New to Java? We'll help you get started with our revised beginner's tutorial, or our free online textbook.

	Get the latest Java books
h t t p : / /w w w . j a v a c o f f e e b r e a k . c o m /

Chapter 11: Join Manager

Contents

• Join Manager
• Jini 1.0 JoinManager
  • Getting information from JoinManager
• Summary

Finding a lookup service involves a common series of steps. Subequent interaction with them also involves common steps. A join manager encapulates these into one convenience class for services.

A service needs to locate lookup services and register the service with them. Locating services can be done using the utility classes of ``Discovery Management''. As each class is discovered, it then needs to be registered, and the lease maintained. The class JoinManager performs all of these tasks. There are two constructors

public class JoinManager {
     public JoinManager(Object obj,
                        Entry[] attrSets,
                        ServiceIDListener callback,
                        DiscoveryManagement discoverMgr,
                        LeaseRenewalManager leaseMgr)    
            throws IOException;

     public JoinManager(Object obj,
                        Entry[] attrSets,
                        ServiceID serviceID,
                        DiscoveryManagement discoverMgr,
                        LeaseRenewalManager leaseMgr)    
            throws IOException;
}

package joinmgr;

import complete.FileClassifierImpl;

import net.jini.lookup.JoinManager;
import net.jini.core.lookup.ServiceID;
import net.jini.discovery.LookupDiscovery;
import net.jini.core.lookup.ServiceRegistrar;
import java.rmi.RemoteException;
import net.jini.lookup.ServiceIDListener;
import net.jini.lease.LeaseRenewalManager;
import net.jini.discovery.LookupDiscoveryManager;
import net.jini.discovery.DiscoveryEvent;
import net.jini.discovery.DiscoveryListener;


/**
 * FileClassifierServer.java
 */

public class FileClassifierServer 
    implements ServiceIDListener {
    
    public static void main(String argv[]) {
	new FileClassifierServer();

        // stay around forever
	Object keepAlive = new Object();
	synchronized(keepAlive) {
	    try {
		keepAlive.wait();
	    } catch(InterruptedException e) {
		// do nothing
	    }
	}
    }

    public FileClassifierServer() {

	JoinManager joinMgr = null;
	try {
	    LookupDiscoveryManager mgr = 
		new LookupDiscoveryManager(LookupDiscovery.ALL_GROUPS,
					   null /* unicast locators */,
					   null /* DiscoveryListener */);
	    joinMgr = new JoinManager(new FileClassifierImpl(), /* service */
				      null /* attr sets */,
				      this /* ServiceIDListener*/,
				      mgr /* DiscoveryManagement */,
				      new LeaseRenewalManager());
	} catch(Exception e) {
	    e.printStackTrace();
	    System.exit(1);
	}
    }

    public void serviceIDNotify(ServiceID serviceID) {
	// called as a ServiceIDListener
	// Should save the id to permanent storage
	System.out.println("got service ID " + serviceID.toString());
    }
    
} // FileClassifierServer

There are a number of other methods in JoinManager, which allow one to modify the state of a service registration.

A version of JoinManager was present in Jini 1.0. At that time no formal specification was given, and it was liable to change. It has, to the version given above. This section describes the old version for those still using Jini 1.0.

There are a number of possible constructors for JoinManager. The simplest is

JoinManager(java.lang.Object obj, 
            Entry[] attrSets, 
            ServiceIDListener callback, 
            LeaseRenewalManager leaseMgr) 

Using this constructor will initiate a search for service locators belonging to the group ``public'', which is defined by a group value of the empty string "". There is no constant for this, and the locators from Sun do not appear to belong to this group, so most applications will need to follow this up immediately with a call to search for locators belonging to any group:

JoinManager joinMgr = new JoinManager(obj, null, null, null);
joinMgr.setGroups(LookupDiscovery.ALL_GROUPS);

The second constructor

JoinManager(java.lang.Object obj, 
            Entry[] attrSets, 
            java.lang.String[] groups,
            LookupLocator[] locators,
            ServiceIDListener callback,
            LeaseRenewalManager leaseMgr) 

JoinManager joinMgr = new JoinManager(obj, null, 
                                      LookupDiscovery.ALL_GROUPS,
                                      null, null, null);

LookupLocator[] locators = new LookupLocator[1];
locators[0] new LookupLocator("http://www.all_about_files.com");
JoinManager joinMgr = new JoinManager(obj, null, 
                                      LookupDiscovery.NO_GROUPS,
                                      locators, null, null);

For example, uploading the complete service of the complete package can become

package joinmgr;

import complete.FileClassifierImpl;

import com.sun.jini.lookup.JoinManager;
import net.jini.core.lookup.ServiceID;
import com.sun.jini.lookup.ServiceIDListener;
import com.sun.jini.lease.LeaseRenewalManager;
import net.jini.discovery.LookupDiscovery;

/**
 * FileClassifierServer1_0.java
 */

public class FileClassifierServer1_0 implements ServiceIDListener {
    
    public static void main(String argv[]) {
	new FileClassifierServer1_0();

        // stay around long enough to receive replies
        try {
            Thread.currentThread().sleep(1000000L);
        } catch(java.lang.InterruptedException e) {
            // do nothing
        }

    }

    public FileClassifierServer1_0() {

	JoinManager joinMgr = null;
	try {
	    /* this is one way of doing it
	    joinMgr = new JoinManager(new FileClassifierImpl(),
				      null,
				      this,
				      new LeaseRenewalManager());
	    joinMgr.setGroups(null);
	    */
	    /* here is another */
	    joinMgr = new JoinManager(new FileClassifierImpl(),
				      null,
				      LookupDiscovery.ALL_GROUPS,
				      null,
				      this,
				      new LeaseRenewalManager());
	} catch(Exception e) {
	    e.printStackTrace();
	    System.exit(1);
	}
    }

    public void serviceIDNotify(ServiceID serviceID) {
	System.out.println("got service ID " + serviceID.toString());
    }
    
} // FileClassifierServer1_0

The JoinManager would appear to be unhelpful in supplying information about the lookup locators it finds. It is available, but by a slightly circuitous route. A service can register a ServiceIDListener to the JoinManager. This will be invoked whenever a new locator is found, by its serviceIDNotify() method. A ServiceID is not particularly useful, so we just ignore it. However, within this method we do know that a new service locator has been found.

The complete set of service locators may be found from the JoinManager method getJoinSet(), whuch returns an array of ServiceRegistrar's. We have met this class before: its method getLocator() will return a LookupLocator, which has information such as host in getHost(). These can be put together as

protected JoinManager joinmgr;
...

joinmgr = new JoinManager(service, null, 
                                          this, new LeaseManager());
...

public  void serviceIDNotify(ServiceID serviceID) {
    ServiceRegistrar registrars = joinmgr.getJoinSet();
    for (int n = 0; n < registrar.length; n++) {
        LookupLocator locator = registrars[n].getLocator();
	String hostName = locator.getHost();
	...
    }
}

If you want to find out which is the latest locator to be found, it looks like you will have to cache the previous set, and find which is new in the array returned. Each call to getJoinSet() will return a new array.

A JoinManager can be used by a server to simplify many of the aspects of locating lookup services, registering one or more services and renewing leases for them.

Back to main