| /* |
| * Written by Doug Lea with assistance from members of JCP JSR-166 |
| * Expert Group and released to the public domain, as explained at |
| * http://creativecommons.org/publicdomain/zero/1.0/ |
| */ |
| |
| package java.util.concurrent; |
| |
| import java.lang.ref.WeakReference; |
| import java.util.AbstractQueue; |
| import java.util.Arrays; |
| import java.util.Collection; |
| import java.util.Iterator; |
| import java.util.NoSuchElementException; |
| import java.util.Objects; |
| import java.util.Spliterator; |
| import java.util.Spliterators; |
| import java.util.concurrent.locks.Condition; |
| import java.util.concurrent.locks.ReentrantLock; |
| |
| // BEGIN android-note |
| // removed link to collections framework docs |
| // END android-note |
| |
| /** |
| * A bounded {@linkplain BlockingQueue blocking queue} backed by an |
| * array. This queue orders elements FIFO (first-in-first-out). The |
| * <em>head</em> of the queue is that element that has been on the |
| * queue the longest time. The <em>tail</em> of the queue is that |
| * element that has been on the queue the shortest time. New elements |
| * are inserted at the tail of the queue, and the queue retrieval |
| * operations obtain elements at the head of the queue. |
| * |
| * <p>This is a classic "bounded buffer", in which a |
| * fixed-sized array holds elements inserted by producers and |
| * extracted by consumers. Once created, the capacity cannot be |
| * changed. Attempts to {@code put} an element into a full queue |
| * will result in the operation blocking; attempts to {@code take} an |
| * element from an empty queue will similarly block. |
| * |
| * <p>This class supports an optional fairness policy for ordering |
| * waiting producer and consumer threads. By default, this ordering |
| * is not guaranteed. However, a queue constructed with fairness set |
| * to {@code true} grants threads access in FIFO order. Fairness |
| * generally decreases throughput but reduces variability and avoids |
| * starvation. |
| * |
| * <p>This class and its iterator implement all of the |
| * <em>optional</em> methods of the {@link Collection} and {@link |
| * Iterator} interfaces. |
| * |
| * @since 1.5 |
| * @author Doug Lea |
| * @param <E> the type of elements held in this queue |
| */ |
| public class ArrayBlockingQueue<E> extends AbstractQueue<E> |
| implements BlockingQueue<E>, java.io.Serializable { |
| |
| /** |
| * Serialization ID. This class relies on default serialization |
| * even for the items array, which is default-serialized, even if |
| * it is empty. Otherwise it could not be declared final, which is |
| * necessary here. |
| */ |
| private static final long serialVersionUID = -817911632652898426L; |
| |
| /** The queued items */ |
| final Object[] items; |
| |
| /** items index for next take, poll, peek or remove */ |
| int takeIndex; |
| |
| /** items index for next put, offer, or add */ |
| int putIndex; |
| |
| /** Number of elements in the queue */ |
| int count; |
| |
| /* |
| * Concurrency control uses the classic two-condition algorithm |
| * found in any textbook. |
| */ |
| |
| /** Main lock guarding all access */ |
| final ReentrantLock lock; |
| |
| /** Condition for waiting takes */ |
| private final Condition notEmpty; |
| |
| /** Condition for waiting puts */ |
| private final Condition notFull; |
| |
| /** |
| * Shared state for currently active iterators, or null if there |
| * are known not to be any. Allows queue operations to update |
| * iterator state. |
| */ |
| transient Itrs itrs; |
| |
| // Internal helper methods |
| |
| /** |
| * Circularly decrements array index i. |
| */ |
| final int dec(int i) { |
| return ((i == 0) ? items.length : i) - 1; |
| } |
| |
| /** |
| * Returns item at index i. |
| */ |
| @SuppressWarnings("unchecked") |
| final E itemAt(int i) { |
| return (E) items[i]; |
| } |
| |
| /** |
| * Inserts element at current put position, advances, and signals. |
| * Call only when holding lock. |
| */ |
| private void enqueue(E x) { |
| // assert lock.getHoldCount() == 1; |
| // assert items[putIndex] == null; |
| final Object[] items = this.items; |
| items[putIndex] = x; |
| if (++putIndex == items.length) putIndex = 0; |
| count++; |
| notEmpty.signal(); |
| } |
| |
| /** |
| * Extracts element at current take position, advances, and signals. |
| * Call only when holding lock. |
| */ |
| private E dequeue() { |
| // assert lock.getHoldCount() == 1; |
| // assert items[takeIndex] != null; |
| final Object[] items = this.items; |
| @SuppressWarnings("unchecked") |
| E x = (E) items[takeIndex]; |
| items[takeIndex] = null; |
| if (++takeIndex == items.length) takeIndex = 0; |
| count--; |
| if (itrs != null) |
| itrs.elementDequeued(); |
| notFull.signal(); |
| return x; |
| } |
| |
| /** |
| * Deletes item at array index removeIndex. |
| * Utility for remove(Object) and iterator.remove. |
| * Call only when holding lock. |
| */ |
| void removeAt(final int removeIndex) { |
| // assert lock.getHoldCount() == 1; |
