1. Instructore: Tasneem Darwish1 University of Palestine Faculty of Applied Engineering and Urban Planning Software Engineering Department Concurrent and Real Time programming Implementing communication paradigms in java

2. Instructore: Tasneem Darwish2 Outlines  Introduction  Semaphores  Signals  Events  Buffers  Blackboards  Broadcasts  Barriers.

3. Instructore: Tasneem Darwish3 Signals  Often a thread needs to wait for a signal from another thread before it can proceed.  There are various types of signals.  A persistent signal is a signal that remains set until a single thread has received it.  A transient signal (or pulse) is a signal that releases one or more waiting threads but is lost if no threads are waiting.

4. Instructore: Tasneem Darwish4 Signals package communicationAbstractions; public interface SignalSender { void send(); } package communicationAbstractions; public interface SignalWaiter { void waits() throws InterruptedException; }

5. Instructore: Tasneem Darwish5 Signals package communicationAbstractions; public abstract class Signal implements SignalSender, SignalWaiter { public synchronized void send() { arrived = true; notify(); } public abstract void waitS() throws InterruptedException; protected boolean arrived = false; }  the implementation of the send operation has code common for all signals

6. Instructore: Tasneem Darwish6 Persistent signals package communicationAbstractions; public interface SignalWaiterOrWatcher extends SignalWaiter { boolean watch(); } package communicationAbstractions; public class PersistentSignal extends Signal implements SignalWaiterOrWatcher { public synchronized void waitS() throws InterruptedException { while(!arrived) wait(); // Wait for a new signal. arrived = false; } public synchronized boolean watch() { // This method never waits. if(!arrived) return false; arrived = false; return true; }

7. Instructore: Tasneem Darwish7 Persistent Signals  Unlike semaphores, send method call will release only one thread no count is maintained.

8. Instructore: Tasneem Darwish8 Transient Signals  A transient signal is a signal that is lost if no threads are currently waiting.  Two types can be recognized:  one that releases a single thread, and  one that releases all threads (called a pulse).  first the simple transient signal that releases just one thread. This extends the abstract Signal class (that implements the SignalSender and SignalWaiter interfaces)

9. Instructore: Tasneem Darwish9 Transient signals package communicationAbstractions; public class TransientSignal extends Signal { public synchronized void send() { // Overrides send in Signal // and implements the SignalSender interface. if(waiting > 0) super.send(); // it will not send a signal unless there is a waiting thread } public synchronized void waitS() throws InterruptedException { // Overrides waitS in Signal and // implements the SignalWaiter interface. try {while(!arrived) { waiting++; wait(); waiting--; } arrived = false; } catch(InterruptedException ie) { waiting--; throw ie;} } protected int waiting = 0; }

10. Instructore: Tasneem Darwish10 Transient signals  If the catch doesn’t decrement the wait variable, the result would be that eventually a transient signal would be made permanent.

11. Instructore: Tasneem Darwish11 Pulses  A Pulse allows all waiting threads to be released. package communicationAbstractions; public class Pulse extends TransientSignal { public synchronized void sendAll() { // A new method. if(waiting > 0) { arrived = true; notifyAll(); }

12. Instructore: Tasneem Darwish12 Pulses public synchronized void waitS() throws InterruptedException { // Overrides waitS in TransientSignal and // implements the SignalWaiter interface. try { while(!arrived) { waiting++; wait(); waiting--; } if(waiting == 0) arrived = false; } catch(InterruptedException ie) { if(--waiting == 0) arrived = false; throw ie; }

13. Instructore: Tasneem Darwish13 Pulses  If more than one thread is waiting for the signal and sendAll is called, all are released.  The last one detects that there are no more threads to be released and sets the boolean flag to false.  Any threads that queue after this point will have to wait for the next signal.

14. Instructore: Tasneem Darwish14 Events  An event is a bivalued variable (UP or DOWN).  A thread can wait for an event to be set or reset.  Events may be created and initialized using the class Event.  The method set causes the event to go into the UP state; the method reset causes it to go into the DOWN state.  The toggle procedure simply changes the state of the event from UP to DOWN, or from DOWN to UP.  The function state returns the current state of the event.  Synchronization with an event is achieved using the await method.

15. Instructore: Tasneem Darwish15 Events package communicationAbstractions; public enum EventState {UP, DOWN}; package communicationAbstractions; public class Event { public Event(EventState initial) { value = initial; } public Event() {value = EventState.DOWN;} public synchronized void await(int state)throws InterruptedException { while(value != state) wait(); } public synchronized void set() { value = EventState.UP; notifyAll(); }

16. Instructore: Tasneem Darwish16 Events public synchronized void reset() { value = EventState.DOWN; notifyAll(); } public synchronized void toggle() { if(value == EventState.DOWN) value = EventState.UP; else value = EventState.DOWN; notifyAll(); }