Instructore: Tasneem Darwish 1

University of PalestineFaculty of Applied Engineering and Urban Planning

Software Engineering Department

Concurrent and Real Time programming

Implementing communication paradigms in java

Instructore: Tasneem Darwish 2

Outlines Introduction

Semaphores

Signals

Events

Buffers

Blackboards

Broadcasts

Barriers.

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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.

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Signals package communicationAbstractions;

public interface SignalSender {

void send();}

package communicationAbstractions;

public interface SignalWaiter {

void waits() throws InterruptedException;}

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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

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Persistent signalspackage 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;

}}

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Persistent Signals

Unlike semaphores, send method call will release only one thread no count is maintained.

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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)

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Transient signalspackage 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; }

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Transient signals

If the catch doesn’t decrement the wait variable, the result would be that eventually a transient signal would be made permanent.

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PulsesA 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();

}}

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Pulsespublic 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;

}}

}

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PulsesIf 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.

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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.

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Eventspackage 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();

}

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Eventspublic synchronized void reset() {

value = EventState.DOWN;notifyAll();

}public synchronized void toggle() {

if(value == EventState.DOWN) value = EventState.UP;else value = EventState.DOWN;notifyAll();

}