Clase Java.util.concurrent.Semaphore en Java

Java

// Java program to demonstrate
// methods of Semaphore class
import java.util.concurrent.*;
  
class MyThread extends Thread {
    Semaphore sem;
    String threadName;
    public MyThread(Semaphore sem, String threadName)
    {
        super(threadName);
        this.sem = sem;
        this.threadName = threadName;
    }
  
    @Override public void run()
    {
  
        // First, get a permit.
        System.out.println(threadName
                           + " is waiting for a permit.");
  
        try {
            // acquire method
            sem.acquire();
        }
        catch (InterruptedException e) {
            e.printStackTrace();
        }
  
        System.out.println(threadName + " gets a permit");
  
        // Now, critical section
        // other waiting threads will wait, until this
        // thread release the lock
        for (int i = 0; i < 2; i++) {
            // hasQueuedThreads() methods
            boolean b = sem.hasQueuedThreads();
            if (b)
                // getQueuedLength() methods
                System.out.println("Length of Queue : "
                                   + sem.getQueueLength());
  
            // Now, allowing a context switch -- if
            // possible.
            try {
                Thread.sleep(10);
            }
            catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
  
        // Release the permit.
        System.out.println(threadName
                           + " releases the permit.");
  
        // release() method
        sem.release();
    }
}
  
// Driver class
public class SemaphoreDemo {
  
    public static void main(String args[])
        throws InterruptedException
    {
        // creating a Semaphore object
        // with number of permits 3 and fairness true
        Semaphore sem = new Semaphore(3, true);
  
        // isFair() method
        System.out.println("is Fairness enabled : "
                           + sem.isFair());
  
        // Main thread try to acquire 2 permits
        // tryAcquire(int permits) method
        sem.tryAcquire(2);
  
        // availablePermits() method
        System.out.println("Available permits : "
                           + sem.availablePermits());
  
        // drainPermits() method
        System.out.println(
            "number of permits drain by Main thread : "
            + sem.drainPermits());
  
        // permit released by Main thread
        sem.release(1);
  
        // creating two threads with name A and B
        MyThread mt1 = new MyThread(sem, "A");
        MyThread mt2 = new MyThread(sem, "B");
  
        // starting threads A
        mt1.start();
  
        // starting threads B
        mt2.start();
  
        // toString method
        System.out.println(sem.toString());
  
        // waiting for threads A and B
        mt1.join();
        mt2.join();
    }
}