Package EDU.oswego.cs.dl.util.concurrent
Class WaiterPreferenceSemaphore
- java.lang.Object
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- EDU.oswego.cs.dl.util.concurrent.Semaphore
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- EDU.oswego.cs.dl.util.concurrent.WaiterPreferenceSemaphore
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- All Implemented Interfaces:
Sync
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.
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Field Summary
Fields Modifier and Type Field Description protected long
waits_
Number of waiting threads-
Fields inherited from interface EDU.oswego.cs.dl.util.concurrent.Sync
ONE_CENTURY, ONE_DAY, ONE_HOUR, ONE_MINUTE, ONE_SECOND, ONE_WEEK, ONE_YEAR
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Constructor Summary
Constructors Constructor Description WaiterPreferenceSemaphore(long initial)
Create a Semaphore with the given initial number of permits.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description void
acquire()
Wait until a permit is available, and take oneboolean
attempt(long msecs)
Wait at most msecs millisconds for a permit.void
release()
Release a permitvoid
release(long n)
Release N permits
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Method Detail
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acquire
public void acquire() throws java.lang.InterruptedException
Description copied from class:Semaphore
Wait until a permit is available, and take one
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attempt
public boolean attempt(long msecs) throws java.lang.InterruptedException
Description copied from class:Semaphore
Wait at most msecs millisconds for a permit.- Specified by:
attempt
in interfaceSync
- Overrides:
attempt
in classSemaphore
- Parameters:
msecs
- the number of milleseconds to wait. An argument less than or equal to zero means not to wait at all. However, this may still require access to a synchronization lock, which can impose unbounded delay if there is a lot of contention among threads.- Returns:
- true if acquired
- Throws:
java.lang.InterruptedException
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release
public void release()
Description copied from class:Semaphore
Release a permit
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