Is Increment and Decrement Operators Atomic?

Question

Is there a need to synchronize increment and decrement operations, or they are atomic? If so, how about pre and post variations?

Answer 1

raychen wrote:
jverd wrote:
Simple rule: If multiple threads are accessing a variable, all access to that variable must be synchronized. There are exceptions, but you'll seldom go wrong by following that rule.I don't synchronize data member accesses in simple setters and getters, as JVM guarantees my read and write are atomic at the memory level. Am I making bad assumptions?Yup, you are. For a couple of reasons.
1. Reading and writing of non-volatile longs and doubles is not guaranteed atomic.
2. It also does NOT guarantee that one thread will ever even see a value written by another thread, however, unless either a) the variable is volatile, in which case threads don't have local copies all reads and writes go against the main copy, or b) access is synchronized and therefore the writing thread's local copy is flushed to main memory upon exiting exiting the sync block and the reading thread gets its first read after entering the sync block from the main copy.
Edited by: jverd on Jul 21, 2009 10:33 AM