| // assert items[removeIndex] != null; |
| // assert removeIndex >= 0 && removeIndex < items.length; |
| final Object[] items = this.items; |
| if (removeIndex == takeIndex) { |
| // removing front item; just advance |
| items[takeIndex] = null; |
| if (++takeIndex == items.length) takeIndex = 0; |
| count--; |
| if (itrs != null) |
| itrs.elementDequeued(); |
| } else { |
| // an "interior" remove |
| |
| // slide over all others up through putIndex. |
| for (int i = removeIndex, putIndex = this.putIndex;;) { |
| int pred = i; |
| if (++i == items.length) i = 0; |
| if (i == putIndex) { |
| items[pred] = null; |
| this.putIndex = pred; |
| break; |
| } |
| items[pred] = items[i]; |
| } |
| count--; |
| if (itrs != null) |
| itrs.removedAt(removeIndex); |
| } |
| notFull.signal(); |
| } |
| |
| /** |
| * Creates an {@code ArrayBlockingQueue} with the given (fixed) |
| * capacity and default access policy. |
| * |
| * @param capacity the capacity of this queue |
| * @throws IllegalArgumentException if {@code capacity < 1} |
| */ |
| public ArrayBlockingQueue(int capacity) { |
| this(capacity, false); |
| } |
| |
| /** |
| * Creates an {@code ArrayBlockingQueue} with the given (fixed) |
| * capacity and the specified access policy. |
| * |
| * @param capacity the capacity of this queue |
| * @param fair if {@code true} then queue accesses for threads blocked |
| * on insertion or removal, are processed in FIFO order; |
| * if {@code false} the access order is unspecified. |
| * @throws IllegalArgumentException if {@code capacity < 1} |
| */ |
| public ArrayBlockingQueue(int capacity, boolean fair) { |
| if (capacity <= 0) |
| throw new IllegalArgumentException(); |
| this.items = new Object[capacity]; |
| lock = new ReentrantLock(fair); |
| notEmpty = lock.newCondition(); |
| notFull = lock.newCondition(); |
| } |
| |
| /** |
| * Creates an {@code ArrayBlockingQueue} with the given (fixed) |
| * capacity, the specified access policy and initially containing the |
| * elements of the given collection, |
| * added in traversal order of the collection's iterator. |
| * |
| * @param capacity the capacity of this queue |
| * @param fair if {@code true} then queue accesses for threads blocked |
| * on insertion or removal, are processed in FIFO order; |
| * if {@code false} the access order is unspecified. |
| * @param c the collection of elements to initially contain |
| * @throws IllegalArgumentException if {@code capacity} is less than |
| * {@code c.size()}, or less than 1. |
| * @throws NullPointerException if the specified collection or any |
| * of its elements are null |
| */ |
| public ArrayBlockingQueue(int capacity, boolean fair, |
| Collection<? extends E> c) { |
| this(capacity, fair); |
| |
| final ReentrantLock lock = this.lock; |
| lock.lock(); // Lock only for visibility, not mutual exclusion |
| try { |
| int i = 0; |
| try { |
| for (E e : c) |
| items[i++] = Objects.requireNonNull(e); |
| } catch (ArrayIndexOutOfBoundsException ex) { |
| throw new IllegalArgumentException(); |
| } |
| count = i; |
| putIndex = (i == capacity) ? 0 : i; |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| /** |
| * Inserts the specified element at the tail of this queue if it is |
| * possible to do so immediately without exceeding the queue's capacity, |
| * returning {@code true} upon success and throwing an |
| * {@code IllegalStateException} if this queue is full. |
| * |
| * @param e the element to add |
| * @return {@code true} (as specified by {@link Collection#add}) |
| * @throws IllegalStateException if this queue is full |
| * @throws NullPointerException if the specified element is null |
| */ |
| public boolean add(E e) { |
| return super.add(e); |
| } |
| |
| /** |
| * Inserts the specified element at the tail of this queue if it is |
| * possible to do so immediately without exceeding the queue's capacity, |
| * returning {@code true} upon success and {@code false} if this queue |
| * is full. This method is generally preferable to method {@link #add}, |
| * which can fail to insert an element only by throwing an exception. |
| * |
| * @throws NullPointerException if the specified element is null |
| */ |
| public boolean offer(E e) { |
| Objects.requireNonNull(e); |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| if (count == items.length) |
| return false; |
| else { |
| enqueue(e); |
| return true; |
| } |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| /** |
| * Inserts the specified element at the tail of this queue, waiting |
| * for space to become available if the queue is full. |
| * |
| * @throws InterruptedException {@inheritDoc} |
| * @throws NullPointerException {@inheritDoc} |
| */ |
| public void put(E e) throws InterruptedException { |
| Objects.requireNonNull(e); |
| final ReentrantLock lock = this.lock; |
| lock.lockInterruptibly(); |
| try { |
| while (count == items.length) |
| notFull.await(); |
| enqueue(e); |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| /** |
