Lookup.java

package helloworld;

import java.io.IOException;

import java.rmi.RemoteException;

import net.jini.core.lookup.ServiceRegistrar;
import net.jini.core.lookup.ServiceTemplate;

import net.jini.discovery.LookupDiscovery;
import net.jini.discovery.DiscoveryListener;
import net.jini.discovery.DiscoveryEvent;

/**
   A class which supports a simple JINI multicast lookup.  It doesn't register
   with any ServiceRegistrars it simply interrogates each one that's
   discovered for a ServiceItem associated with the passed interface class.
   i.e. The service needs to already have registered because we won't notice
   new arrivals. [ServiceRegistrar is the interface implemented by JINI
   lookup services].

   @todo Be more dynamic in our lookups - see above

   @author  Dan Creswell (dan@dancres.org)
   @version 1.00, 7/9/2003
 */
class Lookup implements DiscoveryListener {
    private ServiceTemplate theTemplate;
    private LookupDiscovery theDiscoverer;

    private Object theProxy;

    /**
       @param aServiceInterface the class of the type of service you are
       looking for.  Class is usually an interface class.
     */
    Lookup(Class aServiceInterface) {
        Class[] myServiceTypes = new Class[] {aServiceInterface};
        theTemplate = new ServiceTemplate(null, myServiceTypes, null);
    }

    /**
       Having created a Lookup (which means it now knows what type of service
       you require), invoke this method to attempt to locate a service
       of that type.  The result should be cast to the interface of the
       service you originally specified to the constructor.

       @return proxy for the service type you requested - could be an rmi
       stub or an intelligent proxy.
     */
    Object getService() {
        synchronized(this) {
            if (theDiscoverer == null) {

                try {
                    theDiscoverer =
                        new LookupDiscovery(LookupDiscovery.ALL_GROUPS);
                    theDiscoverer.addDiscoveryListener(this);
                } catch (IOException anIOE) {
                    System.err.println("Failed to init lookup");
                    anIOE.printStackTrace(System.err);
                }
            }
        }

        return waitForProxy();
    }

    /**
       Location of a service causes the creation of some threads.  Call this
       method to shut those threads down either before exiting or after a
       proxy has been returned from getService().
     */
    void terminate() {
        synchronized(this) {
            if (theDiscoverer != null)
                theDiscoverer.terminate();
        }
    }

    /**
       Caller of getService ends up here, blocked until we find a proxy.

       @return the newly downloaded proxy
     */
    private Object waitForProxy() {
        synchronized(this) {
            while (theProxy == null) {

                try {
                    wait();
                } catch (InterruptedException anIE) {
                }
            }

            return theProxy;
        }
    }

    /**
       Invoked to inform a blocked client waiting in waitForProxy that
       one is now available.

       @param aProxy the newly downloaded proxy
     */
    private void signalGotProxy(Object aProxy) {
        synchronized(this) {
            if (theProxy == null) {
                theProxy = aProxy;
                notify();
            }
        }
    }

    /**
       Everytime a new ServiceRegistrar is found, we will be called back on
       this interface with a reference to it.  We then ask it for a service
       instance of the type specified in our constructor.
     */
    public void discovered(DiscoveryEvent anEvent) {
        synchronized(this) {
            if (theProxy != null)
                return;
        }

        ServiceRegistrar[] myRegs = anEvent.getRegistrars();

        for (int i = 0; i < myRegs.length; i++) {
            ServiceRegistrar myReg = myRegs[i];

            Object myProxy = null;

            try {
                myProxy = myReg.lookup(theTemplate);

                if (myProxy != null) {
                    signalGotProxy(myProxy);
                    break;
                }
            } catch (RemoteException anRE) {
                System.err.println("ServiceRegistrar barfed");
                anRE.printStackTrace(System.err);
            }
        }
    }

    /**
       When a ServiceRegistrar "disappears" due to network partition etc.
       we will be advised via a call to this method - as we only care about
       new ServiceRegistrars, we do nothing here.
     */
    public void discarded(DiscoveryEvent anEvent) {
    }
}