org.metasyntactic.thread.concurrent
Class Semaphore

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

All Implemented Interfaces:

Monitor

Direct Known Subclasses:

QueuedSemaphore, WaiterPreferenceSemaphore

public class Semaphore

extends java.lang.Object

implements Monitor

Base class for counting semaphores. Conceptually, a semaphore maintains a set of permits. Each acquire() blocks if necessary until a permit is available, and then takes it. Each release adds a permit. However, no actual permit objects are used; the Semaphore just keeps a count of the number available and acts accordingly.

A semaphore initialized to 1 can serve as a mutual exclusion lock.

Different implementation subclasses may provide different ordering guarantees (or lack thereof) surrounding which threads will be resumed upon a signal.

The default implementation makes NO guarantees about the order in which threads will acquire permits. It is often faster than other implementations.

Field Summary

protected long

permits current number of available 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

Semaphore(long permits)
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.
long	permits() Return the current number of available permits.
void	release() Release a permit
void	release(long n) Release N permits.

Methods inherited from class java.lang.Object

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

Field Detail

permits

protected long permits

current number of available permits

Constructor Detail

Semaphore

public Semaphore(long permits)

Create a Semaphore with the given initial number of permits. Using a seed of one makes the semaphore act as a mutual exclusion lock. Negative seeds are also allowed, in which case no acquires will proceed until the number of releases has pushed the number of permits past 0.

Parameters:

permits - The number of permits to grant. Note! This number can be negative, in which case no permits will be released intil the number of releases pushes this above zero

Method Detail

acquire

public void acquire()
             throws java.lang.InterruptedException

Wait until a permit is available, and take one

Specified by:

acquire in interface Monitor

java.lang.InterruptedException

attempt

public boolean attempt(long msecs)
                throws java.lang.InterruptedException

Wait at most msecs millisconds for a permit.

Specified by:

attempt in interface Monitor

Parameters:

msecs - The number of milliseconds to wait for a permit

Returns:

True/False on whether or not the permit was granted

Throws:

java.lang.InterruptedException

release

public void release()

Release a permit

Specified by:

release in interface Monitor

release

public void release(long n)

Release N permits. release(n) is equivalent in effect to:

  for (int i = 0; i < n; ++i) {
	release();
  }
 

Parameters:

n - The number of permits to release

Throws:

java.lang.IllegalArgumentException - if n is negative.

permits

public long permits()

Return the current number of available permits. Returns an accurate, but possibly unstable value, that may change immediately after returning.

Returns:

The number of permits available.