This chapter delves into some of the more complex things that can happen with Jini applications.
Clients, servers and service locators can use code from a variety of sources. Which source it uses can depend on the structure of a client and a server. This section looks at some of the variations that can occur.
A service may require information about a client before it can
(or will) proceed. For example, a banking service may require
a user id and a PIN number. Using the techniques already discussed,
this could be done by the client collecting the information and
calling suitable methods such as void setName(String name)
in the service (or more likely, in the
service's proxy) running in the client.
public class Client {
String getName();
}
class Service {
void setName(String name);
}
A service may wish to have more control over the setting of names and passwords than this. For example, it may wish to run verification routines based on the pattern of keystroke entries. More mundanely, it may wish to set time limits on the period between entering the name and the password. Or it may wish to enforce some particular user interface to collect this information. Whatever, the service proxy may wish to perform some sort of input processing on the client side before communicating with the real service. This section explores what happens when the service proxy needs to find extra classes in order to perform this processing.
A standalone application to get a user name might use a GUI interface such as
package standalone;
import java.awt.*;
import java.awt.event.*;
/**
* NameEntry.java
*
*
* Created: Sun Mar 28 23:47:02 1999
*
* @author Jan Newmarch
* @version 1.0
*/
public class NameEntry extends Frame {
public NameEntry() {
super("Name Entry");
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {System.exit(0);}
public void windowOpened(WindowEvent e) {}});
Label label = new Label("Name");
TextField name = new TextField(20);
add(label, BorderLayout.WEST);
add(name, BorderLayout.CENTER);
name.addActionListener(new NameHandler());
pack();
}
public static void main(String[] args) {
NameEntry f = new NameEntry();
f.show();
}
} // NameEntry
class NameHandler implements ActionListener {
public void actionPerformed(ActionEvent evt) {
System.out.println("Name was: " + evt.getActionCommand());
}
}
Frame, Label, TextField,
ActionListener, ActionEvent, BorderLayout,
WindowEvent, System.
NameEntry, NameHandler.
In moving to a Jini system, we have already seen that different components may only need access to a subset of the total set of classes. The same will apply here, but it will critically depend on how the application is changed into a Jini system.
We don't want to be overly concerned about program logic of what is done with the user name once it has been entered, as the interesting part in this section is the location of classes. All versions will need an interface definition, which we can make simply as
package common;
/**
* NameEntry.java
*
*
* Created: Mon Mar 29 11:36:25 1999
*
* @author Jan Newmarch
* @version 1.0
*/
public interface NameEntry {
public void show();
} // NameEntry
show()
itself and collect information in whatever way it chooses.
(Note: we don't want to get involved here in the ongoing discussion about the
most appropriate interface definition for GUI classes!)
The first version of an implementation is
package complex;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import com.sun.jini.lookup.JoinManager;
import net.jini.core.lookup.ServiceID;
import com.sun.jini.lookup.ServiceIDListener;
import com.sun.jini.lease.LeaseRenewalManager;
/**
* NameEntryImpl1.java
*
*
* Created: Mon Mar 29 11:38:33 1999
*
* @author Jan Newmarch
* @version 1.0
*/
public class NameEntryImpl1 extends Frame implements common.NameEntry,
ActionListener, java.io.Serializable {
public NameEntryImpl1() {
super("Name Entry");
/*
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {System.exit(0);}
public void windowOpened(WindowEvent e) {}});
*/
setLayout(new BorderLayout());
Label label = new Label("Name");
add(label, BorderLayout.WEST);
TextField name = new TextField(20);
add(name, BorderLayout.CENTER);
name.addActionListener(this);
// don't do this here!
// pack();
}
/**
* method invoked on pressing <return> in the TextField
*/
public void actionPerformed(ActionEvent evt) {
System.out.println("Name was: " + evt.getActionCommand());
}
public void show() {
pack();
super.show();
}
} // NameEntryImpl1
Another problem with this code is that it firstly creates an object
on the server machine that has heavy reliance on environmental factors
on the server. It then removes itself from that environment and has to
re-establish itself on the target client environment. On my current system,
this shows by TextField complaining that it cannot find
a whole bunch of fonts on my server machine. Well it doesn't matter here:
it gets moved to the client machine. (As it happens, the fonts aren't
available there either, so I end with two batches of complaint messages,
from the server and from the client. I should only get the client
complaints.) It could matter if the service died
because of missing stuff on the server side, which would exist
on the client.
