Server Client Program in Java

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?

1) For better help sooner, post an SSCCE. 2) Please use indentation in code. Indentation helps the readability. — Andrew Thompson
– Andrew Thompson, Commented Oct 1, 2011 at 14:38
Good call on the edit. Unfortunately: 1) An SSCCE should ideally be a single source file. 2) That is tricky to do with a server/client set-up (but not impossible). — Andrew Thompson
– Andrew Thompson, Commented Oct 1, 2011 at 15:05

xthexder · Accepted Answer · 2011-10-01 15:06:16Z

2

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.

edited Oct 1, 2011 at 15:06

answered Oct 1, 2011 at 14:58

xthexder

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6 Comments

Andrew Thompson Over a year ago

It is best to link to the latest version of the JavaDocs. See point 2 for tips on how to get that link.

xthexder Over a year ago

My bad, didn't realize that was an old link.

Andrew Thompson Over a year ago

The tip involves finding JavaDoc links using Google. Did you read it?

xthexder Over a year ago

I did read it, but when I first got the link, I was concerned with answering the question, not with how up to date the javadoc was for that class. Classes like these rarely change.

P R Over a year ago

@xthexder- I implemented scheduleAtFixedRate() on the server's side and need to do the same thing on the client's side too. But how do I synchronise them? Any pointers would be welcome.

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