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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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package java.util.concurrent; |
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import java.util.Collection; |
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import java.util.concurrent.locks.AbstractQueuedSynchronizer; |
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/** |
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* A counting semaphore. Conceptually, a semaphore maintains a set of |
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* permits. Each {@link #acquire} blocks if necessary until a permit is |
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* available, and then takes it. Each {@link #release} adds a permit, |
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* potentially releasing a blocking acquirer. |
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* However, no actual permit objects are used; the {@code Semaphore} just |
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* keeps a count of the number available and acts accordingly. |
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* |
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* <p>Semaphores are often used to restrict the number of threads than can |
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* access some (physical or logical) resource. For example, here is |
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* a class that uses a semaphore to control access to a pool of items: |
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* <pre> {@code |
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* class Pool { |
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* private static final int MAX_AVAILABLE = 100; |
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* private final Semaphore available = new Semaphore(MAX_AVAILABLE, true); |
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* |
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* public Object getItem() throws InterruptedException { |
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* available.acquire(); |
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* return getNextAvailableItem(); |
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* } |
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* |
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* public void putItem(Object x) { |
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* if (markAsUnused(x)) |
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* available.release(); |
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* } |
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* |
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* // Not a particularly efficient data structure; just for demo |
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* |
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* protected Object[] items = ... whatever kinds of items being managed |
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* protected boolean[] used = new boolean[MAX_AVAILABLE]; |
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* |
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* protected synchronized Object getNextAvailableItem() { |
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* for (int i = 0; i < MAX_AVAILABLE; ++i) { |
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* if (!used[i]) { |
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* used[i] = true; |
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* return items[i]; |
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* } |
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* } |
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* return null; // not reached |
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* } |
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* |
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* protected synchronized boolean markAsUnused(Object item) { |
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* for (int i = 0; i < MAX_AVAILABLE; ++i) { |
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* if (item == items[i]) { |
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* if (used[i]) { |
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* used[i] = false; |
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* return true; |
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* } else |
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* return false; |
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* } |
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* } |
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* return false; |
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* } |
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* }}</pre> |
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* |
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* <p>Before obtaining an item each thread must acquire a permit from |
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* the semaphore, guaranteeing that an item is available for use. When |
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* the thread has finished with the item it is returned back to the |
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* pool and a permit is returned to the semaphore, allowing another |
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* thread to acquire that item. Note that no synchronization lock is |
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* held when {@link #acquire} is called as that would prevent an item |
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* from being returned to the pool. The semaphore encapsulates the |
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* synchronization needed to restrict access to the pool, separately |
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* from any synchronization needed to maintain the consistency of the |
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* pool itself. |
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* |
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* <p>A semaphore initialized to one, and which is used such that it |
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* only has at most one permit available, can serve as a mutual |
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* exclusion lock. This is more commonly known as a <em>binary |
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* semaphore</em>, because it only has two states: one permit |
