RMI Callbacks in Java

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Callbacks can be easily implemented using RMI The chief difference between a local callback and a remote callback is that some extra initialization must occur, and that both the listener interface and the event source must be implemented as an RMI service For the listener to register itself with the remote event source, it must invoke a remote method and pass an object reference to
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the remote listener interface it defines This sounds complex in theory, but is reasonably straightforward in practice, requiring only a small amount of additional code This next example works through implementing a callback using RMI, and shows how a listener can be a client and a server For the purpose of this example, the server supplies information about changes in temperature, which can be accessed remotely However, it would be inefficient to check the monitor continually, polling for changes, as the time at which a change will occur is not known A temperature monitor registers with a temperature sensor service, and passes a reference to a remote temperature listener object When the temperature does change, a callback is made, notifying registered listeners 111121 Defining the Listener Interface The listener interface defines a remote object with a single method This method should be invoked by an event source whenever an event occurs, so as to act as notification that the event occurred The method signifies a change in temperature, and allows the new temperature to be passed as a parameter
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11, Listing 5 interface TemperatureListener extends javarmiRemote { public void temperatureChanged(double temperature) throws javarmiRemoteException; }
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111122 Defining the Event Source Interface The event source must allow a listener to be registered and unregistered, and may optionally provide additional methods In this case, a method to request the temperature on demand is offered
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11, Listing 6 interface TemperatureSensor extends javarmiRemote { public double getTemperature() throws javarmiRemoteException; public void addTemperatureListener (TemperatureListener listener ) throws javarmiRemoteException; public void removeTemperatureListener (TemperatureListener listener ) throws javarmiRemoteException; }
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111123 Implementing the Event Source Interface Once interfaces have been defined, the next step is to implement them A TemperatureSensorServer class is defined, which acts as an RMI server This server will also notify registered listeners as a client The server must extend UnicastRemoteObject, to offer a service, and implement the Temperature Sensor interface defined earlier After creating an instance of the service and registering it with the rmiregistry, the server launches a new thread, responsible for updating the value of the temperature, based on randomly generated numbers Both the amount (plus or minus 05 degrees) and the time delay between changes are generated randomly As each change occurs, registered listeners are notified, by reading from a list of listeners stored in a javautilVector object This list is modified by the remote addTemperatureListener(TemperatureListener) and removeTemperatureListener(Tempera tureListener) methods Additionally, if a
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remote exception occurs while notifying a listener (indicating the temperature client is inaccessible or has terminated), it will be dropped
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import javautil*; import javarmi*; import javarmiserver*; // 11, Listing 7 public class TemperatureSensorServer extends UnicastRemoteObject implements TemperatureSensor, Runnable { private volatile double temp; private Vector list = new Vector(); public TemperatureSensorServer() throws javarmiRemoteException { // Assign a default setting for the temperature temp = 980; } public double getTemperature() throws javarmiRemoteException { return temp; } public void addTemperatureListener ( TemperatureListener listener ) throws javarmiRemoteException { Systemoutprintln ("adding listener -" + listener); listadd (listener); } public void removeTemperatureListener ( TemperatureListener listener ) throws javarmiRemoteException { Systemoutprintln ("removing listener -" + listener); listremove (listener); } public void run() { Random r = new Random(); for (;;) { try { // Sleep for a random amount of time int duration = rnextInt() % 10000 + 2000; // Check to see if negative, if so, reverse if (duration < 0) duration = duration * -1; Threadsleep(duration); } catch (InterruptedException ie) {
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// Get a number, to see if temp goes up or down int num = rnextInt();
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if (num < 0) { temp += 05; } else { temp -= 05; } // Notify registered listeners notifyListeners(); } } private void notifyListeners() { // Notify every listener in the registered list for (Enumeration e = listelements(); ehasMoreElements(); ) { TemperatureListener listener = (TemperatureListener) enextElement(); // Notify, if possible a listener try { listenertemperatureChanged (temp); } catch (RemoteException re) { Systemoutprintln ("removing listener -" + listener); // Remove the listener listremove( listener ); } } } public static void main(String args[]) { Systemoutprintln ("Loading temperature service"); // Only required for dynamic class loading //SystemsetSecurityManager ( new RMISecurityManager() ); try { // Load the service TemperatureSensorServer sensor = new TemperatureSensorServer(); // Check to see String registry if (argslength { registry = } /service // Note the :port field is optional if a registry was specified = "localhost"; >=1) args[0];
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