| * Inserts the specified element at the tail of this queue, waiting |
| * up to the specified wait time for space to become available if |
| * the queue is full. |
| * |
| * @throws InterruptedException {@inheritDoc} |
| * @throws NullPointerException {@inheritDoc} |
| */ |
| public boolean offer(E e, long timeout, TimeUnit unit) |
| throws InterruptedException { |
| |
| Objects.requireNonNull(e); |
| long nanos = unit.toNanos(timeout); |
| final ReentrantLock lock = this.lock; |
| lock.lockInterruptibly(); |
| try { |
| while (count == items.length) { |
| if (nanos <= 0L) |
| return false; |
| nanos = notFull.awaitNanos(nanos); |
| } |
| enqueue(e); |
| return true; |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| public E poll() { |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| return (count == 0) ? null : dequeue(); |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| public E take() throws InterruptedException { |
| final ReentrantLock lock = this.lock; |
| lock.lockInterruptibly(); |
| try { |
| while (count == 0) |
| notEmpty.await(); |
| return dequeue(); |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
| long nanos = unit.toNanos(timeout); |
| final ReentrantLock lock = this.lock; |
| lock.lockInterruptibly(); |
| try { |
| while (count == 0) { |
| if (nanos <= 0L) |
| return null; |
| nanos = notEmpty.awaitNanos(nanos); |
| } |
| return dequeue(); |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| public E peek() { |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| return itemAt(takeIndex); // null when queue is empty |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| // this doc comment is overridden to remove the reference to collections |
| // greater in size than Integer.MAX_VALUE |
| /** |
| * Returns the number of elements in this queue. |
| * |
| * @return the number of elements in this queue |
| */ |
| public int size() { |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| return count; |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| // this doc comment is a modified copy of the inherited doc comment, |
| // without the reference to unlimited queues. |
| /** |
| * Returns the number of additional elements that this queue can ideally |
| * (in the absence of memory or resource constraints) accept without |
| * blocking. This is always equal to the initial capacity of this queue |
| * less the current {@code size} of this queue. |
| * |
| * <p>Note that you <em>cannot</em> always tell if an attempt to insert |
| * an element will succeed by inspecting {@code remainingCapacity} |
| * because it may be the case that another thread is about to |
| * insert or remove an element. |
| */ |
| public int remainingCapacity() { |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| return items.length - count; |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| /** |
| * Removes a single instance of the specified element from this queue, |
| * if it is present. More formally, removes an element {@code e} such |
| * that {@code o.equals(e)}, if this queue contains one or more such |
| * elements. |
| * Returns {@code true} if this queue contained the specified element |
| * (or equivalently, if this queue changed as a result of the call). |
| * |
| * <p>Removal of interior elements in circular array based queues |
| * is an intrinsically slow and disruptive operation, so should |
| * be undertaken only in exceptional circumstances, ideally |
| * only when the queue is known not to be accessible by other |
| * threads. |
| * |
| * @param o element to be removed from this queue, if present |
| * @return {@code true} if this queue changed as a result of the call |
| */ |
| public boolean remove(Object o) { |
| if (o == null) return false; |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| if (count > 0) { |
| final Object[] items = this.items; |
| final int putIndex = this.putIndex; |
| int i = takeIndex; |
| do { |
| if (o.equals(items[i])) { |
| removeAt(i); |
| return true; |
| } |
| if (++i == items.length) i = 0; |
| } while (i != putIndex); |
| } |
| return false; |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| /** |
| * Returns {@code true} if this queue contains the specified element. |
| * More formally, returns {@code true} if and only if this queue contains |
| * at least one element {@code e} such that {@code o.equals(e)}. |
| * |
| * @param o object to be checked for containment in this queue |
| * @return {@code true} if this queue contains the specified element |
| */ |
| public boolean contains(Object o) { |
| if (o == null) return false; |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| if (count > 0) { |
| final Object[] items = this.items; |
| final int putIndex = this.putIndex; |
| int i = takeIndex; |
| do { |
| if (o.equals(items[i])) |
| return true; |
| if (++i == items.length) i = 0; |
| } while (i != putIndex); |
| } |
| return false; |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| /** |
| * Returns an array containing all of the elements in this queue, in |
