Html表单文件上传:
	<form 
		action="${pageContext.request.contextPath}/UploadServlet" 
		method="POST"
		enctype="multipart/form-data">
			<input type="text" name="desc"/>
			<input type="file" name="testFile1" />
			<input type="file" name="testFile2" />
			<input type="submit" value="上传"/>
	</form>

Http请求post格式:
	POST /xxx HTTP/1.1
	Host: x.x.x
	Content-Length: xxxxxx
	Content-Type: multipart/form-data; boundary=BoundarybXA7KWbsgAx0OB7z

	--BoundarybXA7KWbsgAx0OB7z
	Content-Disposition: form-data; name="desc"

	...文本内容...
	--BoundarybXA7KWbsgAx0OB7z
	Content-Disposition: form-data; name="testFile1"; filename="文件1.zip"
	Content-Type: application/octet-stream

	...文件内容...
	--BoundarybXA7KWbsgAx0OB7z
	Content-Disposition: form-data; name="testFile2"; filename="文件2.zip"
	Content-Type: application/octet-stream

	...文件内容...
	--BoundarybXA7KWbsgAx0OB7z--

@MultipartConfig(
	fileSizeThreshold=(可选), 内存缓冲区大小
	location=(可选), 临时文件目录,调用Part.write(..)会自动清除临时文件
	maxFileSize=(可选), 单个文件最大值, 超过则抛出IllegalStateException异常
	maxRequestSize=(可选) 总上传的最大值, 超过则抛出IllegalStateException异常
				)
public class UploadServlet extends HttpServlet {
	protected void doGet(HttpServletRequest request, HttpServletResponse response){
		Collection<Part> part = request.getParts();
		for (Part p : part) {
			if (p.getSubmittedFileName() == null) {
				// 非文件Part
				StringBuilder value = new StringBuilder();
				BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream(), "utf-8"));
				String line;
				while ((line = br.readLine()) != null) {
					value.append(line);
				}
				System.out.println(p.getName() + ": " + value.toString());
			} else {
				// 文件Part
				String uuidName = UUID.randomUUID().toString() + "_" + p.getSubmittedFileName();
				//  参数是绝对路径+文件名,若只传文件名,则存储在location=目录
				p.write(getServletContext().getRealPath("/WEB-INF/upload")+uuidName);
			}		
		}
	}
}

commons-fileupload和commons-io两个包,fileupload依赖io包	
public class UploadServlet extends HttpServlet {
	protected void doGet(HttpServletRequest request, HttpServletResponse response){
		if (!ServletFileUpload.isMultipartContent(request)) {
			throw new RuntimeException("请用Multipart表单上传!");
		}
		// 1.创建ServletFileUpload,设定内存缓冲区大小(默认10k),设定临时文件目录(默认操作系统临时目录)
		DiskFileItemFactory factory = new DiskFileItemFactory(100*1024,new File(xx/temp));
		ServletFileUpload fileUpload = new ServletFileUpload(factory);
		fileUpload.setFileSizeMax(1024*1024*10); // 单个文件不大于10M
		fileUpload.setSizeMax(1024*1024*100); // 总大小不大于100M
		fileUpload.setHeaderEncoding("utf-8"); // 设置编码集

		// 2.监听文件上传进度
		fileUpload.setProgressListener(new ProgressListener(){
			public void update(long bytesRead, long contentLength, int items) {
				System.out.print("当前是第" + items + "个上传项, 总大小" + length/1024f + "KB, 已经读取" + read/1024f+ "KB");		
				System.out.println();
			}
		});

		// 4.获取所有FileItem
		for (FileItem fileItem : (List<FileItem>) fileUpload.parseRequest(request)) {
			if (fileItem.isFormField()) {
				// 字符项
				String name = fileItem.getFieldName();
				String value = fileItem.getString("utf-8");
				System.out.println(name + ":" + value);
			} else {
				// 文件项
				String uuidName = UUID.randomUUID().toString() + "_" + fileItem.getName();					
				// 生成随机目录path(目录分离)
				char[] hashPath = Integer.toHexString(uuidName.hashCode()).toCharArray();
				String path = getServletContext().getRealPath("WEB-INF/upload");
				for (char p : hashPath)	path += "/" + p;
				new File(path).mkdirs();
				// 存储文件
				InputStream in = fileItem.getInputStream();
				OutputStream out = new FileOutputStream(new File(path, uuidName));
				byte[] b = new byte[100*1024];
				int len;
				while ((len=in.read(b))!=-1) {
					out.write(b, 0, len);
				}
				// 删除临时文件
				fileItem.delete();
			}
		}
	}
}

// Http响应头,要求浏览器以附件形式打开
response.setHeader("Content-Disposition", "attachment;filename="+URLEncoder.encode(filename,"utf-8"));
// MIME类型
response.setContentType(getServletContext().getMimeType(filename));