What is volatile keyword?

Question

in thread class

Answer 1

'Using volatile variables reduces the risk of memory consistency errors, because any write to a volatile variable establishes a happens-before relationship with subsequent reads of that same variable. This means that 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, it sees not just the latest change to the volatile, but also the side effects of the code that led up the change.'
Source:
https://docs.oracle.com/javase/tutorial/essential/concurrency/atomic.html

Answer 2

What is volatile variable in Java and when to use the volatile variable in Java is a famous multi-threading interview question in Java interviews. Though many programmers know what is a volatile variable but they fail on second part i.e. where to use volatile variable in Java as it's not common to have a clear understanding and hands-on on volatile in Java. In this tutorial, we will address this gap by providing a simple example of the volatile variable in Java and discussing some when to use the volatile variable in Java. Anyway, 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.