| * proper sequence. |
| * |
| * <p>The returned array will be "safe" in that no references to it are |
| * maintained by this queue. (In other words, this method must allocate |
| * a new array). The caller is thus free to modify the returned array. |
| * |
| * <p>This method acts as bridge between array-based and collection-based |
| * APIs. |
| * |
| * @return an array containing all of the elements in this queue |
| */ |
| public Object[] toArray() { |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| final Object[] items = this.items; |
| final int end = takeIndex + count; |
| final Object[] a = Arrays.copyOfRange(items, takeIndex, end); |
| if (end != putIndex) |
| System.arraycopy(items, 0, a, items.length - takeIndex, putIndex); |
| return a; |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| /** |
| * Returns an array containing all of the elements in this queue, in |
| * proper sequence; the runtime type of the returned array is that of |
| * the specified array. If the queue fits in the specified array, it |
| * is returned therein. Otherwise, a new array is allocated with the |
| * runtime type of the specified array and the size of this queue. |
| * |
| * <p>If this queue fits in the specified array with room to spare |
| * (i.e., the array has more elements than this queue), the element in |
| * the array immediately following the end of the queue is set to |
| * {@code null}. |
| * |
| * <p>Like the {@link #toArray()} method, this method acts as bridge between |
| * array-based and collection-based APIs. Further, this method allows |
| * precise control over the runtime type of the output array, and may, |
| * under certain circumstances, be used to save allocation costs. |
| * |
| * <p>Suppose {@code x} is a queue known to contain only strings. |
| * The following code can be used to dump the queue into a newly |
| * allocated array of {@code String}: |
| * |
| * <pre> {@code String[] y = x.toArray(new String[0]);}</pre> |
| * |
| * Note that {@code toArray(new Object[0])} is identical in function to |
| * {@code toArray()}. |
| * |
| * @param a the array into which the elements of the queue are to |
| * be stored, if it is big enough; otherwise, a new array of the |
| * same runtime type is allocated for this purpose |
| * @return an array containing all of the elements in this queue |
| * @throws ArrayStoreException if the runtime type of the specified array |
| * is not a supertype of the runtime type of every element in |
| * this queue |
| * @throws NullPointerException if the specified array is null |
| */ |
| @SuppressWarnings("unchecked") |
| public <T> T[] toArray(T[] a) { |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| final Object[] items = this.items; |
| final int count = this.count; |
| final int firstLeg = Math.min(items.length - takeIndex, count); |
| if (a.length < count) { |
| a = (T[]) Arrays.copyOfRange(items, takeIndex, takeIndex + count, |
| a.getClass()); |
| } else { |
| System.arraycopy(items, takeIndex, a, 0, firstLeg); |
| if (a.length > count) |
| a[count] = null; |
| } |
| if (firstLeg < count) |
| System.arraycopy(items, 0, a, firstLeg, putIndex); |
| return a; |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| public String toString() { |
| return Helpers.collectionToString(this); |
| } |
| |
| /** |
| * Atomically removes all of the elements from this queue. |
| * The queue will be empty after this call returns. |
| */ |
| public void clear() { |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| int k = count; |
| if (k > 0) { |
| final Object[] items = this.items; |
| final int putIndex = this.putIndex; |
| int i = takeIndex; |
| do { |
| items[i] = null; |
| if (++i == items.length) i = 0; |
| } while (i != putIndex); |
| takeIndex = putIndex; |
| count = 0; |
| if (itrs != null) |
| itrs.queueIsEmpty(); |
| for (; k > 0 && lock.hasWaiters(notFull); k--) |
| notFull.signal(); |
| } |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| /** |
| * @throws UnsupportedOperationException {@inheritDoc} |
| * @throws ClassCastException {@inheritDoc} |
| * @throws NullPointerException {@inheritDoc} |
| * @throws IllegalArgumentException {@inheritDoc} |
| */ |
| public int drainTo(Collection<? super E> c) { |
| return drainTo(c, Integer.MAX_VALUE); |
| } |
| |
| /** |
| * @throws UnsupportedOperationException {@inheritDoc} |
| * @throws ClassCastException {@inheritDoc} |
| * @throws NullPointerException {@inheritDoc} |
| * @throws IllegalArgumentException {@inheritDoc} |
| */ |
| public int drainTo(Collection<? super E> c, int maxElements) { |
| Objects.requireNonNull(c); |
| if (c == this) |
| throw new IllegalArgumentException(); |
| if (maxElements <= 0) |
| return 0; |
| final Object[] items = this.items; |
| final ReentrantLock lock = this.lock; |
| lock.lock(); |
| try { |
| int n = Math.min(maxElements, count); |
| int take = takeIndex; |
| int i = 0; |
| try { |
| while (i < n) { |
| @SuppressWarnings("unchecked") |
| E x = (E) items[take]; |
| c.add(x); |
| items[take] = null; |
