Component-Based Software Engineering

Component-Based Software Engineering Using the Java Event Model Paul Krause

Using the Java Event Model Contents • Quick Review of the Java Event model • Using this model to notify Customers that Pizzas are ready from a Bakery • This example is taken from JavaBeans by Example, by Henri Jubin, Prentice Hall

Register Event Listener Fire Event Java Event Model Event Source Event Object Event Listener

«interface» OrderListener EventObject Bakery source pizzaStatus(evt) getSource() toString() addOrderListener() removeOrderListener() sendMessage(PizzaEvent) fires PizzaEvent passed to Customer 0..* registers with invokes notifications in 0..* iNumber iSliceNumber run( ) Chili PizzaExpress

EventObject source getSource() toString() PizzaEvent PizzaEvent.java import java.util.EventObject; public class PizzaEvent extends EventObject { public PizzaEvent(Object aSource) { super(aSource); } }

OrderListener pizzaStatus OrderListener.java import java.util.EventListener; public interface OrderListener extends EventListener { public void pizzaStatus(PizzaEvent anEvent); } EventListener

Bakery addOrderListener removeOrderListener sendMessage(PizzaEvent) Bakery.java public class Bakery public Bakery( ) { // constructor of a Bakery instance } public addOrderListener( eL ) { // inserts OrderListeners in some // structure } public removeOrderListener( eL ) { // deletes OrderListeners from that // structure } private void sendMessage( evt ) { // broadcast evt somehow }

Properties of Bakery.java import java.lang.Thread; import java.util.*; public class Bakery implements Runnable { private Vector iCustomers = new Vector( ); private Thread iThread; // methods go here… }

Constructor for Bakery public Bakery ( ) { iThread = new Thread(this); iThread.start( ); } // When a new instance of Bakery is created, // a flow of control owned by it is started.

The main flow of control public void run( ) { while(true) { iThread.sleep(4000); PizzaEvent event = new PizzaEvent(this); sendMessage(event); } } // a Bakery broadcasts a message that Pizza is ready // every 4 seconds

Adding and removing Listeners public void addOrderListener(OrderListener aListener) { iCustomers.addElement(aListener); } // Remember iCustomers is a Vector field in Bakery public void removeOrderListener(OrderListener aListener) { iCustomers.removeElement(aListener); }

Broadcasting the message private void sendMessage(PizzaEvent anEvent) { Vector v; v = iCustomers.clone( ); for (int i = 0; i<v.size( ); i++) { OrderListener ol = v.elementAt(i); ol.pizzaStatus(anEvent); // implement in Customer } System.out.println(“Pizza ready …”); }

Summary for Sources • Record all the references to Listener Objects in a “Vector” • Register Listeners by adding their name to the Vector • Unregister Listeners by removing their name from the Vector • Step through the elements of the Vector to notify all the Listeners

«interface» OrderListener EventObject Bakery source pizzaStatus(evt) getSource() toString() addOrderListener() removeOrderListener() sendMessage(PizzaEvent) fires PizzaEvent passed to Customer 0..* registers with invokes notifications in 0..* iNumber iSliceNumber run( ) The Story so Far

The Customer Class • A Customer also has its own flow of control public class Customer implements OrderListener, Runnable { private int iNumber; // identify customer private boolean iHaveSlice; // something to eat? private Thread iThread; // identifiy flow of control private Randon iRandom; // gaps between bites private int iSliceNumber; // Slices eaten … }

Construct a Customer public Customer(int aNumber) { iNumber = aNumber; iRandom = new Random(aNumber); iThread = new Thread(this); iThread.start( ); } // Construct a Customer with a specified identifier, and // start its own flow of control

Making your Customer Run public void run( ) { while(true) { if (iHaveSlice) { for (int bites=0; bites<4; bites++) { System.out.println(“customer: “ + iNumber + bites + “ slice:” + iSliceNumber); iThread.sleep(iRandom.nextFloat( ) * 3000); } iHaveSlice = false; iThread.suspend( ); } } // Takes 4 bites, with a rest between each, then } // waits for some more Pizza.

Response to PizzaEvents • Remember, we invoked a method called “pizzaStatus” when we broadcast messages from the Bakery. Customer must implement this: public void pizzaStatus(PizzaEvent anEvent) { if ( ! iHaveSlice) { iHaveSlice = true; iSliceNumber++; iThread.resume( ); }

Warning • These slides have simplified the implementation a little bit • We have missed out: • Explicit type conversions; • “Synchronisation” of critical sections in threads • The full implementation can be found on the CSM-15 Web site • This is taken from JavaBeans by Examples, Henri Jubin, Prentice Hall

Running the Bakery public class TestApp { public static void main(String args[ ]) { TestApp t = new TestApp( ); } public TestApp( ) { Bakery b = new Bakery( ); Customer c1 = new Customer( 1 ); Customer c2 = new Customer( 2 ); b.addOrderListener( c1 ); b.addOrderListener( c2 ); } }

Summary • We have explored a simple example of a general Notifier-Observer design pattern • Everything in this example is available in the Java 2 Software Development Kit • The trick has been to use a design pattern that allows as many Observers (Customers, in our case) to be added as required

Component-Based Software Engineering