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* available, or zero permits available. When used in this way, the |
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* binary semaphore has the property (unlike many {@link java.util.concurrent.locks.Lock} |
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* implementations), that the "lock" can be released by a |
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* thread other than the owner (as semaphores have no notion of |
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* ownership). This can be useful in some specialized contexts, such |
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* as deadlock recovery. |
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* |
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* <p>The constructor for this class optionally accepts a |
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* <em>fairness</em> parameter. When set false, this class makes no |
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* guarantees about the order in which threads acquire permits. In |
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* particular, <em>barging</em> is permitted, that is, a thread |
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* invoking {@link #acquire} can be allocated a permit ahead of a |
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* thread that has been waiting - logically the new thread places itself at |
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* the head of the queue of waiting threads. When fairness is set true, the |
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* semaphore guarantees that threads invoking any of the {@link |
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* #acquire() acquire} methods are selected to obtain permits in the order in |
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* which their invocation of those methods was processed |
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* (first-in-first-out; FIFO). Note that FIFO ordering necessarily |
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* applies to specific internal points of execution within these |
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* methods. So, it is possible for one thread to invoke |
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* {@code acquire} before another, but reach the ordering point after |
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* the other, and similarly upon return from the method. |
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* Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not |
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* honor the fairness setting, but will take any permits that are |
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* available. |
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* |
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* <p>Generally, semaphores used to control resource access should be |
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* initialized as fair, to ensure that no thread is starved out from |
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* accessing a resource. When using semaphores for other kinds of |
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* synchronization control, the throughput advantages of non-fair |
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* ordering often outweigh fairness considerations. |
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* |
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* <p>This class also provides convenience methods to {@link |
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* #acquire(int) acquire} and {@link #release(int) release} multiple |
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* permits at a time. Beware of the increased risk of indefinite |
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* postponement when these methods are used without fairness set true. |
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* |
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* <p>Memory consistency effects: Actions in a thread prior to calling |
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* a "release" method such as {@code release()} |
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
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* actions following a successful "acquire" method such as {@code acquire()} |
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* in another thread. |
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* |
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* @since 1.5 |
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* @author Doug Lea |
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*/ |
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public class Semaphore implements java.io.Serializable { |
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private static final long serialVersionUID = -3222578661600680210L; |
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/** All mechanics via AbstractQueuedSynchronizer subclass */ |
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private final Sync sync; |
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/** |
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* Synchronization implementation for semaphore. Uses AQS state |
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* to represent permits. Subclassed into fair and nonfair |
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* versions. |
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*/ |
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abstract static class Sync extends AbstractQueuedSynchronizer { |
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private static final long serialVersionUID = 1192457210091910933L; |
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Sync(int permits) { |
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setState(permits); |
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} |
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final int getPermits() { |
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return getState(); |
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} |
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final int nonfairTryAcquireShared(int acquires) { |
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for (;;) { |
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int available = getState(); |
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int remaining = available - acquires; |
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if (remaining < 0 || |
