volatile

Each multicore processor has its own cache, like the diagram below describes.

It wasn’t a problem when it was just a single core processor (with only one cache), but the problem starts when a value is updated differently for each of the caches that cores have.

To prevent this from happening, there is a keyword volatile. Instead of reading a value from a cache, a variable with volatile keyword always reads from the main system memory.

Quite the same effect is achieved by using ‘synchronized’ keyword; because caches and memories are synchronized when a thread enters and exits the synchronized block.

• Atomization with volatile Because JVM processes data with unit of 4 byte(32 bits), int or any type smaller than int can be processed at once; that being said, there is no possibility of other threads intervening the actual data in the middle of data processing. However, for 8 byte variable types like long and double, it is quite not the case.

    volatile long
    volatile double

For such occasion, using volatile keyword helps, because it guarantees a thread safe operation for the variable at hand.