How do I discover peers and send messages in JXTA-JXSE 2.6?

Question

Using JXTA 2.6 from http://jxse.kenai.com/ I want to create application that can run multiple peers on one or more hosts. The peers should be able to find each other in a group and send direct messages as well as propagate messages.

What would a simple hello world type of application look like that meet these requirements?

I created this question with the intention of supplying a tutorial like answer, an answer I tried very hard to find two months ago when starting to look at JXTA for a uni project. Feel free to add your own answers or improve on mine. I will wait a few days and accept the best one.

Answer 1

Introduction to JXTA 2.6 Peer discovery and pipe messaging

The guide I wish I had 2 months ago =)

After spending a lot of time during a university course building a JXTA p2p application I feel a lot of the frustrations and confusion I went through could have been avoided with a good starting point.

The jar files you will need can be found here:

https://oss.sonatype.org/content/repositories/comkenaijxse-057/com/kenai/jxse/jxse/2.6/jxse-2.6.jar

http://sourceforge.net/projects/practicaljxta/files/lib-dependencies-2.6.zip/download

Throw them into Yourproject/lib, open up eclipse, create a new project "Yourproject" and it should sort out importing the libraries for you.

You will soon come to realize that almost any information on the web is out dated, very out dated. You will also run into a lot of very confusing error messages and most of them can be avoided by going through this check list.

• Is your firewall turned off or at least open for the ports you use? You can disable iptables using "sudo service iptables stop" under Fedora.

• Check spelling! Many times when joining groups or trying to send messages spelling the group name wrong or not using the exact same advertisement when looking for peers and services or opening pipes will cause very confusing messages. Was trying to figure out why my pipe connections timed out when I spotted the group names being "Net info" and "Net_info".

• Are you using a JXTA home directory? One per each peer you run on the same computer?

• Do you really use a unique peer id? The seed provided to IDFactory need to be long enough or else you will get duplicates.

• Turn off SELinux. I have had SELinux turned off during the development but can imagine it causing errors.

While it is common to group all fields together I introduce them as I go to show where they are needed.

Note: This will not work in 2.7. Some issue with PSE membership I think.

public class Hello implements DiscoveryListener, PipeMsgListener {

    // When developing you should handle these exceptions, I don't to lessen the clutter of start()
    public static void main(String[] args) throws PeerGroupException, IOException {

        // JXTA logs a lot, you can configure it setting level here
        Logger.getLogger("net.jxta").setLevel(Level.ALL);


        // Randomize a port to use with a number over 1000 (for non root on unix)
        // JXTA uses TCP for incoming connections which will conflict if more than
        // one Hello runs at the same time on one computer.
        int port = 9000 + new Random().nextInt(100);

        Hello hello = new Hello(port);
        hello.start(); 
        hello.fetch_advertisements();
    }


    private String peer_name;
    private PeerID peer_id;
    private File conf;
    private NetworkManager manager;

    public Hello(int port) {
        // Add a random number to make it easier to identify by name, will also make sure the ID is unique 
        peer_name = "Peer " + new Random().nextInt(1000000); 

        // This is what you will be looking for in Wireshark instead of an IP, hint: filter by "jxta"
        peer_id = IDFactory.newPeerID(PeerGroupID.defaultNetPeerGroupID, peer_name.getBytes());

        // Here the local peer cache will be saved, if you have multiple peers this must be unique
        conf = new File("." + System.getProperty("file.separator") + peer_name);

        // Most documentation you will find use a deprecated network manager setup, use this one instead
        // ADHOC is usually a good starting point, other alternatives include Edge and Rendezvous
        try {
            manager = new NetworkManager(
                    NetworkManager.ConfigMode.ADHOC,
                    peer_name, conf.toURI());
        }
        catch (IOException e) {
            // Will be thrown if you specify an invalid directory in conf
            e.printStackTrace();
        }

