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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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package java.util.concurrent; |
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|
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import java.lang.ref.WeakReference; |
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import java.util.Arrays; |
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import java.util.AbstractQueue; |
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import java.util.Collection; |
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import java.util.Iterator; |
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import java.util.NoSuchElementException; |
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import java.util.concurrent.locks.Condition; |
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import java.util.concurrent.locks.ReentrantLock; |
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|
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/** |
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* A bounded {@linkplain BlockingQueue blocking queue} backed by an |
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* array. This queue orders elements FIFO (first-in-first-out). The |
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* <em>head</em> of the queue is that element that has been on the |
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* queue the longest time. The <em>tail</em> of the queue is that |
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* element that has been on the queue the shortest time. New elements |
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* are inserted at the tail of the queue, and the queue retrieval |
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* operations obtain elements at the head of the queue. |
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* |
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* <p>This is a classic "bounded buffer", in which a |
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* fixed-sized array holds elements inserted by producers and |
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* extracted by consumers. Once created, the capacity cannot be |
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* changed. Attempts to {@code put} an element into a full queue |
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* will result in the operation blocking; attempts to {@code take} an |
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* element from an empty queue will similarly block. |
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* |
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* <p>This class supports an optional fairness policy for ordering |
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* waiting producer and consumer threads. By default, this ordering |
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* is not guaranteed. However, a queue constructed with fairness set |
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* to {@code true} grants threads access in FIFO order. Fairness |
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* generally decreases throughput but reduces variability and avoids |
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* starvation. |
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* |
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* <p>This class and its iterator implement all of the |
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* <em>optional</em> methods of the {@link Collection} and {@link |
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* Iterator} interfaces. |
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* |
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* <p>This class is a member of the |
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* <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
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* Java Collections Framework</a>. |
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* |
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* @since 1.5 |
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* @author Doug Lea |
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* @param <E> the type of elements held in this queue |
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*/ |
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public class ArrayBlockingQueue<E> extends AbstractQueue<E> |
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implements BlockingQueue<E>, java.io.Serializable { |
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|
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/** |
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* Serialization ID. This class relies on default serialization |
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* even for the items array, which is default-serialized, even if |
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* it is empty. Otherwise it could not be declared final, which is |
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* necessary here. |
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*/ |
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private static final long serialVersionUID = -817911632652898426L; |
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|
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/** The queued items */ |
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final Object[] items; |
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|
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/** items index for next take, poll, peek or remove */ |
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int takeIndex; |
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|
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/** items index for next put, offer, or add */ |
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int putIndex; |
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|
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/** Number of elements in the queue */ |
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int count; |
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|
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/* |
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* Concurrency control uses the classic two-condition algorithm |
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* found in any textbook. |
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*/ |
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|
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/** Main lock guarding all access */ |
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final ReentrantLock lock; |
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|
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/** Condition for waiting takes */ |
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private final Condition notEmpty; |
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|
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/** Condition for waiting puts */ |
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private final Condition notFull; |
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|
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/** |
