Synchronization in Java

Overview

Synchronization is the process by which we control the accessibility of multiple threads for a particular resource. With synchronization only one thread at a time can execute a particular method or block of code. Without synchronization we can face the problem of data inconsistency and thread interference. 

Disadvantages of Synchronization

• Synchronization can increase the waiting time of application due to synchronization of multiple threads.
• At times, synchronization can create performance issues.

Types of Synchronization

• Process Synchronization - not present in Java multithreading.
• Thread Synchronization - 
• Mutual Exclusive - It can be achieved by 3 ways
• By synchronized method

 synchronized void bookSeats(int seats) {
    if (seats <= total_seats) {
        System.out.println(seats + " seats are booked successfully!!!");
        total_seats = total_seats - seats;
        System.out.println("Seats left: " + total_seats);
    } else {
        System.out.println("Sorry all the seats are booked");
        System.out.println("Seats left: " + total_seats);
    }
 }

• By synchronized block

 synchronized (this) {
    if (seats <= total_seats) {
        System.out.println(seats + " seats are booked successfully!!!");
        total_seats = total_seats - seats;
        System.out.println("Seats left: " + total_seats);
    } else {
        System.out.println("Sorry all the seats are booked");
        System.out.println("Seats left: " + total_seats);
    }
 }

• By static synchronization
• Cooperation - It can also by achieved by 3 ways
• wait()
• notify()
• notifyAll()

Example of data inconsistency and thread interference.

 public class BookTheatre {
    int total_seats = 10;

    void bookSeats(int seats) {
        if (seats <= total_seats) {
            System.out.println(seats + " seats are booked successfully!!!");
            total_seats = total_seats - seats;
            System.out.println("Seats left: " + total_seats);
        } else {
            System.out.println("Sorry all the seats are booked");
            System.out.println("Seats left: " + total_seats);
        }
    }
 }

 public class MovieBookApp extends Thread {
    private static BookTheatre bookTheatre;
    private int seats;

    @Override
    public void run() {
        bookTheatre.bookSeats(seats);
    }

    public static void main(String[] args) {
        bookTheatre = new BookTheatre(); // Here the property bookTheatre of MovieBookApp is static

        /*Object for Deepak*/
        MovieBookApp deepak = new MovieBookApp();
        deepak.seats = 7;
        deepak.start();

        /*Object for Amit*/
        MovieBookApp amit = new MovieBookApp();
        amit.seats = 8;
        amit.start();

    }
 }

Output - Here Amit shouldn't get 8 seats of the movie because there were total only 10 seats available and out of them 7 seats were booked by Deepak. It's a clear example of miscalculation.   

 7 seats are booked successfully!!!
 8 seats are booked successfully!!!
 Seats left: 3
 Seats left: -5

Eliminating data inconsistency and thread interference through synchronization.

Reference