What’s volatile variable in Java?

The volatile keyword in Java is used as an indicator to Java compiler and Thread that do not cache value of this variable and always read it from main memory.

So if you want to share any variable in which read and write operation is atomic by implementation e.g. read and write in an int or a boolean variable then you can declare them as volatile variable.

The Java volatile keyword cannot be used with method or class and it can only be used with a variable.

Java volatile keyword also guarantees visibility and ordering, after Java 5 write to any volatile variable happens before any read into the volatile variable. By the way use of volatile keyword also prevents compiler or JVM from the reordering of code or moving away them from synchronization barrier.

The Volatile variable Example

To Understand example of volatile keyword in java let’s go back to Singleton pattern in Java and see double checked locking in Singleton with Volatile and without the volatile keyword in java.

public class Singleton{

private static volatile Singleton _instance; //volatile variable 

public static Singleton getInstance(){

   if(_instance == null){

            synchronized(Singleton.class){

              if(_instance == null)

              _instance = new Singleton();

            } 

   }

   return _instance; 

}

If you look at the code carefully you will be able to figure out:

·         We are only creating instance one time

·         We are creating instance lazily at the time of the first request comes.

If we do not make the _instance variable volatile than the Thread which is creating instance of Singleton is not able to communicate other thread, that instance has been created until it comes out of the Singleton block, so if Thread A is creating Singleton instance and just after creation lost the CPU, all other thread will not be able to see value of _instance as not null and they will believe its still null.

So in Summary apart from synchronized keyword in Java, volatile keyword is also used to communicate the content of memory between threads.

When to use Volatile variable in Java

You can use Volatile variable if you want to read and write long and double variable atomically. long and double both are 64 bit data type and by default writing of long and double is not atomic and platform dependence. Many platform perform write in long and double variable 2 step, writing 32 bit in each step, due to this its possible for a Thread to see 32 bit from two different write. You can avoid this issue by making long and double variable volatile in Java.

A volatile variable can be used as an alternative way of achieving synchronization in Java in some cases, like Visibility. With volatile variable, it's guaranteed that all reader thread will see updated value of the volatile variable once write operation completed, without volatile keyword different reader thread may see different values.

 volatile variable can be used to inform the compiler that a particular field is subject to be accessed by multiple threads, which will prevent the compiler from doing any reordering or any kind of optimization which is not desirable in a multi-threaded environment. Without volatile variable compiler can re-order the code, free to cache value of volatile variable instead of always reading from main memory.

 Another place where a volatile variable can be used is to fixing double checked locking in Singleton pattern. As we discussed in Why should you use Enum as Singleton that double checked locking was broken in Java 1.4 environment.

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