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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/licenses/publicdomain |
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*/ |
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|
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package java.util.concurrent; |
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import java.util.*; |
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import java.util.concurrent.locks.*; |
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import java.util.concurrent.atomic.*; |
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|
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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 <tt>Semaphore</tt> 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> |
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* class Pool { |
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* private static final 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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* |
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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 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 accepts a <em>fairness</em> |
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* parameter. When set false, this class makes no guarantees about the |
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* order in which threads acquire permits. In particular, <em>barging</em> is |
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* permitted, that is, a thread invoking {@link #acquire} can be |
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* allocated a permit ahead of a thread that has been waiting. When |
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* fairness is set true, the semaphore guarantees that threads |
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* invoking any of the {@link #acquire() acquire} methods are |
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* allocated permits in the order in which their invocation of those |
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* methods was processed (first-in-first-out; FIFO). Note that FIFO |
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* ordering necessarily applies to specific internal points of |
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* execution within these methods. So, it is possible for one thread |
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* to invoke <tt>acquire</tt> before another, but reach the ordering |
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* point after the other, and similarly upon return from the method. |
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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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* @since 1.5 |
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* @author Doug Lea |
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* |
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*/ |
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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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/** |
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* Synchronization implementation for semaphore. |
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* Uses AQS state to represent permits |
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*/ |
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private final static class Sync extends AbstractQueuedSynchronizer { |
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final boolean fair; |
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Sync(int permits, boolean fair) { |
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this.fair = fair; |
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setState(permits); |
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} |
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|
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public int getPermits() { |
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return getState(); |
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} |
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|
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public int tryAcquireSharedState(boolean isQueued, int acquires) { |
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for (;;) { |
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if (!isQueued && fair && hasQueuedThreads()) |
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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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public boolean releaseSharedState(int releases) { |
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for (;;) { |
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int p = getState(); |
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if (compareAndSetState(p, p + releases)) |
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return true; |
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} |
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} |
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|
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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 (compareAndSetState(current, next)) |
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return; |
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} |
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} |
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} |
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|
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/** |
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* Construct a <tt>Semaphore</tt> with the given number of |
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* permits and the given fairness setting. |
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* @param permits the initial number of permits available. This |
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* value may be negative, in which case releases must |
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* occur before any acquires will be granted. |
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* @param fair true if this semaphore will guarantee first-in |
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* first-out granting of permits under contention, else false. |
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*/ |
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public Semaphore(int permits, boolean fair) { |
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sync = new Sync(permits, fair); |
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} |
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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 {@link 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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* <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 {@link Thread#interrupt interrupts} the current |
