| /* |
| * Written by Doug Lea with assistance from members of JCP JSR-166 |
| * Expert Group and released to the public domain, as explained at |
| * http://creativecommons.org/publicdomain/zero/1.0/ |
| */ |
| |
| package java.util.concurrent; |
| import java.util.*; |
| import java.util.concurrent.locks.*; |
| |
| /** |
| * 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 {@code Semaphore} just |
| * keeps a count of the number available and acts accordingly. |
| * |
| * <p>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: |
| * <pre> {@code |
| * 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; |
| * } |
| * }}</pre> |
| * |
| * <p>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. |
| * |
| * <p>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 <em>binary |
| * semaphore</em>, 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. |
| * |
| * <p> The constructor for this class optionally accepts a |
| * <em>fairness</em> parameter. When set false, this class makes no |
| * guarantees about the order in which threads acquire permits. In |
| * particular, <em>barging</em> is permitted, that is, a thread |
| * invoking {@link #acquire} can be allocated a permit ahead of a |
| * thread that has been waiting - logically the new thread places itself at |
| * the head of the queue of waiting threads. When fairness is set true, the |
| * semaphore guarantees that threads invoking any of the {@link |
| * #acquire() acquire} methods are selected to obtain 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 |
| * {@code acquire} before another, but reach the ordering point after |
| * the other, and similarly upon return from the method. |
| * Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not |
| * honor the fairness setting, but will take any permits that are |
| * available. |
| * |
| * <p>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. |
| * |
| * <p>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. |
| * |
| * <p>Memory consistency effects: Actions in a thread prior to calling |
| * a "release" method such as {@code release()} |
| * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
| * actions following a successful "acquire" method such as {@code acquire()} |
| * in another thread. |
| * |
| * @since 1.5 |
| * @author Doug Lea |
| * |
| */ |
| |
| public class Semaphore implements java.io.Serializable { |
| private static final long serialVersionUID = -3222578661600680210L; |
| /** All mechanics via AbstractQueuedSynchronizer subclass */ |
| private final Sync sync; |
| |
| /** |
| * Synchronization implementation for semaphore. Uses AQS state |
| * to represent permits. Subclassed into fair and nonfair |
| * versions. |
| */ |
| abstract static class Sync extends AbstractQueuedSynchronizer { |
| private static final long serialVersionUID = 1192457210091910933L; |
| |
| Sync(int permits) { |
| setState(permits); |
| } |
| |
| final int getPermits() { |
| return getState(); |
| } |
| |
| final int nonfairTryAcquireShared(int acquires) { |
| for (;;) { |
| int available = getState(); |
| int remaining = available - acquires; |
| if (remaining < 0 || |
| compareAndSetState(available, remaining)) |
| return remaining; |
| } |
| } |
| |
| protected final boolean tryReleaseShared(int releases) { |
| for (;;) { |
| int current = getState(); |
| int next = current + releases; |
| if (next < current) // overflow |
| throw new Error("Maximum permit count exceeded"); |
| if (compareAndSetState(current, next)) |
| return true; |
| } |
| } |
| |
| final void reducePermits(int reductions) { |
| for (;;) { |
| int current = getState(); |
| int next = current - reductions; |
| if (next > current) // underflow |
| throw new Error("Permit count underflow"); |
| if (compareAndSetState(current, next)) |
| return; |
| } |
| } |
| |
| final int drainPermits() { |
| for (;;) { |
| int current = getState(); |
| if (current == 0 || compareAndSetState(current, 0)) |
| return current; |
| } |
| } |
| } |
| |
| /** |
| * NonFair version |
| */ |
| static final class NonfairSync extends Sync { |
| private static final long serialVersionUID = -2694183684443567898L; |
| |
| NonfairSync(int permits) { |
| super(permits); |
| } |
| |
| protected int tryAcquireShared(int acquires) { |
| return nonfairTryAcquireShared(acquires); |
| } |
| } |
| |
| /** |
| * Fair version |
| */ |
| static final class FairSync extends Sync { |
| private static final long serialVersionUID = 2014338818796000944L; |
| |
| FairSync(int permits) { |
| super(permits); |
| } |
| |
| protected int tryAcquireShared(int acquires) { |
| for (;;) { |
| if (hasQueuedPredecessors()) |
| return -1; |
| int available = getState(); |
| int remaining = available - acquires; |
| if (remaining < 0 || |
| compareAndSetState(available, remaining)) |
| return remaining; |
| } |
| } |
| } |
| |
| /** |
| * 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. |
| */ |
| public Semaphore(int permits) { |
| sync = new NonfairSync(permits); |
| } |