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* Shared state for currently active iterators, or null if there |
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* are known not to be any. Allows queue operations to update |
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* iterator state. |
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*/ |
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transient Itrs itrs = null; |
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|
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// Internal helper methods |
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|
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/** |
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* Circularly decrements array index i. |
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*/ |
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final int dec(int i) { |
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return ((i == 0) ? items.length : i) - 1; |
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} |
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|
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/** |
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* Returns item at index i. |
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*/ |
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@SuppressWarnings("unchecked") |
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final E itemAt(int i) { |
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return (E) items[i]; |
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} |
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|
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/** |
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* Inserts element at current put position, advances, and signals. |
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* Call only when holding lock. |
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*/ |
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private void enqueue(E x) { |
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// assert lock.getHoldCount() == 1; |
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// assert items[putIndex] == null; |
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final Object[] items = this.items; |
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items[putIndex] = x; |
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if (++putIndex == items.length) putIndex = 0; |
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count++; |
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notEmpty.signal(); |
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} |
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|
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/** |
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* Extracts element at current take position, advances, and signals. |
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* Call only when holding lock. |
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*/ |
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private E dequeue() { |
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// assert lock.getHoldCount() == 1; |
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// assert items[takeIndex] != null; |
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final Object[] items = this.items; |
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@SuppressWarnings("unchecked") |
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E x = (E) items[takeIndex]; |
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items[takeIndex] = null; |
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if (++takeIndex == items.length) takeIndex = 0; |
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count--; |
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if (itrs != null) |
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itrs.elementDequeued(); |
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notFull.signal(); |
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return x; |
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} |
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|
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/** |
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* Deletes item at array index removeIndex. |
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* Utility for remove(Object) and iterator.remove. |
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* Call only when holding lock. |
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*/ |
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void removeAt(final int removeIndex) { |
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// assert lock.getHoldCount() == 1; |
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// assert items[removeIndex] != null; |
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// assert removeIndex >= 0 && removeIndex < items.length; |
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final Object[] items = this.items; |
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if (removeIndex == takeIndex) { |
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// removing front item; just advance |
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items[takeIndex] = null; |
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if (++takeIndex == items.length) takeIndex = 0; |
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count--; |
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if (itrs != null) |
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itrs.elementDequeued(); |
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} else { |
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// an "interior" remove |
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|
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// slide over all others up through putIndex. |
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for (int i = removeIndex, putIndex = this.putIndex;;) { |
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int pred = i; |
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if (++i == items.length) i = 0; |
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if (i == putIndex) { |
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items[pred] = null; |
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this.putIndex = pred; |
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break; |
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} |
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items[pred] = items[i]; |
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} |
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count--; |
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if (itrs != null) |
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itrs.removedAt(removeIndex); |
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} |
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notFull.signal(); |
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} |
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|
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/** |
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* Creates an {@code ArrayBlockingQueue} with the given (fixed) |
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* capacity and default access policy. |