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* 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 {@link 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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* @see Thread#interrupt |
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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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/** |
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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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* <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 |
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* is {@link Thread#interrupt interrupted} while waiting |
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* for a permit then it will continue to wait, but the time at which |
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* the thread is assigned a permit may change compared to the time it |
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* would have received the permit had no interruption occurred. When the |
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* thread does return from this method its interrupt status will be set. |
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* |
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*/ |
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public void acquireUninterruptibly() { |
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sync.acquireSharedUninterruptibly(1); |
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} |
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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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* <p>Acquires a permit, if one is available and returns immediately, |
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* with the value <tt>true</tt>, |
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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 <tt>false</tt>. |
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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 <tt>tryAcquire()</tt> <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 <tt>true</tt> if a permit was acquired and <tt>false</tt> |
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* otherwise. |
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*/ |
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public boolean tryAcquire() { |
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return sync.tryAcquireSharedState(true, 1) >= 0; |
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} |
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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 |
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* current thread has not been {@link Thread#interrupt interrupted}. |
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* <p>Acquires a permit, if one is available and returns immediately, |
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* with the value <tt>true</tt>, |
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* reducing the number of available permits by one. |
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* <p>If no permit is available then |
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* the current thread becomes disabled for thread scheduling |
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* purposes and lies dormant until 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 {@link Thread#interrupt interrupts} the current |
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* thread; or |
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* <li>The specified waiting time elapses. |
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* </ul> |
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* <p>If a permit is acquired then the value <tt>true</tt> is returned. |
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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 {@link Thread#interrupt interrupted} while waiting to acquire |
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* 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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* <p>If the specified waiting time elapses then the value <tt>false</tt> |
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* is returned. |
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* If the time is less than or equal to zero, the method will not wait |
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* 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 <tt>timeout</tt> argument. |
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* @return <tt>true</tt> if a permit was acquired and <tt>false</tt> |
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* if the waiting time elapsed before a permit was acquired. |
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* |
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* @throws InterruptedException if the current thread is interrupted |
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* |
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* @see Thread#interrupt |
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* |
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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.acquireSharedTimed(1, unit.toNanos(timeout)); |
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} |
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|
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/** |
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* Releases a permit, returning it to the semaphore. |
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* <p>Releases a permit, increasing the number of available permits |
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* by one. |
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* If any threads are blocking trying to acquire a permit, then one |
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* is selected and given the permit that was just released. |
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* That thread is re-enabled for thread scheduling purposes. |
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* <p>There is no requirement that a thread that releases a permit must |
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* have acquired that permit by calling {@link #acquire}. |
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* Correct usage of a semaphore is established by programming convention |
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* in the application. |
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*/ |
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public void release() { |
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sync.releaseShared(1); |
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} |
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|
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/** |
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* Acquires the given number of permits from this semaphore, |
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* blocking until all are available, |
