Question

Guava is very useful.

Now, I want to implement a limit for access counter in a period.(e.g. max 3 times in one hours people can do it)

Use Guava’s mapMarker or cache is very easy….(like using memcached), but I meet some concurrent problem when increase counter in map.

Could Guava support the incr operation like in memcache? when map’s value is integer? or make a new util class for that…

Answer 1

You could store a Semaphore (link) instead of a normal integer. Look at method tryAcquire(). For periodically resetting the number of permits, you can use a combination of drain() and release(). If draining the permit for a brief period is problematic, you could also use AtomicInteger and do something like this:

1) Peek the current value with get()
2) If there is still quota available, attempt to acquire one quota by performing compareAndSet(oldValue, oldValue + 1).
3) If the value is successfully updated (i.e. true is returned), the thread is allowed to proceed. If not, the thread must retry from (1).
4) To reset the number of permits, use set()