A Volatile variable is modified asynchronously by concurrently running threads in a Java application. It is not allowed to have a local copy of a variable that is different from the value currently held in "main" memory. Effectively, a variable declared
volatilemust have its data synchronized across all threads, so that whenever you access or update the variable in any thread, all other threads immediately see the same value. Of course, it is likely that
volatilevariables have a higher access and update overhead than "plain" variables, since the reason threads can have their own copy of data is for better efficiency.
When a field is declared volatile, the compiler and runtime are put on notice that this variable is shared and that operations on it should not be reordered with other memory operations.Volatile variables are not cached in registers or in caches where they are hidden from other processors, so a read of a volatile variable always returns the most recent write by any thread.
The most common use for volatile variables is as a completion, interruption, or status flag. Volatile variables can be used for other kinds of state information, but more care is required when attempting this. For example, the semantics of volatile are not strong enough to make the increment operation (count++) atomic, unless you can guarantee that the variable is written only from a single thread.
Note: Volatile variables can only guarantee visibility not atomicity.