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compareAndSetState(available, remaining)) |
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return remaining; |
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} |
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} |
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protected final boolean tryReleaseShared(int releases) { |
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for (;;) { |
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int current = getState(); |
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int next = current + releases; |
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if (next < current) // overflow |
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throw new Error("Maximum permit count exceeded"); |
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if (compareAndSetState(current, next)) |
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return true; |
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} |
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} |
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final void reducePermits(int reductions) { |
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for (;;) { |
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int current = getState(); |
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int next = current - reductions; |
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if (next > current) // underflow |
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throw new Error("Permit count underflow"); |
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if (compareAndSetState(current, next)) |
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return; |
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} |
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} |
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final int drainPermits() { |
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for (;;) { |
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int current = getState(); |
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if (current == 0 || compareAndSetState(current, 0)) |
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return current; |
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} |
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} |
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} |
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/** |
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* NonFair version |
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*/ |
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static final class NonfairSync extends Sync { |
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private static final long serialVersionUID = -2694183684443567898L; |
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NonfairSync(int permits) { |
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super(permits); |
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} |
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protected int tryAcquireShared(int acquires) { |
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return nonfairTryAcquireShared(acquires); |
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} |
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} |
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/** |
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* Fair version |
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*/ |
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static final class FairSync extends Sync { |
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private static final long serialVersionUID = 2014338818796000944L; |
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FairSync(int permits) { |
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super(permits); |
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} |
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protected int tryAcquireShared(int acquires) { |
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for (;;) { |
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if (hasQueuedPredecessors()) |
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return -1; |
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int available = getState(); |
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int remaining = available - acquires; |
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if (remaining < 0 || |
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compareAndSetState(available, remaining)) |
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return remaining; |
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} |
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} |
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} |
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/** |
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* Creates a {@code Semaphore} with the given number of |
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* permits and nonfair fairness setting. |
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* |
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* @param permits the initial number of permits available. |
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* This value may be negative, in which case releases |
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* must occur before any acquires will be granted. |
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*/ |
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public Semaphore(int permits) { |
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sync = new NonfairSync(permits); |
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} |
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/** |
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* Creates a {@code Semaphore} with the given number of |
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* permits and the given fairness setting. |
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* |
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* @param permits the initial number of permits available. |
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* This value may be negative, in which case releases |
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* must occur before any acquires will be granted. |
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* @param fair {@code true} if this semaphore will guarantee |
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* first-in first-out granting of permits under contention, |
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* else {@code false} |
