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dl |
1.2 |
/* |
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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. Use, modify, and |
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* redistribute this code in any way without acknowledgement. |
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*/ |
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tim |
1.1 |
package java.util.concurrent; |
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dl |
1.16 |
import java.util.*; |
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1.5 |
import java.util.concurrent.locks.*; |
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dl |
1.16 |
import java.util.concurrent.atomic.*; |
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1.1 |
|
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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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dl |
1.16 |
* <p>Semaphores are often used to restrict the number of threads than can |
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tim |
1.1 |
* 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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1.16 |
* private final Semaphore available = new Semaphore(MAX_AVAILABLE, true); |
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1.1 |
* |
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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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1.8 |
* } else |
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1.1 |
* 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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1.16 |
* <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, barging 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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tim |
1.1 |
* |
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* @since 1.5 |
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dl |
1.4 |
* @author Doug Lea |
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tim |
1.1 |
* |
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*/ |
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dl |
1.16 |
|
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dl |
1.2 |
public class Semaphore implements java.io.Serializable { |
122 |
dl |
1.16 |
/* |
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* The underlying algorithm here is a simplified adaptation of |
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* that used for ReentrantLock. See the internal documentation of |
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* ReentrantLock for detailed explanation. |
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*/ |
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private static final long serialVersionUID = -3222578661600680210L; |
129 |
dl |
1.9 |
|
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dl |
1.16 |
/** Node status value to indicate thread has cancelled */ |
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private static final int CANCELLED = 1; |
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/** Node status value to indicate successor needs unparking */ |
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private static final int SIGNAL = -1; |
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/** Node class for waiting threads */ |
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private static class Node { |
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volatile int status; |
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volatile Node prev; |
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volatile Node next; |
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Thread thread; |
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Node(Thread t) { thread = t; } |
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} |
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dl |
1.2 |
|
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dl |
1.16 |
/** Number of available permits held in a separate AtomicInteger */ |
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private final AtomicInteger perms; |
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/** Head of the wait queue, lazily initialized. */ |
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private transient volatile Node head; |
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/** Tail of the wait queue, lazily initialized. */ |
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private transient volatile Node tail; |
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/** true if barging disabled */ |
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private final boolean fair; |
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// Atomic update support |
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private static final |
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AtomicReferenceFieldUpdater<Semaphore, Node> tailUpdater = |
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AtomicReferenceFieldUpdater.<Semaphore, Node>newUpdater |
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(Semaphore.class, Node.class, "tail"); |