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* |
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* @param capacity the capacity of this queue |
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* @throws IllegalArgumentException if {@code capacity < 1} |
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*/ |
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public ArrayBlockingQueue(int capacity) { |
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this(capacity, false); |
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} |
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|
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/** |
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* Creates an {@code ArrayBlockingQueue} with the given (fixed) |
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* capacity and the specified access policy. |
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* |
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* @param capacity the capacity of this queue |
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* @param fair if {@code true} then queue accesses for threads blocked |
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* on insertion or removal, are processed in FIFO order; |
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* if {@code false} the access order is unspecified. |
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* @throws IllegalArgumentException if {@code capacity < 1} |
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*/ |
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public ArrayBlockingQueue(int capacity, boolean fair) { |
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if (capacity <= 0) |
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throw new IllegalArgumentException(); |
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this.items = new Object[capacity]; |
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lock = new ReentrantLock(fair); |
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notEmpty = lock.newCondition(); |
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notFull = lock.newCondition(); |
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} |
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|
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/** |
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* Creates an {@code ArrayBlockingQueue} with the given (fixed) |
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* capacity, the specified access policy and initially containing the |
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* elements of the given collection, |
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* added in traversal order of the collection's iterator. |
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* |
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* @param capacity the capacity of this queue |
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* @param fair if {@code true} then queue accesses for threads blocked |
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* on insertion or removal, are processed in FIFO order; |
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* if {@code false} the access order is unspecified. |
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* @param c the collection of elements to initially contain |
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* @throws IllegalArgumentException if {@code capacity} is less than |
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* {@code c.size()}, or less than 1. |
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* @throws NullPointerException if the specified collection or any |
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* of its elements are null |
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*/ |
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public ArrayBlockingQueue(int capacity, boolean fair, |
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Collection<? extends E> c) { |
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this(capacity, fair); |
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|
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final ReentrantLock lock = this.lock; |
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lock.lock(); // Lock only for visibility, not mutual exclusion |
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try { |
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int i = 0; |
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try { |
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for (E e : c) { |
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if (e == null) throw new NullPointerException(); |
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items[i++] = e; |
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} |
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} catch (ArrayIndexOutOfBoundsException ex) { |
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throw new IllegalArgumentException(); |
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} |
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count = i; |
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putIndex = (i == capacity) ? 0 : i; |
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} finally { |
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lock.unlock(); |
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} |
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} |
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|
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/** |
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* Inserts the specified element at the tail of this queue if it is |
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* possible to do so immediately without exceeding the queue's capacity, |
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* returning {@code true} upon success and throwing an |
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* {@code IllegalStateException} if this queue is full. |
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* |
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* @param e the element to add |
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* @return {@code true} (as specified by {@link Collection#add}) |
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* @throws IllegalStateException if this queue is full |
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* @throws NullPointerException if the specified element is null |
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*/ |
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public boolean add(E e) { |
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return super.add(e); |
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} |
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|
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/** |
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* Inserts the specified element at the tail of this queue if it is |
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* possible to do so immediately without exceeding the queue's capacity, |
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* returning {@code true} upon success and {@code false} if this queue |