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*/ |
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public Semaphore(int permits, boolean fair) { |
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sync = fair ? new FairSync(permits) : new NonfairSync(permits); |
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} |
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/** |
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* Acquires a permit from this semaphore, blocking until one is |
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* available, or the thread is {@linkplain Thread#interrupt interrupted}. |
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* |
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* <p>Acquires a permit, if one is available and returns immediately, |
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* reducing the number of available permits by one. |
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* |
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* <p>If no permit is available then the current thread becomes |
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* disabled for thread scheduling purposes and lies dormant until |
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* one of two things happens: |
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* <ul> |
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* <li>Some other thread invokes the {@link #release} method for this |
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* semaphore and the current thread is next to be assigned a permit; or |
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* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
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* the current thread. |
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* </ul> |
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* |
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* <p>If the current thread: |
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* <ul> |
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* <li>has its interrupted status set on entry to this method; or |
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* <li>is {@linkplain Thread#interrupt interrupted} while waiting |
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* for a permit, |
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* </ul> |
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* then {@link InterruptedException} is thrown and the current thread's |
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* interrupted status is cleared. |
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* |
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* @throws InterruptedException if the current thread is interrupted |
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*/ |
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public void acquire() throws InterruptedException { |
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sync.acquireSharedInterruptibly(1); |
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} |
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/** |
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* Acquires a permit from this semaphore, blocking until one is |
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* available. |
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* |
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* <p>Acquires a permit, if one is available and returns immediately, |
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* reducing the number of available permits by one. |
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* |
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* <p>If no permit is available then the current thread becomes |
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* disabled for thread scheduling purposes and lies dormant until |
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* some other thread invokes the {@link #release} method for this |
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* semaphore and the current thread is next to be assigned a permit. |
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* |
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* <p>If the current thread is {@linkplain Thread#interrupt interrupted} |
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* while waiting for a permit then it will continue to wait, but the |
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* time at which the thread is assigned a permit may change compared to |
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* the time it would have received the permit had no interruption |
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* occurred. When the thread does return from this method its interrupt |
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* status will be set. |
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*/ |
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public void acquireUninterruptibly() { |
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sync.acquireShared(1); |
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} |
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/** |
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* Acquires a permit from this semaphore, only if one is available at the |
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* time of invocation. |
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* |
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* <p>Acquires a permit, if one is available and returns immediately, |
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* with the value {@code true}, |
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* reducing the number of available permits by one. |
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* |
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* <p>If no permit is available then this method will return |
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* immediately with the value {@code false}. |
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* |
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* <p>Even when this semaphore has been set to use a |
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* fair ordering policy, a call to {@code tryAcquire()} <em>will</em> |
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* immediately acquire a permit if one is available, whether or not |
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* other threads are currently waiting. |
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* This "barging" behavior can be useful in certain |
