import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
 * 如果一个表单的类型是post且enctype为multipart/form-date
 * 则所有数据都是以二进制的方式向服务器上传递。
 * 所以req.getParameter("xxx")永远为null。
 * 只可以通过req.getInputStream()来获取数据,获取正文的数据
 * 
 * @author wangxi
 *
 */
public class UpServlet extends HttpServlet {
 public void doPost(HttpServletRequest req, HttpServletResponse resp)
   throws ServletException, IOException {
  req.setCharacterEncoding("UTF-8");
  String txt = req.getParameter("txt");//返回的是null
  System.err.println("txt is :"+txt);
  System.err.println("=========================================");
  InputStream in = req.getInputStream();
//  byte[] b= new byte[1024];
//  int len = 0;
//  while((len=in.read(b))!=-1){
//   String s = new String(b,0,len);
//   System.err.print(s);
//  }
  BufferedReader br = new BufferedReader(new InputStreamReader(in));
  String firstLine = br.readLine();//读取第一行，且第一行是分隔符号，即随机字符串
  String fileName = br.readLine();//第二行文件信息，从中截取出文件名
  fileName = fileName.substring(fileName.lastIndexOf("\\")+1);// xxxx.txt"
  fileName = fileName.substring(0,fileName.length()-1);

  br.readLine();
  br.readLine();
  String data = null;
  //获取当前项目的运行路径
  String projectPath = getServletContext().getRealPath("/up");
  PrintWriter out = new PrintWriter(projectPath+"/"+fileName);
  while((data=br.readLine())!=null){
   if(data.equals(firstLine+"--")){
    break;
   }
   out.println(data);
  }
  out.close();
 }
}

/**
 * DiskFileItemFactory构造的两个参数
 * 第一个参数：sizeThreadHold - 设置缓存(内存)保存多少字节数据，默认为10K
 * 如果一个文件没有大于10K，则直接使用内存直接保存成文件就可以了。
 * 如果一个文件大于10K，就需要将文件先保存到临时目录中去。
 * 第二个参数 File 是指临时目录位置
 *
 */
public class Up2Servlet extends HttpServlet {
 public void doPost(HttpServletRequest req, HttpServletResponse resp)
   throws ServletException, IOException {
  req.setCharacterEncoding("UTf-8");
  //获取项目的路径
  String path = getServletContext().getRealPath("/up");
  //第一步声明diskfileitemfactory工厂类，用于在指的磁盘上设置一个临时目录
  DiskFileItemFactory disk = 
    new DiskFileItemFactory(1024*10,new File("/home/wang/"));
  //第二步：声明ServletFileUpoload，接收上面的临时目录
  ServletFileUpload up = new ServletFileUpload(disk);
  //第三步：解析request
  try {
   List<FileItem> list = up.parseRequest(req);
   //如果就一个文件
   FileItem file = list.get(0);
   //获取文件名,带路径
   String fileName = file.getName();
   fileName = fileName.substring(fileName.lastIndexOf("\\")+1);
   //获取文件的类型
   String fileType = file.getContentType();
   //获取文件的字节码
   InputStream in = file.getInputStream();
   //声明输出字节流
   OutputStream out = new FileOutputStream(path+"/"+fileName);
   //文件copy
   byte[] b = new byte[1024];
   int len = 0;
   while((len=in.read(b))!=-1){
    out.write(b,0,len);
   }
   out.close();

   long size = file.getInputStream().available();

   //删除上传的临时文件
   file.delete();
   //显示数据
   resp.setContentType("text/html;charset=UTf-8");
   PrintWriter op = resp.getWriter();
   op.print("文件上传成功<br/>文件名:"+fileName);
   op.print("<br/>文件类型:"+fileType);
   op.print("<br/>文件大小（bytes）"+size);



