Question

I am little confused…

Is it true that reading\writing from several threads all except long and double are atomic operations and it’s need to use volatile only with long and double?

Answer 1

It sounds like you’re referring to this section of the JLS. It is guaranteed for all primitive types — except double and long — that all threads will see some value that was actually written to that variable. (With double and long, the first four bytes might have been written by one thread, and the last four bytes by another thread, as specified in that section of the JLS.) But they won’t necessarily see the same value at the same time unless the variable is marked volatile.

Even using volatile, x += 3 is not atomic, because it’s x = x + 3, which does a read and a write, and there might be writes to x between the read and the write. That’s why we have things like AtomicInteger and the other utilities in java.util.concurrent.