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* or the thread is {@link Thread#interrupt interrupted}. |
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* |
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* <p>Acquires the given number of permits, if they are available, |
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* and returns immediately, |
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* reducing the number of available permits by the given amount. |
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* |
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* <p>If insufficient permits are 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 one of the {@link #release() release} |
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* methods for this semaphore, the current thread is next to be assigned |
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* permits and the number of available permits satisfies this request; or |
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* <li>Some other thread {@link Thread#interrupt interrupts} the current |
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* 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 {@link 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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* Any permits that were to be assigned to this thread are instead |
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* assigned to the next waiting thread(s), as if |
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* they had been made available by a call to {@link #release()}. |
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* |
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* @param permits the number of permits to acquire |
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* |
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* @throws InterruptedException if the current thread is interrupted |
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* @throws IllegalArgumentException if permits less than zero. |
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* |
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* @see Thread#interrupt |
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*/ |
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public void acquire(int permits) throws InterruptedException { |
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if (permits < 0) throw new IllegalArgumentException(); |
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sync.acquireSharedInterruptibly(permits); |
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} |
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|
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/** |
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* Acquires the given number of permits from this semaphore, |
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* blocking until all are available. |
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* |
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* <p>Acquires the given number of permits, if they are available, |
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* and returns immediately, |
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* reducing the number of available permits by the given amount. |
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* |
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* <p>If insufficient permits are 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 one of the {@link #release() release} |
384 |
* methods for this semaphore, the current thread is next to be assigned |
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* permits and the number of available permits satisfies this request. |
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* |
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* <p>If the current thread |
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* is {@link Thread#interrupt interrupted} while waiting |
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* for permits then it will continue to wait and its position in the |
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* queue is not affected. When the |
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* thread does return from this method its interrupt status will be set. |
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* |
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* @param permits the number of permits to acquire |
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* @throws IllegalArgumentException if permits less than zero. |
395 |
* |
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*/ |
397 |
public void acquireUninterruptibly(int permits) { |
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if (permits < 0) throw new IllegalArgumentException(); |
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sync.acquireSharedUninterruptibly(permits); |
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} |
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|
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/** |
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* Acquires the given number of permits from this semaphore, only if |
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* all are available at the time of invocation. |
405 |
* <p>Acquires the given number of permits, if they are available, and |
406 |
* returns immediately, with the value <tt>true</tt>, |
407 |
* reducing the number of available permits by the given amount. |
408 |
* |
409 |
* <p>If insufficient permits are available then this method will return |
410 |
* immediately with the value <tt>false</tt> and the number of available |
411 |
* permits is unchanged. |
412 |
* |
413 |
* <p>Even when this semaphore has been set to use a fair ordering |
414 |
* policy, a call to <tt>tryAcquire</tt> <em>will</em> |
415 |
* immediately acquire a permit if one is available, whether or |
416 |
* not other threads are currently waiting. This |
417 |
* "barging" behavior can be useful in certain |
418 |
* circumstances, even though it breaks fairness. If you want to |
419 |
* honor the fairness setting, then use {@link #tryAcquire(int, |
420 |
* long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) } |
421 |
* which is almost equivalent (it also detects interruption). |
422 |
* |
423 |
* @param permits the number of permits to acquire |
424 |
* |
425 |
* @return <tt>true</tt> if the permits were acquired and <tt>false</tt> |
426 |
* otherwise. |
427 |
* @throws IllegalArgumentException if permits less than zero. |
428 |
*/ |
429 |
public boolean tryAcquire(int permits) { |
430 |
if (permits < 0) throw new IllegalArgumentException(); |
431 |
return sync.tryAcquireSharedState(true, permits) >= 0; |
432 |
} |
433 |
|
434 |
/** |
435 |
* Acquires the given number of permits from this semaphore, if all |
436 |
* become available within the given waiting time and the |
437 |
* current thread has not been {@link Thread#interrupt interrupted}. |
438 |
* <p>Acquires the given number of permits, if they are available and |
439 |
