第十一章_文件下载
11.1、文件下載概述
1、將響應的內(nèi)容類型設(shè)置為文件的內(nèi)容類型。標頭Content-type用來規(guī)定實體主體中的數(shù)據(jù)類型，包括媒體類型和子類型標識符。
2、加入一個名為Content-Disposition的HTTP響應頭，給它賦值attachment；filename=filename，這里的fileName是指在文件下載對話框中顯示出來的默認文件名稱。
它通常與文件名稱同樣，可是也能夠不同。
比如，下面就是將一個文件發(fā)送到瀏覽器的代碼范例。
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：一定要確保你沒有在無意中發(fā)送超出實際文件內(nèi)容以外的不論什么字符。
這有可能在你毫不知情的情況下發(fā)生。比如，假設(shè)須要在JSP頁面中使用page指令，能夠這么寫：
<%@?page?import=”java.io.FileInputStream”%>
<jsp:useBean?id=”DBBeanId”?scope=”page”?class=”MyBean”>
在你毫不察覺的情況下，page指令后面的回車換行符就會被發(fā)送給瀏覽器。為了防止發(fā)送多余的字符。須要像以下這樣編寫這個指令：
<%@?page?import=”java.io.FileInputStream”
%><jsp:useBean?id=”DBBeanId”?scope=”page”?class=”MyBean”>
?
11.2、范例1：隱藏資源
在以下這個程序中，我們用一個FileDownloadServlet?servlet將secret.pdf文件發(fā)送到瀏覽器?？墒?#xff0c;僅僅有授權(quán)用戶才干瀏覽。假設(shè)用戶沒有登錄，應用程序就會跳轉(zhuǎn)到Login頁面。在這里，用戶能夠在表單中輸入username和password，這些內(nèi)容都將被提交給還有一個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);}}}
login.jsp
<%@ 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>
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
競爭對手非?？赡茉噲D通過跨站引用來“竊取”你的網(wǎng)絡(luò)資產(chǎn)，比如將你的貴重物品顯示在他們的站點上，好像哪些東西就是他們的一樣。假設(shè)通過編程的方式。僅當referer標頭中包括你的域名時才發(fā)送資源，那么就能夠防止上述情況的發(fā)生。當然。那種意志堅定的竊賊還是有可能下載到你的資產(chǎn)，可是那就要費一番功夫了。
以下的應用使用了一個Servlet。當且僅當referer標頭不為空時，才將圖片發(fā)送到瀏覽器。這樣就能夠防止直接在瀏覽器中輸入其網(wǎng)址就能下載到圖片。
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>
這樣就能避免其它站點使用爬蟲等技術(shù)進行圖片下載。直接復制圖片地址是訪問不到圖片的。
轉(zhuǎn)載于:https://www.cnblogs.com/wzzkaifa/p/6794510.html
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