Added semaphore examples

Author: diegoramosb

Concurrency/Semaphores/java/Semaphores/Semaphore.java

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+package Semaphores;
+
+/**
+ * Although java.util.concurrent has its own semaphore class, this is a simpler implementation of a semaphore
+ */
+public class Semaphore {
+
+	/**
+	 * Semaphore's count, it represent how many threads can access the resource at the same time
+	 */
+	private int count;
+
+	public Semaphore(int count) {
+		this.count = count;
+	}
+	
+	public synchronized void acquire() {
+		count--;
+		if(count < 0) {
+			try {
+				wait();
+			} catch (InterruptedException e) {
+				e.printStackTrace();
+			}
+		}
+	}
+	
+	public synchronized void release() {
+		count++;
+		if(count <= 0) {
+			notify();
+		}
+	}
+
+}

Concurrency/Semaphores/java/Semaphores/multiplex/Multiplex.java

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+package Semaphores.multiplex;
+
+import Semaphores.Semaphore;
+
+public class Multiplex {
+	
+	static Semaphore semaphore;
+
+	public static void main(String args[]) {
+		new Multiplex();
+		MultiplexThread[] threads = new MultiplexThread[50];
+		for(int i = 0; i < threads.length; i++){
+			threads[i] = new MultiplexThread();
+		}
+		for(MultiplexThread thread : threads) {
+			thread.start();
+		}
+	}
+	
+	public Multiplex() {
+		//This semaphore ensures that a maximum of three threads are using the resource at the same time
+		semaphore = new Semaphore(3);
+	}
+
+	/**
+	 * You want to limit access to this method with the semaphore
+	 */
+	public static void a() {
+		try {
+			System.out.println("calling a");
+			Thread.sleep(500);
+		} catch (InterruptedException e) {
+			e.printStackTrace();
+		}
+	}
+	
+}

Concurrency/Semaphores/java/Semaphores/multiplex/MultiplexThread.java

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+package Semaphores.multiplex;
+
+public class MultiplexThread extends Thread {
+
+	public MultiplexThread() {
+	}
+
+	public void run() {
+		//Before calling the method, the thread acquires the permit, after calling it, it releases it.
+		Multiplex.semaphore.acquire();
+		Multiplex.a();
+		Multiplex.semaphore.release();
+	}
+}

Concurrency/Semaphores/java/Semaphores/mutex/Mutex.java

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+package Semaphores.mutex;
+
+import Semaphores.Semaphore;
+
+public class Mutex {
+
+    /**
+     * This is the resource you want to limit access to
+	 */
+	private static int n; 
+	
+	static Semaphore semaphore;
+
+	public static void main(String[] args) {
+		
+		new Mutex();
+		MutexThread a = new MutexThread(10, MutexThread.TYPE_A);
+		MutexThread b = new MutexThread(10, MutexThread.TYPE_B);
+
+		//This starts the two threads randomly every time you run the program
+		int order = (int) (Math.random() * 100) % 2;
+
+		if (order == 0) {
+			System.out.println("A starts");
+			a.start();
+			b.start();
+		} else {
+			System.out.println("B starts");
+			b.start();
+			a.start();
+		}
+	}
+	
+	public Mutex() {
+		n = 0;
+		semaphore = new Semaphore(1); //A mutex semaphore always has a count of 1
+	}
+
+	//Methods a and b operate the variable in different ways
+
+	public static void a() {
+		n += 1000;
+		try {
+			System.out.println(n);
+			Thread.sleep(50);
+		} catch (InterruptedException e) {
+			e.printStackTrace();
+		}
+	}
+	
+	public static void b() {
+		n ++;
+		try {
+			System.out.println(n);
+			Thread.sleep(50);
+		} catch (InterruptedException e) {
+			e.printStackTrace();
+		}
+	}
+
+}

Concurrency/Semaphores/java/Semaphores/mutex/MutexThread.java

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+package Semaphores.mutex;
+
+public class MutexThread extends Thread {
+
+	private int times;
+
+	private int type;
+
+	public final static int TYPE_A = 1;
+
+	public final static int TYPE_B = 0;
+
+
+	public MutexThread(int veces, int llamar ) {
+		this.times = veces;
+		this.type = llamar;
+	}
+
+	public void run() {
+		if(type == TYPE_A) {
+			for(int i = 0; i < times; i++){
+				Mutex.semaphore.acquire();
+				Mutex.a();
+				Mutex.semaphore.release();
+			}
+		}
+		if(type == TYPE_B) {
+			for(int i = 0; i < times; i++){
+				Mutex.semaphore.acquire();
+				Mutex.b();
+				Mutex.semaphore.release();
+			}
+		}
+	}
+
+}

