Question

what is the difference in using volatile keyword in java4 and java5 onwards?

and related to that,

Read/write operations on non-atomic variables(long/double) are atomic when they are
declared as volatile.

Is this also true for java4 or it is valid from java5 onwards???

Answer 1

People have provided good points and references responding to my question answering first part.

Going specific to the second part of question, this i read at some forum:

A volatile declared long is atomic (pre-Java 5 also) in the sense that
it guarantees (for all JVM implementations) a read or write go
directly to main memory instead of two 32-bit registers.

And

Pre-Java 5, volatile was supposed to provide such guarantees for long
and double. However things did not work out this way in practice, and
implementations frequently violated this guarantee. As I recall the
issue seemed to get fixed around JDK 1.4, but as they were still
working on the whole memory model thing, they didn’t really make any
clear announcements about it until JDK 5, when the new rules were
announced, and memory guarantees actually meant something.

And this is from Java Language Specification,Second Edition:

17.4 Nonatomic Treatment of double and long

The load, store, read, and write actions on volatile variables are atomic,
even if the type of the variable is double or long.