Contents

• Service attribute entries
• Clients getting and using services
• Service implementation choices
• Example: proxy is the service
• JoinManager - simplifying service code
• Remote events
• User interfaces for distributed services

ServiceRegistrar

• Both lookup methods return a ServiceRegistrar. This is a proxy for the lookup service

        public interface ServiceRegistrar {
            public ServiceRegistration register(ServiceItem i,
                                                long leaseDuration);
  
            public Object lookup(ServiceTemplate tmpl);
            public ServiceMatches lookup(ServiceTemplate tmpl, int maxMatches);
  
            LookupLocator getLocator();
        }
        

• The register() method is used by a service
• The lookup() methods are used by a client. The first returns a single service, the second a set of services
• The getLocator() method can say where the lookup service is

ENTRIES

Entry class

• Entries are additional information about a service, such as location, name, cost
• This information is not part of the service object itself
• The additional information can be used to select between otherwise identical services e.g. a colour printer, a four-slice toaster
• Entries implement Entry

        public interface Entry {}
        

• There are a bunch of convenience implementations such as
  • Address
  • Comment
  • Name
  • ServiceInfo

Creating entries

• You can create your own classes, or use the given ones
• Typical code

        Entry[] entries = new Entry[] {new Comment("best toaster made"),
                                       new Name("Classy Toaster")};
        

Interfaces, implementations and entries

• A client looks for an instance of an interface
• It may qualify the search with entries
• A service exports an implementation of this interface
• A service will also export additional entry information, as much as it can

CLIENT

Problem domain

• Files have a type, that can be described by a MIME type

        public class MIMEType implements java.io.Serializable {
            protected String contentType;
            protected String subType;
  
            public MIMEType(String contentTpe, String subType) {...}
  
            public String toString() {...}
        }
        

  (It may need to be serialized, to move it from one machine to another) The full class is at http://jan.newmarch.name/java/jini/tutorial/SimpleExample.xml
• A file classifier can take a filename and use this to work out the MIME type, using an internal table

        public interface FileClassifier {
            MIMEType getMIMEType(String fileName)
                throws java.rmi.RemoteException;
        }
        

  (The method may be executing remotely, so can throw a remote exception)

Unicast client

import net.jini.core.discovery.LookupLocator;
import net.jini.core.lookup.ServiceRegistrar;
import net.jini.core.lookup.ServiceItem;
import net.jini.core.lookup.ServiceRegistration;
import java.rmi.RMISecurityManager;
import net.jini.core.lookup.ServiceTemplate;

public class TestUnicastFileClassifier {

    public static void main(String argv[]) {
	new TestUnicastFileClassifier();
    }

    public TestUnicastFileClassifier() {
	LookupLocator lookup = null;
	ServiceRegistrar registrar = null;
	FileClassifier classifier = null;

        try {
            lookup = new LookupLocator("jini://www.all_about_files.com");
        } catch(java.net.MalformedURLException e) {
            System.err.println("Lookup failed: " + e.toString());
	    System.exit(1);
        }

	System.setSecurityManager(new RMISecurityManager());

	try {
	    registrar = lookup.getRegistrar();
	} catch (java.io.IOException e) {
            System.err.println("Registrar search failed: " + e.toString());
	    System.exit(1);
	} catch (java.lang.ClassNotFoundException e) {
            System.err.println("Registrar search failed: " + e.toString());
	    System.exit(1);
	}

	Class[] classes = new Class[] {FileClassifier.class};
	ServiceTemplate template = new ServiceTemplate(null, classes, null);
	try {
	    classifier = (FileClassifier) registrar.lookup(template);
	} catch(java.rmi.RemoteException e) {
	    e.printStackTrace();
	    System.exit(1);
	}

	if (classifier == null) {
	    System.out.println("Classifier null");
	    System.exit(2);
	}
	MIMEType type;
	try {
	    type = classifier.getMIMEType("file1.txt");
	    System.out.println("Type is " + type.toString());
	} catch(java.rmi.RemoteException e) {
	    System.err.println(e.toString());
	}
	// System.exit(0);
    }
} // TestUnicastFileClassifier

Client and service locator JVM's

Multicast client

import java.rmi.RMISecurityManager;
import net.jini.discovery.LookupDiscovery;
import net.jini.discovery.DiscoveryListener;
import net.jini.discovery.DiscoveryEvent;
import net.jini.core.lookup.ServiceRegistrar;
import net.jini.core.lookup.ServiceTemplate;

public class TestFileClassifier implements DiscoveryListener {

    public static void main(String argv[]) {
	new TestFileClassifier();

        // stay around long enough to receive replies
        try {
            Thread.currentThread().sleep(100000L);
        } catch(java.lang.InterruptedException e) {
            // do nothing
        }
    }

    public TestFileClassifier() {
	System.setSecurityManager(new RMISecurityManager());

	LookupDiscovery discover = null;
        try {
            discover = new LookupDiscovery(LookupDiscovery.ALL_GROUPS);
        } catch(Exception e) {
            System.err.println(e.toString());
            System.exit(1);
        }

        discover.addDiscoveryListener(this);