  } catch (Exception e) {
   e.printStackTrace();
  }

 }

}

<form action="<c:url value='/Up3Servlet'/>" method="post" enctype="multipart/form-data">
  File1：<input type="file" name="txt"><br/>
  File2：<input type="file" name="txt"><br/>
  <input type="submit"/>
 </form>

public class Up3Servlet extends HttpServlet {
 public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  request.setCharacterEncoding("UTF-8");
  String path = getServletContext().getRealPath("/up");
  //声明disk
  DiskFileItemFactory disk = new DiskFileItemFactory();
  disk.setSizeThreshold(1024*1024);
  disk.setRepository(new File("d:/a"));
  //声明解析requst的servlet
  ServletFileUpload up = new ServletFileUpload(disk);
  try{
   //解析requst
   List<FileItem> list = up.parseRequest(request);
   //声明一个list<map>封装上传的文件的数据
   List<Map<String,String>> ups = new ArrayList<Map<String,String>>();
   for(FileItem file:list){
    Map<String,String> mm = new HashMap<String, String>();
    //获取文件名
    String fileName = file.getName();
    fileName = fileName.substring(fileName.lastIndexOf("\\")+1);
    String fileType = file.getContentType();
    InputStream in = file.getInputStream();
    int size = in.available();
    //使用工具类
    FileUtils.copyInputStreamToFile(in,new File(path+"/"+fileName));
    mm.put("fileName",fileName);
    mm.put("fileType",fileType);
    mm.put("size",""+size);

    ups.add(mm);
    file.delete();
   }
   request.setAttribute("ups",ups);
   //转发
   request.getRequestDispatcher("/jsps/show.jsp").forward(request, response);

  }catch(Exception e){
   e.printStackTrace();
  }
 }
}

public class UpDescServlet extends HttpServlet {
 public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  request.setCharacterEncoding("UTF-8");//可以获取中文的文件名
  String path = getServletContext().getRealPath("/up");
  DiskFileItemFactory disk = 
    new DiskFileItemFactory();
  disk.setRepository(new File("d:/a"));
  try{
   ServletFileUpload up = 
     new ServletFileUpload(disk);
   List<FileItem> list = up.parseRequest(request);
   for(FileItem file:list){
    //第一步：判断是否是普通的表单项
    if(file.isFormField()){
     String fileName = file.getFieldName();//<input type="text" name="desc">=desc
     String value = file.getString("UTF-8");//默认以ISO方式读取数据
     System.err.println(fileName+"="+value);
    }else{//说明是一个文件
     String fileName = file.getName();
     fileName = fileName.substring(fileName.lastIndexOf("\\")+1);
     file.write(new File(path+"/"+fileName));
     System.err.println("文件名是:"+fileName);
     System.err.println("文件大小是："+file.getSize());
     file.delete();
    }
   }
  }catch(Exception e){
   e.printStackTrace();
  }
 }

}

public class FastServlet extends HttpServlet {


 public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  request.setCharacterEncoding("UTF-8");
  String path = getServletContext().getRealPath("/up");
  DiskFileItemFactory disk = 
    new DiskFileItemFactory();
  disk.setRepository(new File("d:/a"));
  try{
   ServletFileUpload up = new ServletFileUpload(disk);
   //以下是迭代器模式
   FileItemIterator it= up.getItemIterator(request);
   while(it.hasNext()){
    FileItemStream item = it.next();
    String fileName = item.getName();
    fileName=fileName.substring(fileName.lastIndexOf("\\")+1);
    InputStream in = item.openStream();
    FileUtils.copyInputStreamToFile(in,new File(path+"/"+fileName));
   }
  }catch(Exception e){
   e.printStackTrace();
  }

 }

}

public void doPost(HttpServletRequest req, HttpServletResponse resp)
   throws ServletException, IOException {
  req.setCharacterEncoding("UTF-8");
  String name = req.getParameter("name");
  //第一步：设置响应的类型
  resp.setContentType("application/force-download");
  //第二读取文件
  String path = getServletContext().getRealPath("/up/"+name);
  InputStream in = new FileInputStream(path);
  //设置响应头
  //对文件名进行url编码
  name = URLEncoder.encode(name, "UTF-8");
  resp.setHeader("Content-Disposition","attachment;filename="+name);
  resp.setContentLength(in.available());