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* is full. This method is generally preferable to method {@link #add}, |
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* which can fail to insert an element only by throwing an exception. |
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* |
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* @throws NullPointerException if the specified element is null |
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*/ |
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public boolean offer(E e) { |
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if (e == null) throw new NullPointerException(); |
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final ReentrantLock lock = this.lock; |
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lock.lock(); |
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try { |
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if (count == items.length) |
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return false; |
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else { |
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enqueue(e); |
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return true; |
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} |
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} finally { |
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lock.unlock(); |
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} |
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} |
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|
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/** |
295 |
* Inserts the specified element at the tail of this queue, waiting |
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* for space to become available if the queue is full. |
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* |
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* @throws InterruptedException {@inheritDoc} |
299 |
* @throws NullPointerException {@inheritDoc} |
300 |
*/ |
301 |
public void put(E e) throws InterruptedException { |
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if (e == null) throw new NullPointerException(); |
303 |
final ReentrantLock lock = this.lock; |
304 |
lock.lockInterruptibly(); |
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try { |
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while (count == items.length) |
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notFull.await(); |
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enqueue(e); |
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} finally { |
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lock.unlock(); |
311 |
} |
312 |
} |
313 |
|
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/** |
315 |
* Inserts the specified element at the tail of this queue, waiting |
316 |
* up to the specified wait time for space to become available if |
317 |
* the queue is full. |
318 |
* |
319 |
* @throws InterruptedException {@inheritDoc} |
320 |
* @throws NullPointerException {@inheritDoc} |
321 |
*/ |
322 |
public boolean offer(E e, long timeout, TimeUnit unit) |
323 |
throws InterruptedException { |
324 |
|
325 |
if (e == null) throw new NullPointerException(); |
326 |
long nanos = unit.toNanos(timeout); |
327 |
final ReentrantLock lock = this.lock; |
328 |
lock.lockInterruptibly(); |
329 |
try { |
330 |
while (count == items.length) { |
331 |
if (nanos <= 0) |
332 |
return false; |
333 |
nanos = notFull.awaitNanos(nanos); |
334 |
} |
335 |
enqueue(e); |
336 |
return true; |
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} finally { |
338 |
lock.unlock(); |
339 |
} |
340 |
} |
341 |
|
342 |
public E poll() { |
343 |
final ReentrantLock lock = this.lock; |
344 |
lock.lock(); |
345 |
try { |
346 |
return (count == 0) ? null : dequeue(); |
347 |
} finally { |
348 |
lock.unlock(); |
349 |
} |
350 |
} |
351 |
|
352 |
public E take() throws InterruptedException { |
353 |
final ReentrantLock lock = this.lock; |
354 |
lock.lockInterruptibly(); |
355 |
try { |
356 |
while (count == 0) |
357 |
notEmpty.await(); |
358 |
return dequeue(); |
359 |
} finally { |
360 |
lock.unlock(); |
361 |
} |
362 |
} |
363 |
|
364 |
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
365 |
long nanos = unit.toNanos(timeout); |
366 |
final ReentrantLock lock = this.lock; |
367 |
lock.lockInterruptibly(); |
368 |
try { |
369 |
while (count == 0) { |
370 |
if (nanos <= 0) |
371 |
return null; |
372 |
nanos = notEmpty.awaitNanos(nanos); |
373 |
} |
374 |
return dequeue(); |
375 |
} finally { |
376 |
lock.unlock(); |
377 |
} |
378 |
} |
379 |
|
380 |
public E peek() { |
381 |
final ReentrantLock lock = this.lock; |
382 |
lock.lock(); |
383 |
try { |
384 |
return itemAt(takeIndex); // null when queue is empty |
385 |
} finally { |
386 |
lock.unlock(); |
387 |
} |
388 |
} |
389 |
|
390 |
// this doc comment is overridden to remove the reference to collections |
391 |
// greater in size than Integer.MAX_VALUE |
392 |
/** |
393 |
* Returns the number of elements in this queue. |
394 |
* |
395 |
* @return the number of elements in this queue |
396 |
*/ |
397 |
public int size() { |
398 |
final ReentrantLock lock = this.lock; |
399 |
lock.lock(); |
400 |
try { |
401 |
return count; |
402 |
} finally { |
403 |
lock.unlock(); |
404 |
} |
405 |
} |
406 |
|
407 |
// this doc comment is a modified copy of the inherited doc comment, |
408 |
// without the reference to unlimited queues. |
409 |
/** |
410 |
* Returns the number of additional elements that this queue can ideally |
411 |
* (in the absence of memory or resource constraints) accept without |
412 |
* blocking. This is always equal to the initial capacity of this queue |
413 |
* less the current {@code size} of this queue. |
414 |
* |
415 |
* <p>Note that you <em>cannot</em> always tell if an attempt to insert |
416 |
* an element will succeed by inspecting {@code remainingCapacity} |
417 |
* because it may be the case that another thread is about to |
418 |
* insert or remove an element. |
419 |
*/ |
420 |
public int remainingCapacity() { |
421 |
final ReentrantLock lock = this.lock; |
422 |
lock.lock(); |
423 |
try { |
424 |
return items.length - count; |
425 |
} finally { |
426 |
lock.unlock(); |
427 |
} |
428 |
} |
429 |
|
430 |
/** |
431 |
* Removes a single instance of the specified element from this queue, |
432 |
* if it is present. More formally, removes an element {@code e} such |
433 |
* that {@code o.equals(e)}, if this queue contains one or more such |
434 |
* elements. |
435 |
* Returns {@code true} if this queue contained the specified element |
436 |
* (or equivalently, if this queue changed as a result of the call). |
437 |
* |
438 |
* <p>Removal of interior elements in circular array based queues |
439 |
* is an intrinsically slow and disruptive operation, so should |
440 |
* be undertaken only in exceptional circumstances, ideally |
441 |
* only when the queue is known not to be accessible by other |
442 |
* threads. |
443 |
* |
444 |
* @param o element to be removed from this queue, if present |
445 |
* @return {@code true} if this queue changed as a result of the call |
446 |
*/ |
447 |
public boolean remove(Object o) { |
448 |
if (o == null) return false; |
449 |
final ReentrantLock lock = this.lock; |
450 |
lock.lock(); |
451 |
try { |
452 |
if (count > 0) { |
453 |
final Object[] items = this.items; |
454 |
final int putIndex = this.putIndex; |
455 |
int i = takeIndex; |
456 |
do { |
457 |
