Simple server/client socket in applets code.

Question

Can anyone please help me with Java sockets ? I want to create an applet that simply checks if it's still online. This means that i'll be using this applet thru modem(that is, dial-up connection). All i want the applet to do is send a signal to the server and the server answer back that it is still online. The server should be able to handle multiple sends and receives. I need just a simple code for me to work on (both server and client). The client doesn't have to send any special message to the server and vice versa.
Thanks.

Answer 1

Below is the code for both Applet and Servlet .I think this will solve your problem.
Applet:
import java.applet.*;
import java.awt.*;
import java.io.*;
import java.net.*;
public class OnlineApplet extends Applet implements Runnable
     private static Thread thread;
     private static HttpURLConnection URLConn;
     public void init()
          try
               //Connect to the URL where servlet is running.
               URL url = new URL("http://localhost/servlet/OnlineServlet");
               URLConn = (HttpURLConnection)url.openConnection();
               URLConn.setDoInput(false);
               URLConn.setDoOutput(true);
          catch (Exception ex)
          thread = new Thread(this);
     public void start()
          thread.start();
     public void paint(Graphics g) {}
     public void stop() { }
     public void destroy() { }
     public void run()
          while(true)
               try
                    thread.sleep(1000 * 60);
                    sendConfirmation();
               catch (Exception ex)
     private void sendConfirmation() throws Exception
          DataOutputStream dos = new DataOutputStream(URLConn.getOutputStream());
          dos.writeChars("Fine");
          dos.flush();
          dos.close();
Servlet:
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
public class OnlineServlet extends HttpServlet implements Runnable
     public static Hashtable hsh;
     private static Thread thread;
     public void init(ServletConfig scon) throws ServletException
          hsh = new Hashtable();
          thread = new Thread(this);
     public void service(HttpServletRequest req, HttpServletResponse res)
          String strHostName = req.getRemoteHost();
          if(hsh.containsKey(strHostName))
               updateHash(strHostName);
          else
               hsh.put(strHostName, System.currentTimeMillis() + "");
     private void updateHash(String strHostName)
          hsh.remove(strHostName);
          hsh.put(strHostName, System.currentTimeMillis() + "");
     public void run()
          while(true)
               try
                    thread.sleep(1000 * 120);
               catch (Exception ex)
               validateUsers(System.currentTimeMillis());
     private void validateUsers(long msec)
          Enumeration keys = hsh.keys();
          int size = hsh.size();
          while(keys.hasMoreElements())
               String strKey = keys.nextElement().toString();
               String strLong = hsh.get(strKey).toString();
               long lg1 = Long.parseLong(strLong);
               if((lg1 - msec) > 100000)
                    // This means there is no response from user . That means he is not online. So you can remove from hashtable.
                    hsh.remove(strKey);