The client needs to know the interface class NameEntry
The server needs to know the class files for
NameEntry
Server1
NameEntryImpl1
The HTTP server needs to know the class files for
NameEntryImpl1
The second implementation minimises the amount of serialised code that
must be shipped around, by creating as much as possible on the client
side. We don't even need to declare the class as a subclass of
Frame as that class also exists on the client side.
The client calls the interface method show(), and all the
GUI creation is moved to there.
package complex;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import com.sun.jini.lookup.JoinManager;
import net.jini.core.lookup.ServiceID;
import com.sun.jini.lookup.ServiceIDListener;
import com.sun.jini.lease.LeaseRenewalManager;
/**
* NameEntryImpl2.java
*
*
* Created: Mon Mar 29 11:38:33 1999
*
* @author Jan Newmarch
* @version 1.0
*/
public class NameEntryImpl2 implements common.NameEntry,
ActionListener, java.io.Serializable {
public NameEntryImpl2() {
}
/**
* method invoked on pressing <return> in the TextField
*/
public void actionPerformed(ActionEvent evt) {
System.out.println("Name was: " + evt.getActionCommand());
}
public void show() {
Frame fr = new Frame("Name Entry");
fr.addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {System.exit(0);}
public void windowOpened(WindowEvent e) {}});
fr.setLayout(new BorderLayout());
Label label = new Label("Name");
fr.add(label, BorderLayout.WEST);
TextField name = new TextField(20);
fr.add(name, BorderLayout.CENTER);
name.addActionListener(this);
fr.pack();
fr.show();
}
} // NameEntryImpl2
There are some standard classes that cannot be serialised: one example
is the Swing JTextArea class (as of Swing 1.1).
This has been frequently logged as a bug against Swing. Until this
is fixed, the only way one of these objects can be
used by a service is to create it on the client.
The client needs to know the interface class NameEntry
The server needs to know the class files for
NameEntry
Server2
NameEntryImpl2
NameEntryImpl2$1
anonymous class, that acts as
the WindowListener. The class file is produced by the
compiler. In version 1, this part of the code was commented out for simplicity.
The HTTP server needs to know the class files for
NameEntryImpl1
NameEntryImpl1$1
Apart from the standard classes and a common interface, the previous implementations just used a single class that was uploaded to the lookup service and then passed on to the client. A more realistic situation might require the uploaded service to access a number of other classes that could not be expected to be on the client machine. It is simple to modify the examples to use a server-side specific class for the action listener, instead of the class itself. This looks like
package complex;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import com.sun.jini.lookup.JoinManager;
import net.jini.core.lookup.ServiceID;
import com.sun.jini.lookup.ServiceIDListener;
import com.sun.jini.lease.LeaseRenewalManager;
/**
* NameEntryImpl3.java
*
*
* Created: Mon Mar 29 11:38:33 1999
*
* @author Jan Newmarch
* @version 1.0
*/
public class NameEntryImpl3 implements common.NameEntry,
java.io.Serializable {
public NameEntryImpl3() {
}
public void show() {
Frame fr = new Frame("Name Entry");
fr.addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {System.exit(0);}
public void windowOpened(WindowEvent e) {}});
fr.setLayout(new BorderLayout());
Label label = new Label("Name");
fr.add(label, BorderLayout.WEST);
TextField name = new TextField(20);
fr.add(name, BorderLayout.CENTER);
name.addActionListener(new NameHandler());
fr.pack();
fr.show();
}
} // NameEntryImpl3
class NameHandler implements ActionListener {
/**
* method invoked on pressing <return> in the TextField
*/
public void actionPerformed(ActionEvent evt) {
System.out.println("Name was: " + evt.getActionCommand());
}
} // NameHandler
This version uses a class NameHandler that only exists on
the server machine. When the client attempts to deserialise the
NameEntryImpl3 instance it will fail to find this class,
and be unable to complete deserialisation. How is this resolved?
Well, in the same way as before, by making it available through the HTTP
server.
The client needs to know the interface class NameEntry
The server needs to know the class files for
NameEntry
Server1
NameEntryImpl1
NameEntryImpl1$1
NameHandler
NameHandler class file is another one produced by the compiler.