        NetworkConfigurator configurator;
        try {
            // Settings Configuration
            configurator = manager.getConfigurator();
            configurator.setTcpPort(port);
            configurator.setTcpEnabled(true);
            configurator.setTcpIncoming(true);
            configurator.setTcpOutgoing(true);
            configurator.setUseMulticast(true);
            configurator.setPeerID(peer_id);
        } 
        catch (IOException e) {
            // Never caught this one but let me know if you do =)
            e.printStackTrace();
        }
    }

    private static final String subgroup_name = "Make sure this is spelled the same everywhere";
    private static final String subgroup_desc = "...";
    private static final PeerGroupID subgroup_id = IDFactory.newPeerGroupID(PeerGroupID.defaultNetPeerGroupID, subgroup_name.getBytes());

    private static final String unicast_name = "This must be spelled the same too";
    private static final String multicast_name = "Or else you will get the wrong PipeID";

    private static final String service_name = "And dont forget it like i did a million times";

    private PeerGroup subgroup;
    private PipeService pipe_service;
    private PipeID unicast_id;
    private PipeID multicast_id;
    private PipeID service_id;
    private DiscoveryService discovery;
    private ModuleSpecAdvertisement mdadv;

    public void start() throws PeerGroupException, IOException {
        // Launch the missiles, if you have logging on and see no exceptions
        // after this is ran, then you probably have at least the jars setup correctly.
        PeerGroup net_group = manager.startNetwork();

        // Connect to our subgroup (all groups are subgroups of Netgroup)
        // If the group does not exist, it will be automatically created
        // Note this is suggested deprecated, not sure what the better way is
        ModuleImplAdvertisement mAdv = null;
        try {
            mAdv = net_group.getAllPurposePeerGroupImplAdvertisement();
        } catch (Exception ex) {
            System.err.println(ex.toString());
        }
        subgroup = net_group.newGroup(subgroup_id, mAdv, subgroup_name, subgroup_desc);

        // A simple check to see if connecting to the group worked
        if (Module.START_OK != subgroup.startApp(new String[0]))
            System.err.println("Cannot start child peergroup");

        // We will spice things up to a more interesting level by sending unicast and multicast messages
        // In order to be able to do that we will create to listeners that will listen for
        // unicast and multicast advertisements respectively. All messages will be handled by Hello in the
        // pipeMsgEvent method. 

        unicast_id = IDFactory.newPipeID(subgroup.getPeerGroupID(), unicast_name.getBytes());
        multicast_id = IDFactory.newPipeID(subgroup.getPeerGroupID(), multicast_name.getBytes());

        pipe_service = subgroup.getPipeService();
        pipe_service.createInputPipe(get_advertisement(unicast_id, false), this);
        pipe_service.createInputPipe(get_advertisement(multicast_id, true), this);

        // In order to for other peers to find this one (and say hello) we will
        // advertise a Hello Service.
        discovery = subgroup.getDiscoveryService();
        discovery.addDiscoveryListener(this);        

        ModuleClassAdvertisement mcadv = (ModuleClassAdvertisement)
        AdvertisementFactory.newAdvertisement(ModuleClassAdvertisement.getAdvertisementType());

        mcadv.setName("STACK-OVERFLOW:HELLO");
        mcadv.setDescription("Tutorial example to use JXTA module advertisement Framework");

        ModuleClassID mcID = IDFactory.newModuleClassID();

        mcadv.setModuleClassID(mcID);

        // Let the group know of this service "module" / collection
        discovery.publish(mcadv);
        discovery.remotePublish(mcadv);

        mdadv = (ModuleSpecAdvertisement)
                AdvertisementFactory.newAdvertisement(ModuleSpecAdvertisement.getAdvertisementType());
        mdadv.setName("STACK-OVERFLOW:HELLO");
        mdadv.setVersion("Version 1.0");
        mdadv.setCreator("sun.com");
        mdadv.setModuleSpecID(IDFactory.newModuleSpecID(mcID));
        mdadv.setSpecURI("http://www.jxta.org/Ex1");

        service_id = IDFactory.newPipeID(subgroup.getPeerGroupID(), service_name.getBytes());
        PipeAdvertisement pipeadv = get_advertisement(service_id, false);
        mdadv.setPipeAdvertisement(pipeadv);