* returns immediately, with the value <tt>true</tt>, |
440 |
* reducing the number of available permits by the given amount. |
441 |
* <p>If insufficient permits are available then |
442 |
* the current thread becomes disabled for thread scheduling |
443 |
* purposes and lies dormant until one of three things happens: |
444 |
* <ul> |
445 |
* <li>Some other thread invokes one of the {@link #release() release} |
446 |
* methods for this semaphore, the current thread is next to be assigned |
447 |
* permits and the number of available permits satisfies this request; or |
448 |
* <li>Some other thread {@link Thread#interrupt interrupts} the current |
449 |
* thread; or |
450 |
* <li>The specified waiting time elapses. |
451 |
* </ul> |
452 |
* <p>If the permits are acquired then the value <tt>true</tt> is returned. |
453 |
* <p>If the current thread: |
454 |
* <ul> |
455 |
* <li>has its interrupted status set on entry to this method; or |
456 |
* <li>is {@link Thread#interrupt interrupted} while waiting to acquire |
457 |
* the permits, |
458 |
* </ul> |
459 |
* then {@link InterruptedException} is thrown and the current thread's |
460 |
* interrupted status is cleared. |
461 |
* Any permits that were to be assigned to this thread, are instead |
462 |
* assigned to the next waiting thread(s), as if |
463 |
* they had been made available by a call to {@link #release()}. |
464 |
* |
465 |
* <p>If the specified waiting time elapses then the value <tt>false</tt> |
466 |
* is returned. |
467 |
* If the time is |
468 |
* less than or equal to zero, the method will not wait at all. |
469 |
* Any permits that were to be assigned to this thread, are instead |
470 |
* assigned to the next waiting thread(s), as if |
471 |
* they had been made available by a call to {@link #release()}. |
472 |
* |
473 |
* @param permits the number of permits to acquire |
474 |
* @param timeout the maximum time to wait for the permits |
475 |
* @param unit the time unit of the <tt>timeout</tt> argument. |
476 |
* @return <tt>true</tt> if all permits were acquired and <tt>false</tt> |
477 |
* if the waiting time elapsed before all permits were acquired. |
478 |
* |
479 |
* @throws InterruptedException if the current thread is interrupted |
480 |
* @throws IllegalArgumentException if permits less than zero. |
481 |
* |
482 |
* @see Thread#interrupt |
483 |
* |
484 |
*/ |
485 |
public boolean tryAcquire(int permits, long timeout, TimeUnit unit) |
486 |
throws InterruptedException { |
487 |
if (permits < 0) throw new IllegalArgumentException(); |
488 |
return sync.acquireSharedTimed(permits, unit.toNanos(timeout)); |
489 |
} |
490 |
|
491 |
/** |
492 |
* Releases the given number of permits, returning them to the semaphore. |
493 |
* <p>Releases the given number of permits, increasing the number of |
494 |
* available permits by that amount. |
495 |
* If any threads are blocking trying to acquire permits, then the |
496 |
* one that has been waiting the longest |
497 |
* is selected and given the permits that were just released. |
498 |
* If the number of available permits satisfies that thread's request |
499 |
* then that thread is re-enabled for thread scheduling purposes; otherwise |
500 |
* the thread continues to wait. If there are still permits available |
501 |
* after the first thread's request has been satisfied, then those permits |
502 |
* are assigned to the next waiting thread. If it is satisfied then it is |
503 |
* re-enabled for thread scheduling purposes. This continues until there |
504 |
* are insufficient permits to satisfy the next waiting thread, or there |
505 |
* are no more waiting threads. |
506 |
* |
507 |
* <p>There is no requirement that a thread that releases a permit must |
508 |
* have acquired that permit by calling {@link Semaphore#acquire acquire}. |
509 |
* Correct usage of a semaphore is established by programming convention |
510 |
* in the application. |
511 |
* |
512 |
* @param permits the number of permits to release |
513 |
* @throws IllegalArgumentException if permits less than zero. |
514 |
*/ |
515 |
public void release(int permits) { |
516 |
if (permits < 0) throw new IllegalArgumentException(); |
517 |
sync.releaseShared(permits); |
518 |
} |
519 |
|
520 |
/** |
521 |
* Return the current number of permits available in this semaphore. |
522 |
* <p>This method is typically used for debugging and testing purposes. |
523 |
* @return the number of permits available in this semaphore. |
524 |
*/ |
525 |
public int availablePermits() { |
526 |
return sync.getPermits(); |
527 |
} |
528 |
|
529 |
/** |
530 |
* Shrink the number of available permits by the indicated |
531 |
* reduction. This method can be useful in subclasses that |
532 |
* use semaphores to track available resources that become |
533 |
* unavailable. This method differs from <tt>acquire</tt> |
534 |
* in that it does not block waiting for permits to become |
535 |
* available. |
536 |
* @param reduction the number of permits to remove |
537 |
* @throws IllegalArgumentException if reduction is negative |
538 |
*/ |
539 |
protected void reducePermits(int reduction) { |
540 |
if (reduction < 0) throw new IllegalArgumentException(); |
541 |
sync.reducePermits(reduction); |
542 |
} |
543 |
|
544 |
/** |
545 |
* Return true if this semaphore has fairness set true. |
546 |
* @return true if this semaphore has fairness set true. |
547 |
*/ |
548 |
public boolean isFair() { |
549 |
return sync.fair; |
550 |
} |
551 |
|
552 |
/** |
553 |
* Queries whether any threads are waiting to acquire. Note that |
554 |
* because cancellations may occur at any time, a <tt>true</tt> |
555 |
* return does not guarantee that any other thread will ever |
556 |
* acquire. This method is designed primarily for use in |
557 |
* monitoring of the system state. |
558 |
* |
559 |
* @return true if there may be other threads waiting to acquire |
560 |
* the lock. |
561 |
*/ |
562 |
public final boolean hasQueuedThreads() { |
563 |
return sync.hasQueuedThreads(); |
564 |
} |
565 |
|
566 |
/** |
567 |
* Returns an estimate of the number of threads waiting to |
568 |
* acquire. The value is only an estimate because the number of |
569 |
* threads may change dynamically while this method traverses |
570 |
* internal data structures. This method is designed for use in |
571 |
* monitoring of the system state, not for synchronization |
572 |
* control. |
573 |
* @return the estimated number of threads waiting for this lock |
574 |
*/ |
575 |
public final int getQueueLength() { |
576 |
return sync.getQueueLength(); |
577 |
} |
578 |
|
579 |
/** |
580 |
* Returns a collection containing threads that may be waiting to |
581 |
* acquire. Because the actual set of threads may change |
582 |
* dynamically while constructing this result, the returned |
583 |
* collection is only a best-effort estimate. The elements of the |
584 |
* returned collection are in no particular order. This method is |
585 |
* designed to facilitate construction of subclasses that provide |
586 |
* more extensive monitoring facilities. |
587 |
* @return the collection of threads |
588 |
*/ |
589 |
protected Collection<Thread> getQueuedThreads() { |
590 |
return sync.getQueuedThreads(); |
591 |
} |
592 |
} |