The HTTP server needs to know the class files for
NameEntryImpl1
NameEntryImpl1$1
NameHandler
In all of the simple examples using explicit registration, a single thread was used. That is, as a service locator was discovered, the registration process commenced in the same thread. Now this registration may take some time, so it would be preferable to do this in a separate thread. This is also recommended in the specification.
Running another thread is not a difficult procedure. Basically we have to
define a new class that extends Thread and move most of
the registration into its run method. This is done in the
following version of the ``option 2'' file classifier server, using an inner class
package complex;
import option2.FileClassifierImpl;
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 com.sun.jini.lease.LeaseRenewalManager;
import com.sun.jini.lease.LeaseListener;
import com.sun.jini.lease.LeaseRenewalEvent;
/**
* FileClassifierServer.java
*
*
* Created: Wed Mar 17 14:23:44 1999
*
* @author Jan Newmarch
* @version 1.1
* added LeaseRenewalManager
* moved sleep() from constructor to main()
*/
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() {
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];
new RegisterThread(registrar).start();
}
}
public void discarded(DiscoveryEvent evt) {
}
public void notify(LeaseRenewalEvent evt) {
System.out.println("Lease expired " + evt.toString());
}
/**
* an inner class to register the service in its own thread
*/
class RegisterThread extends Thread {
ServiceRegistrar registrar;
RegisterThread(ServiceRegistrar registrar) {
this.registrar = registrar;
}
public void run() {
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,
FileClassifierServer.this);
}
}
} // FileClassifierServer
If you use a JoinManager to handle lookup and registration, then
it essentially does this for you: creates a new thread to handle
registration. Thus the examples in the chapter on ``JoinManager'' do not
need any modification, as the JoinManager already uses the
concepts of this section.
It is probably more important to create threads in the client than in the server,
since the client will actually perform some computation
(which may be lengthy) based on the service
it discovers. Again this is a simple matter, moving code into a new class
that implements Thread. Doing this to the
TestFileClassifier results in
package client;
import common.FileClassifier;
import common.MIMEType;
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;
/**
* TestFileClassifierThread.java
*
*
* Created: Tue Apr 17 14:29:15 1999
*
* @author Jan Newmarch
* @version 1.1
* simplified Class.forName to Class.class
*/
public class TestFileClassifierThread implements DiscoveryListener {
public static void main(String argv[]) {
new TestFileClassifierThread();
// stay around long enough to receive replies
try {
Thread.currentThread().sleep(10000L);
} catch(java.lang.InterruptedException e) {
// do nothing
}
}
public TestFileClassifierThread() {
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++) {
System.out.println("Service found");
ServiceRegistrar registrar = registrars[n];
new LookupThread(registrar).start();
}
// System.exit(0);
}
public void discarded(DiscoveryEvent evt) {
// empty
}
class LookupThread extends Thread {
ServiceRegistrar registrar;
LookupThread(ServiceRegistrar registrar) {
this.registrar = registrar;
}
public void run() {
Class[] classes = new Class[] {FileClassifier.class};
FileClassifier classifier = null;
ServiceTemplate template = new ServiceTemplate(null, classes,
null);
try {
classifier = (FileClassifier) registrar.lookup(template);
} catch(java.rmi.RemoteException e) {
e.printStackTrace();
System.exit(2);
}
if (classifier == null) {
System.out.println("Classifier null");
return;
}
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());
}
}
}
} // TestFileClassifier
In the option 3 example of ``Simple Examples'', a proxy of our design was
created. However, when it came to communicating back to the original
service, a remote method call using RMI was used, and for this the proxy
had to locate an RMI stub (or RMI proxy) using Naming.lookup().
Thus we ended up with two proxies on the client!
This section investigates one of the two possibilities in reducing the number of proxies, and that is to reuse the RMI proxy as the only service proxy. To do this, perform the RMI proxy lookup on the server side instead of the client side. Then instead of exporting our own proxy, export the proxy returned from the lookup. There is then no need for our own proxy at all!
