文件下载案例

1. 要求

页面显示超链接
点击超链接后弹出下载提示框
完成图片/视频文件下载

2. 效果

3. 分析

1. 超链接指向的资源如果能够被浏览器解析，则在浏览器中展示，如果不能解析，则弹出下载提示框。不满足需求因为任何资源都必须弹出下载提示框
2. 使用响应头设置资源的打开方式：
	* content-disposition:attachment;filename=xxx
3. 文件中文名乱码

3. 步骤

1. 定义页面，编辑超链接href属性，指向Servlet，传递资源名称filename
2. 定义Servlet
	1. 获取文件名称
	2. 使用字节输入流加载文件进内存
	3. 指定response的响应头： content-disposition:attachment;filename=xxx
	4. 设置传输的数据是什么类型
	5. 将数据写出到response输出流

4. 代码

（1）downLoad.html文件

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <a href="/day12/downLoadServlet?fileName=2.jpg">下载图片</a>
    <br />
    <a href="/day12/downLoadServlet?fileName=1.avi">下载视频</a>
</body>
</html>

（2）DownLoadServlet.java文件

@WebServlet("/downLoadServlet")
public class DownLoadServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.获取参数---文件名称
        String fileName = request.getParameter("fileName");
        //2.将资源加载进内存
        //2.1 获取资源的真实路径
        ServletContext servletContext = request.getServletContext();
        String path = servletContext.getRealPath("/resource/" + fileName);
        //2.2 加载资源
        FileInputStream inputStream = new FileInputStream(path);
        //3.设置响应头-以附件形式发送文件
        response.setHeader("content-disposition", "attachment;filename="+fileName);
        //4.设置传输的数据是什么类型
        String mimeType = servletContext.getMimeType(fileName);
        response.setContentType(mimeType);
        //5.数据传输
        ServletOutputStream outputStream = response.getOutputStream();
        byte[] buff = new byte[1024 * 8]; //字节缓冲数组
        int len = 0;
        while ((len = inputStream.read(buff)) != -1) {
            outputStream.write(buff,0,len);
        }
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    }
}

5. 文件名为中文时乱码的问题

解决方法：
建立下面文件

package cn.wanghao.web.util;

import sun.misc.BASE64Encoder;
import java.io.UnsupportedEncodingException;
import java.net.URLEncoder;


public class DownLoadUtils {

    public static String getFileName(String agent, String filename) throws UnsupportedEncodingException {
        if (agent.contains("MSIE")) {
            // IE浏览器
            filename = URLEncoder.encode(filename, "utf-8");
            filename = filename.replace("+", " ");
        } else if (agent.contains("Firefox")) {
            // 火狐浏览器
            BASE64Encoder base64Encoder = new BASE64Encoder();
            filename = "=?utf-8?B?" + base64Encoder.encode(filename.getBytes("utf-8")) + "?=";
        } else {
            // 其它浏览器
            filename = URLEncoder.encode(filename, "utf-8");
        }
        return filename;
    }
}

在以附件形式发送消息之前，用DownLoadUtils.getFileName()将文件名转换一下。
代码如下：

@WebServlet("/downLoadServlet")
public class DownLoadServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.获取参数---文件名称
        String fileName = request.getParameter("fileName");
        //request.setCharacterEncoding()方法对get请求不起作用
        //将乱码转回用ISO8859-1编码前的二进制位
        byte[] bytes = fileName.getBytes("ISO8859-1");
        //再使用UTF-8进行编码
        fileName = new String(bytes,"UTF-8");
        //2.将资源加载进内存
        //2.1 获取资源的真实路径
        ServletContext servletContext = request.getServletContext();
        String path = servletContext.getRealPath("/resource/" + fileName);
        //2.2 加载资源
        FileInputStream inputStream = new FileInputStream(path);
        //3.设置传输的数据是什么类型
        String mimeType = servletContext.getMimeType(fileName);
        response.setContentType(mimeType);
        //4.设置响应头-以附件形式发送文件
        String ua = request.getHeader("user-agent"); //获取浏览器的用户代理
        System.out.println(ua);
        fileName = DownLoadUtils.getFileName(ua, fileName);
        response.setHeader("content-disposition", "attachment;fileName="+fileName);
        //5.数据传输
        ServletOutputStream outputStream = response.getOutputStream();
        byte[] buff = new byte[1024 * 8]; //字节缓冲数组
        int len = 0;
        while ((len = inputStream.read(buff)) != -1) {
            outputStream.write(buff,0,len);
        }
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    }
}