Need synchronization for an increment-only counter?

java synchronization counter increment
qinsoon picture qinsoon · Oct 4, 2011 · Viewed 19.2k times · Source

I use an integer as counter. The integer will only be increased, and surely more than one thread will increase it at the same time. The value of this counter is read at the end of program execution when no other thread will try to access its value.

I assume that I don't have to use a lock or any kind of synchronization for this kind of increment-only counter. Is this right? I code in Java if that makes any difference.

Answer

Jon Skeet picture Jon Skeet · Oct 4, 2011

If you just used an int or long variable then you would need synchronization - incrementing involves read / increment-locally / write, which is far from an atomic operation. (Even if the variable is volatile to avoid memory model concerns of staleness, you'd still have three distinct operations, with the possibility of being pre-empted between any pair of them.)

Fortunately Java provides AtomicInteger and AtomicLong which can be used without any synchronization:

private final AtomicLong counter = new AtomicLong();

...

counter.incrementAndGet(); // No need for synchronization

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