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private static final |
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AtomicReferenceFieldUpdater<Semaphore, Node> headUpdater = |
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AtomicReferenceFieldUpdater.<Semaphore, Node>newUpdater |
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(Semaphore.class, Node.class, "head"); |
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private static final |
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AtomicIntegerFieldUpdater<Node> statusUpdater = |
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AtomicIntegerFieldUpdater.<Node>newUpdater |
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(Node.class, "status"); |
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dl |
1.2 |
|
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tim |
1.1 |
|
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dl |
1.16 |
/** |
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* Insert node into queue, initializing head and tail if necessary. |
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* @param node the node to insert |
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dl |
1.4 |
*/ |
172 |
dl |
1.16 |
private void enq(Node node) { |
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Node t = tail; |
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if (t == null) { // Must initialize first |
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Node h = new Node(null); |
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while ((t = tail) == null) { |
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if (headUpdater.compareAndSet(this, null, h)) |
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tail = h; |
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} |
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} |
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for (;;) { |
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node.prev = t; // Prev field must be valid before/upon CAS |
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if (tailUpdater.compareAndSet(this, t, node)) { |
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t.next = node; // Next field assignment lags CAS |
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return; |
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} |
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t = tail; |
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} |
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} |
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/** |
193 |
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* Unblock the successor of node |
194 |
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* @param node the node |
195 |
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*/ |
196 |
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private void unparkSuccessor(Node node) { |
197 |
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statusUpdater.compareAndSet(node, SIGNAL, 0); |
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Node s = node.next; |
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if (s == null || s.status == CANCELLED) { |
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s = tail; |
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if (s != null && s != node) { |
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Node p = s.prev; |
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while (p != null && p != node) { |
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if (p.status != CANCELLED) |
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s = p; |
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p = p.prev; |
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} |
208 |
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} |
209 |
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} |
210 |
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if (s != null) |
211 |
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LockSupport.unpark(s.thread); |
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} |
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/** |
216 |
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* Internal version of tryAcquire returning number of remaining |
217 |
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* permits, which is nonnegative only if the acquire succeeded. |
218 |
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* @param permits requested number of permits |
219 |
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* @return remaining number of permits |
220 |
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*/ |
221 |
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private int doTryAcquire(int permits) { |
222 |
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for (;;) { |
223 |
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int available = perms.get(); |
224 |
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int remaining = available - permits; |
225 |
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if (remaining < 0 || |
226 |
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perms.compareAndSet(available, remaining)) |
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return remaining; |
