What does synchronized modifier do in Java?

The synchronized keyword used to indicate that a method can be accessed by only one thread at a time. The synchronized modifier can be applied with any of the four access level modifiers.

Example

Live Demo

 public class TestThread { public static Object Lock1 = new Object(); public static Object Lock2 = new Object(); public static void main(String args[]) { ThreadDemo1 T1 = new ThreadDemo1(); ThreadDemo2 T2 = new ThreadDemo2(); T1.start(); T2.start(); } private static class ThreadDemo1 extends Thread { public void run() { synchronized (Lock1) { System.out.println("Thread 1: Holding lock 1..."); try { Thread.sleep(10); } catch (InterruptedException e) {} System.out.println("Thread 1: Waiting for lock 2..."); synchronized (Lock2) { System.out.println("Thread 1: Holding lock 1 & 2..."); } } } } private static class ThreadDemo2 extends Thread { public void run() { synchronized (Lock2) { System.out.println("Thread 2: Holding lock 2..."); try { Thread.sleep(10); } catch (InterruptedException e) {} System.out.println("Thread 2: Waiting for lock 1..."); synchronized (Lock1) { System.out.println("Thread 2: Holding lock 1 & 2..."); } } } } } 

Output

 Thread 1: Holding lock 1... Thread 2: Holding lock 2... Thread 1: Waiting for lock 2... Thread 2: Waiting for lock 1...