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* circumstances, even though it breaks fairness. If you want to honor |
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* the fairness setting, then use |
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* {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) } |
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* which is almost equivalent (it also detects interruption). |
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* |
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* @return {@code true} if a permit was acquired and {@code false} |
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* otherwise |
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*/ |
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public boolean tryAcquire() { |
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return sync.nonfairTryAcquireShared(1) >= 0; |
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} |
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/** |
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* Acquires a permit from this semaphore, if one becomes available |
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* within the given waiting time and the current thread has not |
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* been {@linkplain Thread#interrupt interrupted}. |
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* |
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* <p>Acquires a permit, if one is available and returns immediately, |
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* with the value {@code true}, |
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* reducing the number of available permits by one. |
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* |
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* <p>If no permit is available then the current thread becomes |
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* disabled for thread scheduling purposes and lies dormant until |
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* one of three things happens: |
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* <ul> |
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* <li>Some other thread invokes the {@link #release} method for this |
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* semaphore and the current thread is next to be assigned a permit; or |
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* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
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* the current thread; or |
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* <li>The specified waiting time elapses. |
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* </ul> |
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* |
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* <p>If a permit is acquired then the value {@code true} is returned. |
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* |
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* <p>If the current thread: |
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* <ul> |
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* <li>has its interrupted status set on entry to this method; or |
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* <li>is {@linkplain Thread#interrupt interrupted} while waiting |
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* to acquire a permit, |
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* </ul> |
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* then {@link InterruptedException} is thrown and the current thread's |
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* interrupted status is cleared. |
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* |
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* <p>If the specified waiting time elapses then the value {@code false} |
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* is returned. If the time is less than or equal to zero, the method |
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* will not wait at all. |
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* |
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* @param timeout the maximum time to wait for a permit |
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* @param unit the time unit of the {@code timeout} argument |
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* @return {@code true} if a permit was acquired and {@code false} |
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* if the waiting time elapsed before a permit was acquired |
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* @throws InterruptedException if the current thread is interrupted |
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*/ |
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public boolean tryAcquire(long timeout, TimeUnit unit) |
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throws InterruptedException { |
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return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); |
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} |
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/** |
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* Releases a permit, returning it to the semaphore. |
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|
|
* |
386 |
|
|
* <p>Releases a permit, increasing the number of available permits by |
387 |
|
|
* one. If any threads are trying to acquire a permit, then one is |
388 |
|
|
* selected and given the permit that was just released. That thread |
389 |
|
|
* is (re)enabled for thread scheduling purposes. |
390 |
|
|
* |
391 |
|
|
* <p>There is no requirement that a thread that releases a permit must |
392 |
|
|
* have acquired that permit by calling {@link #acquire}. |
393 |
|
|
* Correct usage of a semaphore is established by programming convention |
394 |
|
|
* in the application. |
395 |
|
|
*/ |
396 |
|
|
public void release() { |
397 |
|
|
sync.releaseShared(1); |
398 |
|
|
} |
399 |
|
|
|
400 |
|
|
/** |
401 |
|
|
* Acquires the given number of permits from this semaphore, |
402 |
|
|
* blocking until all are available, |
403 |
|
|
* or the thread is {@linkplain Thread#interrupt interrupted}. |
404 |
|
|
* |
405 |
|
|
* <p>Acquires the given number of permits, if they are available, |
406 |
|
|
* and returns immediately, reducing the number of available permits |
407 |
|
|
* by the given amount. |
408 |
|
|
* |
409 |
|
|
* <p>If insufficient permits are available then the current thread becomes |
410 |
|
|