        // Let the group know of the service
        discovery.publish(mdadv);
        discovery.remotePublish(mdadv);

        // Start listening for discovery events, received by the discoveryEvent method
        pipe_service.createInputPipe(pipeadv, this);
    }

    private static PipeAdvertisement get_advertisement(PipeID id, boolean is_multicast) {
        PipeAdvertisement adv = (PipeAdvertisement )AdvertisementFactory.
            newAdvertisement(PipeAdvertisement.getAdvertisementType());
        adv.setPipeID(id);
        if (is_multicast)
            adv.setType(PipeService.PropagateType); 
        else 
            adv.setType(PipeService.UnicastType); 
        adv.setName("This however");
        adv.setDescription("does not really matter");
        return adv;
    }

    @Override public void discoveryEvent(DiscoveryEvent event) {
        // Found another peer! Let's say hello shall we!
        // Reformatting to create a real peer id string
        String found_peer_id = "urn:jxta:" + event.getSource().toString().substring(7);
        send_to_peer("Hello", found_peer_id);
    }


    private void send_to_peer(String message, String found_peer_id) {
        // This is where having the same ID is important or else we wont be
        // able to open a pipe and send messages
        PipeAdvertisement adv = get_advertisement(unicast_id, false);

        // Send message to all peers in "ps", just one in our case
        Set<PeerID> ps = new HashSet<PeerID>();
        try {
            ps.add((PeerID)IDFactory.fromURI(new URI(found_peer_id)));
        } 
        catch (URISyntaxException e) {
            // The JXTA peer ids need to be formatted as proper urns
            e.printStackTrace();
        }

        // A pipe we can use to send messages with
        OutputPipe sender = null;
        try {
            sender = pipe_service.createOutputPipe(adv, ps, 10000);
        } 
        catch (IOException e) {
            // Thrown if there was an error opening the connection, check firewall settings
            e.printStackTrace();
        }

        Message msg = new Message();
        MessageElement fromElem = null;
        MessageElement msgElem = null;
        try {
            fromElem = new ByteArrayMessageElement("From", null, peer_id.toString().getBytes("ISO-8859-1"), null);
            msgElem = new ByteArrayMessageElement("Msg", null, message.getBytes("ISO-8859-1"), null);
        } catch (UnsupportedEncodingException e) {
            // Yepp, you want to spell ISO-8859-1 correctly
            e.printStackTrace();
        }


         msg.addMessageElement(fromElem);
         msg.addMessageElement(msgElem);

         try {
            sender.send(msg);
        } catch (IOException e) {
            // Check, firewall, settings.
            e.printStackTrace();
        }
    }

    @Override public void pipeMsgEvent(PipeMsgEvent event) {
        // Someone is sending us a message!
        try {
            Message msg = event.getMessage();
            byte[] msgBytes = msg.getMessageElement("Msg").getBytes(true);  
            byte[] fromBytes = msg.getMessageElement("From").getBytes(true); 
            String from = new String(fromBytes);
            String message = new String(msgBytes);
            System.out.println(message + " says " + from);
        }
        catch (Exception e) {
            // You will notice that JXTA is not very specific with exceptions...
            e.printStackTrace();
        }
    }

    /**
     * We will not find anyone if we are not regularly looking
     */
    private void fetch_advertisements() {
      new Thread("fetch advertisements thread") {
         public void run() {
            while(true) {
                discovery.getRemoteAdvertisements(null, DiscoveryService.ADV, "Name", "STACK-OVERFLOW:HELLO", 1, null);
                try {
                    sleep(10000);

                }
                catch(InterruptedException e) {} 
            }
         }
      }.start();
   }
}