Volatile

If a variable is declared with the volatile keyword then it is guaranteed that any thread that reads the field will see the most recently written value.

The volatile keyword will not perform any mutual exclusive lock on the variable.

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.

The 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.

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 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.

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.

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.

• 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.

• 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 the 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.

• Another place where volatile variable can be used is to fixing double checked locking in Singleton pattern

Important points on Volatile keyword in Java

1. The 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

1. The 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.