How can multiple threads get access to a limited resource pool

In this Q&A, how threads can obtain access to limited resources in a pool using Semaphore

Conceptually Semaphore maintains permits.  Threads can acquire permits or release unused permits.  Thread trying to acquire a permit will wait until one is available.

See demo code below.

package com.javahowdoi.thread;

import java.util.concurrent.*;

public class SemphareDemo {

    private static class SemaphoreTask implements Runnable{

        Semaphore sem;

        public SemaphoreTask(Semaphore sem) {

            this.sem = sem;

        }

        @Override

        public void run() {

            try {

                System.out.println( Thread.currentThread().getName() + " start" );

                // acquire a permit

                sem.acquire();

                Thread.sleep(1000);

                System.out.println( Thread.currentThread().getName() + " done" );

            } catch (InterruptedException e) {

                e.printStackTrace();

            }

            finally {

                sem.release();

            }

        }

    }

    public static void main(String[] args ) throws InterruptedException {

        // 5 threads competing for two resources

        Semaphore se  = new Semaphore(2);

        ExecutorService es  = Executors.newFixedThreadPool(5);

        for(int i = 0; i < 5; ++i) {

            es.submit( new SemaphoreTask(se));

        }

        es.shutdown();

    }

}