if (o.equals(items[i])) { |
458 |
removeAt(i); |
459 |
return true; |
460 |
} |
461 |
if (++i == items.length) i = 0; |
462 |
} while (i != putIndex); |
463 |
} |
464 |
return false; |
465 |
} finally { |
466 |
lock.unlock(); |
467 |
} |
468 |
} |
469 |
|
470 |
/** |
471 |
* Returns {@code true} if this queue contains the specified element. |
472 |
* More formally, returns {@code true} if and only if this queue contains |
473 |
* at least one element {@code e} such that {@code o.equals(e)}. |
474 |
* |
475 |
* @param o object to be checked for containment in this queue |
476 |
* @return {@code true} if this queue contains the specified element |
477 |
*/ |
478 |
public boolean contains(Object o) { |
479 |
if (o == null) return false; |
480 |
final ReentrantLock lock = this.lock; |
481 |
lock.lock(); |
482 |
try { |
483 |
if (count > 0) { |
484 |
final Object[] items = this.items; |
485 |
final int putIndex = this.putIndex; |
486 |
int i = takeIndex; |
487 |
do { |
488 |
if (o.equals(items[i])) |
489 |
return true; |
490 |
if (++i == items.length) i = 0; |
491 |
} while (i != putIndex); |
492 |
} |
493 |
return false; |
494 |
} finally { |
495 |
lock.unlock(); |
496 |
} |
497 |
} |
498 |
|
499 |
/** |
500 |
* Returns an array containing all of the elements in this queue, in |
501 |
* proper sequence. |
502 |
* |
503 |
* <p>The returned array will be "safe" in that no references to it are |
504 |
* maintained by this queue. (In other words, this method must allocate |
505 |
* a new array). The caller is thus free to modify the returned array. |
506 |
* |
507 |
* <p>This method acts as bridge between array-based and collection-based |
508 |
* APIs. |
509 |
* |
510 |
* @return an array containing all of the elements in this queue |
511 |
*/ |
512 |
public Object[] toArray() { |
513 |
final ReentrantLock lock = this.lock; |
514 |
lock.lock(); |
515 |
try { |
516 |
final Object[] items = this.items; |
517 |
final int end = takeIndex + count; |
518 |
final Object[] a = Arrays.copyOfRange(items, takeIndex, end); |
519 |
if (end != putIndex) |
520 |
System.arraycopy(items, 0, a, items.length - takeIndex, putIndex); |
521 |
return a; |
522 |
} finally { |
523 |
lock.unlock(); |
524 |
} |
525 |
} |
526 |
|
527 |
/** |
528 |
* Returns an array containing all of the elements in this queue, in |
529 |
* proper sequence; the runtime type of the returned array is that of |
530 |
* the specified array. If the queue fits in the specified array, it |
531 |
* is returned therein. Otherwise, a new array is allocated with the |
532 |
* runtime type of the specified array and the size of this queue. |
533 |
* |
534 |
* <p>If this queue fits in the specified array with room to spare |
535 |
* (i.e., the array has more elements than this queue), the element in |
536 |
* the array immediately following the end of the queue is set to |
537 |
* {@code null}. |
538 |
* |
539 |
* <p>Like the {@link #toArray()} method, this method acts as bridge between |
540 |
* array-based and collection-based APIs. Further, this method allows |
541 |
* precise control over the runtime type of the output array, and may, |
542 |
* under certain circumstances, be used to save allocation costs. |
543 |
* |
544 |
* <p>Suppose {@code x} is a queue known to contain only strings. |
545 |
* The following code can be used to dump the queue into a newly |
546 |
* allocated array of {@code String}: |
547 |
* |
548 |
* <pre> {@code String[] y = x.toArray(new String[0]);}</pre> |
549 |
* |
550 |
* Note that {@code toArray(new Object[0])} is identical in function to |
551 |
* {@code toArray()}. |
552 |
* |
553 |
* @param a the array into which the elements of the queue are to |
554 |
* be stored, if it is big enough; otherwise, a new array of the |
555 |
* same runtime type is allocated for this purpose |
556 |
* @return an array containing all of the elements in this queue |
557 |
* @throws ArrayStoreException if the runtime type of the specified array |
558 |
* is not a supertype of the runtime type of every element in |
559 |
* this queue |
560 |
* @throws NullPointerException if the specified array is null |
561 |
*/ |
562 |
@SuppressWarnings("unchecked") |
563 |
public <T> T[] toArray(T[] a) { |
564 |
final ReentrantLock lock = this.lock; |
565 |
lock.lock(); |
566 |
try { |
567 |
final Object[] items = this.items; |
568 |
final int count = this.count; |
569 |
final int firstLeg = Math.min(items.length - takeIndex, count); |
570 |
if (a.length < count) { |
571 |
a = (T[]) Arrays.copyOfRange(items, takeIndex, takeIndex + count, |
572 |
a.getClass()); |
573 |
} else { |
574 |
System.arraycopy(items, takeIndex, a, 0, firstLeg); |
575 |
if (a.length > count) |
576 |
a[count] = null; |
577 |
} |
578 |
if (firstLeg < count) |
579 |
System.arraycopy(items, 0, a, firstLeg, putIndex); |
580 |
return a; |
581 |
} finally { |
582 |
lock.unlock(); |
583 |
} |
584 |
} |
585 |
|
586 |
public String toString() { |
587 |
final ReentrantLock lock = this.lock; |
588 |
lock.lock(); |
589 |
try { |
590 |
int k = count; |
591 |
if (k == 0) |
592 |
return "[]"; |
593 |
|
594 |
final Object[] items = this.items; |
595 |
StringBuilder sb = new StringBuilder(); |
596 |
sb.append('['); |
597 |
for (int i = takeIndex; ; ) { |
598 |
Object e = items[i]; |
599 |
sb.append(e == this ? "(this Collection)" : e); |
600 |
if (--k == 0) |
601 |
return sb.append(']').toString(); |
602 |
sb.append(',').append(' '); |
603 |
if (++i == items.length) i = 0; |
604 |
} |
605 |
} finally { |
606 |
lock.unlock(); |
607 |
} |
608 |
} |
609 |
|
610 |
/** |
611 |
* Atomically removes all of the elements from this queue. |
612 |
* The queue will be empty after this call returns. |
613 |
*/ |
614 |
public void clear() { |
615 |
final Object[] items = this.items; |
616 |
final ReentrantLock lock = this.lock; |
617 |
lock.lock(); |
618 |
try { |
619 |
int k = count; |
620 |
if (k > 0) { |
621 |
final int putIndex = this.putIndex; |
622 |
int i = takeIndex; |
623 |
do { |
624 |
items[i] = null; |
625 |
if (++i == items.length) i = 0; |
626 |
} while (i != putIndex); |
627 |
takeIndex = putIndex; |
628 |
count = 0; |
629 |
if (itrs != null) |
630 |
itrs.queueIsEmpty(); |
631 |
for (; k > 0 && lock.hasWaiters(notFull); k--) |
632 |
notFull.signal(); |
633 |
} |
634 |
} finally { |
635 |
lock.unlock(); |
636 |
} |
637 |
} |
638 |
|
639 |
/** |
640 |
* @throws UnsupportedOperationException {@inheritDoc} |
641 |
* @throws ClassCastException {@inheritDoc} |
642 |
* @throws NullPointerException {@inheritDoc} |
643 |
* @throws IllegalArgumentException {@inheritDoc} |
644 |
*/ |
645 |
public int drainTo(Collection<? super E> c) { |
646 |
return drainTo(c, Integer.MAX_VALUE); |
647 |
} |
648 |
|
649 |
/** |
650 |
* @throws UnsupportedOperationException {@inheritDoc} |
651 |
* @throws ClassCastException {@inheritDoc} |
652 |
* @throws NullPointerException {@inheritDoc} |
653 |
* @throws IllegalArgumentException {@inheritDoc} |
654 |
*/ |
655 |
public int drainTo(Collection<? super E> c, int maxElements) { |
656 |
if (c == null) throw new NullPointerException(); |
657 |
if (c == this) |
658 |
throw new IllegalArgumentException(); |
659 |
if (maxElements <= 0) |
660 |
return 0; |
661 |
final Object[] items = this.items; |
662 |
final ReentrantLock lock = this.lock; |
663 |
lock.lock(); |
664 |
try { |
665 |
int n = Math.min(maxElements, count); |
666 |
int take = takeIndex; |
667 |
int i = 0; |
668 |
try { |
669 |
while (i < n) { |
670 |
@SuppressWarnings("unchecked") |
671 |
