Long primitive or AtomicLong for a counter?
I have a need for a counter of type long with the following requirements/facts:
- Incrementing the counter should take as little time as possible.
- The counter will only be written to by one thread.
- Reading from the counter will be done in another thread.
- The counter will be incremented regularly (as much as a few thousand times per second), but will only be read once every five seconds.
- Precise accuracy isn't essential, only a rough idea of the size of the counter is good enough.
- The counter is never cleared, decremented.
Based upon these requirements, how would you choose to implement your counter? As a simple long, as a volatile long or using an AtomicLong? Why?
At the moment I have a volatile long but was wondering whether another approach would be better. I am also incrementing my long by doing ++counter as opposed to counter++. Is this really any more efficient (as I have been led to believe elsewhere) because there is no assignment being done?
Answer
Given these sets of requirements, I think that a volatile long should be sufficient. The counter wouldn't be incorrect with a non-volatile long, but the reader might be reading stale information in that case.
One problem is that reads and writes to a long are not required to be atomic, by the JVM specification if it is not declared volatile. That would mean that the reading thread could get a pretty much fictive value if it reads the value while the writing thread has updated one part of the value, but not the other one.
The difference between ++counter and counter++ is probably irrelevant, as the JVM will realize that the value of the expression is not used any more and the two are equivalent in this case.