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What is the volatile keyword in Java? How and why should we use it?

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What is the volatile keyword in Java? How and why should we use it?
posted Feb 6, 2017 by Dhaval Vaghela

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1 Answer

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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.

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 from 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.

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.

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

  2. volatile keyword in Java guarantees that value of the 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 the 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 the use of volatile keyword in Java.

  7. An access to a volatile variable in Java never has a 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 mean 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, you don't need to use volatile keyword with that variable.

answer Feb 7, 2017 by Karthick.c
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