| |
| /** |
| * 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} |
| */ |
| public Semaphore(int permits, boolean fair) { |
| sync = fair ? new FairSync(permits) : new NonfairSync(permits); |
| } |
| |
| /** |
| * Acquires a permit from this semaphore, blocking until one is |
| * available, or the thread is {@linkplain Thread#interrupt interrupted}. |
| * |
| * <p>Acquires a permit, if one is available and returns immediately, |
| * reducing the number of available permits by one. |
| * |
| * <p>If no permit is available then the current thread becomes |
| * disabled for thread scheduling purposes and lies dormant until |
| * one of two things happens: |
| * <ul> |
| * <li>Some other thread invokes the {@link #release} method for this |
| * semaphore and the current thread is next to be assigned a permit; or |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| * the current thread. |
| * </ul> |
| * |
| * <p>If the current thread: |
| * <ul> |
| * <li>has its interrupted status set on entry to this method; or |
| * <li>is {@linkplain Thread#interrupt interrupted} while waiting |
| * for a permit, |
| * </ul> |
| * then {@link InterruptedException} is thrown and the current thread's |
| * interrupted status is cleared. |
| * |
| * @throws InterruptedException if the current thread is interrupted |
| */ |
| public void acquire() throws InterruptedException { |
| sync.acquireSharedInterruptibly(1); |
| } |
| |
| /** |
| * Acquires a permit from this semaphore, blocking until one is |
| * available. |
| * |
| * <p>Acquires a permit, if one is available and returns immediately, |
| * reducing the number of available permits by one. |
| * |
| * <p>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. |
| * |
| * <p>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. |
| */ |
| public void acquireUninterruptibly() { |
| sync.acquireShared(1); |
| } |
| |
| /** |
| * Acquires a permit from this semaphore, only if one is available at the |
| * time of invocation. |
| * |
| * <p>Acquires a permit, if one is available and returns immediately, |
| * with the value {@code true}, |
| * reducing the number of available permits by one. |
| * |
| * <p>If no permit is available then this method will return |
| * immediately with the value {@code false}. |
| * |
| * <p>Even when this semaphore has been set to use a |
| * fair ordering policy, a call to {@code tryAcquire()} <em>will</em> |
| * 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 |
| */ |
| public boolean tryAcquire() { |
| return sync.nonfairTryAcquireShared(1) >= 0; |
| } |
| |
| /** |
| * 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}. |
| * |
| * <p>Acquires a permit, if one is available and returns immediately, |
| * with the value {@code true}, |
| * reducing the number of available permits by one. |
| * |
| * <p>If no permit is available then the current thread becomes |
| * disabled for thread scheduling purposes and lies dormant until |
| * one of three things happens: |
| * <ul> |
| * <li>Some other thread invokes the {@link #release} method for this |
| * semaphore and the current thread is next to be assigned a permit; or |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| * the current thread; or |
| * <li>The specified waiting time elapses. |
| * </ul> |
| * |
| * <p>If a permit is acquired then the value {@code true} is returned. |
| * |
| * <p>If the current thread: |
| * <ul> |
| * <li>has its interrupted status set on entry to this method; or |
| * <li>is {@linkplain Thread#interrupt interrupted} while waiting |
| * to acquire a permit, |
| * </ul> |
| * then {@link InterruptedException} is thrown and the current thread's |
| * interrupted status is cleared. |
| * |
| * <p>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 |
| */ |
| public boolean tryAcquire(long timeout, TimeUnit unit) |
| throws InterruptedException { |
| return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); |
| } |
| |
| /** |
| * Releases a permit, returning it to the semaphore. |
| * |
| * <p>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. |
| * |
| * <p>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() { |
| sync.releaseShared(1); |
| } |
| |
| /** |
| * Acquires the given number of permits from this semaphore, |
| * blocking until all are available, |
| * or the thread is {@linkplain Thread#interrupt interrupted}. |
| * |
| * <p>Acquires the given number of permits, if they are available, |
| * and returns immediately, reducing the number of available permits |
| * by the given amount. |
| * |
| * <p>If insufficient permits are available then the current thread becomes |
| * disabled for thread scheduling purposes and lies dormant until |
| * one of two things happens: |
| * <ul> |
| * <li>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 |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| * the current thread. |
| * </ul> |
| * |
| * <p>If the current thread: |
| * <ul> |
| * <li>has its interrupted status set on entry to this method; or |
