Response download file for javaweb programming

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About HttpServletResponse

When the web server receives an http request from the client, the server will create an object representing the request, called HttpServletRequest; An object representing the response, called HttpServletResponse;

The simple understanding is:

• If you need to obtain the parameters requested by the client, use HttpServletRequest;

• If you want to provide some response information to the client, use HttpServletResponse.

A simple classification of HttpServletResponse:

• Methods responsible for sending some data to the client browser:

 ServletOutputStream getOutputStream() throws IOException;
​
  PrintWriter getWriter() throws IOException;

• Methods responsible for sending some response headers to the browser:

 void sendError(int var1, String var2) throws IOException;
​
    void sendError(int var1) throws IOException;
​
    void sendRedirect(String var1) throws IOException;
​
    void setDateHeader(String var1, long var2);
​
    void addDateHeader(String var1, long var2);
​
    void setHeader(String var1, String var2);
​
    void addHeader(String var1, String var2);
​
    void setIntHeader(String var1, int var2);
​
    void addIntHeader(String var1, int var2);
​
    void setStatus(int var1);

• Status code responsible for response

int SC_CONTINUE = 100;
    int SC_SWITCHING_PROTOCOLS = 101;
    int SC_OK = 200;        //common
    int SC_CREATED = 201;
    int SC_ACCEPTED = 202;
    int SC_NON_AUTHORITATIVE_INFORMATION = 203;
    int SC_NO_CONTENT = 204;
    int SC_RESET_CONTENT = 205;
    int SC_PARTIAL_CONTENT = 206;
    int SC_MULTIPLE_CHOICES = 300;      //common
    int SC_MOVED_PERMANENTLY = 301;
    int SC_MOVED_TEMPORARILY = 302;
    int SC_FOUND = 302;
    int SC_SEE_OTHER = 303;
    int SC_NOT_MODIFIED = 304;
    int SC_USE_PROXY = 305;
    int SC_TEMPORARY_REDIRECT = 307;
    int SC_BAD_REQUEST = 400;
    int SC_UNAUTHORIZED = 401;
    int SC_PAYMENT_REQUIRED = 402;
    int SC_FORBIDDEN = 403;
    int SC_NOT_FOUND = 404;     //common
    int SC_METHOD_NOT_ALLOWED = 405;
    int SC_NOT_ACCEPTABLE = 406;
    int SC_PROXY_AUTHENTICATION_REQUIRED = 407;
    int SC_REQUEST_TIMEOUT = 408;
    int SC_CONFLICT = 409;
    int SC_GONE = 410;
    int SC_LENGTH_REQUIRED = 411;
    int SC_PRECONDITION_FAILED = 412;
    int SC_REQUEST_ENTITY_TOO_LARGE = 413;
    int SC_REQUEST_URI_TOO_LONG = 414;
    int SC_UNSUPPORTED_MEDIA_TYPE = 415;
    int SC_REQUESTED_RANGE_NOT_SATISFIABLE = 416;
    int SC_EXPECTATION_FAILED = 417;
    int SC_INTERNAL_SERVER_ERROR = 500;     //common
    int SC_NOT_IMPLEMENTED = 501;
    int SC_BAD_GATEWAY = 502;
    int SC_SERVICE_UNAVAILABLE = 503;
    int SC_GATEWAY_TIMEOUT = 504;
    int SC_HTTP_VERSION_NOT_SUPPORTED = 505;

About response status codes: 404 errors, 500 errors, etc. are often encountered during web browsing. These are called response status codes.

Common applications of HttpServletResponse

• Input some data to the browser;

• Download File

  • Prepare the path to download the file

  • Prepare the file name of the file to be downloaded

  • Set up browser support Downloads

  • Get input stream to downloaded file

  • Create a buffer to write to the stream

  • Get OutputStream object

  • Writes the stream of the FileOutputStream object to the buffer buffer created previously

  • Use the OutputStream object to output the data in the buffer to your own client

Example:

Configuration xml:

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0"
         metadata-complete="true">
​
<servlet>
    <servlet-name>fileDownload</servlet-name>
    <servlet-class>psl.wong.servlet.FileServlet</servlet-class>
</servlet>
    <servlet-mapping>
        <servlet-name>fileDownload</servlet-name>
        <url-pattern>/down</url-pattern>
    </servlet-mapping>
​
</web-app>

Primary code:

package psl.wong.servlet;
​
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.URLEncoder;
​
public class FileServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
​
//Download File
//- Prepare the path to download the file
        String realPath = "D:\\Program Files\\ideaIU\\IdeaWorkspace\\Javaweb_02_Servlet\\Response\\src\\main\\resources\\tencent.PNG";   //source file URL
        System.out.println("Path to download file:"+realPath);
//- Prepare the file name of the file to be downloaded
        String fileName = realPath.substring(realPath.lastIndexOf("\\")+1); //Get to last "/"The following data is the file name.
//- Set up browser support Downloads
        resp.setHeader("Content-Disposition", "attachment;filename="+ URLEncoder.encode(fileName,"UTF-8"));
//- Get input stream to downloaded file
        FileInputStream in = new FileInputStream(realPath);
​
//- Create a buffer to write to the stream
        int len=0;
        byte[] bytes = new byte[1024];
​
//- obtain OutputStream object
        ServletOutputStream outputStream = resp.getOutputStream();
​
//- take FileOutputStream Object to the previously created buffer In buffer,apply OutputStream Object will buffer The data in the buffer is output to its own client
        while ((len=in.read(bytes))>0){
            outputStream.write(bytes,0,len);
        }
        outputStream.close();
        in.close();
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}