| if (++take == items.length) take = 0; |
| i++; |
| } |
| return n; |
| } finally { |
| // Restore invariants even if c.add() threw |
| if (i > 0) { |
| count -= i; |
| takeIndex = take; |
| if (itrs != null) { |
| if (count == 0) |
| itrs.queueIsEmpty(); |
| else if (i > take) |
| itrs.takeIndexWrapped(); |
| } |
| for (; i > 0 && lock.hasWaiters(notFull); i--) |
| notFull.signal(); |
| } |
| } |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| /** |
| * Returns an iterator over the elements in this queue in proper sequence. |
| * The elements will be returned in order from first (head) to last (tail). |
| * |
| * <p>The returned iterator is |
| * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
| * |
| * @return an iterator over the elements in this queue in proper sequence |
| */ |
| public Iterator<E> iterator() { |
| return new Itr(); |
| } |
| |
| /** |
| * Shared data between iterators and their queue, allowing queue |
| * modifications to update iterators when elements are removed. |
| * |
| * This adds a lot of complexity for the sake of correctly |
| * handling some uncommon operations, but the combination of |
| * circular-arrays and supporting interior removes (i.e., those |
| * not at head) would cause iterators to sometimes lose their |
| * places and/or (re)report elements they shouldn't. To avoid |
| * this, when a queue has one or more iterators, it keeps iterator |
| * state consistent by: |
| * |
| * (1) keeping track of the number of "cycles", that is, the |
| * number of times takeIndex has wrapped around to 0. |
| * (2) notifying all iterators via the callback removedAt whenever |
| * an interior element is removed (and thus other elements may |
| * be shifted). |
| * |
| * These suffice to eliminate iterator inconsistencies, but |
| * unfortunately add the secondary responsibility of maintaining |
| * the list of iterators. We track all active iterators in a |
| * simple linked list (accessed only when the queue's lock is |
| * held) of weak references to Itr. The list is cleaned up using |
| * 3 different mechanisms: |
| * |
| * (1) Whenever a new iterator is created, do some O(1) checking for |
| * stale list elements. |
| * |
| * (2) Whenever takeIndex wraps around to 0, check for iterators |
| * that have been unused for more than one wrap-around cycle. |
| * |
| * (3) Whenever the queue becomes empty, all iterators are notified |
| * and this entire data structure is discarded. |
| * |
| * So in addition to the removedAt callback that is necessary for |
| * correctness, iterators have the shutdown and takeIndexWrapped |
| * callbacks that help remove stale iterators from the list. |
| * |
| * Whenever a list element is examined, it is expunged if either |
| * the GC has determined that the iterator is discarded, or if the |
| * iterator reports that it is "detached" (does not need any |
| * further state updates). Overhead is maximal when takeIndex |
| * never advances, iterators are discarded before they are |
| * exhausted, and all removals are interior removes, in which case |
| * all stale iterators are discovered by the GC. But even in this |
| * case we don't increase the amortized complexity. |
| * |
| * Care must be taken to keep list sweeping methods from |
| * reentrantly invoking another such method, causing subtle |
| * corruption bugs. |
| */ |
| class Itrs { |
| |
| /** |
| * Node in a linked list of weak iterator references. |
| */ |
| private class Node extends WeakReference<Itr> { |
| Node next; |
| |
| Node(Itr iterator, Node next) { |
| super(iterator); |
| this.next = next; |
| } |
| } |
| |
| /** Incremented whenever takeIndex wraps around to 0 */ |
| int cycles; |
| |
| /** Linked list of weak iterator references */ |
| private Node head; |
| |
| /** Used to expunge stale iterators */ |
| private Node sweeper; |
| |
| private static final int SHORT_SWEEP_PROBES = 4; |
| private static final int LONG_SWEEP_PROBES = 16; |
| |
| Itrs(Itr initial) { |
| register(initial); |
| } |
| |
| /** |
| * Sweeps itrs, looking for and expunging stale iterators. |
| * If at least one was found, tries harder to find more. |
| * Called only from iterating thread. |
| * |
| * @param tryHarder whether to start in try-harder mode, because |
| * there is known to be at least one iterator to collect |
| */ |
| void doSomeSweeping(boolean tryHarder) { |
| // assert lock.getHoldCount() == 1; |
| // assert head != null; |
| int probes = tryHarder ? LONG_SWEEP_PROBES : SHORT_SWEEP_PROBES; |
| Node o, p; |
| final Node sweeper = this.sweeper; |
| boolean passedGo; // to limit search to one full sweep |
| |
| if (sweeper == null) { |
| o = null; |
| p = head; |
| passedGo = true; |
| } else { |
| o = sweeper; |
| p = o.next; |
| passedGo = false; |
| } |
| |
| for (; probes > 0; probes--) { |
| if (p == null) { |
| if (passedGo) |
| break; |
| o = null; |
| p = head; |
| passedGo = true; |
| } |
| final Itr it = p.get(); |
| final Node next = p.next; |
| if (it == null || it.isDetached()) { |
