ConnectionLimiter.java

package semaphore;

import java.util.concurrent.Semaphore;

/**
 * This is a small program to demo how semaphore can create trouble for us.
 * One rule of thumb is:
 * Always release what you acquire!
 * Copied from the Internet.
 */
//this is a bad public class name, it doesn't apply to 
//what this class really do, it's a name from a different class that I copied earlier.
public class ConnectionLimiter {

    private static class DoubleResourceGrabber implements Runnable {

        private Semaphore first;
        private Semaphore second;

        public DoubleResourceGrabber(Semaphore s1, Semaphore s2) {
            first = s1;
            second = s2;
        }

        @Override
        public void run() {
            Thread t = Thread.currentThread();

            try {
                first.acquire();
                System.out.println(t.getName() + " acquired " + first);

//				Thread.sleep(20);//to demo a deadlock

//				second.acquire();
//				System.out.println(t.getName() + " acquired " + second);

//				second.release();
//				System.out.println(t.getName() + " released " + second);

                first.release();
                System.out.println(t.getName() + " released " + first);

            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * @param args
     * @throws InterruptedException
     */
    public static void main(String[] args) throws InterruptedException {
        Semaphore s1 = new Semaphore(1);//give it only 1 permit
        Semaphore s2 = new Semaphore(1);//give it only 1 permit as well
        Thread t1 = new Thread(new DoubleResourceGrabber(s1, s2));
        //now reverse them, here comes the trouble
        Thread t2 = new Thread(new DoubleResourceGrabber(s2, s1));

        t1.start();
        t2.start();

        t1.join();
        t2.join();
        System.out.println("We got lucky!");

    }

}