依赖2个jar包：commons-fileupload，commons-io。

代码如下：

    package com.example.web;  
      
    import java.io.File;  
    import java.io.FileOutputStream;  
    import java.io.IOException;  
    import java.io.InputStream;  
    import java.io.OutputStream;  
    import java.io.PrintWriter;  
    import java.text.DateFormat;  
    import java.text.SimpleDateFormat;  
    import java.util.Date;  
    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.disk.DiskFileItemFactory;  
    import org.apache.commons.fileupload.servlet.ServletFileUpload;  
    import org.apache.commons.io.IOUtils;  
      
    public class FileUpload extends HttpServlet {  
      
        public void doGet(HttpServletRequest request, HttpServletResponse response)  
                throws ServletException, IOException {  
              
            request.setCharacterEncoding("UTF-8");  
            response.setContentType("text/html;charset=utf-8");  
            PrintWriter out=response.getWriter();  
            //指定文件存放目录  
            String storePath=getServletContext().getRealPath("WEB-INF/files");  
            File file=new File(storePath);  
            //如果目录不存在，需要创建  
            if(!file.exists()){  
                file.mkdirs();  
            }  
            //要检查是不是enctype=multipart/form-data提交过来的数据  
            boolean isMultipart=ServletFileUpload.isMultipartContent(request);  
            //如果不是，需要处理  
            if(!isMultipart){  
                out.write("请将enctype设置成multipart/form-data类型");  
                response.setHeader("Refresh", "1;URL="+request.getContextPath());  
                return;  
            }  
            //去生成一个文件上传的FileUpload对象  
            DiskFileItemFactory factory=new DiskFileItemFactory();  
            //设置一个临时目录  
            //factory.setRepository(new File("E:/temp"));temp文件需要临时创建，我已经删掉了  
              
            ServletFileUpload upload=new ServletFileUpload(factory);  
            //将用户的请求转化为List<FileItem>  
            try {  
                List<FileItem> items=upload.parseRequest(request);  
                //遍历List<FileItem>中的数据  
                for (FileItem item : items) {  
                    //如果部分文件没有上传  
                    if("".equals(item.getName())){  
                        continue;  
                    }  
                    //判断是普通字段  
                    if(item.isFormField()){  
                        String fieldName=item.getFieldName();  
                        String fieldValue=item.getString("UTF-8");  
                        System.out.println(fieldName+":"+fieldValue);  
                    }else{  
                        //是上传的文件  
                        //需要获取文件名,有可能是路径  
                        String fileName=item.getName();  
                        fileName=fileName.substring(fileName.lastIndexOf(File.separator)+1);  
                        //文件名同名问题  
                        fileName=UUID.randomUUID().toString()+"_"+fileName;  
                        //解决一个文件夹下的文件过多的问题  
                        //用时间来生成子目录  
                        //String childPath=generateDir(storePath);  
                        //用hashcode来生成子目录  
                        String childPath=generateDir(storePath,fileName);  
                        //获取上传文件的数据  
                        InputStream is=item.getInputStream();  
                        OutputStream os=new FileOutputStream(storePath+File.separator+childPath+File.separator+fileName);  
                        IOUtils.copy(is, os);  
                        os.close();  
                        is.close();  
                        //item.delete();//清空临时文件  
                    }  
                }  
                out.write("文件上传成功<a href='"+request.getContextPath()+"'>继续上传</a>");  
            } catch (FileUploadException e) {  
                e.printStackTrace();  
            }  
        }  
      
      
        private String generateDir(String storePath, String fileName) {  
            int hashcode=fileName.hashCode();  
            int dir1=hashcode &  0xf;  
            int dir2=(hashcode & 0xf0)>>4;  
            String childPath=dir1+File.separator+dir2;  
            File file=new File(storePath,childPath);  
            if(!file.exists()){  
                file.mkdirs();  
            }  
            return childPath;  
        }  
      
      
        private String generateDir(String storePath) {  
            DateFormat df=new SimpleDateFormat("yyyyMMddhhmmss");  
            String childPath=df.format(new Date());  
            File file=new File(storePath, childPath);  
            if(!file.exists()){  
                file.mkdirs();  
            }  
            return childPath;  
        }  
      
      
        public void doPost(HttpServletRequest request, HttpServletResponse response)  
                throws ServletException, IOException {  
            this.doGet(request, response);  
      
        }  
      
    }