java下载文件案例（带源码）

以你之姓@ 2022-06-15 11:41 431阅读 0赞

#### 前言： ####

web开发中上传和下载是最基本的内容了。前几天写了一篇上传的案例，今天总结一下下载。

#### 案例说明： ####

1.本案例是使用myeclipse编写
    2.需要下载的文件存放的 /download_demo/WebRoot/WEB-INF/attachment  目录下面---这里我只是放进去了测试文件

#### 下载文件思路： ####

1.得到服务器上面的文件File
    2.得到输入流，将文件流化
    3.得到servlet输出流，将文件已流的方式写到客户端

#### 看懂案例中的需要的知识点： ####

1.servlet的配置
    2.filter的配置
    3.java文件的基础知识
    4.java流的知识

#### 前台页面代码：一、index.jsp页面 ####

功能：点击按钮，进入下载文件页面

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
    <% String path = request.getContextPath(); String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/"; %>
    
    <!DOCTYPE HTML>
    <html>
      <head>
        <base href="<%=basePath%>">
    
        <title>download demo</title>
        <meta charset="UTF-8">  
        <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">
        
    
        <script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-3.2.1.js"></script>
        <script type="text/javascript"> $(function(){ $("#download").on("click",function(){ window.open("download/show"); }); }); </script>
      </head>
    
      <body>
            <input type="button" value="点我进入下载页面" id="download"/>
      </body>
    </html>

#### 前台页面：二、download.jsp 页面 ####

作用：下载文件界面：

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
    <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
    <% String path = request.getContextPath(); String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/"; %>
    
    <!DOCTYPE HTML>
    <html>
      <head>
        <base href="<%=basePath%>">
    
        <title>download demo</title>
        <meta charset="UTF-8">  
        <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">
        
    
        <script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-3.2.1.js"></script>
    
      </head>
    
      <body>
            
            <c:forEach var="file" items="${files}" varStatus="status">
                <span>${file.name}</span><a href="download/downloadfile?fileName=${file.name}">下载</a>
                <br>
            </c:forEach>
      </body>
    </html>

#### 后端代码：一、下载控制器（servlet） —核心代码 ####

package cn.cupcat.controller;
    
    import java.io.BufferedInputStream;
    import java.io.File;
    import java.io.FileInputStream;
    import java.io.IOException;
    import java.net.URLEncoder;
    
    import javax.servlet.ServletException;
    import javax.servlet.ServletOutputStream;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    
    public class DownloadController extends HttpServlet { 
    
        /** * */
        private static final long serialVersionUID = 1L;
    
    
        @Override
        protected void doPost(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            System.out.println("我进入了post方法！");
    
            /*测试开始*/
            /* String uri = request.getRequestURI();// /download_demo/download/show StringBuffer url = request.getRequestURL();//http://127.0.0.1:8080/download_demo/download/show String servletPath = request.getServletPath();// /download String serverName = request.getServerName();// 127.0.0.1 String servletPath = request.getPathInfo();// /show */
    
            /*测试结束*/
            String servletPath = request.getPathInfo();// /show
    
    
            String relativePath = File.separatorChar + "WEB-INF"+ File.separatorChar + "attachment";
            String absolutePath = request.getServletContext().getRealPath(relativePath);
    
            if(servletPath.equalsIgnoreCase("/show")){
                File file = new File(absolutePath);
    
                if(file.exists()){
                    if(file.isDirectory()){
                        File[] files = file.listFiles();
                        request.setAttribute("files", files);
    
                    }
                }
    
                request.getRequestDispatcher("/download.jsp").forward(request, response);
    
            }else if(servletPath.equalsIgnoreCase("/downloadfile")){
                String fileName = request.getParameter("fileName");
                String fileAbosultePath = absolutePath +File.separatorChar+ fileName;
    
                File file = new File(fileAbosultePath);
                if(file.exists()){
       //file存在
                    if(file.isFile()){
       //file是一个文件
    
                        FileInputStream fis = new FileInputStream(file);
                        BufferedInputStream bis = new BufferedInputStream(fis);
    
