SrvletContext オブジェクトとファイル ダウンロードの実装コードの概要

package cn.itcast.utils;
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;
    }
}

@WebServlet(name = "DownloadServlet",urlPatterns = "/download3")
public class DownloadServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request,response);
    }
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 1.获取文件名
        String filename = request.getParameter("filename");
        // 2.获取真实路径
        // 2.1 获取ServletContext
        ServletContext servletContext = this.getServletContext();
        // 2.2 根据文件存储的位置,选择路径,本人的路径是web下的upload下的文件
        String realPath = servletContext.getRealPath("/upload/" + filename);
        // 3.设置响应头
        // 3.1 设置content-type
        response.setHeader("content-type",servletContext.getMimeType(filename));
        // 3.2 解决中文乱码
        String agent = request.getHeader("user-agent");
        filename = DownLoadUtils.getFileName(agent,filename);
        // 3.3 设置content-disposition
        response.setHeader("content-disposition","attachment;filename=" + filename);
        // 4.标准的IO流
        FileInputStream fis = new FileInputStream(realPath);
        ServletOutputStream sos = response.getOutputStream();
        byte[] b = new byte[1024 * 8];
        int len = 0;
        while((len = fis.read(b)) != -1){
            sos.write(b,0,len);
        }
        fis.close();
        sos.close();
    }
}