Differences between synchronized and volatile keyword in detail in java

Its very important question from interview perspective.

  1. Volatile can be used as a keyword against the variable, we cannot use volatile against method declaration.
volatile void method1(){} //it’s illegal, compilation error.
volatile int i; //legal

While synchronization can be used in method declaration or we can create synchronization blocks. Variables cannot be synchronized.
Synchronized method:
synchronized void method2(){} //legal

Synchronized block:
   void method2(){
      synchronized (this) {
          //code inside synchronized block.

Synchronized variable (illegal):
synchronized int i; //it’s illegal, compilation error.

  1. Volatile does not acquire any lock on variable or object, but Synchronization acquires lock on method or block in which it is used.

  1. Volatile variables are not cached, but variables used inside synchronized method or block are cached.

  1. When volatile is used will never create deadlock in program, as volatile never obtains any kind of lock . But in case if synchronization is not done properly, we might end up creating dedlock in program.

  1. Synchronization may cost us performance issues, as one thread might be waiting for another thread to release lock on object. But volatile is never expensive in terms of performance.

Full program to show that how synchronization block can cost us performance,
we will create two threads on same object, then start both threads, either of the thread will acquire object lock and other thread will wait to acquire lock until lock is released by thread which acquired it>
/** Copyright (c), AnkitMittal JavaMadeSoEasy.com */
public class MyClass implements Runnable{
   public void run() {
          System.out.println(Thread.currentThread().getName()+" is waiting for lock");
          synchronized (this) {
                 try {
                       System.out.println(Thread.currentThread().getName()+" has acquired lock");
                       Thread.sleep(2000); //sleep for 2 seconds
                 } catch (InterruptedException e) {
                 System.out.println(Thread.currentThread().getName()+" has ended");
   public static void main(String[] args) {
                 MyClass obj=new MyClass();
      Thread thread1=new Thread(obj,"Thread-1");
      Thread thread2=new Thread(obj,"Thread-2");
Thread-1 is waiting for lock
Thread-2 is waiting for lock
Thread-1 has acquired lock
Thread-1 has ended
Thread-2 has acquired lock
Thread-2 has ended

What happened in above program ?
Thread1 acquired object lock by entering in synchronized block & Thread2 was waiting for Thread1 to release lock and enter in synchronization block, it costed us performance.

Here i have shown example code that how synchronized block can cost us performance, likewise you may call any synchronized method from run() method, to see how synchronized method can cost us performance.


Volatile keyword vs synchronized>

Volatile keyword in java- difference between synchronized and volatile, 10 key points about volatile keyword, why volatile variables are not cached in memory

Differences between important thread methods >

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Difference between notify() and notifyAll() methods, with program

Difference between wait() and wait(long timeout) -What will be Thread states

Thread Pool, Thread local, Busy Spin >

Implement Thread pool in java

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Busy Spin - What is busy spin? Consumer Producer problem with busy spin and solution to busy spin.

Guidelines to threadsafe code >

Interviews >

THREADS - Top 80 interview questions and answers (detailed explanation with programs) Set-1 >  Q1- Q60

THREADS - Top 80 interview questions and answers, important interview OUTPUT questions and answers, Set-2 > Q61- Q80

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