记录一下，以免以下几个包导错

import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.FileUploadException;
import org.apache.commons.fileupload.ProgressListener;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;

源码：

FileSerlvet类

package com.chen;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;
import java.util.UUID;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.FileUploadException;
import org.apache.commons.fileupload.ProgressListener;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;

/**
 * Servlet implementation class FileSerlvet
 */
public class FileSerlvet extends HttpServlet {
	private static final long serialVersionUID = 1L;

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
	 *      response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		// TODO Auto-generated method stub
		// response.getWriter().append("Served at: ").append(request.getContextPath());
		
		// 判断上传的文件普通表单还是带文件的表单
		if (!ServletFileUpload.isMultipartContent(request)) {
			return;//终止方法运行,说明这是一个普通的表单,直接返回
		}
	    //创建上传文件的保存路径,建议在WEB-INF路径下,安全,用户无法直接访间上传的文件;
	    String uploadPath =this.getServletContext().getRealPath("/WEB-INF/upload");
	    File uploadFile = new File(uploadPath);
	    if (!uploadFile.exists()){
	    uploadFile.mkdir(); //创建这个月录
	    }
		
		// 创建上传文件的保存路径，建议在WEB-INF路径下，安全，用户无法直接访问上传的文件
		String tmpPath = this.getServletContext().getRealPath("/WEB-INF/tmp");
		File file = new File(tmpPath);
		if (!file.exists()) {
			file.mkdir();//创建临时目录
		}

		// 处理上传的文件,一般都需要通过流来获取,我们可以使用 request, getInputstream(),原生态的文件上传流获取,十分麻烦
		// 但是我们都建议使用 Apache的文件上传组件来实现, common-fileupload,它需要旅 commons-io组件;
		try {
			// 创建DiskFileItemFactory对象，处理文件路径或者大小限制
			DiskFileItemFactory factory = getDiskFileItemFactory(file);
			/*
			 * //通过这个工厂设置一个缓冲区,当上传的文件大于这个缓冲区的时候,将他放到临时文件 factory.setSizeThreshold(1024 *
			 * 1024); //缓存区大小为1M factory.setRepository (file);//临时目录的保存目录,需要一个File
			 */

			// 2、获取ServletFileUpload
			ServletFileUpload upload = getServletFileUpload(factory);

			// 3、处理上传文件
			// 把前端请求解析，封装成FileItem对象，需要从ServletFileUpload对象中获取
			String msg = uploadParseRequest(upload, request, uploadPath);
			
			// Servlet请求转发消息
			System.out.println(msg);
			if(msg == "文件上传成功!") {
				// Servlet请求转发消息
				request.setAttribute("msg",msg);
				request.getRequestDispatcher("info.jsp").forward(request, response);
			}else {
				msg ="请上传文件";
				request.setAttribute("msg",msg);
				request.getRequestDispatcher("info.jsp").forward(request, response);
			}

		} catch (FileUploadException e) {
			// TODO 自动生成的 catch 块
			e.printStackTrace();
		}
	}

	public static DiskFileItemFactory getDiskFileItemFactory(File file) {
		DiskFileItemFactory factory = new DiskFileItemFactory();
		// 通过这个工厂设置一个缓冲区,当上传的文件大于这个缓冲区的时候,将他放到临时文件中;
		factory.setSizeThreshold(1024 * 1024);// 缓冲区大小为1M
		factory.setRepository(file);// 临时目录的保存目录,需要一个file
		return factory;
	}

	public static ServletFileUpload getServletFileUpload(DiskFileItemFactory factory) {
		ServletFileUpload upload = new ServletFileUpload(factory);
		// 监听长传进度
		upload.setProgressListener(new ProgressListener() {

			// pBYtesRead:已读取到的文件大小
			// pContextLength:文件大小
			public void update(long pBytesRead, long pContentLength, int pItems) {
				System.out.println("总大小：" + pContentLength + "已上传：" + pBytesRead);
			}
		});

		// 处理乱码问题
		upload.setHeaderEncoding("UTF-8");
		// 设置单个文件的最大值
		upload.setFileSizeMax(1024 * 1024 * 10);
		// 设置总共能够上传文件的大小
		// 1024 = 1kb * 1024 = 1M * 10 = 10м

		return upload;
	}

	public static String uploadParseRequest(ServletFileUpload upload, HttpServletRequest request, String uploadPath)
			throws FileUploadException, IOException {

