ServletContext object

• Lifecycle (it can be seen from the lifecycle that this is a global object)
  • Created when the project starts
  • Destroyed when the project is closed
• Concept: represents the entire web application and can communicate with the program's container (server).
• Obtain
  1. Get through the request object: request.getServletContext();
  2. Get through HttpServlet: this.getServletContext();
• function
  1. Get MIME type (type of file)
     • MIME type: a file data type defined in the process of Internet communication
       • Format: large type / small type
     • String getMimeType(String file): determine the type of the file according to its suffix
  2. Domain objects (sharing data for all users)
     • setAttribute(String name,Object obj)
     • getAttribute(String name)
     • removeAttribute(String name)
  3. Get the real (server) path of the file
     • File under web: getRealPath("/ filename");
     • Under web/WEB-INF: getRealPath("/WEB-INF / filename");
     • Under src: getRealPath("/WEB-INF/classes / filename");

There is a problem of Chinese code scrambling when downloading files, so let's start with a tool class

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 browser
            filename = URLEncoder.encode(filename, "utf-8");
            filename = filename.replace("+", " ");
        } else if (agent.contains("Firefox")) {
            // Firefox
            BASE64Encoder base64Encoder = new BASE64Encoder();
            filename = "=?utf-8?B?" + base64Encoder.encode(filename.getBytes("utf-8")) + "?=";
        } else {
            // Other browsers
            filename = URLEncoder.encode(filename, "utf-8");
        }
        return filename;
    }
}

Next is the implementation of servlet class:

@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. Get file name
        String filename = request.getParameter("filename");
        // 2. Get the real path
        // 2.1 get ServletContext
        ServletContext servletContext = this.getServletContext();
        // 2.2 select the path according to the location of file storage. My path is the file under upload under the web
        String realPath = servletContext.getRealPath("/upload/" + filename);
        // 3. Set response head
        // 3.1 setting content type
        response.setHeader("content-type",servletContext.getMimeType(filename));
        // 3.2 solve Chinese code disorder
        String agent = request.getHeader("user-agent");
        filename = DownLoadUtils.getFileName(agent,filename);
        // 3.3 setting content disposition
        response.setHeader("content-disposition","attachment;filename=" + filename);
        // 4. Standard IO flow
        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();
    }
}