A version of the option 3 file classifier to do this is
package complex;
import option3.FileClassifierImpl;
import option3.RemoteFileClassifier;
import java.rmi.Naming;
import java.net.InetAddress;
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 com.sun.jini.lease.LeaseRenewalManager;
import com.sun.jini.lease.LeaseListener;
import com.sun.jini.lease.LeaseRenewalEvent;
import java.rmi.RMISecurityManager;
/**
* FileClassifierServer.java
*
*
* Created: Wed Mar 17 14:23:44 1999
*
* @author Jan Newmarch
* @version 1.1
* added LeaseRenewalManager
* moved sleep() from constructor to main()
*/
public class FileClassifierServerRMI implements DiscoveryListener, LeaseListener {
// this is just a name - can be anything
// impl object forces search for Stub
static final String serviceName = "FileClassifier";
String registeredName;
protected FileClassifierImpl impl;
protected LeaseRenewalManager leaseManager = new LeaseRenewalManager();
public static void main(String argv[]) {
new FileClassifierServerRMI();
// no need to keep server alive, RMI will do that
}
public FileClassifierServerRMI() {
try {
impl = new FileClassifierImpl();
} catch(Exception e) {
System.err.println("New impl: " + e.toString());
System.exit(1);
}
// register this with RMI registry
System.setSecurityManager(new RMISecurityManager());
/*
try {
Naming.rebind("rmi://localhost/" + serviceName, impl);
} catch(java.net.MalformedURLException e) {
System.err.println("Binding: " + e.toString());
System.exit(1);
} catch(java.rmi.RemoteException e) {
System.err.println("Binding: " + e.toString());
System.exit(1);
}
*/
System.out.println("bound");
// find where we are running
String address = null;
try {
address = InetAddress.getLocalHost().getHostName();
} catch(java.net.UnknownHostException e) {
System.err.println("Address: " + e.toString());
System.exit(1);
}
registeredName = "//" + address + "/" + serviceName;
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();
RemoteFileClassifier service;
/*
try {
Object obj = Naming.lookup(registeredName);
service = (RemoteFileClassifier) obj;
} catch(Exception e) {
e.printStackTrace();
return;
}
*/
for (int n = 0; n < registrars.length; n++) {
ServiceRegistrar registrar = registrars[n];
// export the proxy service
ServiceItem item = new ServiceItem(null,
impl, //service,
null);
ServiceRegistration reg = null;
try {
reg = registrar.register(item, Lease.FOREVER);
} catch(java.rmi.RemoteException e) {
System.err.print("Register exception: ");
e.printStackTrace();
// System.exit(2);
continue;
}
try {
System.out.println("service registered at " +
registrar.getLocator().getHost());
} catch(Exception e) {
}
leaseManager.renewFor(reg.getLease(), Lease.FOREVER, this);
}
}
public void discarded(DiscoveryEvent evt) {
}
public void notify(LeaseRenewalEvent evt) {
System.out.println("Lease expired " + evt.toString());
}
} // FileClassifierServer
Suppose you have a printer service that prints at 30 pages per minute. A client wishes to find a printer that will print at least 24 pages per minute. How will this client find the service? The standard Jini pattern matching will either be for an exact match on an attribute or an ignored match on an attribute. So the only way a client can find this printer is to ignore the speed attribute and perform a later selection among all the printers that it sees.
