Call in order- Using Sempahores

In previous post, we’ve learnt Calling methods in order using low level wait-notify.

Below we’re using Semarphore concept to achieve the same.

For problem statement , please refer previous post.

Problem Statement : Making three threads working in way that thread1 calling first(), thread2 calling second() and thread3 calling third()

public class MyThread extends Thread {

    private String method;

    private Fooo foo;

    public MyThread(Fooo foo, String method) {

         this.method = method;

         this.foo = foo;

    }

    /*t1,t2,t3 calling run()*/

    public void run() {

         if (method == "first") {

             foo.first();

         } else if (method == "second") {

             foo.second();

         } else if (method == "third") {

             foo.third();

         }

    }

}

package threads;

import java.util.concurrent.Semaphore;

public class Fooo {

    public int pauseTime = 0;

    public Semaphore sem1;

    public Semaphore sem2;

    public Fooo() {

         try {

             sem1 = new Semaphore(1);

             sem2 = new Semaphore(1);

             sem1.acquire();

             System.out.println("Semaphore 1->"+sem1.availablePermits());

             sem2.acquire();

             System.out.println("Semaphore 2->"+sem2.availablePermits());

            

         } catch (InterruptedException e) {

             e.printStackTrace();

         }

    }

    public void first() {

         try {

             System.out.println(Thread.currentThread().getName()+ " Started Executing 1");

             Thread.sleep(pauseTime);

             System.out.println(Thread.currentThread().getName()+ " Finished Executing 1");

             sem1.release();

         } catch (Exception ex) {

             ex.printStackTrace();

         }

    }

    public void second() {

         try {

             /* block until thread1 release semaphore sem1*/

             sem1.acquire();

             sem1.release();

             System.out.println(Thread.currentThread().getName()+ " Started Executing 2");

             Thread.sleep(pauseTime);

             System.out.println(Thread.currentThread().getName()+ " Finished Executing 2");

             sem2.release();

         } catch (Exception ex) {

             ex.printStackTrace();

         }

    }

    public void third() {

         try {

             /* we've called acquire() so that third() wait till second() finish

              * and second() finish only when thread2 release semaphore sem2*/

             sem2.acquire();

             sem2.release();

             System.out.println(Thread.currentThread().getName()+ " Started Executing 3");

             Thread.sleep(pauseTime);

             System.out.println(Thread.currentThread().getName()+ " Finished Executing 3");

         } catch (Exception ex) {

             ex.printStackTrace();

         }

    }

    public static void main(String[] args) {

         Fooo foo = new Fooo();

         MyThread t1 = new MyThread(foo, "first");

         MyThread t2 = new MyThread(foo, "second");

         MyThread t3 = new MyThread(foo, "third");

         t3.start();

         t2.start();

         t1.start();

    }

}

Output

Thread-0 Started Executing 1

Thread-0 Finished Executing 1

Thread-1 Started Executing 2

Thread-1 Finished Executing 2

Thread-2 Started Executing 3

Thread-2 Finished Executing 3