| * <li>is {@linkplain Thread#interrupt interrupted} while waiting |
| * for a permit, |
| * </ul> |
| * 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 |
| */ |
| public void acquire(int permits) throws InterruptedException { |
| if (permits < 0) throw new IllegalArgumentException(); |
| sync.acquireSharedInterruptibly(permits); |
| } |
| |
| /** |
| * Acquires the given number of permits from this semaphore, |
| * blocking until all are available. |
| * |
| * <p>Acquires the given number of permits, if they are available, |
| * and returns immediately, reducing the number of available permits |
| * by the given amount. |
| * |
| * <p>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. |
| * |
| * <p>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 |
| * |
| */ |
| public void acquireUninterruptibly(int permits) { |
| if (permits < 0) throw new IllegalArgumentException(); |
| sync.acquireShared(permits); |
| } |
| |
| /** |
| * Acquires the given number of permits from this semaphore, only |
| * if all are available at the time of invocation. |
| * |
| * <p>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. |
| * |
| * <p>If insufficient permits are available then this method will return |
| * immediately with the value {@code false} and the number of available |
| * permits is unchanged. |
| * |
| * <p>Even when this semaphore has been set to use a fair ordering |
| * policy, a call to {@code tryAcquire} <em>will</em> |
| * 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 |
| */ |
| public boolean tryAcquire(int permits) { |
| if (permits < 0) throw new IllegalArgumentException(); |
| return sync.nonfairTryAcquireShared(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 {@linkplain Thread#interrupt interrupted}. |
| * |
| * <p>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. |
| * |
| * <p>If insufficient permits are available then |
| * the current thread becomes disabled for thread scheduling |
| * purposes and lies dormant until one of three things happens: |
| * <ul> |
| * <li>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 |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| * the current thread; or |
| * <li>The specified waiting time elapses. |
| * </ul> |
| * |
| * <p>If the permits are acquired then the value {@code true} is returned. |
| * |
| * <p>If the current thread: |
| * <ul> |
| * <li>has its interrupted status set on entry to this method; or |
| * <li>is {@linkplain Thread#interrupt interrupted} while waiting |
| * to acquire the permits, |
| * </ul> |
| * 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()}. |
| * |
| * <p>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 |
| */ |
| public boolean tryAcquire(int permits, long timeout, TimeUnit unit) |
| throws InterruptedException { |
| if (permits < 0) throw new IllegalArgumentException(); |
| return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout)); |
| } |
| |
| /** |
| * Releases the given number of permits, returning them to the semaphore. |
| * |
| * <p>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. |
| * |
| * <p>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 |
| */ |
| public void release(int permits) { |
| if (permits < 0) throw new IllegalArgumentException(); |
| sync.releaseShared(permits); |
| } |
| |
| /** |
| * Returns the current number of permits available in this semaphore. |
| * |
| * <p>This method is typically used for debugging and testing purposes. |
| * |
| * @return the number of permits available in this semaphore |
| */ |
| public int availablePermits() { |
| return sync.getPermits(); |
| } |
| |
| /** |
| * Acquires and returns all permits that are immediately available. |
| * |
| * @return the number of permits acquired |
| */ |
| public int drainPermits() { |
| return sync.drainPermits(); |
| } |
| |
| /** |
| * 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 |
| */ |
| protected void reducePermits(int reduction) { |
| if (reduction < 0) throw new IllegalArgumentException(); |
| sync.reducePermits(reduction); |
| } |
| |
| /** |
| * Returns {@code true} if this semaphore has fairness set true. |
| * |
| * @return {@code true} if this semaphore has fairness set true |
| */ |
| public boolean isFair() { |
| return sync instanceof FairSync; |
| } |
| |
| /** |
| * 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 |
| */ |
| public final boolean hasQueuedThreads() { |
| return sync.hasQueuedThreads(); |
| } |
| |
| /** |
| * 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 |
| */ |
| public final int getQueueLength() { |
| return sync.getQueueLength(); |
| } |
| |
| /** |
| * 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 |
| */ |
| protected Collection<Thread> getQueuedThreads() { |
| return sync.getQueuedThreads(); |
| } |
| |
| /** |
| * 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 |
| */ |
| public String toString() { |
| return super.toString() + "[Permits = " + sync.getPermits() + "]"; |
| } |
| } |
