Java Semaphore example 2017-07-24 02:52

Semaphore is used to control the number of available threads. Let's say you have a lot of workers and only 5 computers. When workers finish his work he has to release the computer for next worker. Semaphore is good at handle this case. The example of Semaphore is here.

public class Worker implements Runnable {
    private int num;
    private Semaphore semaphore;

    public Worker(int num, Semaphore semaphore) {
        this.num = num;
        this.semaphore = semaphore;

    public void run() {
        try {
            System.out.println("thread" + this.num + " acquire a computer...");
            System.out.println("thread" + this.num + " release the computer.");
        } catch (InterruptedException e) {

public class SemaphoreClient {
    public static void main(String[] args) {
        int N = 8;
        Semaphore semaphore = new Semaphore(5);
        for (int i = 0; i < N; i++) {
            new Thread(new Worker(i, semaphore)).start();


thread0 acquire a computer...
thread2 acquire a computer...
thread1 acquire a computer...
thread4 acquire a computer...
thread3 acquire a computer...
thread1 release the computer
thread2 release the computer
thread4 release the computer
thread3 release the computer
thread0 release the computer
thread7 acquire a computer...
thread6 acquire a computer...
thread5 acquire a computer...
thread7 release the computer
thread6 release the computer
thread5 release the computer