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Threads safety and Synchronization in Java

Thread safety is an utmost important attribute to consider while designing multithreaded applications. Let's understand this with an example


 PlaneReservationSystem:

Suppose two persons "John" and "Jack" want to book the seats on the same flight on the same day.

Let's assume that only two seats left on the plane and both John and Jack wants to book two seats as well.

Now suppose John and Jack try to book the tickets at the same time, and both of them could see that available seats are 2, but John will get Success of booking message and Jack will get Sorry message.

This will create confusion for Jack as he saw available seats 2 but he cannot book the tickets. The reason behind this is that both the person threads are trying to access the same data at the same time causing the problem.


Code Synchronization:

To avoid this problem we have a feature called code synchronization which restricts multiple threads to access the same code and data at the same time.

Synchronization uses the locking concept to avoid threads working simultaneously.

Threads that uses synchronized code should acquire a lock of object file or classes respectively

Synchronization can only be applied to methods or blocks of code.

It cannot be applied to classes or variables.


 Syntax:

public synchronized void methodName();


 Program to show the use of code synchronization in PlaneReservationSystem:

 ThreadSyncMain.java

public class ThreadSyncMain {

    public static void main(String[] args){
        PlaneReservation pr = new PlaneReservation();
        PersonThread john = new PersonThread(2, pr, "John");
        PersonThread jack = new PersonThread(2, pr, "Jack");
        john.start();
        jack.start();
    }

}


PersonThread.java

public class PersonThread extends Thread {

    private int seatsNeeded;

    public PersonThread(int seats, Runnable target, String name){
        super(target, name);
        this.seatsNeeded = seats;
    }

    public int getSeatsInput() {
        return seatsNeeded;
    }

}

PlaneReservation.java

public class PlaneReservation implements Runnable {

    private static int totalSeatsAvailable=2;

    public void run(){
        PersonThread bk = (PersonThread) Thread.currentThread();
        boolean isTicketBooked = this.ticketsBooking(bk.getSeatsInput(), bk.getName());
        if (isTicketBooked){

            System.out.println("Congrats Mr." + bk.getName() + " " + "your" + " " + bk.getSeatsInput() + " " + "seats is/are booked");
        }
        else{
            System.out.println("Unfortunately the seats are Booked");
        }
    }

    public synchronized boolean ticketsBooking(int seatsNeeded, String name){
        System.out.println("Welcome Mr." + " " + Thread.currentThread().getName() + " " + "to AirFinn Airlines " + " ");
        System.out.println("Available seats are" + " " + this.totalSeatsAvailable + " " + "for Booking");
        if(seatsNeeded > totalSeatsAvailable){
            return false;
        }
        else{
            totalSeatsAvailable = totalSeatsAvailable - seatsNeeded;
            return true;
        }
    }

}

For Threads basics in Java click here Threads Basics
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