| An implementation of counting Semaphores that
enforces enough fairness for applications that
need to avoid indefinite overtaking without
necessarily requiring FIFO ordered access.
Empirically, very little is paid for this property
unless there is a lot of contention among threads
or very unfair JVM scheduling.
The acquire method waits even if there are permits
available but have not yet been claimed by threads that have
been notified but not yet resumed. This makes the semaphore
almost as fair as the underlying Java primitives allow.
So, if synch lock entry and notify are both fair
so is the semaphore -- almost: Rewaits stemming
from timeouts in attempt, along with potentials for
interrupted threads to be notified can compromise fairness,
possibly allowing later-arriving threads to pass before
later arriving ones. However, in no case can a newly
entering thread obtain a permit if there are still others waiting.
Also, signalling order need not coincide with
resumption order. Later-arriving threads might get permits
and continue before other resumable threads are actually resumed.
However, all of these potential fairness breaches are
very rare in practice unless the underlying JVM
performs strictly LIFO notifications (which has, sadly enough,
been known to occur) in which case you need to use
a FIFOSemaphore to maintain a reasonable approximation
of fairness.
[ Introduction to this package. ]
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