		String msg = "";
		
		// 把前端请求解析，封装成FileItem对象
		List<FileItem> fileItems = upload.parseRequest(request);
		for (FileItem fileItem : fileItems) {
			if (fileItem.isFormField()) {// 判断上传的文件是普通的表单还是带文件的表单
				// getFieldName指的是前端表单控件的name;
				String name = fileItem.getFieldName();
				String value = fileItem.getString("UTF-8"); // 处理乱码
				System.out.println(name + ": " + value);
			} else {// 判断它是上传的文件
				
				// ============处理文件==============

				// 拿到文件名
				String uploadFileName = fileItem.getName();
				System.out.println("上传的文件名: " + uploadFileName);
				if (uploadFileName.trim().equals("") || uploadFileName == null) {
					continue;
				}

				// 获得上传的文件名/images/girl/paojie.png
				String fileName = uploadFileName.substring(uploadFileName.lastIndexOf("/") + 1);
				// 获得文件的后缀名
				String fileExtName = uploadFileName.substring(uploadFileName.lastIndexOf(".") + 1);

				/*
				 * 如果文件后缀名fileExtName不是我们所需要的 就直按return.不处理,告诉用户文件类型不对。
				 */

				System.out.println("文件信息[件名: " + fileName + " ---文件类型" + fileExtName + "]");
				// 可以使用UID（唯一识别的通用码),保证文件名唯
				// 0UID. randomUUID(),随机生一个唯一识别的通用码;
				String uuidPath = UUID.randomUUID().toString();
				
				// ================处理文件完毕==============

				// 存到哪? uploadPath
				// 文件真实存在的路径realPath
				String realPath = uploadPath + "/" + uuidPath;
				// 给每个文件创建一个对应的文件夹
				File realPathFile = new File(realPath);
				if (!realPathFile.exists()) {
					realPathFile.mkdir();
				}
				// ==============存放地址完毕==============
				
				
				// 获得文件上传的流
				InputStream inputStream = fileItem.getInputStream();
				// 创建一个文件输出流
				// realPath =真实的文件夹;
				// 差了一个文件;加上翰出文件的名产"/"+uuidFileName
				FileOutputStream fos = new FileOutputStream(realPath + "/" + fileName);

				// 创建一个缓冲区
				byte[] buffer = new byte[1024 * 1024];
				// 判断是否读取完毕
				int len = 0;
				// 如果大于0说明还存在数据;
				while ((len = inputStream.read(buffer)) > 0) {
					fos.write(buffer, 0, len);
				}
				// 关闭流
				fos.close();
				inputStream.close();
				
				msg = "文件上传成功!";
				fileItem.delete(); // 上传成功,清除临时文件
				//=============文件传输完成=============
			}
		}
		return msg;

	}
}

注册xml

<servlet>
  	<servlet-name>upload</servlet-name>
  	<servlet-class>com.chen.FileSerlvet</servlet-class>
  </servlet>
  <servlet-mapping>
  	<servlet-name>upload</servlet-name>
  	<url-pattern>/upload.do</url-pattern>

导入依赖的jar包

<!-- https://mvnrepository.com/artifact/commons-fileupload/commons-fileupload -->
	<dependency>
		<groupId>commons-fileupload</groupId>
		<artifactId>commons-fileupload</artifactId>
		<version>1.4</version>
	</dependency>
<!-- https://mvnrepository.com/artifact/commons-io/commons-io -->
	<dependency>
		<groupId>commons-io</groupId>
		<artifactId>commons-io</artifactId>
		<version>2.6</version>
	</dependency>

index.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
	<body>
		<%--
		GET：上传文件大小有限制
		POST：上传文件大小没有限制
		 ${pageContext.request.contextPath}
		 --%>
		<form action="upload.do" enctype="multipart/form-data"  method="post">
			上传用户：<input type="text" name="username"><br/>
			<P><input type="file" name="file1"></P>
			<P><input type="file" name="file1"></P>
			<P><input type="submit" value="提交"> | <input type="reset"></P>
		</form>
	</body>
</html>

info.jsp

<%@ page language="java" contentType="text/html;charset=UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<title>Insert title here</title>
</head>
<body>

<%=request.getAttribute("msg")%>

</body>
</html>