Servlet实现web站点文件下载功能示例

package com.hyman.view;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class SharedFilesServlet
 */
public class SharedFilesServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public SharedFilesServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request,HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException {
        // TODO Auto-generated method stub
        response.setContentType("text/html; charset=utf-8"); 
        PrintWriter out = response.getWriter();
        out.println("<script type='text/javascript' language='javascript'>"
                + "function fileDown(obj){"
                + "var resourceName = '';"
                + "var buttonId = obj.getAttribute('id');"
                + "var eleName = buttonId.substring(0,3) + 'File';"            
                + "resourceName = document.getElementsByName(eleName)[0].value;"
                + "var url = '/SharingSite/DownFileServlet?fileName=' + resourceName;"
                + "window.location.href=url;"
                + "}"
                + "</script>");
        out.println("<h1>文件共享网站</h1>");
        out.println("mp3资源：<input type='text' name='mp3File' />&nbsp;&nbsp;&nbsp;&nbsp;<input type='submit' value ='点击下载' id='mp3Down' onclick='fileDown(this)' /><br/>");
        out.println("avi资源：<input type='text' name='aviFile' />&nbsp;&nbsp;&nbsp;&nbsp;<input type='submit' value ='点击下载' id='aviDown' onclick='fileDown(this)' /><br/>");
        out.println("jpg资源：<input type='text' name='jpgFile' />&nbsp;&nbsp;&nbsp;&nbsp;<input type='submit' value ='点击下载' id='jpgDown' onclick='fileDown(this)' /><br/>");
        out.println("txt资源：<input type='text' name='txtFile' />&nbsp;&nbsp;&nbsp;&nbsp;<input type='submit' value ='点击下载' id='txtDown' onclick='fileDown(this)' /><br/>");
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request,HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request,IOException {
        // TODO Auto-generated method stub
        this.doGet(request,response);
    }

}

package com.hyman.controller;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStream;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class DownFileServlet
 */
public class DownFileServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public DownFileServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request,IOException {
        // TODO Auto-generated method stub
        response.setContentType("text/html; charset=utf-8");
        String referer = request.getHeader("Referer");
        if (referer == null || !referer.startsWith("http://localhost:8080/SharingSite/")) {
            response.sendRedirect("/SharingSite/AntiPirateLink");
        } else {
            String fileName = request.getParameter("fileName");
            response.setHeader("Content-Disposition","attachment; filename="+fileName);
            String path = this.getServletContext().getRealPath("/resources/"+fileName);
            FileInputStream fis = null;
            OutputStream os = null;
            try {
                fis = new FileInputStream(new File(path));
                byte buff[] = new byte[1024];
                int len = 0;
                os = response.getOutputStream();
                while ((len = fis.read(buff)) > 0) {
                    os.write(buff,len);
                }
            } catch (Exception e) {
                e.printStackTrace();
                throw new RuntimeException();
            } finally {
                if (os != null) os.close();
                if (fis != null) fis.close();
            }
        }
        
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request,response);
    }

}

package com.hyman.view;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class AntiPirateLink
 */
public class AntiPirateLink extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public AntiPirateLink() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request,IOException {
        // TODO Auto-generated method stub
        response.setContentType("text/html; charset=utf-8");
        PrintWriter out = response.getWriter();
        out.println("<script type='text/javascript' language='javascript'>"
                + "var i = 10;"
                + "function showTime(){"
                + "if(i > 0) {document.getElementById('time').innerHTML = '请不要盗链下载... ' + i +' 秒后自动跳转到下载首页...';"
                + "i--;}"
                + "}"
                + "setInterval('showTime()',1000);"
                + "</script>");
        out.println("<span id='time'></span>");
        response.setHeader("Refresh","10; url=/SharingSite/SharedFilesServlet");
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request,response);
    }

}

【分析】文件下载部分实现步骤：

1、response.setHeader("Content-Disposition","attachment; filename="+fileName);在http回复消息头中的Content-Disposition中指明下文件的文件名；
2、调用this.getServletContext().getRealPath("/resources/"+fileName);函数获取到文件的全路径；
3、使用文件流FileInputStream将文件读入到服务器内存，重要要用到缓存处理，然后再调用response的getOutputStream()方法获得向客户端写文件的输出文件流；

来自： http://www.cnblogs.com/page-of-Hyman/p/4321523.html