Concurrency/Semaphores/java/Semaphores/rendezVouz/RendezVouz.java

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+package Semaphores.rendezVouz;
+
+import Semaphores.Semaphore;
+
+/**
+ * You use semaphores this way when you want methods in different threads to happen in a certain order
+ */
+public class RendezVouz {
+
+	/**
+	 * In the end, n = ((3*8) + (4*7))/2 = 26
+	 */
+	private static int n;
+	
+	static Semaphore s1;
+	
+	static Semaphore s2;
+
+
+	public static void main(String args[]) {
+		new RendezVouz();
+		RendezVouzThread a = new RendezVouzThread(RendezVouzThread.TYPE_A);
+		RendezVouzThread b = new RendezVouzThread(RendezVouzThread.TYPE_B);
+
+		int order = (int) (Math.random() * 100) % 2;
+		if (order == 0) {
+			System.out.println("B starts");
+			a.start();
+			b.start();
+		} else {
+			System.out.println("A starts");
+			b.start();
+			a.start();
+		}
+	}
+	
+	public RendezVouz() {
+		n = 0;
+		//In this case, semaphores start in 0, you want them to lock the first thread that calls acquire()
+		s1 = new Semaphore(0);
+		s2 = new Semaphore(0);
+	}
+	
+	public static void xA() {
+		n += 3*8;
+	}
+	
+	public static void xB() {
+		n += 4*7; 
+	}
+	
+	public static void yA() {
+		System.out.println(n);
+	}
+	
+	public static void yB() {
+		n = n/2;
+	}
+	
+}

Concurrency/Semaphores/java/Semaphores/rendezVouz/RendezVouzThread.java

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+package Semaphores.rendezVouz;
+
+public class RendezVouzThread extends Thread {
+
+    private int type;
+
+    public final static int TYPE_A = 1;
+
+    public final static int TYPE_B = 0;
+
+    public RendezVouzThread(int type ) {
+        this.type = type;
+    }
+
+
+    @Override
+    public void run() {
+        /*
+         * xA() and xB() can be called in any order, so each threads calls the one it corresponds to them first. Then,
+         * one of the thread calls release() on one of the semaphores, so when the other one calls acquire() it doesn't
+         * block. Then, the first thread calls acquire() on the other semaphore to wait for the other thread. Finally,
+         * the second thread calls release() on the semaphore that the firs thread is blocked. This way, yB() is always
+         * called before yA(). Independently of the order that the threads started
+         */
+        if(type == TYPE_A){
+            RendezVouz.xA();
+            RendezVouz.s1.release();
+            RendezVouz.s2.acquire();
+            RendezVouz.yA();
+        }
+        else if(type == TYPE_B){
+            RendezVouz.xB();
+            RendezVouz.s1.acquire();
+            RendezVouz.yB();
+            RendezVouz.s2.release();
+        }
+    }
+
+}

Concurrency/Semaphores/java/Semaphores/signaling/Signaling.java

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+package Semaphores.signaling;
+
+import Semaphores.Semaphore;
+
+/**
+ * You can also use semaphores when you have a method that you always want to call before another.
+ */
+public class Signaling {
+	
+	public static double n;
+
+	static Semaphore semaphore;
+	
+	public static void main(String[] args) {
+		
+		new Signaling();
+		SignalingThread a = new SignalingThread(SignalingThread.TYPE_A);
+		SignalingThread b = new SignalingThread(SignalingThread.TYPE_B);
+		
+		int orden = (int) (Math.random() * 100) % 2;
+
+		if (orden == 0) {
+			System.out.println("A starts");
+			a.start();
+			b.start();
+		} else {
+			System.out.println("B starts");
+			b.start();
+			a.start();
+		}
+		
+	}
+	
+	public Signaling() {
+		n = 0;
+		semaphore = new Semaphore(0); //You want the first thread to call acquire() to block;
+	}
+
+	//In this case, you always want a to be called before b to avoid getting an exception.
+
+	public static void a() {
+		n = 2;
+		try {
+			Thread.sleep(500);
+		} catch (InterruptedException e) {
+			e.printStackTrace();
+		}
+	}
+	
+	public static void b() {
+		System.out.println(0 / n);
+		try {
+			Thread.sleep(500);
+		} catch (InterruptedException e) {
+			e.printStackTrace();
+		}
+	}
+
+}

Concurrency/Semaphores/java/Semaphores/signaling/SignalingThread.java

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+package Semaphores.signaling;
+
+public class SignalingThread extends Thread {
+	
+	private int type;
+
+	public final static int TYPE_A = 1;
+
+	public final static int TYPE_B = 0;
+	
+	public SignalingThread(int tipo) {
+		this.type = tipo;
+	}
+	
+	public void run() {
+		//As you always want a() to be called first, the thread that calls it releases the permit after calling a.
+		if(type == TYPE_A) {
+			Signaling.a();
+			Signaling.semaphore.release();
+		}
+		if(type == TYPE_B) {
+			Signaling.semaphore.acquire();
+			Signaling.b();
+		}
+	}
+}