How to recognize peers and send messages in JXTA-JXSE 2.6?

Question

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

What would a simple peaceful type of application look like to meet these requirements?

I created this question with the goal of providing a tutorial, such as an answer, an answer that I tried very hard to find two months ago when I started looking at JXTA for the uni project. Feel free to add your own answers or improve mine. I will wait a few days and accept the best.

Answer 1

Introduction to JXTA 2.6 Peer Detection and Pipe Messaging

The guide I would like to have 2 months ago =)

After spending a lot of time while creating a university course, the JXTA p2p app, I feel a lot of frustrations and confusion that I encountered could be avoided with a good starting point.

The jar files you 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

Drop them into your / lib project, open eclipse, create a new project "Yourproject" and it should figure it out by importing the libraries for you.

You will soon realize that almost any information on the Internet is out of date, very out of date. You will also come across many very confusing error messages, and most of them can be avoided through this checklist.

• Is your firewall disabled or at least open to the ports you are using? You can disable iptables with "sudo service iptables stop" in Fedora.

• Check your spelling! Many times, when you join groups or try to send messages by saying the wrong group name or not using exactly the same advertising when looking for peers and services or opening pipes, it will cause very confusing messages. I tried to find out why my pipe connections ended when I noticed group names that are "Net info" and "Net_info".

• Do you use the JXTA home directory? One for each peer computer that you run on the same computer?

• Are you really using a unique peer identifier? The seed provided by IDFactory must be long enough, otherwise you will get duplicates.

• Disable SELinux. I had SELinux turned off during development, but might imagine that it is causing errors.

While it is usual to group all fields together, I introduce them when I go to show where they are needed.

Note. This will not work in version 2.7. I think there is some issue with PSE membership.

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 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(); } }