File	Doc	Category	Size	Date	Package
Semaphore.java	API Doc	Java SE 6 API	27788	Tue Jun 10 00:25:56 BST 2008	java.util.concurrent

Semaphore

• java.lang.Object

class Pool {
private static final int MAX_AVAILABLE = 100;
private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);

public Object getItem() throws InterruptedException {
available.acquire();
return getNextAvailableItem();
}

public void putItem(Object x) {
if (markAsUnused(x))
available.release();
}

// Not a particularly efficient data structure; just for demo

protected Object[] items = ... whatever kinds of items being managed
protected boolean[] used = new boolean[MAX_AVAILABLE];

protected synchronized Object getNextAvailableItem() {
for (int i = 0; i < MAX_AVAILABLE; ++i) {
if (!used[i]) {
used[i] = true;
return items[i];
}
}
return null; // not reached
}

protected synchronized boolean markAsUnused(Object item) {
for (int i = 0; i < MAX_AVAILABLE; ++i) {
if (item == items[i]) {
if (used[i]) {
used[i] = false;
return true;
} else
return false;
}
}
return false;
}

}

Fields Summary
private static final long	serialVersionUID
private final Sync	sync All mechanics via AbstractQueuedSynchronizer subclass

Constructors Summary
public Semaphore(int permits) Creates a {@code Semaphore} with the given number of permits and nonfair fairness setting. param permits the initial number of permits available. This value may be negative, in which case releases must occur before any acquires will be granted. sync = new NonfairSync(permits);
public Semaphore(int permits, boolean fair) Creates a {@code Semaphore} with the given number of permits and the given fairness setting. param permits the initial number of permits available. This value may be negative, in which case releases must occur before any acquires will be granted. param fair {@code true} if this semaphore will guarantee first-in first-out granting of permits under contention, else {@code false} sync = (fair)? new FairSync(permits) : new NonfairSync(permits);

