Semaphore

• A Semaphore is a thread synchronization construct that can be used either to send signals between threads to avoid missed signals, or to guard a critical section like you would with a lock
• Simple Semaphore implementation:
• The take() method sends a signal which is stored internally in the Semaphore. The release() method waits for a signal. When received the signal flag is cleared again, and the release() method exited.
• // Prototype pattern public abstract class Prototype implements Cloneable { public Prototype clone() throws CloneNotSupportedException{ return (Prototype) super.clone(); } } public class ConcretePrototype1 extends Prototype { @Override public Prototype clone() throws CloneNotSupportedException { return (ConcretePrototype1)super.clone(); } } public class ConcretePrototype2 extends Prototype { @Override public Prototype clone() throws CloneNotSupportedException { return (ConcretePrototype2)super.clone(); } }
• Using a semaphore like this you can avoid missed signals. You will call take() instead of notify() and release() instead of wait().
• Using Semaphores for Signaling
• Here is a simplified example of two threads signaling each other using a Semaphore:
• MainApp.java
Semaphore semaphore = new Semaphore(); SendingThread sender = new SendingThread(semaphore); ReceivingThread receiver = new ReceivingThread(semaphore); receiver.start(); sender.start();
• SendingThread.java
public class SendingThread { Semaphore semaphore = null; public SendingThread(Semaphore semaphore){ this.semaphore = semaphore; } public void run(){ while(true){ //do something, then signal this.semaphore.take(); } } }
• RecevingThread.java
public class RecevingThread { Semaphore semaphore = null; public ReceivingThread(Semaphore semaphore){ this.semaphore = semaphore; } public void run(){ while(true){ this.semaphore.release(); //receive signal, then do something... } } }
• Semaphore to block threads
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