td>
                <td><input type="file" name="myfile" /></td>
            </tr>
            <tr>
                <td colspan="2"><input type="submit" value="上传" /></td>
            </tr>
        </table>
    </form>
</body>
</html>

创建Servlet：

package cn.itcast.fileupload;
import java.io.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.*;
import org.apache.commons.fileupload.*;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;
@WebServlet("/UploadServlet")
//上传文件的Servlet类
public class UploadServlet extends HttpServlet {
   private static final long serialVersionUID = 1L;
   public void doGet(HttpServletRequest request, HttpServletResponse response)
           throws ServletException, IOException {
       try {
       //设置ContentType字段值
       response.setContentType("text/html;charset=utf-8");
           // 创建DiskFileItemFactory工厂对象
           DiskFileItemFactory factory = new DiskFileItemFactory();
           //设置文件缓存目录，如果该目录不存在则新创建一个
           File f = new File("d:\\TempFolder");
           if (!f.exists()) {
               f.mkdirs();
           }
           // 设置文件的缓存路径
           factory.setRepository(f);
           // 创建 ServletFileUpload对象
           ServletFileUpload fileupload = new ServletFileUpload(factory);
           //设置字符编码
           fileupload.setHeaderEncoding("utf-8");
           // 解析 request，得到上传文件的FileItem对象
           List<FileItem> fileitems = fileupload.parseRequest(request);
           //获取字符流
           PrintWriter writer = response.getWriter();
           // 遍历集合
           for (FileItem fileitem : fileitems) {
               // 判断是否为普通字段
               if (fileitem.isFormField()) {
                   // 获得字段名和字段值
                   String name = fileitem.getFieldName();
                   if(name.equals("name")){
                       //如果文件不为空，将其保存在value中
                       if(!fileitem.getString().equals("")){
                           String value = fileitem.getString("utf-8");
                           writer.print("上传者：" + value + "<br>");
                       }
                   }
               } else {
                   // 获取上传的文件名
                   String filename = fileitem.getName();
                   //处理上传文件
                   if(filename != null && !filename.equals("")){
                       writer.print("上传的文件名称是：" + filename + "<br>");
                       // 截取出文件名
                       filename = filename.substring(filename.lastIndexOf("\\") + 1);
                       // 文件名需要唯一
                       filename = UUID.randomUUID().toString() + "_" + filename;
                       // 在服务器创建同名文件
                       String webPath = "/upload/";
                       //将服务器中文件夹路径与文件名组合成完整的服务器端路径
                       String filepath = getServletContext().getRealPath(webPath + filename);
                       // 创建文件
                       File file = new File(filepath);
                       file.getParentFile().mkdirs();
                       file.createNewFile();
                       // 获得上传文件流
                       InputStream in = fileitem.getInputStream();
                       // 使用FileOutputStream打开服务器端的上传文件
                       FileOutputStream out = new FileOutputStream(file);
                       // 流的对拷
                       byte[] buffer = new byte[1024];//每次读取1个字节
                       int len;
                       //开始读取上传文件的字节，并将其输出到服务端的上传文件输出流中
                       while ((len = in.read(buffer)) > 0)
                           out.write(buffer, 0, len);
                       // 关闭流
                       in.close();
                       out.close();
                       // 删除临时文件
                       fileitem.delete();
                       writer.print("上传文件成功！<br>");
                   }
               }
           }
       } catch (Exception e) {
           throw new RuntimeException(e);
       }
   }
   public void doPost(HttpServletRequest request, HttpServletResponse response)
           throws ServletException, IOException {
       doGet(request, response);
   }
}

文件下载：

创建下载页面：

<%@ page language="java" contentType="text/html; charset=UTF-8"
   pageEncoding="UTF-8"%>
<%@page import="java.net.URLEncoder"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>文件下载</title>
</head>
<body>
   

   <a href="/chapter12/DownloadServlet?filename=猫.jpg">文件</a>

   
</body>
</html>

创建Servlet：

package cn.itcast.fileupload;
import java.io.*;

import java.net.URLEncoder;
import javax.servlet.*;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.*;
@WebServlet("/DownloadServlet")
public class DownloadServlet extends HttpServlet {
   private static final long serialVersionUID = 1L;

   public void doGet(HttpServletRequest request, HttpServletResponse
       response) throws ServletException, IOException {
       //设置ContentType字段值
       response.setContentType("text/html;charset=utf-8");
       //设置相应消息编码
       response.setCharacterEncoding("utf-8");
       //设置请求消息编码
       request.setCharacterEncoding("utf-8");
       //获取所要下载的文件名称
       String filename = request.getParameter("filename");
       //对文件名称编码
       filename = new String(filename.trim().getBytes("iso8859-1"),"UTF-8");
//下载文件所在目录
String folder = "/download/";
// 通知浏览器以下载的方式打开
       response.addHeader("Content-Type", "application/octet-stream");
       response.addHeader("Content-Disposition",
       "attachment;filename="+URLEncoder.encode(filename,"utf-8"));
       // 通过文件流读取文件
       InputStream in = getServletContext().getResourceAsStream(
               folder+filename);
       // 获取response对象的输出流
       OutputStream out = response.getOutputStream();
       byte[] buffer = new byte[1024];
       int len;
       //循环取出流中的数据
       while ((len = in.read(buffer)) != -1) {
           out.write(buffer, 0, len);
       }

   }
   public void doPost(HttpServletRequest request, HttpServletResponse
       response) throws ServletException, IOException {
       doGet(request, response);
   }
}