E x = (E) items[take]; |
672 |
c.add(x); |
673 |
items[take] = null; |
674 |
if (++take == items.length) take = 0; |
675 |
i++; |
676 |
} |
677 |
return n; |
678 |
} finally { |
679 |
// Restore invariants even if c.add() threw |
680 |
if (i > 0) { |
681 |
count -= i; |
682 |
takeIndex = take; |
683 |
if (itrs != null) { |
684 |
if (count == 0) |
685 |
itrs.queueIsEmpty(); |
686 |
else if (i > take) |
687 |
itrs.takeIndexWrapped(); |
688 |
} |
689 |
for (; i > 0 && lock.hasWaiters(notFull); i--) |
690 |
notFull.signal(); |
691 |
} |
692 |
} |
693 |
} finally { |
694 |
lock.unlock(); |
695 |
} |
696 |
} |
697 |
|
698 |
/** |
699 |
* Returns an iterator over the elements in this queue in proper sequence. |
700 |
* The elements will be returned in order from first (head) to last (tail). |
701 |
* |
702 |
* <p>The returned iterator is |
703 |
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
704 |
* |
705 |
* @return an iterator over the elements in this queue in proper sequence |
706 |
*/ |
707 |
public Iterator<E> iterator() { |
708 |
return new Itr(); |
709 |
} |
710 |
|
711 |
/** |
712 |
* Shared data between iterators and their queue, allowing queue |
713 |
* modifications to update iterators when elements are removed. |
714 |
* |
715 |
* This adds a lot of complexity for the sake of correctly |
716 |
* handling some uncommon operations, but the combination of |
717 |
* circular-arrays and supporting interior removes (i.e., those |
718 |
* not at head) would cause iterators to sometimes lose their |
719 |
* places and/or (re)report elements they shouldn't. To avoid |
720 |
* this, when a queue has one or more iterators, it keeps iterator |
721 |
* state consistent by: |
722 |
* |
723 |
* (1) keeping track of the number of "cycles", that is, the |
724 |
* number of times takeIndex has wrapped around to 0. |
725 |
* (2) notifying all iterators via the callback removedAt whenever |
726 |
* an interior element is removed (and thus other elements may |
727 |
* be shifted). |
728 |
* |
729 |
* These suffice to eliminate iterator inconsistencies, but |
730 |
* unfortunately add the secondary responsibility of maintaining |
731 |
* the list of iterators. We track all active iterators in a |
732 |
* simple linked list (accessed only when the queue's lock is |
733 |
* held) of weak references to Itr. The list is cleaned up using |
734 |
* 3 different mechanisms: |
735 |
* |
736 |
* (1) Whenever a new iterator is created, do some O(1) checking for |
737 |
* stale list elements. |
738 |
* |
739 |
* (2) Whenever takeIndex wraps around to 0, check for iterators |
740 |
* that have been unused for more than one wrap-around cycle. |
741 |
* |
742 |
* (3) Whenever the queue becomes empty, all iterators are notified |
743 |
* and this entire data structure is discarded. |
744 |
* |
745 |
* So in addition to the removedAt callback that is necessary for |
746 |
* correctness, iterators have the shutdown and takeIndexWrapped |
747 |
* callbacks that help remove stale iterators from the list. |
748 |
* |
749 |
* Whenever a list element is examined, it is expunged if either |
750 |
* the GC has determined that the iterator is discarded, or if the |
751 |
* iterator reports that it is "detached" (does not need any |
752 |
* further state updates). Overhead is maximal when takeIndex |
753 |
* never advances, iterators are discarded before they are |
754 |
* exhausted, and all removals are interior removes, in which case |
755 |
* all stale iterators are discovered by the GC. But even in this |
756 |
* case we don't increase the amortized complexity. |
757 |
* |
758 |
* Care must be taken to keep list sweeping methods from |
759 |
* reentrantly invoking another such method, causing subtle |
760 |
* corruption bugs. |
761 |
*/ |
762 |
class Itrs { |
763 |
|
764 |
/** |
765 |
* Node in a linked list of weak iterator references. |
766 |
*/ |
767 |
private class Node extends WeakReference<Itr> { |
768 |
Node next; |
769 |
|
770 |
Node(Itr iterator, Node next) { |
771 |
super(iterator); |
772 |
this.next = next; |
773 |
} |
774 |
} |
775 |
|
776 |
/** Incremented whenever takeIndex wraps around to 0 */ |
777 |
int cycles; |
778 |
|
779 |
/** Linked list of weak iterator references */ |
780 |
private Node head; |
781 |
|
782 |
/** Used to expunge stale iterators */ |
783 |
private Node sweeper; |
784 |
|
785 |
private static final int SHORT_SWEEP_PROBES = 4; |
786 |
private static final int LONG_SWEEP_PROBES = 16; |
787 |
|
788 |
Itrs(Itr initial) { |
789 |
register(initial); |
790 |
} |
791 |
|
792 |
/** |
793 |
* Sweeps itrs, looking for and expunging stale iterators. |
794 |
* If at least one was found, tries harder to find more. |
795 |
* Called only from iterating thread. |
796 |
* |
797 |
* @param tryHarder whether to start in try-harder mode, because |
798 |
* there is known to be at least one iterator to collect |
799 |
*/ |
800 |
void doSomeSweeping(boolean tryHarder) { |
801 |
// assert lock.getHoldCount() == 1; |
802 |
// assert head != null; |
803 |
int probes = tryHarder ? LONG_SWEEP_PROBES : SHORT_SWEEP_PROBES; |
804 |
Node o, p; |
805 |
final Node sweeper = this.sweeper; |
806 |
boolean passedGo; // to limit search to one full sweep |
807 |
|
808 |
if (sweeper == null) { |
809 |
o = null; |
810 |
p = head; |
811 |
passedGo = true; |
812 |
} else { |
813 |
o = sweeper; |
814 |
p = o.next; |
815 |
passedGo = false; |
816 |
} |
817 |
|
818 |
for (; probes > 0; probes--) { |
819 |
if (p == null) { |
820 |
if (passedGo) |
821 |
break; |
822 |
o = null; |
823 |
p = head; |
824 |
passedGo = true; |
825 |
} |
826 |
final Itr it = p.get(); |
827 |
final Node next = p.next; |
828 |
if (it == null || it.isDetached()) { |
829 |
// found a discarded/exhausted iterator |
830 |
probes = LONG_SWEEP_PROBES; // "try harder" |
831 |
// unlink p |
832 |
p.clear(); |
833 |
p.next = null; |
834 |
if (o == null) { |
835 |
head = next; |
836 |
if (next == null) { |
837 |
// We've run out of iterators to track; retire |
838 |
itrs = null; |
839 |
return; |
840 |
} |
841 |
} |
842 |
else |
843 |
o.next = next; |
844 |
} else { |
845 |
o = p; |
846 |
} |
847 |
p = next; |
848 |
} |
849 |
|
850 |
this.sweeper = (p == null) ? null : o; |
851 |
} |
852 |
|
853 |
/** |
854 |
* Adds a new iterator to the linked list of tracked iterators. |
855 |
*/ |
856 |
void register(Itr itr) { |
857 |
// assert lock.getHoldCount() == 1; |
858 |
head = new Node(itr, head); |
859 |
} |
860 |
|
861 |
/** |
862 |
* Called whenever takeIndex wraps around to 0. |
863 |
* |
864 |
* Notifies all iterators, and expunges any that are now stale. |
865 |
*/ |
866 |
void takeIndexWrapped() { |
867 |
// assert lock.getHoldCount() == 1; |
868 |
cycles++; |
869 |
for (Node o = null, p = head; p != null;) { |
870 |
final Itr it = p.get(); |
871 |
final Node next = p.next; |
872 |
if (it == null || it.takeIndexWrapped()) { |
873 |
// unlink p |
874 |
// assert it == null || it.isDetached(); |
875 |
p.clear(); |
876 |
p.next = null; |
877 |
if (o == null) |
878 |
head = next; |
879 |
else |
880 |
o.next = next; |
881 |
} else { |
882 |
o = p; |
883 |
} |
884 |
p = next; |
885 |
} |
886 |
if (head == null) // no more iterators to track |
887 |
itrs = null; |
888 |
} |
889 |
|
890 |
/** |
891 |
* Called whenever an interior remove (not at takeIndex) occurred. |
892 |
* |
893 |
* Notifies all iterators, and expunges any that are now stale. |
894 |
*/ |
895 |
void removedAt(int removedIndex) { |
896 |
for (Node o = null, p = head; p != null;) { |