    }
    
    public void discovered(DiscoveryEvent evt) {

        ServiceRegistrar[] registrars = evt.getRegistrars();
	Class [] classes = new Class[] {FileClassifier.class};
	FileClassifier classifier = null;
	ServiceTemplate template = new ServiceTemplate(null, classes, 
						       null);
 
        for (int n = 0; n < registrars.length; n++) {
	    System.out.println("Service found");
            ServiceRegistrar registrar = registrars[n];
	    try {
		classifier = (FileClassifier) registrar.lookup(template);
	    } catch(java.rmi.RemoteException e) {
		e.printStackTrace();
		System.exit(2);
	    }
	    if (classifier == null) {
		System.out.println("Classifier null");
		continue;
	    }
	    MIMEType type;
	    try {
		type = classifier.getMIMEType("file1.txt");
		System.out.println("Type is " + type.toString());
	    } catch(java.rmi.RemoteException e) {
		System.err.println(e.toString());
	    }
	    // System.exit(0);
	}
    }

    public void discarded(DiscoveryEvent evt) {
	// empty
    }
} // TestFileClassifier

PROXY IMPLEMENTATION CHOICES

Proxy is the Service

• The complete service may be uploaded, so there is no separate proxy
  
• The client uses FileClassifier
• The server creates a FileClassifierImpl
• The server exports the FileClassifierImpl
• This is a "fat" proxy model, with no processing being done on the server side
• This is suitable for software services that do not need to maintain state on the server side

RMI Proxies

• If RMI stubs are used to implement the proxy, it looks like
  
• The client uses FileClassifier
• The server creates a FileClassifierImpl
• The RMI runtime ensures that a FileClassifierimpl_Stub is exported to the service locator
• This is a "thin" proxy, with no processing being done on the client side
• This is closest to the standard RPC model between client and service

Non-RMI Proxy

• If RMI is not used, the proxy must be explicitly created
  
• The client uses FileClassifier (like before - it doesn't care what the proxy is)
• The server creates a FileClassifierImpl and a FileClassifierProxy
• The server exports a FileClassifierProxy
• The proxy and service communicate using some protocol
• This can have processing done on both sides
• This is good for client-server problems communicating on their own ports with their own protocol. The proxy is the heart of the client

RMI and Non-RMI Proxies

• You can use both RMI and a non-RMI proxy
• The client uses FileClassifier
• The server creates a FileClassifierImpl and a FileClassifierProxy
• The server exports a FileClassifierProxy and a FileClassifierImpl_Stub
• The proxy uses the stub locally (on the client), which uses RMI to talk back to the service
• This can have processing done on both sides, using an RPC-like model for communication

PROXY IS THE SERVICE

Service implementation

public class FileClassifierImpl implements FileClassifier, java.io.Serializable {

    public MIMEType getMIMEType(String fileName) {
        if (fileName.endsWith(".gif")) {
            return new MIMEType("image", "gif");
        } else if (fileName.endsWith(".jpeg")) {
            return new MIMEType("image", "jpeg");
        } else if (fileName.endsWith(".mpg")) {
            return new MIMEType("video", "mpeg");
        } else if (fileName.endsWith(".txt")) {
            return new MIMEType("text", "plain");
        } else if (fileName.endsWith(".html")) {
            return new MIMEType("text", "html");
        } else
            // fill in lots of other types,
            // but eventually give up and
            return null;
    }

    public FileClassifierImpl() {
	// empty
    }
    
} // FileClassifierImpl

File classifier server

import java.rmi.RMISecurityManager;
import net.jini.discovery.LookupDiscovery;
import net.jini.discovery.DiscoveryListener;
import net.jini.discovery.DiscoveryEvent;
import net.jini.core.lookup.ServiceRegistrar;
import net.jini.core.lookup.ServiceItem;
import net.jini.core.lookup.ServiceRegistration;
import net.jini.core.lease.Lease;
import net.jini.lease.LeaseRenewalManager;
import net.jini.lease.LeaseListener;
import net.jini.lease.LeaseRenewalEvent;

public class FileClassifierServer implements DiscoveryListener, 
                                             LeaseListener {
    
    protected LeaseRenewalManager leaseManager = new LeaseRenewalManager();

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

        // keep server running forever to 
	// - allow time for locator discovery and
	// - keep re-registering the lease
        try {
            Thread.currentThread().sleep(Lease.FOREVER);
        } catch(java.lang.InterruptedException e) {
            // do nothing
        }
    }

    public FileClassifierServer() {
	System.setSecurityManager(new RMISecurityManager());

	LookupDiscovery discover = null;
        try {
            discover = new LookupDiscovery(LookupDiscovery.ALL_GROUPS);
        } catch(Exception e) {
            System.err.println(e.toString());
            System.exit(1);
        }

        discover.addDiscoveryListener(this);
    }
    
    public void discovered(DiscoveryEvent evt) {

        ServiceRegistrar[] registrars = evt.getRegistrars();

        for (int n = 0; n < registrars.length; n++) {
            ServiceRegistrar registrar = registrars[n];

	    ServiceItem item = new ServiceItem(null,
					       new FileClassifierImpl(), 
					       null);
	    ServiceRegistration reg = null;
	    try {
		reg = registrar.register(item, Lease.FOREVER);
	    } catch(java.rmi.RemoteException e) {
		System.err.println("Register exception: " + e.toString());
	    }
	    System.out.println("service registered");

	    // set lease renewal in place
	    leaseManager.renewUntil(reg.getLease(), Lease.FOREVER, this);
	}
    }

    public void discarded(DiscoveryEvent evt) {

    }

    public void notify(LeaseRenewalEvent evt) {
	System.out.println("Lease expired " + evt.toString());
    }   
    
} // FileClassifierServer

All the JVM's

References

Jan Newmarch A Programmers Guide to Jini