* disabled for thread scheduling purposes and lies dormant until |
411 |
|
|
* one of two things happens: |
412 |
|
|
* <ul> |
413 |
|
|
* <li>Some other thread invokes one of the {@link #release() release} |
414 |
|
|
* methods for this semaphore, the current thread is next to be assigned |
415 |
|
|
* permits and the number of available permits satisfies this request; or |
416 |
|
|
* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
417 |
|
|
* the current thread. |
418 |
|
|
* </ul> |
419 |
|
|
* |
420 |
|
|
* <p>If the current thread: |
421 |
|
|
* <ul> |
422 |
|
|
* <li>has its interrupted status set on entry to this method; or |
423 |
|
|
* <li>is {@linkplain Thread#interrupt interrupted} while waiting |
424 |
|
|
* for a permit, |
425 |
|
|
* </ul> |
426 |
|
|
* then {@link InterruptedException} is thrown and the current thread's |
427 |
|
|
* interrupted status is cleared. |
428 |
|
|
* Any permits that were to be assigned to this thread are instead |
429 |
|
|
* assigned to other threads trying to acquire permits, as if |
430 |
|
|
* permits had been made available by a call to {@link #release()}. |
431 |
|
|
* |
432 |
|
|
* @param permits the number of permits to acquire |
433 |
|
|
* @throws InterruptedException if the current thread is interrupted |
434 |
|
|
* @throws IllegalArgumentException if {@code permits} is negative |
435 |
|
|
*/ |
436 |
|
|
public void acquire(int permits) throws InterruptedException { |
437 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
438 |
|
|
sync.acquireSharedInterruptibly(permits); |
439 |
|
|
} |
440 |
|
|
|
441 |
|
|
/** |
442 |
|
|
* Acquires the given number of permits from this semaphore, |
443 |
|
|
* blocking until all are available. |
444 |
|
|
* |
445 |
|
|
* <p>Acquires the given number of permits, if they are available, |
446 |
|
|
* and returns immediately, reducing the number of available permits |
447 |
|
|
* by the given amount. |
448 |
|
|
* |
449 |
|
|
* <p>If insufficient permits are available then the current thread becomes |
450 |
|
|
* disabled for thread scheduling purposes and lies dormant until |
451 |
|
|
* some other thread invokes one of the {@link #release() release} |
452 |
|
|
* methods for this semaphore, the current thread is next to be assigned |
453 |
|
|
* permits and the number of available permits satisfies this request. |
454 |
|
|
* |
455 |
|
|
* <p>If the current thread is {@linkplain Thread#interrupt interrupted} |
456 |
|
|
* while waiting for permits then it will continue to wait and its |
457 |
|
|
* position in the queue is not affected. When the thread does return |
458 |
|
|
* from this method its interrupt status will be set. |
459 |
|
|
* |
460 |
|
|
* @param permits the number of permits to acquire |
461 |
|
|
* @throws IllegalArgumentException if {@code permits} is negative |
462 |
|
|
*/ |
463 |
|
|
public void acquireUninterruptibly(int permits) { |
464 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
465 |
|
|
sync.acquireShared(permits); |
466 |
|
|
} |
467 |
|
|
|
468 |
|
|
/** |
469 |
|
|
* Acquires the given number of permits from this semaphore, only |
470 |
|
|
* if all are available at the time of invocation. |
471 |
|
|
* |
472 |
|
|
* <p>Acquires the given number of permits, if they are available, and |
473 |
|
|
* returns immediately, with the value {@code true}, |
474 |
|
|
* reducing the number of available permits by the given amount. |
475 |
|
|
* |
476 |
|
|
* <p>If insufficient permits are available then this method will return |
477 |
|
|
* immediately with the value {@code false} and the number of available |
478 |
|
|
* permits is unchanged. |
479 |
|
|
* |
480 |
|
|
* <p>Even when this semaphore has been set to use a fair ordering |
481 |
|
|
* policy, a call to {@code tryAcquire} <em>will</em> |
482 |
|
|
* immediately acquire a permit if one is available, whether or |
483 |
|
|
* not other threads are currently waiting. This |
484 |
|
|
* "barging" behavior can be useful in certain |
485 |
|
|
* circumstances, even though it breaks fairness. If you want to |
486 |
|
|
* honor the fairness setting, then use {@link #tryAcquire(int, |
487 |
|
|
* long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) } |
488 |
|
|
* which is almost equivalent (it also detects interruption). |
489 |
|
|
* |
490 |
|
|
* @param permits the number of permits to acquire |
491 |
|
|
* @return {@code true} if the permits were acquired and |
492 |
|
|
* {@code false} otherwise |
493 |
|
|
* @throws IllegalArgumentException if {@code permits} is negative |
494 |
|
|
*/ |
495 |
|
|
public boolean tryAcquire(int permits) { |
496 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
497 |
|
|
return sync.nonfairTryAcquireShared(permits) >= 0; |
498 |
|
|
} |
499 |
|
|
|
500 |
|
|
/** |
501 |
|
|
* Acquires the given number of permits from this semaphore, if all |
502 |
|
|
* become available within the given waiting time and the current |
503 |
|
|
* thread has not been {@linkplain Thread#interrupt interrupted}. |
504 |
|
|
* |
505 |
|
|
* <p>Acquires the given number of permits, if they are available and |
506 |
|
|
* returns immediately, with the value {@code true}, |
507 |
|
|
* reducing the number of available permits by the given amount. |
508 |
|
|
* |
509 |
|
|
* <p>If insufficient permits are available then |
510 |
|
|
* the current thread becomes disabled for thread scheduling |
511 |
|
|
* purposes and lies dormant until one of three things happens: |
512 |
|
|
* <ul> |
513 |
|
|
* <li>Some other thread invokes one of the {@link #release() release} |
514 |
|
|
* methods for this semaphore, the current thread is next to be assigned |
515 |
|
|
* permits and the number of available permits satisfies this request; or |
516 |
|
|
* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
517 |
|
|
* the current thread; or |
518 |
|
|
* <li>The specified waiting time elapses. |
519 |
|
|
* </ul> |
520 |
|
|
* |
521 |
|
|
* <p>If the permits are acquired then the value {@code true} is returned. |
522 |
|
|
* |
523 |
|
|
* <p>If the current thread: |
524 |
|
|
* <ul> |
525 |
|
|
* <li>has its interrupted status set on entry to this method; or |
526 |
|
|
* <li>is {@linkplain Thread#interrupt interrupted} while waiting |
527 |
|
|
* to acquire the permits, |
528 |
|
|
* </ul> |
529 |
|
|
* then {@link InterruptedException} is thrown and the current thread's |
530 |
|
|
* interrupted status is cleared. |
531 |
|
|
* Any permits that were to be assigned to this thread, are instead |
532 |
|
|
* assigned to other threads trying to acquire permits, as if |
533 |
|
|
* the permits had been made available by a call to {@link #release()}. |
534 |
|
|
* |
535 |
|
|
* <p>If the specified waiting time elapses then the value {@code false} |
536 |
|
|