Methods Summary
public void	acquire() Acquires a permit from this semaphore, blocking until one is available, or the thread is {@linkplain Thread#interrupt interrupted}. Acquires a permit, if one is available and returns immediately, reducing the number of available permits by one. If no permit is available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happens: Some other thread invokes the {@link #release} method for this semaphore and the current thread is next to be assigned a permit; or Some other thread {@linkplain Thread#interrupt interrupts} the current thread. If the current thread: has its interrupted status set on entry to this method; or is {@linkplain Thread#interrupt interrupted} while waiting for a permit, then {@link InterruptedException} is thrown and the current thread's interrupted status is cleared. throws InterruptedException if the current thread is interrupted sync.acquireSharedInterruptibly(1);
public void	acquire(int permits) Acquires the given number of permits from this semaphore, blocking until all are available, or the thread is {@linkplain Thread#interrupt interrupted}. Acquires the given number of permits, if they are available, and returns immediately, reducing the number of available permits by the given amount. If insufficient permits are available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happens: Some other thread invokes one of the {@link #release() release} methods for this semaphore, the current thread is next to be assigned permits and the number of available permits satisfies this request; or Some other thread {@linkplain Thread#interrupt interrupts} the current thread. If the current thread: has its interrupted status set on entry to this method; or is {@linkplain Thread#interrupt interrupted} while waiting for a permit, then {@link InterruptedException} is thrown and the current thread's interrupted status is cleared. Any permits that were to be assigned to this thread are instead assigned to other threads trying to acquire permits, as if permits had been made available by a call to {@link #release()}. param permits the number of permits to acquire throws InterruptedException if the current thread is interrupted throws IllegalArgumentException if {@code permits} is negative if (permits < 0) throw new IllegalArgumentException(); sync.acquireSharedInterruptibly(permits);
public void	acquireUninterruptibly() Acquires a permit from this semaphore, blocking until one is available. Acquires a permit, if one is available and returns immediately, reducing the number of available permits by one. If no permit is available then the current thread becomes disabled for thread scheduling purposes and lies dormant until some other thread invokes the {@link #release} method for this semaphore and the current thread is next to be assigned a permit. If the current thread is {@linkplain Thread#interrupt interrupted} while waiting for a permit then it will continue to wait, but the time at which the thread is assigned a permit may change compared to the time it would have received the permit had no interruption occurred. When the thread does return from this method its interrupt status will be set. sync.acquireShared(1);
public void	acquireUninterruptibly(int permits) Acquires the given number of permits from this semaphore, blocking until all are available. Acquires the given number of permits, if they are available, and returns immediately, reducing the number of available permits by the given amount. If insufficient permits are available then the current thread becomes disabled for thread scheduling purposes and lies dormant until some other thread invokes one of the {@link #release() release} methods for this semaphore, the current thread is next to be assigned permits and the number of available permits satisfies this request. If the current thread is {@linkplain Thread#interrupt interrupted} while waiting for permits then it will continue to wait and its position in the queue is not affected. When the thread does return from this method its interrupt status will be set. param permits the number of permits to acquire throws IllegalArgumentException if {@code permits} is negative if (permits < 0) throw new IllegalArgumentException(); sync.acquireShared(permits);
public int	availablePermits() Returns the current number of permits available in this semaphore. This method is typically used for debugging and testing purposes. return the number of permits available in this semaphore return sync.getPermits();
public int	drainPermits() Acquires and returns all permits that are immediately available. return the number of permits acquired return sync.drainPermits();
public final int	getQueueLength() Returns an estimate of the number of threads waiting to acquire. The value is only an estimate because the number of threads may change dynamically while this method traverses internal data structures. This method is designed for use in monitoring of the system state, not for synchronization control. return the estimated number of threads waiting for this lock return sync.getQueueLength();
protected java.util.Collection	getQueuedThreads() Returns a collection containing threads that may be waiting to acquire. Because the actual set of threads may change dynamically while constructing this result, the returned collection is only a best-effort estimate. The elements of the returned collection are in no particular order. This method is designed to facilitate construction of subclasses that provide more extensive monitoring facilities. return the collection of threads return sync.getQueuedThreads();
public final boolean	hasQueuedThreads() Queries whether any threads are waiting to acquire. Note that because cancellations may occur at any time, a {@code true} return does not guarantee that any other thread will ever acquire. This method is designed primarily for use in monitoring of the system state. return {@code true} if there may be other threads waiting to acquire the lock return sync.hasQueuedThreads();
public boolean	isFair() Returns {@code true} if this semaphore has fairness set true. return {@code true} if this semaphore has fairness set true return sync instanceof FairSync;
protected void	reducePermits(int reduction) Shrinks the number of available permits by the indicated reduction. This method can be useful in subclasses that use semaphores to track resources that become unavailable. This method differs from {@code acquire} in that it does not block waiting for permits to become available. param reduction the number of permits to remove throws IllegalArgumentException if {@code reduction} is negative if (reduction < 0) throw new IllegalArgumentException(); sync.reducePermits(reduction);
public void	release(int permits) Releases the given number of permits, returning them to the semaphore. Releases the given number of permits, increasing the number of available permits by that amount. If any threads are trying to acquire permits, then one is selected and given the permits that were just released. If the number of available permits satisfies that thread's request then that thread is (re)enabled for thread scheduling purposes; otherwise the thread will wait until sufficient permits are available. If there are still permits available after this thread's request has been satisfied, then those permits are assigned in turn to other threads trying to acquire permits. There is no requirement that a thread that releases a permit must have acquired that permit by calling {@link Semaphore#acquire acquire}. Correct usage of a semaphore is established by programming convention in the application. param permits the number of permits to release throws IllegalArgumentException if {@code permits} is negative if (permits < 0) throw new IllegalArgumentException(); sync.releaseShared(permits);