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} |
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} |
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/** |
232 |
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* Main code for untimed acquires. |
233 |
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* @param permits number of permits requested |
234 |
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* @param interrupts interrupt control: -1 for abort on interrupt, |
235 |
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* 0 for continue on interrupt |
236 |
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* @return true if lock acquired (can be false only if interruptible) |
237 |
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*/ |
238 |
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private boolean doAcquire(int permits, int interrupts) { |
239 |
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// Fast path bypasses queue |
240 |
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if ((!fair || head == tail) && doTryAcquire(permits) >= 0) |
241 |
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return true; |
242 |
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Thread current = Thread.currentThread(); |
243 |
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Node node = new Node(current); |
244 |
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// Retry fast path before enqueuing |
245 |
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if (!fair && doTryAcquire(permits) >= 0) |
246 |
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return true; |
247 |
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enq(node); |
248 |
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249 |
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for (;;) { |
250 |
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Node p = node.prev; |
251 |
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if (p == head) { |
252 |
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int remaining = doTryAcquire(permits); |
253 |
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if (remaining >= 0) { |
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p.next = null; |
255 |
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node.thread = null; |
256 |
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node.prev = null; |
257 |
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head = node; |
258 |
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// if still some permits left, wake up successor |
259 |
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if (remaining > 0 && node.status < 0) |
260 |
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unparkSuccessor(node); |
261 |
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if (interrupts > 0) // Re-interrupt on normal exit |
262 |
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current.interrupt(); |
263 |
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return true; |
264 |
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} |
265 |
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} |
266 |
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int status = p.status; |
267 |
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if (status == 0) |
268 |
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statusUpdater.compareAndSet(p, 0, SIGNAL); |
269 |
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else if (status == CANCELLED) |
270 |
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node.prev = p.prev; |
271 |
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else { |
272 |
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assert (status == SIGNAL); |
273 |
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LockSupport.park(); |
274 |
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if (Thread.interrupted()) { |
275 |
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if (interrupts < 0) { |
276 |
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node.thread = null; |
277 |
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node.status = CANCELLED; |
278 |
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unparkSuccessor(node); |
279 |
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return false; |
280 |
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} |
281 |
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interrupts = 1; // set to re-interrupt on exit |
282 |
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} |
283 |
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} |
284 |
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} |
285 |
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} |
286 |
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287 |
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/** |
288 |
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* Main code for timed acquires. Same as doAcquire but with |
289 |
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* interspersed time checks. |
290 |
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* @param permits number of permits requested |
291 |
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* @param nanos timeout in nanosecs |
292 |
