第十一章_文件下载

来源：互联网发布：重庆手机数据恢复编辑：程序博客网时间：2024/05/17 05:08

11.1、文件下载概述

1、将响应的内容类型设置为文件的内容类型。标头Content-type用来规定实体主体中的数据类型，包含媒体类型和子类型标识符。

2、添加一个名为Content-Disposition的HTTP响应头，给它赋值attachment；filename=filename，这里的fileName是指在文件下载对话框中显示出来的默认文件名。它通常与文件名相同，但是也可以不同。

例如，以下就是将一个文件发送到浏览器的代码范例。

FileInputStream fis = new FileInputStream(file) ;

BufferedInputStream bis = new BufferedInputStream(fis) ;

byte[] bytes = new byte[bis.available()] ;

response.setContentType(contentType) ;

OutputStream os = response.getOutputStream() ;

bis.read(bytes) ;

os.write(bytes) ;

警告：一定要确保你没有在无意中发送超出实际文件内容以外的任何字符。这有可能在你毫不知情的情况下发生。例如，如果需要在JSP页面中使用page指令，可以这么写：

<%@ page import=”java.io.FileInputStream”%>

<jsp:useBean id=”DBBeanId” scope=”page” class=”MyBean”>

在你毫不察觉的情况下，page指令后面的回车换行符就会被发送给浏览器。为了防止发送多余的字符，需要像下面这样编写这个指令：

<%@ page import=”java.io.FileInputStream”

%><jsp:useBean id=”DBBeanId” scope=”page” class=”MyBean”>

11.2、范例1：隐藏资源

在下面这个程序中，我们用一个FileDownloadServlet servlet将secret.pdf文件发送到浏览器。但是，只有授权用户才能浏览。如果用户没有登录，应用程序就会跳转到Login页面。在这里，用户可以在表单中输入用户名和密码，这些内容都将被提交给另一个Servlet：LoginServlet。

LoginServlet.java

package filedownloaded;import java.io.IOException;import javax.servlet.RequestDispatcher;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpSession;@WebServlet(urlPatterns = {"/login"})public class LoginServlet extends HttpServlet{private static final long serialVersionUID = 1L;public void doPost(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException {String userName = request.getParameter("userName") ;String password = request.getParameter("password") ;if(userName != null && userName.equals("ken")&& password != null && password.equals("secret")){HttpSession session = request.getSession(true) ;session.setAttribute("loggedIn", Boolean.TRUE);response.sendRedirect("download");return ;}else{RequestDispatcher dispatcher = request.getRequestDispatcher("/login.jsp") ;dispatcher.forward(request, response);}}}

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><%String path = request.getContextPath();String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html>  <head>    <base href="<%=basePath%>">        <title>My JSP 'login.jsp' starting page</title>    <meta http-equiv="pragma" content="no-cache"><meta http-equiv="cache-control" content="no-cache"><meta http-equiv="expires" content="0">    <meta http-equiv="keywords" content="keyword1,keyword2,keyword3"><meta http-equiv="description" content="This is my page"><!--<link rel="stylesheet" type="text/css" href="styles.css">-->  </head>    <body>    <form action="login" method="post">    <table>    <tr>    <td>User name: </td>    <td><input name="userName"/></td>    </tr>    <tr>    <td>Password: </td>    <td><input name="password" type="password"/></td>    </tr>    <tr>    <td colspan="2">    <input type="submit" value="login"/>    </td>    </tr>    </table>    </form>  </body></html>

FileDownloadServlet.java

package filedownloaded;import java.io.BufferedInputStream;import java.io.File;import java.io.FileInputStream;import java.io.IOException;import java.io.OutputStream;import javax.servlet.RequestDispatcher;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpSession;@WebServlet(urlPatterns = {"/download"})public class FileDownloadServlet extends HttpServlet{private static final long serialVersionUID = 1L;@Overridepublic void doGet(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {HttpSession session = req.getSession() ;if(session == null || session.getAttribute("loggedIn") == null){RequestDispatcher dispatcher = req.getRequestDispatcher("/login.jsp") ;dispatcher.forward(req, resp); return ;}String dataDirectory = req.getServletContext().getRealPath("/WEB-INF/data") ;File file = new File(dataDirectory, "secret.pdf") ;if(file.exists()){resp.setContentType("application/pdf");resp.addHeader("Content-Disposition", "attachment; filename=secret.pdf");byte[] buffer = new byte[1024] ;try(FileInputStream fis = new FileInputStream(file) ;BufferedInputStream bis = new BufferedInputStream(fis);OutputStream os = resp.getOutputStream()){int i = bis.read(buffer) ;while(i != -1){os.write(buffer, 0, i);i = bis.read(buffer) ;}}catch(IOException e){e.printStackTrace();}}}}

12.3、范例2：防止跨站引用

竞争对手很可能试图通过跨站引用来“窃取”你的网络资产，例如将你的贵重物品显示在他们的网站上，好像哪些东西就是他们的一样，如果通过编程的方式，仅当referer标头中包含你的域名时才发送资源，那么就可以防止上述情况的发生。当然，那种意志坚定的窃贼还是有可能下载到你的资产，但是那就要费一番功夫了。

下面的应用使用了一个Servlet，当且仅当referer标头不为空时，才将图片发送到浏览器。这样就可以防止直接在浏览器中输入其网址就能下载到图片。

ImageServlet.java

package filedownloaded;import java.io.BufferedInputStream;import java.io.File;import java.io.FileInputStream;import java.io.IOException;import java.io.OutputStream;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;@WebServlet(urlPatterns = {"/getImage"})public class ImageServlet extends HttpServlet{private static final long serialVersionUID = 1L;public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException,IOException{String referer = request.getHeader("referer") ;if(referer != null){String imageId = request.getParameter("id") ;String imageDirectory = request.getServletContext().getRealPath("/WEB-INF/image") ;File file = new File(imageDirectory, imageId + ".jpg") ;if(file.exists()){response.setContentType("image/jpg");byte[] buffer = new byte[1024] ;try(FileInputStream fis = new FileInputStream(file) ;BufferedInputStream bis = new BufferedInputStream(fis);OutputStream os = response.getOutputStream()){int i = bis.read(buffer) ;while(i != -1){os.write(buffer, 0, i);i = bis.read(buffer) ;}}catch(IOException e){e.printStackTrace();}}}}}

images.html

<!DOCTYPE html><html>  <head>    <title>images.html</title>    <meta name="keywords" content="keyword1,keyword2,keyword3">    <meta name="description" content="this is my page">    <meta name="content-type" content="text/html; charset=UTF-8">        <!--<link rel="stylesheet" type="text/css" href="./styles.css">-->  </head>    <body>    <img src="getImage?id=1"/>    <img src="getImage?id=2"/>    <img src="getImage?id=3"/>    <img src="getImage?id=4"/>    <img src="getImage?id=5"/>    <img src="getImage?id=6"/>    <img src="getImage?id=7"/>    <img src="getImage?id=8"/>    <img src="getImage?id=9"/>    <img src="getImage?id=10"/>  </body></html>

这样就能避免其他网站使用爬虫等技术进行图片下载。直接复制图片地址是访问不到图片的。

0 0