/* * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain. Use, modify, and * redistribute this code in any way without acknowledgement. */ package java.util.concurrent; import java.util.*; import java.util.concurrent.locks.*; import java.util.concurrent.atomic.*; /** * A counting semaphore. Conceptually, a semaphore maintains a set of * permits. Each {@link #acquire} blocks if necessary until a permit is * available, and then takes it. Each {@link #release} adds a permit, * potentially releasing a blocking acquirer. * However, no actual permit objects are used; the Semaphore just * keeps a count of the number available and acts accordingly. * *
Semaphores are often used to restrict the number of threads than can * access some (physical or logical) resource. For example, here is * a class that uses a semaphore to control access to a pool of items: *
* class Pool { * private static final 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; * } * * } ** *
Before obtaining an item each thread must acquire a permit from * the semaphore, guaranteeing that an item is available for use. When * the thread has finished with the item it is returned back to the * pool and a permit is returned to the semaphore, allowing another * thread to acquire that item. Note that no synchronization lock is * held when {@link #acquire} is called as that would prevent an item * from being returned to the pool. The semaphore encapsulates the * synchronization needed to restrict access to the pool, separately * from any synchronization needed to maintain the consistency of the * pool itself. * *
A semaphore initialized to one, and which is used such that it * only has at most one permit available, can serve as a mutual * exclusion lock. This is more commonly known as a binary * semaphore, because it only has two states: one permit * available, or zero permits available. When used in this way, the * binary semaphore has the property (unlike many {@link Lock} * implementations), that the "lock" can be released by a * thread other than the owner (as semaphores have no notion of * ownership). This can be useful in some specialized contexts, such * as deadlock recovery. * *
The constructor for this class accepts a fairness * parameter. When set false, this class makes no guarantees about the * order in which threads acquire permits. In particular, barging is * permitted, that is, a thread invoking {@link #acquire} can be * allocated a permit ahead of a thread that has been waiting. When * fairness is set true, the semaphore guarantees that threads * invoking any of the {@link #acquire() acquire} methods are * allocated permits in the order in which their invocation of those * methods was processed (first-in-first-out; FIFO). Note that FIFO * ordering necessarily applies to specific internal points of * execution within these methods. So, it is possible for one thread * to invoke acquire before another, but reach the ordering * point after the other, and similarly upon return from the method. * *
Generally, semaphores used to control resource access should be * initialized as fair, to ensure that no thread is starved out from * accessing a resource. When using semaphores for other kinds of * synchronization control, the throughput advantages of non-fair * ordering often outweigh fairness considerations. * *
This class also provides convenience methods to {@link
* #acquire(int) acquire} and {@link #release(int) release} multiple
* permits at a time. Beware of the increased risk of indefinite
* postponement when these methods are used without fairness set true.
*
* @since 1.5
* @author Doug Lea
*
*/
public class Semaphore implements java.io.Serializable {
/*
* The underlying algorithm here is a simplified adaptation of
* that used for ReentrantLock. See the internal documentation of
* ReentrantLock for detailed explanation.
*/
private static final long serialVersionUID = -3222578661600680210L;
/** Node status value to indicate thread has cancelled */
private static final int CANCELLED = 1;
/** Node status value to indicate successor needs unparking */
private static final int SIGNAL = -1;
/** Node class for waiting threads */
private static class Node {
volatile int status;
volatile Node prev;
volatile Node next;
Thread thread;
Node(Thread t) { thread = t; }
}
/** Number of available permits held in a separate AtomicInteger */
private final AtomicInteger perms;
/** Head of the wait queue, lazily initialized. */
private transient volatile Node head;
/** Tail of the wait queue, lazily initialized. */
private transient volatile Node tail;
/** true if barging disabled */
private final boolean fair;
// Atomic update support
private static final
AtomicReferenceFieldUpdater 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:
* If the current thread:
* 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 {@link 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.
*
*/
public void acquireUninterruptibly() {
doAcquire(1, 0);
}
/**
* 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 true,
* reducing the number of available permits by one.
*
* If no permit is available then this method will return
* immediately with the value false.
*
* @return true if a permit was acquired and false
* otherwise.
*/
public boolean tryAcquire() {
return doTryAcquire(1) >= 0;
}
/**
* Acquires a permit from this semaphore, if one becomes available
* within the given waiting time and the
* current thread has not been {@link Thread#interrupt interrupted}.
* Acquires a permit, if one is available and returns immediately,
* with the value 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:
* If a permit is acquired then the value true is returned.
* If the current thread:
* If the specified waiting time elapses then the value 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 timeout argument.
