JmDNS service discovery in client-server

Question

I'm trying to enable service discovery in my client-server application using JmDNS. I fully understand service registry on the server side, with code that resembles this:

JmDNS jmdns = JmDNS.create(localhost);
jmdns.register(serviceInfo);

However, I'm having trouble figuring out how to have my client retrieve the port number and IP address from the registered service and use this data to open a TCP connection. I've searched for examples of how to use JmDNS but to no avail. Can anyone here give me some basic examples? Or if anyone has any links to good resources/tutorials on JmDNS could they please provide them?

Note: JmDNS is a neccessity here, so I would appreciate answers pertaining only to JmDNS and not offering an alternative. Also, I have looked at the JmDNS API extensively, and still can't figure it out, so please don't post a link to the docs as a resource.

Thanks.

Antonia

Answer 1

Server side:

mdnsServer = JmDNS.create(localhost);
// Register a test service.
ServiceInfo testService = ServiceInfo.create("my-service-type", "Test Service", 6666, "test service");
mdnsServer.registerService(testService);

Client side:

private ServiceListener mdnsServiceListener = new ServiceListener() {
  public void serviceAdded(ServiceEvent serviceEvent) {
    // Test service is discovered. requestServiceInfo() will trigger serviceResolved() callback.
    mdnsService.requestServiceInfo(Constants.mdnsServiceType, serviceEvent.getName());
  }

  public void serviceRemoved(ServiceEvent serviceEvent) {
    // Test service is disappeared.
  }

  public void serviceResolved(ServiceEvent serviceEvent) {
    // Test service info is resolved.
    String serviceUrl = serviceEvent.getInfo().getURL();
    // serviceURL is usually something like http://192.168.11.2:6666/my-service-name
};

mdnsService = JmDNS.create();
mdnsService.addServiceListener("my-service-type", mdnsServiceListener);
ServiceInfo[] infos = mdnsService.list(Constants.mdnsServiceType);

// Retrieve service info from either ServiceInfo[] returned here or listener callback method above.
mdnsService.removeServiceListener("my-service-type", mdnsServiceListener);
mdnsService.close();

Once you have the URL http://192.168.11.2:6666/myTestService, you can parse/use it open socket connection. Hope that help.

Inquisitionist

Thanks, this really makes things a lot clearer for me. Being completely unfamiliar with callbacks in Java (or in any language, for that matter) how would I go about retrieving the service info from the callback as opposed to the ServiceInfo[]? Is it just a matter of declaring the serviceUrl string outside the serviceListener implemenation so I can access it globally? – Antonia

Yes, usually define ServiceListener as a inner class and define serviceUrl as a instance variable of outer class. – Inquisitionist

@Inquisitionist Any chance you could expand the code sample to also show how to parse it and open the socket? I cannot find the proper client side socket connect code. – Housework

what is Contstants.mdnsServiceType here? Is it the same thing as "my-service-type" ? These methods in jmdns have no comments and the param names are completely useless.. – Microstructure

@Erix, see sample code in this answer. – Inquisitionist

Do I really need to call: mdnsService.requestServiceInfo() in serviceAdded() ? – Osmose