897 |
final Itr it = p.get(); |
898 |
final Node next = p.next; |
899 |
if (it == null || it.removedAt(removedIndex)) { |
900 |
// unlink p |
901 |
// assert it == null || it.isDetached(); |
902 |
p.clear(); |
903 |
p.next = null; |
904 |
if (o == null) |
905 |
head = next; |
906 |
else |
907 |
o.next = next; |
908 |
} else { |
909 |
o = p; |
910 |
} |
911 |
p = next; |
912 |
} |
913 |
if (head == null) // no more iterators to track |
914 |
itrs = null; |
915 |
} |
916 |
|
917 |
/** |
918 |
* Called whenever the queue becomes empty. |
919 |
* |
920 |
* Notifies all active iterators that the queue is empty, |
921 |
* clears all weak refs, and unlinks the itrs datastructure. |
922 |
*/ |
923 |
void queueIsEmpty() { |
924 |
// assert lock.getHoldCount() == 1; |
925 |
for (Node p = head; p != null; p = p.next) { |
926 |
Itr it = p.get(); |
927 |
if (it != null) { |
928 |
p.clear(); |
929 |
it.shutdown(); |
930 |
} |
931 |
} |
932 |
head = null; |
933 |
itrs = null; |
934 |
} |
935 |
|
936 |
/** |
937 |
* Called whenever an element has been dequeued (at takeIndex). |
938 |
*/ |
939 |
void elementDequeued() { |
940 |
// assert lock.getHoldCount() == 1; |
941 |
if (count == 0) |
942 |
queueIsEmpty(); |
943 |
else if (takeIndex == 0) |
944 |
takeIndexWrapped(); |
945 |
} |
946 |
} |
947 |
|
948 |
/** |
949 |
* Iterator for ArrayBlockingQueue. |
950 |
* |
951 |
* To maintain weak consistency with respect to puts and takes, we |
952 |
* read ahead one slot, so as to not report hasNext true but then |
953 |
* not have an element to return. |
954 |
* |
955 |
* We switch into "detached" mode (allowing prompt unlinking from |
956 |
* itrs without help from the GC) when all indices are negative, or |
957 |
* when hasNext returns false for the first time. This allows the |
958 |
* iterator to track concurrent updates completely accurately, |
959 |
* except for the corner case of the user calling Iterator.remove() |
960 |
* after hasNext() returned false. Even in this case, we ensure |
961 |
* that we don't remove the wrong element by keeping track of the |
962 |
* expected element to remove, in lastItem. Yes, we may fail to |
963 |
* remove lastItem from the queue if it moved due to an interleaved |
964 |
* interior remove while in detached mode. |
965 |
*/ |
966 |
private class Itr implements Iterator<E> { |
967 |
/** Index to look for new nextItem; NONE at end */ |
968 |
private int cursor; |
969 |
|
970 |
/** Element to be returned by next call to next(); null if none */ |
971 |
private E nextItem; |
972 |
|
973 |
/** Index of nextItem; NONE if none, REMOVED if removed elsewhere */ |
974 |
private int nextIndex; |
975 |
|
976 |
/** Last element returned; null if none or not detached. */ |
977 |
private E lastItem; |
978 |
|
979 |
/** Index of lastItem, NONE if none, REMOVED if removed elsewhere */ |
980 |
private int lastRet; |
981 |
|
982 |
/** Previous value of takeIndex, or DETACHED when detached */ |
983 |
private int prevTakeIndex; |
984 |
|
985 |
/** Previous value of iters.cycles */ |
986 |
private int prevCycles; |
987 |
|
988 |
/** Special index value indicating "not available" or "undefined" */ |
989 |
private static final int NONE = -1; |
990 |
|
991 |
/** |
992 |
* Special index value indicating "removed elsewhere", that is, |
993 |
* removed by some operation other than a call to this.remove(). |
994 |
*/ |
995 |
private static final int REMOVED = -2; |
996 |
|
997 |
/** Special value for prevTakeIndex indicating "detached mode" */ |
998 |
private static final int DETACHED = -3; |
999 |
|
1000 |
Itr() { |
1001 |
// assert lock.getHoldCount() == 0; |
1002 |
lastRet = NONE; |
1003 |
final ReentrantLock lock = ArrayBlockingQueue.this.lock; |
1004 |
lock.lock(); |
1005 |
try { |
1006 |
if (count == 0) { |
1007 |
// assert itrs == null; |
1008 |
cursor = NONE; |
1009 |
nextIndex = NONE; |
1010 |
prevTakeIndex = DETACHED; |
1011 |
} else { |
1012 |
final int takeIndex = ArrayBlockingQueue.this.takeIndex; |
1013 |
prevTakeIndex = takeIndex; |
1014 |
nextItem = itemAt(nextIndex = takeIndex); |
1015 |
cursor = incCursor(takeIndex); |
1016 |
if (itrs == null) { |
1017 |
itrs = new Itrs(this); |
1018 |
} else { |
1019 |
itrs.register(this); // in this order |
1020 |
itrs.doSomeSweeping(false); |
1021 |
} |
1022 |
prevCycles = itrs.cycles; |
1023 |
// assert takeIndex >= 0; |
1024 |
// assert prevTakeIndex == takeIndex; |
1025 |
// assert nextIndex >= 0; |
1026 |
// assert nextItem != null; |
1027 |
} |
1028 |
} finally { |
1029 |
lock.unlock(); |
1030 |
} |
1031 |
} |
1032 |
|
1033 |
boolean isDetached() { |
1034 |
// assert lock.getHoldCount() == 1; |
1035 |
return prevTakeIndex < 0; |
1036 |
} |
1037 |
|
1038 |
private int incCursor(int index) { |
1039 |
// assert lock.getHoldCount() == 1; |
1040 |
if (++index == items.length) index = 0; |
1041 |
if (index == putIndex) index = NONE; |
1042 |
return index; |
1043 |
} |
1044 |
|
1045 |
/** |
1046 |
* Returns true if index is invalidated by the given number of |
1047 |
* dequeues, starting from prevTakeIndex. |
1048 |
*/ |
1049 |
private boolean invalidated(int index, int prevTakeIndex, |
1050 |
long dequeues, int length) { |
1051 |
if (index < 0) |
1052 |
return false; |
1053 |
int distance = index - prevTakeIndex; |
1054 |
if (distance < 0) |
1055 |
distance += length; |
1056 |
return dequeues > distance; |
1057 |
} |
1058 |
|
1059 |
/** |
1060 |
* Adjusts indices to incorporate all dequeues since the last |
1061 |
* operation on this iterator. Call only from iterating thread. |
1062 |
*/ |
1063 |
private void incorporateDequeues() { |
1064 |
// assert lock.getHoldCount() == 1; |
1065 |
// assert itrs != null; |
1066 |
// assert !isDetached(); |
1067 |
// assert count > 0; |
1068 |
|
1069 |
final int cycles = itrs.cycles; |
1070 |
final int takeIndex = ArrayBlockingQueue.this.takeIndex; |
1071 |
final int prevCycles = this.prevCycles; |
1072 |
final int prevTakeIndex = this.prevTakeIndex; |
1073 |
|
1074 |
if (cycles != prevCycles || takeIndex != prevTakeIndex) { |
1075 |
final int len = items.length; |
1076 |
// how far takeIndex has advanced since the previous |
1077 |
// operation of this iterator |
1078 |
long dequeues = (cycles - prevCycles) * len |
1079 |
+ (takeIndex - prevTakeIndex); |
1080 |
|
1081 |
// Check indices for invalidation |
1082 |
if (invalidated(lastRet, prevTakeIndex, dequeues, len)) |
1083 |
lastRet = REMOVED; |
1084 |
if (invalidated(nextIndex, prevTakeIndex, dequeues, len)) |
1085 |
nextIndex = REMOVED; |
1086 |
if (invalidated(cursor, prevTakeIndex, dequeues, len)) |
1087 |
cursor = takeIndex; |
1088 |
|
1089 |
if (cursor < 0 && nextIndex < 0 && lastRet < 0) |
1090 |
detach(); |
1091 |
else { |
1092 |
this.prevCycles = cycles; |
1093 |
this.prevTakeIndex = takeIndex; |
1094 |
} |
1095 |
} |
1096 |
} |
1097 |
|
1098 |
/** |
1099 |
* Called when itrs should stop tracking this iterator, either |
1100 |
* because there are no more indices to update (cursor < 0 && |
1101 |
* nextIndex < 0 && lastRet < 0) or as a special exception, when |
1102 |
* lastRet >= 0, because hasNext() is about to return false for the |
1103 |
* first time. Call only from iterating thread. |
1104 |
*/ |
1105 |
private void detach() { |
1106 |
// Switch to detached mode |
1107 |
// assert lock.getHoldCount() == 1; |
1108 |
// assert cursor == NONE; |
1109 |
// assert nextIndex < 0; |
1110 |
// assert lastRet < 0 || nextItem == null; |
1111 |
// assert lastRet < 0 ^ lastItem != null; |
1112 |
if (prevTakeIndex >= 0) { |
