Question

I’m implementing a program where the controller(server) calls the agent(client) periodically and sends it IP address.

Controller.java

public class Controller {
         static int discoveryInterval;
         NetworkDiscovery n1;

   Controller(){        
         discoveryInterval=6000;
}
public static void main(String[] args) throws IOException {
         Timer t1=new Timer();
         t1.schedule(new NetworkDiscovery(), discoveryInterval);
}
}

NetworkDiscovery.java-

import java.io.*;
public class NetworkDiscovery extends TimerTask {
        protected DatagramSocket socket = null;
        protected BufferedReader in = null;

        public NetworkDiscovery() throws IOException {
               this("NetworkDiscovery");
        }
public NetworkDiscovery(String name) throws IOException {
        super(name);
        socket = new DatagramSocket(4445);
}

public void run() {
        try {
        byte[] buf = new byte[256];

       // receive request
       DatagramPacket packet = new DatagramPacket(buf, buf.length);
       socket.receive(packet);

       // figure out response
       String dString = InetAddress.getLocalHost().toString();
       buf = dString.getBytes();

       // send the response to the client at "address" and "port"
       InetAddress address = packet.getAddress();
       int port = packet.getPort();

       packet = new DatagramPacket(buf, buf.length, address, port);
       socket.send(packet);
} catch (IOException e) {
       e.printStackTrace();
}
socket.close();
}
}

On the Agent(client’s side)-
Agent.java

public class Agent {
      ackDiscovery ackDisc=new ackDiscovery();
      public static void main(String[] args) throws  SocketException,UnknownHostException,IOException {
             ackDiscovery ackDisc=new ackDiscovery();
             ackDisc.ackInfo();
             }
}

And ackDiscovery.java-

public class ackDiscovery {
            int agentListenPort;
            void ackDiscovery(){
                      agentListenPort=4455;
}

public void ackInfo() throws SocketException, UnknownHostException, IOException{
              // get a datagram socket
              DatagramSocket socket = new DatagramSocket();

              // send request
              byte[] buf = new byte[256];
              InetAddress address = InetAddress.getByName(MY_IP);
              DatagramPacket packet = new DatagramPacket(buf, buf.length, address,      4445);
              socket.send(packet);
              // get response
              packet = new DatagramPacket(buf, buf.length);
              socket.receive(packet);

              // display response
              String received = new String(packet.getData());
              System.out.println("Data received:"+ received);
              socket.close();
}
}

When I run the Controller(Server), the Agent’s(client’s) side get executed only once though the Controller is still listening. Also, if I re-run the agent, nothing happens. Can someone please help me?

Answer 1

If you look at the definitions of the schedule method here:

http://download.oracle.com/javase/7/docs/api/java/util/Timer.html

You can see that the method with a single long parameter, will only execute once.
What you’re looking for is the one with two long parameters. This will wait for delay ms and then execute every period ms.

The one you are using will only execute once after delay ms.

You may also want to look at either using non-blocking io (java.nio.*) for receiving connections or using a receive timeout. This way you won’t have multiple threads running at once.