EDU.oswego.cs.dl.util.concurrent

Class WaiterPreferenceSemaphore

java.lang.Object

EDU.oswego.cs.dl.util.concurrent.Semaphore

EDU.oswego.cs.dl.util.concurrent.WaiterPreferenceSemaphore

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.

[ Introduction to this package. ]

Field Summary

Fields

Modifier and Type	Field and Description
protected long	waits_ Number of waiting threads

Fields inherited from class EDU.oswego.cs.dl.util.concurrent.Semaphore

permits_

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

Constructor Summary

Constructors

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

Method Summary

Modifier and Type	Method and Description
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

permits

Methods inherited from class java.lang.Object

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

Field Detail

waits_

protected long waits_

Number of waiting threads

Constructor Detail

WaiterPreferenceSemaphore

public WaiterPreferenceSemaphore(long initial)

Create a Semaphore with the given initial number of permits.

Method Detail

acquire

public void acquire()
             throws java.lang.InterruptedException

Description copied from class: Semaphore

Wait until a permit is available, and take one

Specified by:

acquire in interface Sync

Overrides:

acquire in class Semaphore

Throws:

java.lang.InterruptedException

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 interface Sync

Overrides:

attempt in class Semaphore

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

release

public void release()

Description copied from class: Semaphore

Release a permit

Specified by:

release in interface Sync

Overrides:

release in class Semaphore

release

public void release(long n)

Release N permits

Overrides:

release in class Semaphore