一、文件上传

所需jar包

首先是commons-fileupload.jar包
其次是commons-IO.jar包
前者的使用依赖后者，两者缺一不可

实现

前台要求

在前台提交的form表单请求方式必须为:post,编码类型需设置为:multipart/form-data

后台实现

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.setCharacterEncoding("utf-8");
        response.setCharacterEncoding("utf-8");
        response.setContentType("text/html;charset=utf-8");
        try {
            boolean isMultipart = ServletFileUpload.isMultipartContent(request);
            if(isMultipart) {//判断前台请求form表单的编码类型是否为multipart/form-data
                FileItemFactory factory = new DiskFileItemFactory();
                ServletFileUpload upload = new ServletFileUpload(factory);
                List<FileItem> items = upload.parseRequest(request);//解析请求,获取form表单所提交的每一项
                for(FileItem item:items) {
                    if(!item.isFormField()) {//判断是否为普通form表单项,若不普通，则认为是文件，进行上传操作
                        String fileName = item.getName();//获取文件名
                        String path = "D:\\MyDevelope\\ServerFiles";//设置文件在服务器中的存储路径
                        File file = new File(path,fileName);
                        item.write(file);//将文件写入指定路径
                    }
                }
            }   
        }catch (Exception e) {
            e.printStackTrace();
        }
    }

二、文件下载

于文件上传所不同的是，文件下载不需要导包，但是他需要解决中文乱码问题

解决中文乱码

对于Firefox、opera、chrome，可以将content-Disposition消息头设置为：

    response.addHeader("content-Disposition","attachment;filename==?UTF-8?B?"+new String(Base64.encodeBase64(fileName.getBytes("UTF-8")))+"?=");

而对于edge，则需要将content-Disposition消息头设置为：

    response.addHeader("content-Disposition","attachment;filename="+URLEncoder.encode(fileName,"UTF-8"));

获取客户端浏览器类型

可以使用

    request.getHeader("User-Agent");

获取包含浏览器类型的消息头，接下来使用String的contains()方法即可判断客户端是否为某种类型

实现

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            request.setCharacterEncoding("utf-8");
            response.setCharacterEncoding("utf-8");
            response.addHeader("content-Type", "application/octet-stream");//设置下载的文件的MIME类型
            String fileName = request.getParameter("fileName");
            String path = "D:\\MyDevelope\\ServerFiles\\";//设置要下载的文件在服务器中的目录
            /**
             *解析浏览器类型，设置不同的“content-Disposition”消息头来解决中文乱码问题
             */
            String agent = request.getHeader("User-Agent").toLowerCase();
            String contentDisposition = "attachement;filename="+fileName;
            System.out.println(agent);
            if(agent.contains("edge")) {
                System.out.println("edge");
                contentDisposition = "attachment;filename="+URLEncoder.encode(fileName,"UTF-8");
            }else if(agent.contains("firefox")||agent.contains("opr")||agent.contains("chrome")) {
                contentDisposition = "attachment;filename==?UTF-8?B?"+new String(Base64.encodeBase64(fileName.getBytes("UTF-8")))+"?=";
            }
            response.addHeader("content-Disposition",contentDisposition);   
            //InputStream in = getServletContext().getResourceAsStream(path+fileName);//此种方式只能下载项目根目录中的文件
            /**
             * 通过IO操作将文件输出给用户
             */
            InputStream in = new FileInputStream(path+fileName);
            ServletOutputStream out = response.getOutputStream();
            byte[] bs = new byte[10];
            int len = -1;
            while((len=in.read(bs))!=-1) {
                out.write(bs,0,bs.length);
            }
            out.close();
            in.close();
        }