| // found a discarded/exhausted iterator |
| probes = LONG_SWEEP_PROBES; // "try harder" |
| // unlink p |
| p.clear(); |
| p.next = null; |
| if (o == null) { |
| head = next; |
| if (next == null) { |
| // We've run out of iterators to track; retire |
| itrs = null; |
| return; |
| } |
| } |
| else |
| o.next = next; |
| } else { |
| o = p; |
| } |
| p = next; |
| } |
| |
| this.sweeper = (p == null) ? null : o; |
| } |
| |
| /** |
| * Adds a new iterator to the linked list of tracked iterators. |
| */ |
| void register(Itr itr) { |
| // assert lock.getHoldCount() == 1; |
| head = new Node(itr, head); |
| } |
| |
| /** |
| * Called whenever takeIndex wraps around to 0. |
| * |
| * Notifies all iterators, and expunges any that are now stale. |
| */ |
| void takeIndexWrapped() { |
| // assert lock.getHoldCount() == 1; |
| cycles++; |
| for (Node o = null, p = head; p != null;) { |
| final Itr it = p.get(); |
| final Node next = p.next; |
| if (it == null || it.takeIndexWrapped()) { |
| // unlink p |
| // assert it == null || it.isDetached(); |
| p.clear(); |
| p.next = null; |
| if (o == null) |
| head = next; |
| else |
| o.next = next; |
| } else { |
| o = p; |
| } |
| p = next; |
| } |
| if (head == null) // no more iterators to track |
| itrs = null; |
| } |
| |
| /** |
| * Called whenever an interior remove (not at takeIndex) occurred. |
| * |
| * Notifies all iterators, and expunges any that are now stale. |
| */ |
| void removedAt(int removedIndex) { |
| for (Node o = null, p = head; p != null;) { |
| final Itr it = p.get(); |
| final Node next = p.next; |
| if (it == null || it.removedAt(removedIndex)) { |
| // unlink p |
| // assert it == null || it.isDetached(); |
| p.clear(); |
| p.next = null; |
| if (o == null) |
| head = next; |
| else |
| o.next = next; |
| } else { |
| o = p; |
| } |
| p = next; |
| } |
| if (head == null) // no more iterators to track |
| itrs = null; |
| } |
| |
| /** |
| * Called whenever the queue becomes empty. |
| * |
| * Notifies all active iterators that the queue is empty, |
| * clears all weak refs, and unlinks the itrs datastructure. |
| */ |
| void queueIsEmpty() { |
| // assert lock.getHoldCount() == 1; |
| for (Node p = head; p != null; p = p.next) { |
| Itr it = p.get(); |
| if (it != null) { |
| p.clear(); |
| it.shutdown(); |
| } |
| } |
| head = null; |
| itrs = null; |
| } |
| |
| /** |
| * Called whenever an element has been dequeued (at takeIndex). |
| */ |
| void elementDequeued() { |
| // assert lock.getHoldCount() == 1; |
| if (count == 0) |
| queueIsEmpty(); |
| else if (takeIndex == 0) |
| takeIndexWrapped(); |
| } |
| } |
| |
| /** |
| * Iterator for ArrayBlockingQueue. |
| * |
| * To maintain weak consistency with respect to puts and takes, we |
| * read ahead one slot, so as to not report hasNext true but then |
| * not have an element to return. |
| * |
| * We switch into "detached" mode (allowing prompt unlinking from |
| * itrs without help from the GC) when all indices are negative, or |
| * when hasNext returns false for the first time. This allows the |
| * iterator to track concurrent updates completely accurately, |
| * except for the corner case of the user calling Iterator.remove() |
| * after hasNext() returned false. Even in this case, we ensure |
| * that we don't remove the wrong element by keeping track of the |
| * expected element to remove, in lastItem. Yes, we may fail to |
| * remove lastItem from the queue if it moved due to an interleaved |
| * interior remove while in detached mode. |
| */ |
| private class Itr implements Iterator<E> { |
| /** Index to look for new nextItem; NONE at end */ |
| private int cursor; |
| |
| /** Element to be returned by next call to next(); null if none */ |
| private E nextItem; |
| |
| /** Index of nextItem; NONE if none, REMOVED if removed elsewhere */ |
| private int nextIndex; |
| |
| /** Last element returned; null if none or not detached. */ |
| private E lastItem; |
| |
| /** Index of lastItem, NONE if none, REMOVED if removed elsewhere */ |
| private int lastRet; |
| |
| /** Previous value of takeIndex, or DETACHED when detached */ |
| private int prevTakeIndex; |
| |
| /** Previous value of iters.cycles */ |
| private int prevCycles; |
| |
| /** Special index value indicating "not available" or "undefined" */ |
| private static final int NONE = -1; |
| |
| /** |
| * Special index value indicating "removed elsewhere", that is, |
| * removed by some operation other than a call to this.remove(). |
| */ |
| private static final int REMOVED = -2; |
| |
| /** Special value for prevTakeIndex indicating "detached mode" */ |
| private static final int DETACHED = -3; |
| |
| Itr() { |
| // assert lock.getHoldCount() == 0; |
| lastRet = NONE; |
| final ReentrantLock lock = ArrayBlockingQueue.this.lock; |
| lock.lock(); |
| try { |
| if (count == 0) { |
| // assert itrs == null; |
| cursor = NONE; |
| nextIndex = NONE; |
| prevTakeIndex = DETACHED; |
| } else { |
| final int takeIndex = ArrayBlockingQueue.this.takeIndex; |
| prevTakeIndex = takeIndex; |