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* @return true if lock acquired |
293 |
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*/ |
294 |
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private boolean doTimedAcquire(int permits, long nanos) throws InterruptedException { |
295 |
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if ((!fair || head == tail) && doTryAcquire(permits) >= 0) |
296 |
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return true; |
297 |
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Thread current = Thread.currentThread(); |
298 |
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long lastTime = System.nanoTime(); |
299 |
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Node node = new Node(current); |
300 |
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// Retry fast path before enqueuing |
301 |
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if (!fair && doTryAcquire(permits) >= 0) |
302 |
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return true; |
303 |
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enq(node); |
304 |
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305 |
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for (;;) { |
306 |
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Node p = node.prev; |
307 |
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if (p == head) { |
308 |
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int remaining = doTryAcquire(permits); |
309 |
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if (remaining >= 0) { |
310 |
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p.next = null; |
311 |
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node.thread = null; |
312 |
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node.prev = null; |
313 |
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head = node; |
314 |
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if (remaining > 0 && node.status < 0) |
315 |
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unparkSuccessor(node); |
316 |
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return true; |
317 |
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} |
318 |
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} |
319 |
|
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if (nanos <= 0L) { |
320 |
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node.thread = null; |
321 |
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node.status = CANCELLED; |
322 |
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unparkSuccessor(node); |
323 |
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return false; |
324 |
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} |
325 |
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|
326 |
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int status = p.status; |
327 |
|
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if (status == 0) |
328 |
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statusUpdater.compareAndSet(p, 0, SIGNAL); |
329 |
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else if (status == CANCELLED) |
330 |
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node.prev = p.prev; |
331 |
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else { |
332 |
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LockSupport.parkNanos(nanos); |
333 |
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if (Thread.interrupted()) { |
334 |
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node.thread = null; |
335 |
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node.status = CANCELLED; |
336 |
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unparkSuccessor(node); |
337 |
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throw new InterruptedException(); |
338 |
|
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} |
339 |
|
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long now = System.nanoTime(); |
340 |
|
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nanos -= now - lastTime; |
341 |
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lastTime = now; |
342 |
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} |
343 |
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} |
344 |
|
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} |
345 |
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|
346 |
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/** |
347 |
|
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* Internal version of release |
348 |
|
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*/ |
349 |
|
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private void doRelease(int permits) { |
350 |
|
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for (;;) { |
351 |
|
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int p = perms.get(); |
352 |
|
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if (perms.compareAndSet(p, p + permits)) { |
353 |
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Node h = head; |
354 |
|
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if (h != null && h.status < 0) |
355 |
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unparkSuccessor(h); |
356 |
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return; |
357 |
|
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} |