* is returned. If the time is less than or equal to zero, the method |
537 |
|
|
* will not wait at all. Any permits that were to be assigned to this |
538 |
|
|
* thread, are instead assigned to other threads trying to acquire |
539 |
|
|
* permits, as if the permits had been made available by a call to |
540 |
|
|
* {@link #release()}. |
541 |
|
|
* |
542 |
|
|
* @param permits the number of permits to acquire |
543 |
|
|
* @param timeout the maximum time to wait for the permits |
544 |
|
|
* @param unit the time unit of the {@code timeout} argument |
545 |
|
|
* @return {@code true} if all permits were acquired and {@code false} |
546 |
|
|
* if the waiting time elapsed before all permits were acquired |
547 |
|
|
* @throws InterruptedException if the current thread is interrupted |
548 |
|
|
* @throws IllegalArgumentException if {@code permits} is negative |
549 |
|
|
*/ |
550 |
|
|
public boolean tryAcquire(int permits, long timeout, TimeUnit unit) |
551 |
|
|
throws InterruptedException { |
552 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
553 |
|
|
return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout)); |
554 |
|
|
} |
555 |
|
|
|
556 |
|
|
/** |
557 |
|
|
* Releases the given number of permits, returning them to the semaphore. |
558 |
|
|
* |
559 |
|
|
* <p>Releases the given number of permits, increasing the number of |
560 |
|
|
* available permits by that amount. |
561 |
|
|
* If any threads are trying to acquire permits, then one |
562 |
|
|
* is selected and given the permits that were just released. |
563 |
|
|
* If the number of available permits satisfies that thread's request |
564 |
|
|
* then that thread is (re)enabled for thread scheduling purposes; |
565 |
|
|
* otherwise the thread will wait until sufficient permits are available. |
566 |
|
|
* If there are still permits available |
567 |
|
|
* after this thread's request has been satisfied, then those permits |
568 |
|
|
* are assigned in turn to other threads trying to acquire permits. |
569 |
|
|
* |
570 |
|
|
* <p>There is no requirement that a thread that releases a permit must |
571 |
|
|
* have acquired that permit by calling {@link Semaphore#acquire acquire}. |
572 |
|
|
* Correct usage of a semaphore is established by programming convention |
573 |
|
|
* in the application. |
574 |
|
|
* |
575 |
|
|
* @param permits the number of permits to release |
576 |
|
|
* @throws IllegalArgumentException if {@code permits} is negative |
577 |
|
|
*/ |
578 |
|
|
public void release(int permits) { |
579 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
580 |
|
|
sync.releaseShared(permits); |
581 |
|
|
} |
582 |
|
|
|
583 |
|
|
/** |
584 |
|
|
* Returns the current number of permits available in this semaphore. |
585 |
|
|
* |
586 |
|
|
* <p>This method is typically used for debugging and testing purposes. |
587 |
|
|
* |
588 |
|
|
* @return the number of permits available in this semaphore |
589 |
|
|
*/ |
590 |
|
|
public int availablePermits() { |
591 |
|
|
return sync.getPermits(); |
592 |
|
|
} |
593 |
|
|
|
594 |
|
|
/** |
595 |
|
|
* Acquires and returns all permits that are immediately available. |
596 |
|
|
* |
597 |
|
|
* @return the number of permits acquired |
598 |
|
|
*/ |
599 |
|
|
public int drainPermits() { |
600 |
|
|
return sync.drainPermits(); |
601 |
|
|
} |
602 |
|
|
|
603 |
|
|
/** |
604 |
|
|
* Shrinks the number of available permits by the indicated |
605 |
|
|
* reduction. This method can be useful in subclasses that use |
606 |
|
|
* semaphores to track resources that become unavailable. This |
607 |
|
|
* method differs from {@code acquire} in that it does not block |
608 |
|
|
* waiting for permits to become available. |
609 |
|
|
* |
610 |
|
|
* @param reduction the number of permits to remove |
611 |
|
|
* @throws IllegalArgumentException if {@code reduction} is negative |
612 |
|
|
*/ |
613 |
|
|
protected void reducePermits(int reduction) { |
614 |
|
|
if (reduction < 0) throw new IllegalArgumentException(); |
615 |
|
|
sync.reducePermits(reduction); |
616 |
|
|
} |
617 |
|
|
|
618 |
|
|
/** |
619 |
|
|
* Returns {@code true} if this semaphore has fairness set true. |
620 |
|
|
* |
621 |
|
|
* @return {@code true} if this semaphore has fairness set true |
622 |
|
|
*/ |
623 |
|
|
public boolean isFair() { |
624 |
|
|
return sync instanceof FairSync; |
625 |
|
|
} |
626 |
|
|
|
627 |
|
|
/** |
628 |
|
|
* Queries whether any threads are waiting to acquire. Note that |
629 |
|
|
* because cancellations may occur at any time, a {@code true} |
630 |
|
|
* return does not guarantee that any other thread will ever |
631 |
|
|
* acquire. This method is designed primarily for use in |
632 |
|
|
* monitoring of the system state. |
633 |
|
|
* |
634 |
|
|
* @return {@code true} if there may be other threads waiting to |
635 |
|
|
* acquire the lock |
636 |
|
|
*/ |
637 |
|
|
public final boolean hasQueuedThreads() { |
638 |
|
|
return sync.hasQueuedThreads(); |
639 |
|
|
} |
640 |
|
|
|
641 |
|
|
/** |
642 |
|
|
* Returns an estimate of the number of threads waiting to acquire. |
643 |
|
|
* The value is only an estimate because the number of threads may |
644 |
|
|
* change dynamically while this method traverses internal data |
645 |
|
|
* structures. This method is designed for use in monitoring of the |
646 |
|
|
* system state, not for synchronization control. |
647 |
|
|
* |
648 |
|
|
* @return the estimated number of threads waiting for this lock |
649 |
|
|
*/ |
650 |
|
|
public final int getQueueLength() { |
651 |
|
|
return sync.getQueueLength(); |
652 |
|
|
} |
653 |
|
|
|
654 |
|
|
/** |
655 |
|
|
* Returns a collection containing threads that may be waiting to acquire. |
656 |
|
|
* Because the actual set of threads may change dynamically while |
657 |
|
|
* constructing this result, the returned collection is only a best-effort |
658 |
|
|
* estimate. The elements of the returned collection are in no particular |
659 |
|
|
* order. This method is designed to facilitate construction of |
660 |
|
|
* subclasses that provide more extensive monitoring facilities. |
661 |
|
|
* |
662 |
|
|
* @return the collection of threads |
663 |
|
|
*/ |
664 |
|
|
protected Collection<Thread> getQueuedThreads() { |
665 |
|
|
return sync.getQueuedThreads(); |
666 |
|
|
} |
667 |
|
|
|
668 |
|
|
/** |
669 |
|
|
* Returns a string identifying this semaphore, as well as its state. |
670 |
|
|
* The state, in brackets, includes the String {@code "Permits ="} |
671 |
|
|
* followed by the number of permits. |
672 |
|
|
* |
673 |
|
|
* @return a string identifying this semaphore, as well as its state |
674 |
|
|
*/ |
675 |
|
|
public String toString() { |
676 |
|
|
return super.toString() + "[Permits = " + sync.getPermits() + "]"; |
677 |
|
|
} |
678 |
|
|
} |