1113 |
// assert itrs != null; |
1114 |
prevTakeIndex = DETACHED; |
1115 |
// try to unlink from itrs (but not too hard) |
1116 |
itrs.doSomeSweeping(true); |
1117 |
} |
1118 |
} |
1119 |
|
1120 |
/** |
1121 |
* For performance reasons, we would like not to acquire a lock in |
1122 |
* hasNext in the common case. To allow for this, we only access |
1123 |
* fields (i.e. nextItem) that are not modified by update operations |
1124 |
* triggered by queue modifications. |
1125 |
*/ |
1126 |
public boolean hasNext() { |
1127 |
// assert lock.getHoldCount() == 0; |
1128 |
if (nextItem != null) |
1129 |
return true; |
1130 |
noNext(); |
1131 |
return false; |
1132 |
} |
1133 |
|
1134 |
private void noNext() { |
1135 |
final ReentrantLock lock = ArrayBlockingQueue.this.lock; |
1136 |
lock.lock(); |
1137 |
try { |
1138 |
// assert cursor == NONE; |
1139 |
// assert nextIndex == NONE; |
1140 |
if (!isDetached()) { |
1141 |
// assert lastRet >= 0; |
1142 |
incorporateDequeues(); // might update lastRet |
1143 |
if (lastRet >= 0) { |
1144 |
lastItem = itemAt(lastRet); |
1145 |
// assert lastItem != null; |
1146 |
detach(); |
1147 |
} |
1148 |
} |
1149 |
// assert isDetached(); |
1150 |
// assert lastRet < 0 ^ lastItem != null; |
1151 |
} finally { |
1152 |
lock.unlock(); |
1153 |
} |
1154 |
} |
1155 |
|
1156 |
public E next() { |
1157 |
// assert lock.getHoldCount() == 0; |
1158 |
final E x = nextItem; |
1159 |
if (x == null) |
1160 |
throw new NoSuchElementException(); |
1161 |
final ReentrantLock lock = ArrayBlockingQueue.this.lock; |
1162 |
lock.lock(); |
1163 |
try { |
1164 |
if (!isDetached()) |
1165 |
incorporateDequeues(); |
1166 |
// assert nextIndex != NONE; |
1167 |
// assert lastItem == null; |
1168 |
lastRet = nextIndex; |
1169 |
final int cursor = this.cursor; |
1170 |
if (cursor >= 0) { |
1171 |
nextItem = itemAt(nextIndex = cursor); |
1172 |
// assert nextItem != null; |
1173 |
this.cursor = incCursor(cursor); |
1174 |
} else { |
1175 |
nextIndex = NONE; |
1176 |
nextItem = null; |
1177 |
} |
1178 |
} finally { |
1179 |
lock.unlock(); |
1180 |
} |
1181 |
return x; |
1182 |
} |
1183 |
|
1184 |
public void remove() { |
1185 |
// assert lock.getHoldCount() == 0; |
1186 |
final ReentrantLock lock = ArrayBlockingQueue.this.lock; |
1187 |
lock.lock(); |
1188 |
try { |
1189 |
if (!isDetached()) |
1190 |
incorporateDequeues(); // might update lastRet or detach |
1191 |
final int lastRet = this.lastRet; |
1192 |
this.lastRet = NONE; |
1193 |
if (lastRet >= 0) { |
1194 |
if (!isDetached()) |
1195 |
removeAt(lastRet); |
1196 |
else { |
1197 |
final E lastItem = this.lastItem; |
1198 |
// assert lastItem != null; |
1199 |
this.lastItem = null; |
1200 |
if (itemAt(lastRet) == lastItem) |
1201 |
removeAt(lastRet); |
1202 |
} |
1203 |
} else if (lastRet == NONE) |
1204 |
throw new IllegalStateException(); |
1205 |
// else lastRet == REMOVED and the last returned element was |
1206 |
// previously asynchronously removed via an operation other |
1207 |
// than this.remove(), so nothing to do. |
1208 |
|
1209 |
if (cursor < 0 && nextIndex < 0) |
1210 |
detach(); |
1211 |
} finally { |
1212 |
lock.unlock(); |
1213 |
// assert lastRet == NONE; |
1214 |
// assert lastItem == null; |
1215 |
} |
1216 |
} |
1217 |
|
1218 |
/** |
1219 |
* Called to notify the iterator that the queue is empty, or that it |
1220 |
* has fallen hopelessly behind, so that it should abandon any |
1221 |
* further iteration, except possibly to return one more element |
1222 |
* from next(), as promised by returning true from hasNext(). |
1223 |
*/ |
1224 |
void shutdown() { |
1225 |
// assert lock.getHoldCount() == 1; |
1226 |
cursor = NONE; |
1227 |
if (nextIndex >= 0) |
1228 |
nextIndex = REMOVED; |
1229 |
if (lastRet >= 0) { |
1230 |
lastRet = REMOVED; |
1231 |
lastItem = null; |
1232 |
} |
1233 |
prevTakeIndex = DETACHED; |
1234 |
// Don't set nextItem to null because we must continue to be |
1235 |
// able to return it on next(). |
1236 |
// |
1237 |
// Caller will unlink from itrs when convenient. |
1238 |
} |
1239 |
|
1240 |
private int distance(int index, int prevTakeIndex, int length) { |
1241 |
int distance = index - prevTakeIndex; |
1242 |
if (distance < 0) |
1243 |
distance += length; |
1244 |
return distance; |
1245 |
} |
1246 |
|
1247 |
/** |
1248 |
* Called whenever an interior remove (not at takeIndex) occurred. |
1249 |
* |
1250 |
* @return true if this iterator should be unlinked from itrs |
1251 |
*/ |
1252 |
boolean removedAt(int removedIndex) { |
1253 |
// assert lock.getHoldCount() == 1; |
1254 |
if (isDetached()) |
1255 |
return true; |
1256 |
|
1257 |
final int takeIndex = ArrayBlockingQueue.this.takeIndex; |
1258 |
final int prevTakeIndex = this.prevTakeIndex; |
1259 |
final int len = items.length; |
1260 |
// distance from prevTakeIndex to removedIndex |
1261 |
final int removedDistance = |
1262 |
len * (itrs.cycles - this.prevCycles |
1263 |
+ ((removedIndex < takeIndex) ? 1 : 0)) |
1264 |
+ (removedIndex - prevTakeIndex); |
1265 |
// assert itrs.cycles - this.prevCycles >= 0; |
1266 |
// assert itrs.cycles - this.prevCycles <= 1; |
1267 |
// assert removedDistance > 0; |
1268 |
// assert removedIndex != takeIndex; |
1269 |
int cursor = this.cursor; |
1270 |
if (cursor >= 0) { |
1271 |
int x = distance(cursor, prevTakeIndex, len); |
1272 |
if (x == removedDistance) { |
1273 |
if (cursor == putIndex) |
1274 |
this.cursor = cursor = NONE; |
1275 |
} |
1276 |
else if (x > removedDistance) { |
1277 |
// assert cursor != prevTakeIndex; |
1278 |
this.cursor = cursor = dec(cursor); |
1279 |
} |
1280 |
} |
1281 |
int lastRet = this.lastRet; |
1282 |
if (lastRet >= 0) { |
1283 |
int x = distance(lastRet, prevTakeIndex, len); |
1284 |
if (x == removedDistance) |
1285 |
this.lastRet = lastRet = REMOVED; |
1286 |
else if (x > removedDistance) |
1287 |
this.lastRet = lastRet = dec(lastRet); |
1288 |
} |
1289 |
int nextIndex = this.nextIndex; |
1290 |
if (nextIndex >= 0) { |
1291 |
int x = distance(nextIndex, prevTakeIndex, len); |
1292 |
if (x == removedDistance) |
1293 |
this.nextIndex = nextIndex = REMOVED; |
1294 |
else if (x > removedDistance) |
1295 |
this.nextIndex = nextIndex = dec(nextIndex); |
1296 |
} |
1297 |
if (cursor < 0 && nextIndex < 0 && lastRet < 0) { |
1298 |
this.prevTakeIndex = DETACHED; |
1299 |
return true; |
1300 |
} |
1301 |
return false; |
1302 |
} |
1303 |
|
1304 |
/** |
1305 |
* Called whenever takeIndex wraps around to zero. |
1306 |
* |
1307 |
* @return true if this iterator should be unlinked from itrs |
1308 |
*/ |
1309 |
boolean takeIndexWrapped() { |
1310 |
// assert lock.getHoldCount() == 1; |
1311 |
if (isDetached()) |
1312 |
return true; |
1313 |
if (itrs.cycles - prevCycles > 1) { |
1314 |
// All the elements that existed at the time of the last |
1315 |
// operation are gone, so abandon further iteration. |
1316 |
shutdown(); |
1317 |
return true; |
1318 |
} |
1319 |
return false; |
1320 |
} |
1321 |
|
1322 |
// /** Uncomment for debugging. */ |
1323 |
// public String toString() { |
1324 |
// return ("cursor=" + cursor + " " + |
1325 |
// "nextIndex=" + nextIndex + " " + |
1326 |
// "lastRet=" + lastRet + " " + |
1327 |
// "nextItem=" + nextItem + " " + |
1328 |
// "lastItem=" + lastItem + " " + |
1329 |
// "prevCycles=" + prevCycles + " " + |
1330 |
// "prevTakeIndex=" + prevTakeIndex + " " + |
1331 |
// "size()=" + size() + " " + |
1332 |
// "remainingCapacity()=" + remainingCapacity()); |
1333 |
// } |
1334 |
} |
1335 |
|
1336 |
} |