JavaWeb上传和下载原理及实现

//name的意思是文本框里面name的属性值，而admin是我们输入的文本值
Content-Disposition: form-data; name="username"
admin

//filename的意思是：我们上传的文件名称，content-Type的意思是：MIME类型，asdasdas的意思是：文件里面的内容
Content-Disposition: form-data; name="upload"; filename="a.txt"
Content-Type: text/plain
asdasdas

 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 判断表单的enctype值是不是"multipart/form-data"
        boolean isMultipartContent = ServletFileUpload
                .isMultipartContent(request);
        if (!isMultipartContent) {
            throw new RuntimeException("your form is not multipart/form-data");
        }
        // 创建一个DiskFileItemfactory工厂类
        DiskFileItemFactory factory = new DiskFileItemFactory();
        // 创建一个ServletFileUpload核心对象
        ServletFileUpload sfu = new ServletFileUpload(factory);
        // 解析request对象，并得到一个表单项的集合
        try {
            List<FileItem> fileItems = sfu.parseRequest(request);
            // 遍历表单项数据
            for (FileItem fileitem : fileItems) {
                if (fileitem.isFormField()) {
                    // 普通表单项
                    String fieldName = fileitem.getFieldName();
                    String fieldValue = fileitem.getString();
                    System.out.println(fieldName + "=====" + fieldValue);
                } else {
                    //上传表单项
                    //得到文件输入流
                    InputStream is = fileitem.getInputStream();
                    //创建文件存储目录
                    String directoryRealPath = this.getServletContext().getRealPath("WEB-INF/upload");
                    //创建文件路径
                    File storeDirectory = new File(directoryRealPath + File.separator + fileitem.getName());
                    //使用apache commons-io包，将输入流转成文件
                    FileUtils.copyInputStreamToFile(is, storeDirectory);
                }
            }

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

    // 下载
    public class DownloadServlet extends HttpServlet{
        public void doGet(HttpServletRequest request, HttpServletResponse response)
                throws ServletException,IOException{
            //根据文件名获取 MIME 类型
            String contentType = this.getServletContext().getMimeType("测试.xls");

            //设置content-Type头
            response.setHeader("Content-Type",contentType);

            //解决下载框为中文名称出现乱码问题
            String fileName;

            //获取用户浏览器
            String agent=request.getHeader("user-agent")

            // 火狐浏览器
            if (agent.contains("Firefox")) { 
                filename = "=?UTF-8?B?"
                        + new BASE64Encoder().encode(filename.getBytes("utf-8"))
                        + "?=";
                filename = filename.replaceAll("\r\n", "");
            // IE及其他浏览器
            } else { 
                filename = URLEncoder.encode(filename, "utf-8");
                filename = filename.replace("+"," ");
            }
            //设置Content-Disposition头
            resp.setHeader("Content-Disposition","attachment; filename=" + filename + "");

            // 输入流
            FileInputStream fis= new FileInputStream("文件路径");
            // 获取输出流
            ServletOutputStream output = response.getOutputStream();
            // 把输入流中的数据写入到输出流中
            byte[] b = new byte[fis.available()];
            fis.read(b);
            //输出
            output .write(b);
            output .flush();
        }
    }