WaiterPreferenceSemaphore

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org.metasyntactic.thread.concurrent
Class WaiterPreferenceSemaphore

java.lang.Object
  |
  +--org.metasyntactic.thread.concurrent.Semaphore
        |
        +--org.metasyntactic.thread.concurrent.WaiterPreferenceSemaphore

All Implemented Interfaces:: Monitor

public final class WaiterPreferenceSemaphore
extends Semaphore

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.

Field Summary

Fields inherited from class org.metasyntactic.thread.concurrent.Semaphore

permits

Fields inherited from interface org.metasyntactic.thread.concurrent.Monitor

ONE_CENTURY, ONE_DAY, ONE_HOUR, ONE_MINUTE, ONE_SECOND, ONE_WEEK, ONE_YEAR

Constructor Summary

WaiterPreferenceSemaphore(long initial)
          Create a Semaphore with the given initial number of permits.

Method Summary

void acquire()
          Wait until a permit is available, and take one

boolean attempt(long msecs)
          Wait at most msecs millisconds for a permit.

void release()
          Release a permit

void release(long n)
          Release N permits

Methods inherited from class org.metasyntactic.thread.concurrent.Semaphore

permits

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail