DiningPhilosophersProblem.java

package rafael.q.g.petri_network_examples;

import java.util.concurrent.Semaphore;
import java.util.concurrent.ThreadLocalRandom;
//REF: https://www.javatpoint.com/dining-philosophers-problem-and-solution-in-java#:~:text=The%20Solution%20to%20the%20Dining%20Philosophers%20Problem&text=There%20is%20a%20drawback%20of,situation%20leads%20to%20a%20deadlock.
public class DiningPhilosophersProblem {
//defining the number of philosophers  

    static int philosopher = 5;
//initializing an array of philosophers with the number of philosophers  
    static Philosopher philosophers[] = new Philosopher[philosopher];
//initializing an array of chosticks with the number of philosophers  
    static Chopstick chopsticks[] = new Chopstick[philosopher];

    static class Chopstick {
//creating a constructor of the Semaphore class that accepts the number permits  

        public Semaphore mutex = new Semaphore(1);
//the method grabs the chopstick  

        void grab() {
            try {
//acquires a permit from the semaphore  
                mutex.acquire();
            } catch (Exception e) {
                e.printStackTrace(System.out);
            }
        }
//release the chopstick  

        void release() {
//releases an acquire a permit and increases the number of available permits by one  
            mutex.release();
        }
//checks if the chopstick is free or not  

        boolean isFree() {
//the method returns the current number of permits available in the semaphore  
//returns true if available permits is greater than 0, else returns false  
            return mutex.availablePermits() > 0;
        }
    }  //end of Chopstick class  

    static class Philosopher extends Thread {

        public int number;
//represents left chopstick  
        public Chopstick leftchopstick;
//represents right chopstick  
        public Chopstick rightchopstick;
//creating a constructor of the Philosopher class  

        Philosopher(int num, Chopstick left, Chopstick right) {
            number = num;
            leftchopstick = left;
            rightchopstick = right;
        }

        public void run() {
            while (true) {
//philosopher grabs the chopsticks if both are free  
                leftchopstick.grab();
                System.out.println("Philosopher " + (number + 1) + " grabs left chopstick.");
                rightchopstick.grab();
                System.out.println("Philosopher " + (number + 1) + " grabs right chopstick.");
//hunger philosopher starts eating   
                eat();
//releases left and right chopsticks when philosopher is not hunger  
                leftchopstick.release();
                System.out.println("Philosopher " + (number + 1) + " releases left chopstick.");
                rightchopstick.release();
                System.out.println("Philosopher " + (number + 1) + " releases right chopstick.");
            }  //end of while loop  
        } //end of run() method  
//the method invokes after grabbing both the chopsticks (left and right)  

        void eat() {
            try {
//determines the pseudorandom number between 0 to 1000 that represents the sleep time in milli seconds  
                int sleepTime = ThreadLocalRandom.current().nextInt(0, 1000);
                System.out.println("Philosopher " + (number + 1) + " eats for " + sleepTime + "ms"); //sleeps the thread for a specified time  
                Thread.sleep(sleepTime);
            } catch (Exception e) {
                e.printStackTrace(System.out);
            }
        }// end of eat() method  
    }

    public static void main(String args[]) {
//loop iterates over chopsticks  
        for (int i = 0; i < philosopher; i++) {
            chopsticks[i] = new Chopstick();
        } //end of for loop  
//loop iterates over philosopher  
        for (int i = 0; i < philosopher; i++) {
            philosophers[i] = new Philosopher(i, chopsticks[i], chopsticks[(i + 1) % philosopher]);
//begins the execution of the thread  
            philosophers[i].start();
        } //end of for loop  
        while (true) {
            try {
//thread sleep for 1 sec or 1000ms  
                Thread.sleep(1000);
//check for deadlock condition  
                boolean deadlock = true;
//for each loop iterates over chopsticks  
                for (Chopstick cs : chopsticks) {
//checks if chopstick is free or not   
                    if (cs.isFree()) {
                        deadlock = false;
                        break;
                    } //end of if  
                } //end of for loop  
//deadlock occurs if sleep time is 1000ms it means each philosopher is eating  
                if (deadlock) {
                    Thread.sleep(1000);
                    System.out.println("Everyone Eats");
                    break;
                } //end of if  
            } catch (Exception e) {
                e.printStackTrace(System.out);
            }
        }
        System.out.println("Exit The Program!");
        System.exit(0);
    }
}