358 |
|
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} |
359 |
dl |
1.2 |
} |
360 |
tim |
1.1 |
|
361 |
|
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/** |
362 |
|
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* Construct a <tt>Semaphore</tt> with the given number of |
363 |
dl |
1.16 |
* permits and the given fairness setting |
364 |
|
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* @param permits the initial number of permits available. This |
365 |
|
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* value may be negative, in which case releases must |
366 |
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* occur before any acquires will be granted. |
367 |
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* @param fair true if this semaphore will guarantee first-in |
368 |
|
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* first-out granting of permits under contention, else false. |
369 |
tim |
1.1 |
*/ |
370 |
dl |
1.16 |
public Semaphore(int permits, boolean fair) { |
371 |
|
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this.fair = fair; |
372 |
|
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perms = new AtomicInteger(permits); |
373 |
tim |
1.1 |
} |
374 |
|
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|
375 |
|
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/** |
376 |
|
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* Acquires a permit from this semaphore, blocking until one is |
377 |
|
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* available, or the thread is {@link Thread#interrupt interrupted}. |
378 |
|
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* |
379 |
|
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* <p>Acquires a permit, if one is available and returns immediately, |
380 |
|
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* reducing the number of available permits by one. |
381 |
|
|
* <p>If no permit is available then the current thread becomes |
382 |
|
|
* disabled for thread scheduling purposes and lies dormant until |
383 |
|
|
* one of two things happens: |
384 |
|
|
* <ul> |
385 |
|
|
* <li>Some other thread invokes the {@link #release} method for this |
386 |
dl |
1.16 |
* semaphore and the current thread is next to be assigned a permit; or |
387 |
tim |
1.1 |
* <li>Some other thread {@link Thread#interrupt interrupts} the current |
388 |
|
|
* thread. |
389 |
|
|
* </ul> |
390 |
|
|
* |
391 |
|
|
* <p>If the current thread: |
392 |
|
|
* <ul> |
393 |
|
|
* <li>has its interrupted status set on entry to this method; or |
394 |
|
|
* <li>is {@link Thread#interrupt interrupted} while waiting |
395 |
|
|
* for a permit, |
396 |
|
|
* </ul> |
397 |
|
|
* then {@link InterruptedException} is thrown and the current thread's |
398 |
|
|
* interrupted status is cleared. |
399 |
|
|
* |
400 |
|
|
* @throws InterruptedException if the current thread is interrupted |
401 |
|
|
* |
402 |
|
|
* @see Thread#interrupt |
403 |
|
|
*/ |
404 |
dl |
1.2 |
public void acquire() throws InterruptedException { |
405 |
dl |
1.16 |
if (Thread.interrupted() || !doAcquire(1, -1)) |
406 |
|
|
throw new InterruptedException(); |
407 |
dl |
1.2 |
} |
408 |
tim |
1.1 |
|
409 |
|
|
/** |
410 |
|
|
* Acquires a permit from this semaphore, blocking until one is |
411 |
|
|
* available. |
412 |
|
|
* |
413 |
|
|
* <p>Acquires a permit, if one is available and returns immediately, |
414 |
|
|
* reducing the number of available permits by one. |
415 |
|
|
* <p>If no permit is available then the current thread becomes |
416 |
|
|
* disabled for thread scheduling purposes and lies dormant until |
417 |
|
|
* some other thread invokes the {@link #release} method for this |
418 |
dl |
1.16 |
* semaphore and the current thread is next to be assigned a permit. |
419 |
tim |
1.1 |
* |
420 |
|
|
* <p>If the current thread |
421 |
|
|
* is {@link Thread#interrupt interrupted} while waiting |
422 |
|
|
* for a permit then it will continue to wait, but the time at which |
423 |
|
|
* the thread is assigned a permit may change compared to the time it |
424 |
|
|
* would have received the permit had no interruption occurred. When the |
425 |
|
|
* thread does return from this method its interrupt status will be set. |
426 |
|
|
* |
427 |
|
|
*/ |
428 |
dl |
1.2 |
public void acquireUninterruptibly() { |
429 |
dl |
1.16 |
doAcquire(1, 0); |
430 |
dl |
1.2 |
} |
431 |
tim |
1.1 |
|
432 |
|
|
/** |
433 |
|
|
* Acquires a permit from this semaphore, only if one is available at the |
434 |
|
|
* time of invocation. |
435 |
|
|
* <p>Acquires a permit, if one is available and returns immediately, |
436 |
|
|
* with the value <tt>true</tt>, |
437 |
|
|
* reducing the number of available permits by one. |
438 |
|
|
* |
439 |
|
|
* <p>If no permit is available then this method will return |
440 |
|
|
* immediately with the value <tt>false</tt>. |
441 |
|
|
* |
442 |
|
|
* @return <tt>true</tt> if a permit was acquired and <tt>false</tt> |
443 |
|
|
* otherwise. |
444 |
|
|
*/ |
445 |
|
|
public boolean tryAcquire() { |
446 |
dl |
1.16 |
return doTryAcquire(1) >= 0; |
447 |
tim |
1.1 |
} |
448 |
|
|
|
449 |
|
|
/** |
450 |
|
|
* Acquires a permit from this semaphore, if one becomes available |
451 |
|
|
* within the given waiting time and the |
452 |