We can define a printer interface that will allow us to access printer speed (plus other capabilities) as
package common;
import java.io.Serializable;
/**
* Printer.java
*
*
* Created: Tue Apr 20 21:16:22 1999
*
* @author Jan Newmarch
* @version 1.0
*/
public interface Printer extends Serializable {
public void print(String str);
public int getSpeed();
} // Printer
Given this, a client can choose a suitably fast printer in a two-step process
package client;
import common.Printer;
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;
import net.jini.core.lookup.ServiceMatches;
/**
* TestPrinterSpeed.java
*
*
* Created: Mon Mar 29 12:03:51 1999
*
* @author Jan Newmarch
* @version 1.1
* simplified Class.forName to Class.class
*/
public class TestPrinterSpeed implements DiscoveryListener {
public TestPrinterSpeed() {
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[] {Printer.class};
ServiceTemplate template = new ServiceTemplate(null, classes,
null);
for (int n = 0; n < registrars.length; n++) {
ServiceRegistrar registrar = registrars[n];
ServiceMatches matches;
try {
matches = registrar.lookup(template, 10);
} catch(java.rmi.RemoteException e) {
e.printStackTrace();
continue;
}
// NB: matches.totalMatches may be greater than matches.items.length
for (int m = 0; m < matches.items.length; m++) {
Printer printer = (Printer) matches.items[m].service;
// Inexact matching is not performed by lookup()
// we have to do it ourselves on each printer
// we get
int speed = printer.getSpeed();
if (speed >= 24) {
// this one is okay, use its print() method
printer.print("fast enough printer");
} else {
// we can't use this printer, so just say so
System.out.println("Printer too slow at " + speed);
}
}
}
}
public void discarded(DiscoveryEvent evt) {
// empty
}
public static void main(String[] args) {
TestPrinterSpeed f = new TestPrinterSpeed();
// stay around long enough to receive replies
try {
Thread.currentThread().sleep(10000L);
} catch(java.lang.InterruptedException e) {
// do nothing
}
}
} // TestPrinterSpeed
When a user connects their laptop into a brand-new network, they will probably know little about the environment they have joined. If they want to use services in this network, they will probably want to use general terms and have them translated into specific terms for this new environment. For example, the user may want to print a file on a nearby printer. In this situation, there is little likelihood that the new user knows how to work out the distance between themselves and the printers. However, a local service could be running which does know how to calculate physical distances between objects on the network.
Finding a ``close enough'' printer then becomes a matter of querying service locators both for printers and for a distance service. As each printer is found, the distance service can be asked to calculate the distance between itself and the laptop (or camera, or any other device that wants to print).
The complexity of the task to be done by clients is growing: a client has to find two sets of services, and when it finds one (a printer) invoke the other (the distance service). This calls for lookup processing to be handled in separate threads. In addition, as each locator is found, it may know about printers, it may know about distance services, both, or none! When the client starts up, it will be discovering these services in arbitrary order, and the code must be structured to deal with this.
Some of the cases that may arise are
Vector, and
a distance service in a single variable.
In searching for printers, we only want to find those that have location information.
However, we do not want to match on any particular values. The client will have to use
wildcard patterns in a location object. The location information of a printer
will need to be retrieved along with the printer, so it can be used. So instead
of just storing printers, we need to store ServiceItem's, which
carry the attribute information as well as the objects.
Of course, for this to work, the client also needs to know where it is!
A client satisfying these requirements is
package client;
import common.Printer;
import common.Distance;
import java.util.Vector;
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;
import net.jini.lookup.entry.Location;
import net.jini.core.lookup.ServiceItem;
import net.jini.core.lookup.ServiceMatches;
import net.jini.core.entry.Entry;
/**
* TestPrinterDistance.java
*
*
* Created: Tue Apr 17 14:29:15 1999
*
* @author Jan Newmarch
* @version 1.1
* simplified Class.forName to Class.class
*/
public class TestPrinterDistance implements DiscoveryListener {
protected Distance distance = null;
protected Object distanceLock = new Object();
protected Vector printers = new Vector();
public static void main(String argv[]) {
new TestPrinterDistance();
// stay around long enough to receive replies
try {
Thread.currentThread().sleep(10000L);
} catch(java.lang.InterruptedException e) {
// do nothing
}
}
public TestPrinterDistance() {
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++) {
