asked on

How to Server Push when Event has occured?

X Hello!

Does anyone got a better solution for this?

What I need to create is a servlet that uses Server side Push/Server sid Reload. When something happens a page should be reloaded.

Suppose that you got a visit counter at a page in a browser and if that counter is updated from an other page in an other browser all other page in all browsers with that counter needs to be updated.

I have created a ugly hack that produce this, but I'm not satisfied with it. My hack does a file access every second and I do not want that.

What I would like is some Listener that listens if perhaps the file is changed or some other Event has occur on the server side but I do not know if that is possible.

Below is my ugly hack divided in 2 Servlets. My FileManager just reads a number from a file, nothing magic.

So if any one got a nice solution please let me know!

BTW Server side Push doesn't work in Internet Explorer I use a old NetScape at Linux Mandrake 8.0 that works fine. I have heard that it is possible with FireFox as well.

Best regards
Fredrik

The servlets that checks for changes:

package my.servlets;

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;

import my.servlets.*;
import my.tools.*;

import com.oreilly.servlet.*;
import com.oreilly.servlet.multipart.*;

public class ServerReload extends GeneralServlet
{
public void start(HttpServletRequest request, HttpServletResponse response) throws IOException
{
ServletOutputStream out = response.getOutputStream();
MultipartResponse multi = new MultipartResponse(response);

int counter = 0;
while(counter < 100)
{
String output = FileManager.getStringFor("counter.txt");
int tempCounter = Integer.parseInt(output);

if(counter != tempCounter)
{
multi.startResponse("text/plain");
counter = tempCounter;
out.println(counter);
multi.endResponse();
}

try
{
Thread.sleep(1000);
}
catch (InterruptedException e)
{
}
}
multi.finish();
out.close();
}
}

The servlet that updates the counter.txt file:

package my.servlets.assignments1;

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

import my.servlets.*;
import my.tools.*;

public class SecureFileAccess extends GeneralServlet
{
public void start(HttpServletRequest request, HttpServletResponse response) throws IOException
{
response.setContentType("text/plain");
out = new PrintWriter(response.getOutputStream());

String output = null;
synchronized(this)
{
try
{
output = FileManager.getStringFor("counter.txt");
int counter = Integer.parseInt(output);
output = ""+(++counter);
FileManager.writeString("counter.txt", output, false);
}
catch(Exception e)
{
output = "1";
FileManager.writeString("counter.txt", output, false);
}
}

printString(output);
flush();
}
}

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kiranhk

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fredand44

ASKER

Hello Mates!
I solved it like this, not the best way but I guess that it will work.

public class ServerReload_1g extends GeneralServlet implements HttpSessionAttributeListener
{
      static boolean send = false;

      public void start(HttpServletRequest request, HttpServletResponse response) throws IOException
      {
            ServletOutputStream servletOutputStream = response.getOutputStream();
            MultipartResponse multipartResponse = new MultipartResponse(response);

            send = true;

            int counter = 0;

            while(counter < 100)
            {
                  if(send)
                  {
                        String output = FileManager.getStringFor("applications/int_2/int_2/templates/1a.txt");
                        counter = Integer.parseInt(output);
                        multipartResponse.startResponse("text/plain");
                        servletOutputStream.println(counter);
                        multipartResponse.endResponse();
                        send = false;
                  }

                  try
                  {
                        Thread.sleep(1000);
                        counter++;
                  }
                  catch (InterruptedException e)
                  {
                  }
            }
            multipartResponse.finish();
            servletOutputStream.close();

      }

      public void attributeAdded(HttpSessionBindingEvent se)
      {
            send = true;
      }

      public void attributeRemoved(HttpSessionBindingEvent se)
      {
      }

      public void attributeReplaced(HttpSessionBindingEvent se)
      {
      }
}

public class SecureFileAccess_1a extends GeneralServlet
{
      public void start(HttpServletRequest request, HttpServletResponse response) throws IOException
      {
            response.setContentType("text/plain");
            out = new PrintWriter(response.getOutputStream());

            String output = null;
            synchronized(this)
            {
                  try
                  {
                        output = FileManager.getStringFor("applications/int_2/int_2/templates/1a.txt");
                        int counter = Integer.parseInt(output);
                        output = ""+(++counter);
                        FileManager.writeString("applications/int_2/int_2/templates/1a.txt", output, false);
                  }
                  catch(Exception e)
                  {
                        output = "1";
                        FileManager.writeString("applications/int_2/int_2/templates/1a.txt", output, false);
                  }
            }

            //Just notify the listener in ServerReload_1g that we have updated the counter
            session.setAttribute("ServerReload_1g", "ServerReload_1g");
            session.removeAttribute("ServerReload_1g");
            //It is possible to work around the listeners with using the static variable send direct.
            //ServerReload_1g.send = true;

            printString(output);
            flush();
      }
}

Thanks for your time!
Fredrik