                        byte[] buff = new byte[4096];
                        int readSize ;
                        ServletOutputStream outputStream = response.getOutputStream();
    
                        while((readSize = bis.read(buff)) != -1){
                            outputStream.write(buff, 0, readSize);
                        }
    
                         //1.设置文件ContentType类型，这样设置，会自动判断下载文件类型 
                        response.setContentType("multipart/form-data"); 
    
                        //2.设置文件头：最后一个参数是设置下载文件名(假如我们叫a.pdf) 
                        response.setHeader("Content-Disposition", "attachment;fileName="+ URLEncoder.encode(file.getName(), "UTF-8"));  
    
    
                        bis.close();
    
                        outputStream.close();
                    }
                }
    
            }else{
                System.out.println("没有用到！");
            }
    
    
    
    
    
    
    
    
    
        }
    
    
    
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp)
                throws ServletException, IOException {
            System.out.println("我进入了get方法！");
            this.doPost(req, resp);
    
        }
    
    
    
    
    }

#### 后端代码：二、过滤器（filter） —同一编码 ####

package cn.cupcat.filter;
    
    import java.io.IOException;
    
    import javax.servlet.Filter;
    import javax.servlet.FilterChain;
    import javax.servlet.FilterConfig;
    import javax.servlet.ServletException;
    import javax.servlet.ServletRequest;
    import javax.servlet.ServletResponse;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    
    public class UnicodeFilter implements Filter{ 
    
        @Override
        public void destroy() {
    
    
        }
    
        @Override
        public void doFilter(ServletRequest arg0, ServletResponse arg1,
                FilterChain arg2) throws IOException, ServletException {
            if(arg0 instanceof HttpServletRequest){
                System.out.println("进入了过滤器");
                HttpServletRequest request = (HttpServletRequest)arg0;
                request.setCharacterEncoding("UTF-8"); //设置编码，防止后台得到中文乱码
            }
            if (arg1 instanceof HttpServletResponse){
                HttpServletResponse response = (HttpServletResponse)arg1;
                response.setCharacterEncoding("UTF-8");//设置编码，防止返回客户端乱码
            }
            arg2.doFilter(arg0, arg1);
        }
    
        @Override
        public void init(FilterConfig arg0) throws ServletException {
    
        }
    
    }

#### 配置文件：web.xml —配置servlet、filter ####

<?xml version="1.0" encoding="UTF-8"?>
    <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
      <display-name>download_demo</display-name>
    
      
      <servlet>
            <servlet-name>download</servlet-name>
            <servlet-class>cn.cupcat.controller.DownloadController</servlet-class>
      </servlet>
      <servlet-mapping>
            <servlet-name>download</servlet-name>
            <url-pattern>/download/*</url-pattern>
      </servlet-mapping>
      
    
    
    
    
      
      <filter>
            <filter-name>unicode</filter-name>
            <filter-class>cn.cupcat.filter.UnicodeFilter</filter-class>
      </filter>
      <filter-mapping>
            <filter-name>unicode</filter-name>
                
            <url-pattern>/*</url-pattern>
      </filter-mapping>
       
    
    
      <welcome-file-list>
        <welcome-file>index.html</welcome-file>
        <welcome-file>index.htm</welcome-file>
        <welcome-file>index.jsp</welcome-file>
        <welcome-file>default.html</welcome-file>
        <welcome-file>default.htm</welcome-file>
        <welcome-file>default.jsp</welcome-file>
      </welcome-file-list>
    </web-app>

#### 注意： ####

//1.设置文件ContentType类型，这样设置，会自动判断下载文件类型  
    response.setContentType("multipart/form-data");
    
    
     //2.设置文件头：最后一个参数是设置下载文件名(假如我们叫a.pdf)  
     response.setHeader("Content-Disposition", "attachment;fileName="+ URLEncoder.encode(file.getName(), "UTF-8")); 
    
    
    
    说明：
        1.第一句是设置返回客户端的二进制文件，客户端可以直接下载
        2.第二句是设置返下载后的文件名

#### 项目源码下载地址： ####

http://download.csdn.net/detail/xinghuo0007/9858034