|
|
* current thread has not been {@link Thread#interrupt interrupted}. |
453 |
|
|
* <p>Acquires a permit, if one is available and returns immediately, |
454 |
|
|
* with the value <tt>true</tt>, |
455 |
|
|
* reducing the number of available permits by one. |
456 |
|
|
* <p>If no permit is available then |
457 |
|
|
* the current thread becomes disabled for thread scheduling |
458 |
|
|
* purposes and lies dormant until one of three things happens: |
459 |
|
|
* <ul> |
460 |
|
|
* <li>Some other thread invokes the {@link #release} method for this |
461 |
dl |
1.16 |
* semaphore and the current thread is next to be assigned a permit; or |
462 |
tim |
1.1 |
* <li>Some other thread {@link Thread#interrupt interrupts} the current |
463 |
|
|
* thread; or |
464 |
|
|
* <li>The specified waiting time elapses. |
465 |
|
|
* </ul> |
466 |
|
|
* <p>If a permit is acquired then the value <tt>true</tt> is returned. |
467 |
|
|
* <p>If the current thread: |
468 |
|
|
* <ul> |
469 |
|
|
* <li>has its interrupted status set on entry to this method; or |
470 |
|
|
* <li>is {@link Thread#interrupt interrupted} while waiting to acquire |
471 |
|
|
* a permit, |
472 |
|
|
* </ul> |
473 |
|
|
* then {@link InterruptedException} is thrown and the current thread's |
474 |
|
|
* interrupted status is cleared. |
475 |
|
|
* <p>If the specified waiting time elapses then the value <tt>false</tt> |
476 |
|
|
* is returned. |
477 |
dl |
1.16 |
* If the time is less than or equal to zero, the method will not wait |
478 |
|
|
* at all. |
479 |
tim |
1.1 |
* |
480 |
|
|
* @param timeout the maximum time to wait for a permit |
481 |
dl |
1.13 |
* @param unit the time unit of the <tt>timeout</tt> argument. |
482 |
tim |
1.1 |
* @return <tt>true</tt> if a permit was acquired and <tt>false</tt> |
483 |
|
|
* if the waiting time elapsed before a permit was acquired. |
484 |
|
|
* |
485 |
|
|
* @throws InterruptedException if the current thread is interrupted |
486 |
|
|
* |
487 |
|
|
* @see Thread#interrupt |
488 |
|
|
* |
489 |
|
|
*/ |
490 |
dl |
1.16 |
public boolean tryAcquire(long timeout, TimeUnit unit) |
491 |
tim |
1.1 |
throws InterruptedException { |
492 |
dl |
1.16 |
if (unit == null) |
493 |
|
|
throw new NullPointerException(); |
494 |
|
|
if (Thread.interrupted()) |
495 |
|
|
throw new InterruptedException(); |
496 |
|
|
return doTimedAcquire(1, unit.toNanos(timeout)); |
497 |
tim |
1.1 |
} |
498 |
|
|
|
499 |
|
|
/** |
500 |
|
|
* Releases a permit, returning it to the semaphore. |
501 |
|
|
* <p>Releases a permit, increasing the number of available permits |
502 |
|
|
* by one. |
503 |
|
|
* If any threads are blocking trying to acquire a permit, then one |
504 |
|
|
* is selected and given the permit that was just released. |
505 |
|
|
* That thread is re-enabled for thread scheduling purposes. |
506 |
|
|
* <p>There is no requirement that a thread that releases a permit must |
507 |
|
|
* have acquired that permit by calling {@link #acquire}. |
508 |
|
|
* Correct usage of a semaphore is established by programming convention |
509 |
|
|
* in the application. |
510 |
|
|
*/ |
511 |
dl |
1.2 |
public void release() { |
512 |
dl |
1.16 |
doRelease(1); |
513 |
|
|
} |
514 |
|
|
|
515 |
|
|
/** |
516 |
|
|
* Acquires the given number of permits from this semaphore, |
517 |
|
|
* blocking until all are available, |
518 |
|
|
* or the thread is {@link Thread#interrupt interrupted}. |
519 |
|
|
* |
520 |
|
|
* <p>Acquires the given number of permits, if they are available, |
521 |
|
|
* and returns immediately, |
522 |
|
|
* reducing the number of available permits by the given amount. |
523 |
|
|
* |
524 |
|
|
* <p>If insufficient permits are available then the current thread becomes |
525 |
|
|
* disabled for thread scheduling purposes and lies dormant until |
526 |
|
|
* one of two things happens: |
527 |
|
|
* <ul> |
528 |
|
|
* <li>Some other thread invokes one of the {@link #release() release} |
529 |
|
|
* methods for this semaphore, the current thread is next to be assigned |
530 |
|
|
* permits and the number of available permits satisfies this request; or |
531 |
|
|
* <li>Some other thread {@link Thread#interrupt interrupts} the current |
532 |
|
|
* thread. |
533 |
|
|
* </ul> |
534 |
|
|
* |
535 |
|
|
* <p>If the current thread: |
536 |
|
|
* <ul> |
537 |
|
|
* <li>has its interrupted status set on entry to this method; or |
538 |
|
|
* <li>is {@link Thread#interrupt interrupted} while waiting |
539 |
|
|
* for a permit, |
540 |
|
|
* </ul> |
541 |
|
|
* then {@link InterruptedException} is thrown and the current thread's |
542 |
|
|
* interrupted status is cleared. |
543 |
|
|
* Any permits that were to be assigned to this thread, are instead |
544 |
|
|
* assigned to the next waiting thread(s), as if |
545 |
|
|
* they had been made available by a call to {@link #release()}. |
546 |
|
|
* |
547 |
|
|
* @param permits the number of permits to acquire |
548 |
|
|
* |
549 |
|
|
* @throws InterruptedException if the current thread is interrupted |
550 |
|
|
* @throws IllegalArgumentException if permits less than zero. |
551 |
|
|
* |
552 |
|
|
* @see Thread#interrupt |
553 |
|
|
*/ |
554 |
|
|
public void acquire(int permits) throws InterruptedException { |
555 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
556 |
|
|
if (Thread.interrupted() || !doAcquire(permits, -1)) |