public void	release() Releases a permit, returning it to the semaphore. Releases a permit, increasing the number of available permits by one. If any threads are trying to acquire a permit, then one is selected and given the permit that was just released. That thread is (re)enabled for thread scheduling purposes. There is no requirement that a thread that releases a permit must have acquired that permit by calling {@link #acquire}. Correct usage of a semaphore is established by programming convention in the application. sync.releaseShared(1);
public java.lang.String	toString() Returns a string identifying this semaphore, as well as its state. The state, in brackets, includes the String {@code "Permits ="} followed by the number of permits. return a string identifying this semaphore, as well as its state return super.toString() + "[Permits = " + sync.getPermits() + "]";
public boolean	tryAcquire(int permits) Acquires the given number of permits from this semaphore, only if all are available at the time of invocation. Acquires the given number of permits, if they are available, and returns immediately, with the value {@code true}, reducing the number of available permits by the given amount. If insufficient permits are available then this method will return immediately with the value {@code false} and the number of available permits is unchanged. Even when this semaphore has been set to use a fair ordering policy, a call to {@code tryAcquire} will immediately acquire a permit if one is available, whether or not other threads are currently waiting. This "barging" behavior can be useful in certain circumstances, even though it breaks fairness. If you want to honor the fairness setting, then use {@link #tryAcquire(int, long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) } which is almost equivalent (it also detects interruption). param permits the number of permits to acquire return {@code true} if the permits were acquired and {@code false} otherwise throws IllegalArgumentException if {@code permits} is negative if (permits < 0) throw new IllegalArgumentException(); return sync.nonfairTryAcquireShared(permits) >= 0;
public boolean	tryAcquire(int permits, long timeout, java.util.concurrent.TimeUnit unit) Acquires the given number of permits from this semaphore, if all become available within the given waiting time and the current thread has not been {@linkplain Thread#interrupt interrupted}. Acquires the given number of permits, if they are available and returns immediately, with the value {@code true}, reducing the number of available permits by the given amount. If insufficient permits are available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens: Some other thread invokes one of the {@link #release() release} methods for this semaphore, the current thread is next to be assigned permits and the number of available permits satisfies this request; or Some other thread {@linkplain Thread#interrupt interrupts} the current thread; or The specified waiting time elapses. If the permits are acquired then the value {@code true} is returned. If the current thread: has its interrupted status set on entry to this method; or is {@linkplain Thread#interrupt interrupted} while waiting to acquire the permits, then {@link InterruptedException} is thrown and the current thread's interrupted status is cleared. Any permits that were to be assigned to this thread, are instead assigned to other threads trying to acquire permits, as if the permits had been made available by a call to {@link #release()}. If the specified waiting time elapses then the value {@code false} is returned. If the time is less than or equal to zero, the method will not wait at all. Any permits that were to be assigned to this thread, are instead assigned to other threads trying to acquire permits, as if the permits had been made available by a call to {@link #release()}. param permits the number of permits to acquire param timeout the maximum time to wait for the permits param unit the time unit of the {@code timeout} argument return {@code true} if all permits were acquired and {@code false} if the waiting time elapsed before all permits were acquired throws InterruptedException if the current thread is interrupted throws IllegalArgumentException if {@code permits} is negative if (permits < 0) throw new IllegalArgumentException(); return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
public boolean	tryAcquire() Acquires a permit from this semaphore, only if one is available at the time of invocation. Acquires a permit, if one is available and returns immediately, with the value {@code true}, reducing the number of available permits by one. If no permit is available then this method will return immediately with the value {@code false}. Even when this semaphore has been set to use a fair ordering policy, a call to {@code tryAcquire()} will immediately acquire a permit if one is available, whether or not other threads are currently waiting. This "barging" behavior can be useful in certain circumstances, even though it breaks fairness. If you want to honor the fairness setting, then use {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) } which is almost equivalent (it also detects interruption). return {@code true} if a permit was acquired and {@code false} otherwise return sync.nonfairTryAcquireShared(1) >= 0;
public boolean	tryAcquire(long timeout, java.util.concurrent.TimeUnit unit) Acquires a permit from this semaphore, if one becomes available within the given waiting time and the current thread has not been {@linkplain Thread#interrupt interrupted}. Acquires a permit, if one is available and returns immediately, with the value {@code true}, reducing the number of available permits by one. If no permit is available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens: Some other thread invokes the {@link #release} method for this semaphore and the current thread is next to be assigned a permit; or Some other thread {@linkplain Thread#interrupt interrupts} the current thread; or The specified waiting time elapses. If a permit is acquired then the value {@code true} is returned. If the current thread: has its interrupted status set on entry to this method; or is {@linkplain Thread#interrupt interrupted} while waiting to acquire a permit, then {@link InterruptedException} is thrown and the current thread's interrupted status is cleared. If the specified waiting time elapses then the value {@code false} is returned. If the time is less than or equal to zero, the method will not wait at all. param timeout the maximum time to wait for a permit param unit the time unit of the {@code timeout} argument return {@code true} if a permit was acquired and {@code false} if the waiting time elapsed before a permit was acquired throws InterruptedException if the current thread is interrupted return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));