Upload & Download

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Upload Demo：

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("UTF-8");
        resp.setContentType("text/html;charset=UTF-8");
        PrintWriter out = resp.getWriter();
        boolean isMulti = ServletFileUpload.isMultipartContent(req);
        if (isMulti) {
            FileItemFactory factory = new DiskFileItemFactory();
            ServletFileUpload upload = new ServletFileUpload(factory);
            try {
                Iterator items = upload.parseRequest(new ServletRequestContext(req)).iterator();
                while (items.hasNext()) {
                    FileItem item = (FileItem) items.next();
                    if (!item.isFormField()) {
                        String name = item.getName();
                        String fileName = name.substring(name.lastIndexOf('\\') + 1);
                        String path = req.getRealPath("file") + File.separatorChar + fileName;
                        File uploadedFile = new File(path);
                        String foo = path.substring(0, path.lastIndexOf('\\'));
                        File bar = new File(foo);
                        if (!bar.exists())
                            bar.mkdirs();
                        item.write(uploadedFile);
//                        PrintWriter out = resp.getWriter();
                        out.print(" 上传的文件为: " + name + "<br>");
                        out.print("保存的地址: " + path );
                    }
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

Download Demo：

Content-disposition 是 MIME 协议的扩展。MIME 协议指示 MIME 用户代理如何显示附加的文件。Content-disposition其实可以控制用户请求所得的内容存为一个文件的时候提供一个默认的文件名，文件直接在浏览器上显示或者在访问时弹出文件下载对话框。
关键在于

resp.setContentType("application/force-download");

随后通过流操作即可完成文件操作。

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        try {
            String fileName = "test.txt";
            resp.setHeader("Content-Disposition", "attachment; filename=" + fileName);
            resp.setContentType("application/force-download");
            OutputStream out; //输出流
            InputStream in; //输入流
            out = resp.getOutputStream();
            int byteSent;
            byte[] buff = new byte[512];
            in = new FileInputStream(new File(req.getRealPath(fileName)));
            while ((byteSent = in.read(buff)) != -1)
                out.write(buff, 0, byteSent);
            in.close();
            out.close();
        } catch (Exception e) {
            System.out.println(e);
            e.printStackTrace();
        }
    }

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
    <welcome-file-list>
        <welcome-file>UploadFileDemo.jsp</welcome-file>
    </welcome-file-list>
    <filter>
        <filter-name>SetChar</filter-name>
        <filter-class>SetCharacterEncodingFilter</filter-class>
        <init-param>
            <param-name>encoding</param-name>
            <param-value>utf-8</param-value>
        </init-param>
        <init-param>
            <param-name>enable</param-name>
            <param-value>true</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>SetChar</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
    <servlet>
        <servlet-name>ArbitraryName_Whatever</servlet-name>
        <servlet-class>MyServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>ArbitraryName_Whatever</servlet-name>
        <url-pattern>/FormActionName</url-pattern>
    </servlet-mapping>
</web-app>

关于文件路径的注意事项

这四行代码得到的结果分别为：

目录结构：