557 |
|
|
throw new InterruptedException(); |
558 |
|
|
} |
559 |
|
|
|
560 |
|
|
/** |
561 |
|
|
* Acquires the given number of permits from this semaphore, |
562 |
|
|
* blocking until all are available. |
563 |
|
|
* |
564 |
|
|
* <p>Acquires the given number of permits, if they are available, |
565 |
|
|
* and returns immediately, |
566 |
|
|
* reducing the number of available permits by the given amount. |
567 |
|
|
* |
568 |
|
|
* <p>If insufficient permits are available then the current thread becomes |
569 |
|
|
* disabled for thread scheduling purposes and lies dormant until |
570 |
|
|
* some other thread invokes one of the {@link #release() release} |
571 |
|
|
* methods for this semaphore, the current thread is next to be assigned |
572 |
|
|
* permits and the number of available permits satisfies this request. |
573 |
|
|
* |
574 |
|
|
* <p>If the current thread |
575 |
|
|
* is {@link Thread#interrupt interrupted} while waiting |
576 |
|
|
* for permits then it will continue to wait and its position in the |
577 |
|
|
* queue is not affected. When the |
578 |
|
|
* thread does return from this method its interrupt status will be set. |
579 |
|
|
* |
580 |
|
|
* @param permits the number of permits to acquire |
581 |
|
|
* @throws IllegalArgumentException if permits less than zero. |
582 |
|
|
* |
583 |
|
|
*/ |
584 |
|
|
public void acquireUninterruptibly(int permits) { |
585 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
586 |
|
|
doAcquire(permits, 0); |
587 |
|
|
} |
588 |
|
|
|
589 |
|
|
/** |
590 |
|
|
* Acquires the given number of permits from this semaphore, only if |
591 |
|
|
* all are available at the time of invocation. |
592 |
|
|
* <p>Acquires the given number of permits, if they are available, and |
593 |
|
|
* returns immediately, with the value <tt>true</tt>, |
594 |
|
|
* reducing the number of available permits by the given amount. |
595 |
|
|
* |
596 |
|
|
* <p>If insufficient permits are available then this method will return |
597 |
|
|
* immediately with the value <tt>false</tt> and the number of available |
598 |
|
|
* permits is unchanged. |
599 |
|
|
* |
600 |
|
|
* @param permits the number of permits to acquire |
601 |
|
|
* |
602 |
|
|
* @return <tt>true</tt> if the permits were acquired and <tt>false</tt> |
603 |
|
|
* otherwise. |
604 |
|
|
* @throws IllegalArgumentException if permits less than zero. |
605 |
|
|
*/ |
606 |
|
|
public boolean tryAcquire(int permits) { |
607 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
608 |
|
|
return doTryAcquire(permits) >= 0; |
609 |
|
|
} |
610 |
|
|
|
611 |
|
|
/** |
612 |
|
|
* Acquires the given number of permits from this semaphore, if all |
613 |
|
|
* become available within the given waiting time and the |
614 |
|
|
* current thread has not been {@link Thread#interrupt interrupted}. |
615 |
|
|
* <p>Acquires the given number of permits, if they are available and |
616 |
|
|
* returns immediately, with the value <tt>true</tt>, |
617 |
|
|
* reducing the number of available permits by the given amount. |
618 |
|
|
* <p>If insufficient permits are available then |
619 |
|
|
* the current thread becomes disabled for thread scheduling |
620 |
|
|
* purposes and lies dormant until one of three things happens: |
621 |
|
|
* <ul> |
622 |
|
|
* <li>Some other thread invokes one of the {@link #release() release} |
623 |
|
|
* methods for this semaphore, the current thread is next to be assigned |
624 |
|
|
* permits and the number of available permits satisfies this request; or |
625 |
|
|
* <li>Some other thread {@link Thread#interrupt interrupts} the current |
626 |
|
|
* thread; or |
627 |
|
|
* <li>The specified waiting time elapses. |
628 |
|
|
* </ul> |
629 |
|
|
* <p>If the permits are acquired then the value <tt>true</tt> is returned. |
630 |
|
|
* <p>If the current thread: |
631 |
|
|
* <ul> |
632 |
|
|
* <li>has its interrupted status set on entry to this method; or |
633 |
|
|
* <li>is {@link Thread#interrupt interrupted} while waiting to acquire |
634 |
|
|
* the permits, |
635 |
|
|
* </ul> |
636 |
|
|
* then {@link InterruptedException} is thrown and the current thread's |
637 |
|
|
* interrupted status is cleared. |
638 |
|
|
* Any permits that were to be assigned to this thread, are instead |
639 |
|
|
* assigned to the next waiting thread(s), as if |
640 |
|
|
* they had been made available by a call to {@link #release()}. |
641 |
|
|
* |
642 |
|
|
* <p>If the specified waiting time elapses then the value <tt>false</tt> |
643 |
|
|
* is returned. |
644 |
|
|
* If the time is |
645 |
|
|
* less than or equal to zero, the method will not wait at all. |
646 |
|
|
* Any permits that were to be assigned to this thread, are instead |
647 |
|
|
* assigned to the next waiting thread(s), as if |
648 |
|
|
* they had been made available by a call to {@link #release()}. |
649 |
|
|
* |
650 |
|
|
* @param permits the number of permits to acquire |
651 |
|
|
* @param timeout the maximum time to wait for the permits |
652 |
|
|
* @param unit the time unit of the <tt>timeout</tt> argument. |
653 |
|
|
* @return <tt>true</tt> if all permits were acquired and <tt>false</tt> |
654 |
|
|
* if the waiting time elapsed before all permits were acquired. |
655 |
|
|
* |
656 |
|
|
* @throws InterruptedException if the current thread is interrupted |
657 |
|