System.out.println("Service found");
ServiceRegistrar registrar = registrars[n];
new LookupThread(registrar).start();
}
// System.exit(0);
}
public void discarded(DiscoveryEvent evt) {
// empty
}
class LookupThread extends Thread {
ServiceRegistrar registrar;
LookupThread(ServiceRegistrar registrar) {
this.registrar = registrar;
}
public void run() {
synchronized(distanceLock) {
// only look for one distance service
if (distance == null) {
lookupDistance();
}
if (distance != null) {
// found a new distance service
// process any previously found printers
synchronized(printers) {
for (int n = 0; n < printers.size(); n++) {
ServiceItem item = (ServiceItem) printers.elementAt(n);
reportDistance(item);
}
}
}
}
ServiceMatches matches = lookupPrinters();
for (int n = 0; n < matches.items.length; n++) {
if (matches.items[n] != null) {
synchronized(distanceLock) {
if (distance != null) {
reportDistance(matches.items[n]);
} else {
synchronized(printers) {
printers.addElement(matches.items[n]);
}
}
}
}
}
}
/*
* We must be protected by the lock on distanceLock here
*/
void lookupDistance() {
// If we don't have a distance service, see if this
// locator knows of one
Class[] classes = new Class[] {Distance.class};
ServiceTemplate template = new ServiceTemplate(null, classes,
null);
try {
distance = (Distance) registrar.lookup(template);
} catch(java.rmi.RemoteException e) {
e.printStackTrace();
}
}
ServiceMatches lookupPrinters() {
// look for printers with
// wildcard matching on all fields of Location
Entry[] entries = new Entry[] {new Location(null, null, null)};
Class[] classes = new Class[1];
try {
classes[0] = Class.forName("common.Printer");
} catch(ClassNotFoundException e) {
System.err.println("Class not found");
System.exit(1);
}
ServiceTemplate template = new ServiceTemplate(null, classes,
entries);
ServiceMatches matches = null;
try {
matches = registrar.lookup(template, 10);
} catch(java.rmi.RemoteException e) {
e.printStackTrace();
}
return matches;
}
/**
* report on the distance of the printer from
* this client
*/
void reportDistance(ServiceItem item) {
Location whereAmI = getMyLocation();
Location whereIsPrinter = getPrinterLocation(item);
if (whereIsPrinter != null) {
int dist = distance.getDistance(whereAmI, whereIsPrinter);
System.out.println("Found a printer at " + dist +
" units of length away");
}
}
Location getMyLocation() {
return new Location("1", "1", "Building 1");
}
Location getPrinterLocation(ServiceItem item) {
Entry[] entries = item.attributeSets;
for (int n = 0; n < entries.length; n++) {
if (entries[n] instanceof Location) {
return (Location) entries[n];
}
}
return null;
}
}
} // TestFileClassifier
A number of services will need to be running. At least one distance service will
be needed, implementing the interface Distance
package common;
import net.jini.lookup.entry.Location;
/**
* Distance.java
*
*
* Created: Wed Apr 21 13:12:35 1999
*
* @author Jan Newmarch
* @version 1.0
*/
public interface Distance extends java.io.Serializable {
int getDistance(Location loc1, Location loc2);
} // Distance
package complex;
import net.jini.lookup.entry.Location;
/**
* DistanceImpl.java
*
*
* Created: Wed Apr 21 15:24:15 1999
*
* @author Jan Newmarch
* @version 1.0
*/
public class DistanceImpl implements common.Distance {
public DistanceImpl() {
}
/**
* A very naive distance metric
*/
public int getDistance(Location loc1, Location loc2) {
int room1, room2;
try {
room1 = Integer.parseInt(loc1.room);
room2 = Integer.parseInt(loc2.room);
} catch(Exception e) {
return -1;
}
int value = room1 - room2;
return (value > 0 ? value : -value);
}
} // DistanceImpl
package complex;
import printer.Printer30;
import printer.Printer20;
import complex.DistanceImpl;
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;
import net.jini.lookup.entry.Location;
import net.jini.core.entry.Entry;
/**
* PrinterServerLocation.java
*
*
* Created: Wed Mar 17 14:23:44 1999
*
* @author Jan Newmarch
* @version 1.0
*/
public class PrinterServerLocation implements ServiceIDListener {
public static void main(String argv[]) {
new PrinterServerLocation();
// stay around long enough to receive replies
try {
Thread.currentThread().sleep(1000000L);
} catch(java.lang.InterruptedException e) {
// do nothing
}
}
public PrinterServerLocation() {
JoinManager joinMgr = null;
try {
// distance service
joinMgr = new JoinManager(new DistanceImpl(),
null,
this,
new LeaseRenewalManager());
joinMgr.setGroups(null);
// slow printer in room 20
joinMgr = new JoinManager(new Printer20(),
new Entry[] {new Location("1", "20",
"Building 1")},
this,
new LeaseRenewalManager());
joinMgr.setGroups(null);
// fast printer in room 30
joinMgr = new JoinManager(new Printer30(),
new Entry[] {new Location("1", "30",
"Building 1")},
this,
new LeaseRenewalManager());
joinMgr.setGroups(null);
} catch(Exception e) {
e.printStackTrace();
System.exit(1);
}
}
public void serviceIDNotify(ServiceID serviceID) {
System.out.println("got service ID " + serviceID.toString());
}
} // PrinterServerLocation