| nextItem = itemAt(nextIndex = takeIndex); |
| cursor = incCursor(takeIndex); |
| if (itrs == null) { |
| itrs = new Itrs(this); |
| } else { |
| itrs.register(this); // in this order |
| itrs.doSomeSweeping(false); |
| } |
| prevCycles = itrs.cycles; |
| // assert takeIndex >= 0; |
| // assert prevTakeIndex == takeIndex; |
| // assert nextIndex >= 0; |
| // assert nextItem != null; |
| } |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| boolean isDetached() { |
| // assert lock.getHoldCount() == 1; |
| return prevTakeIndex < 0; |
| } |
| |
| private int incCursor(int index) { |
| // assert lock.getHoldCount() == 1; |
| if (++index == items.length) index = 0; |
| if (index == putIndex) index = NONE; |
| return index; |
| } |
| |
| /** |
| * Returns true if index is invalidated by the given number of |
| * dequeues, starting from prevTakeIndex. |
| */ |
| private boolean invalidated(int index, int prevTakeIndex, |
| long dequeues, int length) { |
| if (index < 0) |
| return false; |
| int distance = index - prevTakeIndex; |
| if (distance < 0) |
| distance += length; |
| return dequeues > distance; |
| } |
| |
| /** |
| * Adjusts indices to incorporate all dequeues since the last |
| * operation on this iterator. Call only from iterating thread. |
| */ |
| private void incorporateDequeues() { |
| // assert lock.getHoldCount() == 1; |
| // assert itrs != null; |
| // assert !isDetached(); |
| // assert count > 0; |
| |
| final int cycles = itrs.cycles; |
| final int takeIndex = ArrayBlockingQueue.this.takeIndex; |
| final int prevCycles = this.prevCycles; |
| final int prevTakeIndex = this.prevTakeIndex; |
| |
| if (cycles != prevCycles || takeIndex != prevTakeIndex) { |
| final int len = items.length; |
| // how far takeIndex has advanced since the previous |
| // operation of this iterator |
| long dequeues = (cycles - prevCycles) * len |
| + (takeIndex - prevTakeIndex); |
| |
| // Check indices for invalidation |
| if (invalidated(lastRet, prevTakeIndex, dequeues, len)) |
| lastRet = REMOVED; |
| if (invalidated(nextIndex, prevTakeIndex, dequeues, len)) |
| nextIndex = REMOVED; |
| if (invalidated(cursor, prevTakeIndex, dequeues, len)) |
| cursor = takeIndex; |
| |
| if (cursor < 0 && nextIndex < 0 && lastRet < 0) |
| detach(); |
| else { |
| this.prevCycles = cycles; |
| this.prevTakeIndex = takeIndex; |
| } |
| } |
| } |
| |
| /** |
| * Called when itrs should stop tracking this iterator, either |
| * because there are no more indices to update (cursor < 0 && |
| * nextIndex < 0 && lastRet < 0) or as a special exception, when |
| * lastRet >= 0, because hasNext() is about to return false for the |
| * first time. Call only from iterating thread. |
| */ |
| private void detach() { |
| // Switch to detached mode |
| // assert lock.getHoldCount() == 1; |
| // assert cursor == NONE; |
| // assert nextIndex < 0; |
| // assert lastRet < 0 || nextItem == null; |
| // assert lastRet < 0 ^ lastItem != null; |
| if (prevTakeIndex >= 0) { |
| // assert itrs != null; |
| prevTakeIndex = DETACHED; |
| // try to unlink from itrs (but not too hard) |
| itrs.doSomeSweeping(true); |
| } |
| } |
| |
| /** |
| * For performance reasons, we would like not to acquire a lock in |
| * hasNext in the common case. To allow for this, we only access |
| * fields (i.e. nextItem) that are not modified by update operations |
| * triggered by queue modifications. |
| */ |
| public boolean hasNext() { |
| // assert lock.getHoldCount() == 0; |
| if (nextItem != null) |
| return true; |
| noNext(); |
| return false; |
| } |
| |
| private void noNext() { |
| final ReentrantLock lock = ArrayBlockingQueue.this.lock; |
| lock.lock(); |
| try { |
| // assert cursor == NONE; |
| // assert nextIndex == NONE; |
| if (!isDetached()) { |
| // assert lastRet >= 0; |
| incorporateDequeues(); // might update lastRet |
| if (lastRet >= 0) { |
| lastItem = itemAt(lastRet); |
| // assert lastItem != null; |
| detach(); |
| } |
| } |
| // assert isDetached(); |
| // assert lastRet < 0 ^ lastItem != null; |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| public E next() { |
| // assert lock.getHoldCount() == 0; |
| final E x = nextItem; |
| if (x == null) |
| throw new NoSuchElementException(); |
| final ReentrantLock lock = ArrayBlockingQueue.this.lock; |
| lock.lock(); |
| try { |
| if (!isDetached()) |
| incorporateDequeues(); |
| // assert nextIndex != NONE; |
| // assert lastItem == null; |
| lastRet = nextIndex; |
| final int cursor = this.cursor; |
| if (cursor >= 0) { |
| nextItem = itemAt(nextIndex = cursor); |
| // assert nextItem != null; |
| this.cursor = incCursor(cursor); |
| } else { |
| nextIndex = NONE; |
| nextItem = null; |
| } |
| } finally { |
| lock.unlock(); |
| } |
| return x; |
| } |
| |
| public void remove() { |
| // assert lock.getHoldCount() == 0; |
| final ReentrantLock lock = ArrayBlockingQueue.this.lock; |
| lock.lock(); |
| try { |
| if (!isDetached()) |
| incorporateDequeues(); // might update lastRet or detach |
| final int lastRet = this.lastRet; |