|
* @throws IllegalArgumentException if permits less than zero. |
658 |
|
|
* |
659 |
|
|
* @see Thread#interrupt |
660 |
|
|
* |
661 |
|
|
*/ |
662 |
|
|
public boolean tryAcquire(int permits, long timeout, TimeUnit unit) |
663 |
|
|
throws InterruptedException { |
664 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
665 |
|
|
if (unit == null) |
666 |
|
|
throw new NullPointerException(); |
667 |
|
|
if (Thread.interrupted()) |
668 |
|
|
throw new InterruptedException(); |
669 |
|
|
return doTimedAcquire(permits, unit.toNanos(timeout)); |
670 |
|
|
} |
671 |
|
|
|
672 |
|
|
|
673 |
|
|
/** |
674 |
|
|
* Releases the given number of permits, returning them to the semaphore. |
675 |
|
|
* <p>Releases the given number of permits, increasing the number of |
676 |
|
|
* available permits by that amount. |
677 |
|
|
* If any threads are blocking trying to acquire permits, then the |
678 |
|
|
* one that has been waiting the intest |
679 |
|
|
* is selected and given the permits that were just released. |
680 |
|
|
* If the number of available permits satisfies that thread's request |
681 |
|
|
* then that thread is re-enabled for thread scheduling purposes; otherwise |
682 |
|
|
* the thread continues to wait. If there are still permits available |
683 |
|
|
* after the first thread's request has been satisfied, then those permits |
684 |
|
|
* are assigned to the next waiting thread. If it is satisfied then it is |
685 |
|
|
* re-enabled for thread scheduling purposes. This continues until there |
686 |
|
|
* are insufficient permits to satisfy the next waiting thread, or there |
687 |
|
|
* are no more waiting threads. |
688 |
|
|
* |
689 |
|
|
* <p>There is no requirement that a thread that releases a permit must |
690 |
|
|
* have acquired that permit by calling {@link Semaphore#acquire acquire}. |
691 |
|
|
* Correct usage of a semaphore is established by programming convention |
692 |
|
|
* in the application. |
693 |
|
|
* |
694 |
|
|
* @param permits the number of permits to release |
695 |
|
|
* @throws IllegalArgumentException if permits less than zero. |
696 |
|
|
*/ |
697 |
|
|
public void release(int permits) { |
698 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
699 |
|
|
doRelease(permits); |
700 |
dl |
1.2 |
} |
701 |
tim |
1.1 |
|
702 |
|
|
/** |
703 |
|
|
* Return the current number of permits available in this semaphore. |
704 |
|
|
* <p>This method is typically used for debugging and testing purposes. |
705 |
|
|
* @return the number of permits available in this semaphore. |
706 |
|
|
*/ |
707 |
dl |
1.16 |
public int availablePermits() { |
708 |
|
|
return perms.get(); |
709 |
tim |
1.1 |
} |
710 |
dl |
1.15 |
|
711 |
|
|
/** |
712 |
|
|
* Shrink the number of available permits by the indicated |
713 |
|
|
* reduction. This method can be useful in subclasses that |
714 |
|
|
* use semaphores to track available resources that become |
715 |
|
|
* unavailable. This method differs from <tt>acquire</tt> |
716 |
|
|
* in that it does not block waiting for permits to become |
717 |
|
|
* available. |
718 |
|
|
* @param reduction the number of permits to remove |
719 |
|
|
* @throws IllegalArgumentException if reduction is negative |
720 |
|
|
*/ |
721 |
dl |
1.16 |
protected void reducePermits(int reduction) { |
722 |
dl |
1.15 |
if (reduction < 0) throw new IllegalArgumentException(); |
723 |
dl |
1.16 |
perms.getAndAdd(-reduction); |
724 |
|
|
} |
725 |
|
|
|
726 |
|
|
/** |
727 |
|
|
* Return true if this semaphore has fairness set true. |
728 |
|
|
* @return true if this semaphore has fairness set true. |
729 |
|
|
*/ |
730 |
|
|
public boolean isFair() { |
731 |
|
|
return fair; |
732 |
dl |
1.15 |
} |
733 |
dl |
1.16 |
|
734 |
|
|
/** |
735 |
|
|
* Returns an estimate of the number of threads waiting to acquire |
736 |
|
|
* a permit. The value is only an estimate because the number of |
737 |
|
|
* threads may change dynamically while this method traverses |
738 |
|
|
* internal data structures. This method is designed for use in |
739 |
|
|
* monitoring of the system state, not for synchronization |
740 |
|
|
* control. |
741 |
|
|
* @return the estimated number of threads waiting for a permit |
742 |
|
|
*/ |
743 |
|
|
public int getQueueLength() { |
744 |
|
|
int n = 0; |
745 |
|
|
for (Node p = tail; p != null && p != head; p = p.prev) |
746 |
|
|
++n; |
747 |
|
|
return n; |
748 |
|
|
} |
749 |
|
|
|
750 |
|
|
/** |
751 |
|
|
* Returns a collection containing threads that may be waiting to |
752 |
|
|
* acquire a permit. Because the actual set of threads may |
753 |
|
|
* change dynamically while constructing this result, the returned |
754 |
|
|
* collection is only a best-effort estimate. The elements of the |
755 |
|
|
* returned collection are in no particular order. This method is |
756 |
|
|
* designed to facilitate construction of subclasses that provide |
757 |
|
|
* more extensive monitoring facilities. |
758 |
|
|
* @return the collection of threads |
759 |
|
|
*/ |
760 |
|
|
protected Collection<Thread> getQueuedThreads() { |
761 |
|
|
ArrayList<Thread> list = new ArrayList<Thread>(); |
762 |
|
|
for (Node p = tail; p != null; p = p.prev) { |
763 |
|
|
Thread t = p.thread; |
764 |
|
|
if (t != null) |
765 |
|
|
list.add(t); |
766 |
|
|
} |
767 |
|
|
return list; |
768 |
|
|
} |
769 |
|
|
|
770 |
tim |
1.1 |
} |