Raja PLECI: How Volatile in Java works? Example of volatile keyword in Java

How to use Volatile keyword in Java
What is volatile variable in Java and when to use Volatile variable in Java is famous multi-threading interview question in Java interviews. Though many programmer knows what is a volatile variable but they fail on second part i.e. where to use volatile variable in Java as its not common to have clear understanding and hands-on on volatile in Java. In this tutorial we will address this gap by providing simple example of volatile variable in Java and discussing some when to use Volatile variable in Java. Any way 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 int or boolean variable you can declare them as volatile variable. From Java 5 along with major changes like Autoboxing, Enum, Generics and Variable arguments , Java introduces some change in Java Memory Model (JMM), Which guarantees visibility of changes made by one thread to another also as "happens-before" which solves the problem of memory writes that happen in one thread can "leak through" and be seen by another thread. Java volatile keyword cannot be used with method or class and it can only be used with variable. Java volatile keyword also guarantees visibility and ordering , after Java 5 write to any volatile variable happens before any read into volatile variable. By the way use of volatile keyword also prevents compiler or JVM from reordering of code or moving away them from synchronization barrier.

Volatile variable Example in Java

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 volatile keyword in java.

/**
 * Java program to demonstrate where to use Volatile keyword in Java.
 * In this example Singleton Instance is declared as volatile variable to ensure
 * every thread see updated value for _instance.
 * 
 * @author Javin Paul
 */
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:
1) We are only creating instance one time
2) We are creating instance lazily at the time of first request comes.

If we do not make _instance variable volatile then 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.

Why? because reader threads are not doing any locking and until writer thread comes out of synchronized block, memory will not be synchronized and value of _instance will not be updated in main memory. With Volatile keyword in Java this is handled by Java himself and such updates will be visible by all reader threads.

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

Let’s see another example of volatile keyword in Java

most of the time while writing game we use a variable bExit to check whether user has pressed exit button or not, value of this variable is updated in event thread and checked in game thread , So if we don't use volatile keyword with this variable , Game Thread might miss update from event handler thread if its not synchronized in java already. volatile keyword in java guarantees that value of volatile variable will always be read from main memory and "happens-before" relationship in Java Memory model will ensure that content of memory will be communicated to different threads.

private boolean bExit;

 while(!bExit) {
    checkUserPosition();
    updateUserPosition();
 }

In this code example One Thread (Game Thread) can cache the value of "bExit" instead of getting it from main memory every time and if in between any other thread (Event handler Thread) changes the value; it would not be visible to this thread. Making boolean variable "bExit" as volatile in java ensures this will not happen.

When to use Volatile variable in Java

One of the most important thing in learning of volatile keyword is understanding when to use volatile variable in Java. Many programmer knows what is volatile variable and How does it work but they never really used volatile for any practical purpose. Here are couple of example to demonstrate when to use Volatile keyword in Java:

1) 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.

2) Volatile variable can be used as an alternative way of achieving synchronization in Java in some cases, like Visibility. with volatile variable its guaranteed that all reader thread will see updated value of volatile variable once write operation completed, without volatile keyword different reader thread may see different values.

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

private boolean isActive = thread;
public void printMessage(){
  while(isActive){
     System.out.println("Thread is Active");
  }
}

without volatile modifier its not guaranteed that one Thread see the updated value of isActive from other thread. compiler is also free to cache value of isActive instead of reading it from main memory in every iteration. By making isActive a volatile variable you avoid these issue.

4) Another place where 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.

Important points on Volatile keyword in Java

1. volatile keyword in Java is only application to variable and using volatile keyword with class and method is illegal.

2. volatile keyword in Java guarantees that value of volatile variable will always be read from main memory and not from Thread's local cache.

3. In Java reads and writes are atomic for all variables declared using Java volatile keyword (including long and double variables).

4. Using Volatile keyword in Java on variables reduces the risk of memory consistency errors, because any write to a volatile variable in Java establishes a happens-before relationship with subsequent reads of that same variable.

5. From Java 5 changes to a volatile variable are always visible to other threads. What’s more it also means that when a thread reads a volatile variable in Java, it sees not just the latest change to the volatile variable but also the side effects of the code that led up the change.

6. Reads and writes are atomic for reference variables are for most primitive variables (all types except long and double) even without use of volatile keyword in Java.

7. An access to a volatile variable in Java never has chance to block, since we are only doing a simple read or write, so unlike a synchronized block we will never hold on to any lock or wait for any lock.

8. Java volatile variable that is an object reference may be null.

9. Java volatile keyword doesn't means atomic, its common misconception that after declaring volatile ++ will be atomic, to make the operation atomic you still need to ensure exclusive access using synchronized method or block in Java.

10. If a variable is not shared between multiple threads no need to use volatile keyword with that variable.

Difference between synchronized and volatile keyword in Java

What is difference between volatile and synchronized is another popular core Java question asked in multi-threading and concurrency interviews. Remember volatile is not a replacement of synchronized keyword but can be used as an alternative in certain cases. Here are few differences between volatile and synchronized keyword in Java.

1. Volatile keyword in Java is a field modifier, while synchronized modifies code blocks and methods.

2. Synchronized obtains and releases lock on monitor’s Java volatile keyword doesn't require that.

3. Threads in Java can be blocked for waiting any monitor in case of synchronized, that is not the case with volatile keyword in Java.

4. Synchronized method affects performance more than volatile keyword in Java.

5. Since volatile keyword in Java only synchronizes the value of one variable between Thread memory and "main" memory while synchronized synchronizes the value of all variable between thread memory and "main" memory and locks and releases a monitor to boot. Due to this reason synchronized keyword in Java is likely to have more overhead than volatile.

6. You can not synchronize on null object but your volatile variable in java could be null.

7. From Java 5 Writing into a volatile field has the same memory effect as a monitor release, and reading from a volatile field has the same memory effect as a monitor acquire

In Summary volatile keyword in Java is not a replacement of synchronized block or method but in some situation is very handy and can save performance overhead which comes with use of synchronization in Java

if you like to know more about volatile I would also suggest to go thorough FAQ on Java Memory Model here which explains happens-before operations quite well.

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