* @return true if a permit was acquired and false
* if the waiting time elapsed before a permit was acquired.
*
* @throws InterruptedException if the current thread is interrupted
*
* @see Thread#interrupt
*
*/
public boolean tryAcquire(long timeout, TimeUnit unit)
throws InterruptedException {
if (unit == null)
throw new NullPointerException();
if (Thread.interrupted())
throw new InterruptedException();
return doTimedAcquire(1, unit.toNanos(timeout));
}
/**
* Releases a permit, returning it to the semaphore.
* Releases a permit, increasing the number of available permits
* by one.
* If any threads are blocking 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.
*/
public void release() {
doRelease(1);
}
/**
* Acquires the given number of permits from this semaphore,
* blocking until all are available,
* or the thread is {@link 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:
* If the current thread:
* 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 {@link 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 permits less than zero.
*
*/
public void acquireUninterruptibly(int permits) {
if (permits < 0) throw new IllegalArgumentException();
doAcquire(permits, 0);
}
/**
* 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 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 false and the number of available
* permits is unchanged.
*
* @param permits the number of permits to acquire
*
* @return true if the permits were acquired and false
* otherwise.
* @throws IllegalArgumentException if permits less than zero.
*/
public boolean tryAcquire(int permits) {
if (permits < 0) throw new IllegalArgumentException();
return doTryAcquire(permits) >= 0;
}
/**
* 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 {@link Thread#interrupt interrupted}.
* Acquires the given number of permits, if they are available and
* returns immediately, with the value 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:
* If the permits are acquired then the value true is returned.
* If the current thread:
* If the specified waiting time elapses then the value 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 the next waiting thread(s), as if
* they 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 timeout argument.
* @return true if all permits were acquired and false
* if the waiting time elapsed before all permits were acquired.
*
* @throws InterruptedException if the current thread is interrupted
* @throws IllegalArgumentException if permits less than zero.
*
* @see Thread#interrupt
*
*/
public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
if (unit == null)
throw new NullPointerException();
if (Thread.interrupted())
throw new InterruptedException();
return doTimedAcquire(permits, unit.toNanos(timeout));
}
/**
* 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 blocking trying to acquire permits, then the
* one that has been waiting the longest
* 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 continues to wait. If there are still permits available
* after the first thread's request has been satisfied, then those permits
* are assigned to the next waiting thread. If it is satisfied then it is
* re-enabled for thread scheduling purposes. This continues until there
* are insufficient permits to satisfy the next waiting thread, or there
* are no more waiting threads.
*
* 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 permits less than zero.
*/
public void release(int permits) {
if (permits < 0) throw new IllegalArgumentException();
doRelease(permits);
}
/**
* Return 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.
*/
public int availablePermits() {
return perms.get();
}
/**
* Shrink the number of available permits by the indicated
* reduction. This method can be useful in subclasses that
* use semaphores to track available resources that become
* unavailable. This method differs from acquire
* in that it does not block waiting for permits to become
* available.
* @param reduction the number of permits to remove
* @throws IllegalArgumentException if reduction is negative
*/
protected void reducePermits(int reduction) {
if (reduction < 0) throw new IllegalArgumentException();
perms.getAndAdd(-reduction);
}
/**
* Return true if this semaphore has fairness set true.
* @return true if this semaphore has fairness set true.
*/
public boolean isFair() {
return fair;
}
/**
* Returns an estimate of the number of threads waiting to acquire
* a permit. 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 a permit
*/
public int getQueueLength() {
int n = 0;
for (Node p = tail; p != null && p != head; p = p.prev)
++n;
return n;
}
/**
* Returns a collection containing threads that may be waiting to
* acquire a permit. 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
*/
protected Collection
*
*
*
*
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* @throws InterruptedException if the current thread is interrupted
*
* @see Thread#interrupt
*/
public void acquire() throws InterruptedException {
if (Thread.interrupted() || !doAcquire(1, -1))
throw new InterruptedException();
}
/**
* Acquires a permit from this semaphore, blocking until one is
* available.
*
*
*
*
*
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
*
*
*
*
* 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 the next waiting thread(s), as if
* they 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 permits less than zero.
*
* @see Thread#interrupt
*/
public void acquire(int permits) throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
if (Thread.interrupted() || !doAcquire(permits, -1))
throw new InterruptedException();
}
/**
* Acquires the given number of permits from this semaphore,
* blocking until all are available.
*
*
*
*
*
* 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 the next waiting thread(s), as if
* they had been made available by a call to {@link #release()}.
*
*