| this.lastRet = NONE; |
| if (lastRet >= 0) { |
| if (!isDetached()) |
| removeAt(lastRet); |
| else { |
| final E lastItem = this.lastItem; |
| // assert lastItem != null; |
| this.lastItem = null; |
| if (itemAt(lastRet) == lastItem) |
| removeAt(lastRet); |
| } |
| } else if (lastRet == NONE) |
| throw new IllegalStateException(); |
| // else lastRet == REMOVED and the last returned element was |
| // previously asynchronously removed via an operation other |
| // than this.remove(), so nothing to do. |
| |
| if (cursor < 0 && nextIndex < 0) |
| detach(); |
| } finally { |
| lock.unlock(); |
| // assert lastRet == NONE; |
| // assert lastItem == null; |
| } |
| } |
| |
| /** |
| * Called to notify the iterator that the queue is empty, or that it |
| * has fallen hopelessly behind, so that it should abandon any |
| * further iteration, except possibly to return one more element |
| * from next(), as promised by returning true from hasNext(). |
| */ |
| void shutdown() { |
| // assert lock.getHoldCount() == 1; |
| cursor = NONE; |
| if (nextIndex >= 0) |
| nextIndex = REMOVED; |
| if (lastRet >= 0) { |
| lastRet = REMOVED; |
| lastItem = null; |
| } |
| prevTakeIndex = DETACHED; |
| // Don't set nextItem to null because we must continue to be |
| // able to return it on next(). |
| // |
| // Caller will unlink from itrs when convenient. |
| } |
| |
| private int distance(int index, int prevTakeIndex, int length) { |
| int distance = index - prevTakeIndex; |
| if (distance < 0) |
| distance += length; |
| return distance; |
| } |
| |
| /** |
| * Called whenever an interior remove (not at takeIndex) occurred. |
| * |
| * @return true if this iterator should be unlinked from itrs |
| */ |
| boolean removedAt(int removedIndex) { |
| // assert lock.getHoldCount() == 1; |
| if (isDetached()) |
| return true; |
| |
| final int takeIndex = ArrayBlockingQueue.this.takeIndex; |
| final int prevTakeIndex = this.prevTakeIndex; |
| final int len = items.length; |
| // distance from prevTakeIndex to removedIndex |
| final int removedDistance = |
| len * (itrs.cycles - this.prevCycles |
| + ((removedIndex < takeIndex) ? 1 : 0)) |
| + (removedIndex - prevTakeIndex); |
| // assert itrs.cycles - this.prevCycles >= 0; |
| // assert itrs.cycles - this.prevCycles <= 1; |
| // assert removedDistance > 0; |
| // assert removedIndex != takeIndex; |
| int cursor = this.cursor; |
| if (cursor >= 0) { |
| int x = distance(cursor, prevTakeIndex, len); |
| if (x == removedDistance) { |
| if (cursor == putIndex) |
| this.cursor = cursor = NONE; |
| } |
| else if (x > removedDistance) { |
| // assert cursor != prevTakeIndex; |
| this.cursor = cursor = dec(cursor); |
| } |
| } |
| int lastRet = this.lastRet; |
| if (lastRet >= 0) { |
| int x = distance(lastRet, prevTakeIndex, len); |
| if (x == removedDistance) |
| this.lastRet = lastRet = REMOVED; |
| else if (x > removedDistance) |
| this.lastRet = lastRet = dec(lastRet); |
| } |
| int nextIndex = this.nextIndex; |
| if (nextIndex >= 0) { |
| int x = distance(nextIndex, prevTakeIndex, len); |
| if (x == removedDistance) |
| this.nextIndex = nextIndex = REMOVED; |
| else if (x > removedDistance) |
| this.nextIndex = nextIndex = dec(nextIndex); |
| } |
| if (cursor < 0 && nextIndex < 0 && lastRet < 0) { |
| this.prevTakeIndex = DETACHED; |
| return true; |
| } |
| return false; |
| } |
| |
| /** |
| * Called whenever takeIndex wraps around to zero. |
| * |
| * @return true if this iterator should be unlinked from itrs |
| */ |
| boolean takeIndexWrapped() { |
| // assert lock.getHoldCount() == 1; |
| if (isDetached()) |
| return true; |
| if (itrs.cycles - prevCycles > 1) { |
| // All the elements that existed at the time of the last |
| // operation are gone, so abandon further iteration. |
| shutdown(); |
| return true; |
| } |
| return false; |
| } |
| |
| // /** Uncomment for debugging. */ |
| // public String toString() { |
| // return ("cursor=" + cursor + " " + |
| // "nextIndex=" + nextIndex + " " + |
| // "lastRet=" + lastRet + " " + |
| // "nextItem=" + nextItem + " " + |
| // "lastItem=" + lastItem + " " + |
| // "prevCycles=" + prevCycles + " " + |
| // "prevTakeIndex=" + prevTakeIndex + " " + |
| // "size()=" + size() + " " + |
| // "remainingCapacity()=" + remainingCapacity()); |
| // } |
| } |
| |
| /** |
| * Returns a {@link Spliterator} over the elements in this queue. |
| * |
| * <p>The returned spliterator is |
| * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
| * |
| * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT}, |
| * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}. |
| * |
| * @implNote |
| * The {@code Spliterator} implements {@code trySplit} to permit limited |
| * parallelism. |
| * |
| * @return a {@code Spliterator} over the elements in this queue |
| * @since 1.8 |
| */ |
| public Spliterator<E> spliterator() { |
| return Spliterators.spliterator |
| (this, (Spliterator.ORDERED | |
| Spliterator.NONNULL | |
| Spliterator.CONCURRENT)); |
| } |
| |
| } |
