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java.lang.Object EDU.oswego.cs.dl.util.concurrent.Semaphore EDU.oswego.cs.dl.util.concurrent.WaiterPreferenceSemaphore
public final class WaiterPreferenceSemaphore
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. ]
Field Summary | |
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protected long |
waits_
Number of waiting threads |
Fields inherited from class EDU.oswego.cs.dl.util.concurrent.Semaphore |
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permits_ |
Fields inherited from interface EDU.oswego.cs.dl.util.concurrent.Sync |
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ONE_CENTURY, ONE_DAY, ONE_HOUR, ONE_MINUTE, ONE_SECOND, ONE_WEEK, ONE_YEAR |
Constructor Summary | |
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WaiterPreferenceSemaphore(long initial)
Create a Semaphore with the given initial number of permits. |
Method Summary | |
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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 EDU.oswego.cs.dl.util.concurrent.Semaphore |
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permits |
Methods inherited from class java.lang.Object |
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clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Field Detail |
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protected long waits_
Constructor Detail |
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public WaiterPreferenceSemaphore(long initial)
Method Detail |
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public void acquire() throws java.lang.InterruptedException
Semaphore
acquire
in interface Sync
acquire
in class Semaphore
java.lang.InterruptedException
public boolean attempt(long msecs) throws java.lang.InterruptedException
Semaphore
attempt
in interface Sync
attempt
in class Semaphore
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.
java.lang.InterruptedException
public void release()
Semaphore
release
in interface Sync
release
in class Semaphore
public void release(long n)
release
in class Semaphore
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