  //第三步：开始文件copy
  OutputStream out = resp.getOutputStream();
  byte[] b = new byte[1024];
  int len = 0;
  while((len=in.read(b))!=-1){
   out.write(b,0,len);
  }
  out.close();
  in.close();
 }

在使用J2EE流行框架时

<form action="fileUpload.action" method="post" enctype="multipart/form-data">
 　　
  username: <input type="text" name="username"><br>
  file: <input type="file" name="file"><br>

  <input type="submit" value="submit">
 </form>

public class FileUploadAction extends ActionSupport
{
 private String username;

　　 //注意，file并不是指前端jsp上传过来的文件本身，而是文件上传过来存放在临时文件夹下面的文件
 private File file;

 //提交过来的file的名字
 //struts会自动截取上次文件的名字注入给该属性
 private String fileFileName;
 //getter和setter此时为了节约篇幅省掉
  @Override
 public String execute() throws Exception
 {
 //保存上传文件的路径
  String root = ServletActionContext.getServletContext().getRealPath("/upload");
  //获取临时文件输入流
  InputStream is = new FileInputStream(file);
  //输出文件
  OutputStream os = new FileOutputStream(new File(root, fileFileName));
  //打印出上传的文件的文件名
  System.out.println("fileFileName: " + fileFileName);
　　　　// 因为file是存放在临时文件夹的文件，我们可以将其文件名和文件路径打印出来，看和之前的fileFileName是否相同
  System.out.println("file: " + file.getName());
  System.out.println("file: " + file.getPath());

  byte[] buffer = new byte[1024];
  int length = 0;

  while(-1 != (length = is.read(buffer, 0, buffer.length)))
  {
   os.write(buffer);
  }

  os.close();
  is.close();

  return SUCCESS;
 }
}

public class FileDownloadAction extends ActionSupport
{
 //要下载文件在服务器上的路径
 private String path;
 //要下载文件的文件名
 private String downloadFileName;
 //写入getter和setter
 public InputStream getDownloadFile()
 {
  return ServletActionContext.getServletContext().getResourceAsStream(path);
 }

 @Override
 public String execute() throws Exception
 {
 //当前action默认在valuestack的栈顶
  setDownloadFileName(xxx);
  return SUCCESS;
 }
}

 <action name="fileDownload" class="com.struts2.FileDownloadAction">
    <result name="download" type="stream">
  <param name="contentDisposition">attachment;fileName="${downloadFileName}"</param>
  <param name="inputName">downloadFile</param>
   </result>
 </action>

@Controller
@RequestMapping(value="fileOperate")
public class FileOperateAction {
 @RequestMapping(value="upload")
 public String upload(HttpServletRequest request,@RequestParam("file") MultipartFile photoFile){
 //上传文件保存的路径
  String dir = request.getSession().getServletContext().getRealPath("/")+"upload";
  //原始的文件名
  String fileName = photoFile.getOriginalFilename();      //获取文件扩展名
  String extName = fileName.substring(fileName.lastIndexOf(".")); 
 //防止文件名冲突，把名字小小修改一下
 fileName = fileName.substring(0,fileName.lastIndexOf(".")) + System.nanoTime() + extName; 
  FileUtils.writeByteArrayToFile(new File(dir,fileName),photoFile.getBytes());
  return "success";
 }
 }

 @RequestMapping("/download")
 public String download(String fileName, HttpServletRequest request,
   HttpServletResponse response) {
  response.setCharacterEncoding("utf-8");
  response.setContentType("multipart/form-data");
  response.setHeader("Content-Disposition", "attachment;fileName="
    + fileName);
  try {
   InputStream inputStream = new FileInputStream(new File(文件的路径);

   OutputStream os = response.getOutputStream();
   byte[] b = new byte[2048];
   int length;
   while ((length = inputStream.read(b)) > 0) {
    os.write(b, 0, length);
   }

    // 这里主要关闭。
   os.close();

   inputStream.close();
  } catch (FileNotFoundException e) {
   e.printStackTrace();
  } catch (IOException e) {
   e.printStackTrace();
  }
   // 返回值要注意，要不然就出现下面这句错误！
   //java+getOutputStream() has already been called for this response
  return null;
 }