CODEWITHSUNDEEP
javascriptjavajspservlets
Fancy
Fancy

Reputation: 11

How to update the output in the JSP file, without refreshing the page?

I am trying to use a Servlet with my JSP file using the MVC model, to be able to continuously update the UI without refreshing the page, when the output of the doGet and doSet change.

I have tried to use a timer inside the Servlet (didn't work) and also I tried to use a timer in the JSP file but it didn't work either. I thought it would help to call 'doGet' every 2 seconds to update the output I see on the screen when I run the program. But the only time the output changes is when i manually refresh the website. I expect for this easy program to be able to change and see when I run the program, between the strings "blue" and "red".

The Servlet :

public class Servlet2 extends HttpServlet implements Servlet {

     String msg = "blue";

   protected void processRequest(HttpServletRequest request, HttpServletResponse response)
           throws ServletException, IOException {
       response.setContentType("text/html");
       request.setAttribute("state", changeColor()); 
       request.getRequestDispatcher("/test.jsp").forward(request, response);

   }

   public String changeColor(){
       if(msg.equals("red"))
           msg = "blue";
       else msg = "red";
       return msg;
   }

   @Override
   protected void doGet(
           HttpServletRequest request, HttpServletResponse response)
           throws ServletException, IOException {

       processRequest(request, response);
   }

   @Override
   protected void doPost(
           HttpServletRequest request, HttpServletResponse response)
           throws ServletException, IOException {

       processRequest(request, response);
   }

}

And the JSP file:

<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html> 
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>JSP Page</title>
    </head>
    <body>
            ID:  <%=(String)request.getAttribute("state")%><br>

    </body>
</html>

Upvotes: 1

Views: 2229

Answers (1)

SirFartALot
SirFartALot

Reputation: 1225

As you already found out, this is not possible.

  1. To know why this is not possible you have to know, what happens, when you access a servlet: First after you entered "http://host/yourServlet" your browser will open a connection to the host and asks for the yourServlet part.
  2. The Webserver (Tomcat?) will then execute the servlet's code (doGet) and returns anything which is outputted from the servlet code or jsp.
  3. Your browser receives the output and renders the page.
  4. After that, the connection is closed and nothing more happens.

To achieve your change of "red" to "blue", you may reload your page automatically by using a html header (meta http-equiv="refresh")/javascript (How to reload a page using JavaScript) or you utilize javascript to ask your servlet for the current value of state (jQuery Ajax POST example with PHP).

Upvotes: 2

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