ArrayBlockingQueue

/* * @(#)ArrayBlockingQueue.java1.14 06/06/01 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */package java.util.concurrent;import java.util.concurrent.locks.*;import java.util.*;/** * 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 * increased. Attempts to <tt>put</tt> an element into a full queue * will result in the operation blocking; attempts to <tt>take</tt> 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 <tt>true</tt> 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. * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html" mce_href="technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @since 1.5 * @author Doug Lea * @param <E> the type of elements held in this collection */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 */ private final E[] items; /** items index for next take, poll or remove */ private int takeIndex; /** items index for next put, offer, or add. */ private int putIndex; /** Number of items in the queue */ private int count; /* * Concurrency control uses the classic two-condition algorithm * found in any textbook. */ /** Main lock guarding all access */ private final ReentrantLock lock; /** Condition for waiting takes */ private final Condition notEmpty; /** Condition for waiting puts */ private final Condition notFull; // Internal helper methods /** * Circularly increment i. */ final int inc(int i) { return (++i == items.length)? 0 : i; } /** * Inserts element at current put position, advances, and signals. * Call only when holding lock. */ private void insert(E x) { items[putIndex] = x; putIndex = inc(putIndex); ++count; notEmpty.signal(); } /** * Extracts element at current take position, advances, and signals. * Call only when holding lock. */ private E extract() { final E[] items = this.items; E x = items[takeIndex]; items[takeIndex] = null; takeIndex = inc(takeIndex); --count; notFull.signal(); return x; } /** * Utility for remove and iterator.remove: Delete item at position i. * Call only when holding lock. */ void removeAt(int i) { final E[] items = this.items; // if removing front item, just advance if (i == takeIndex) { items[takeIndex] = null; takeIndex = inc(takeIndex); } else { // slide over all others up through putIndex. for (;;) { int nexti = inc(i); if (nexti != putIndex) { items[i] = items[nexti]; i = nexti; } else { items[i] = null; putIndex = i; break; } } } --count; notFull.signal(); } /** * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed) * capacity and default access policy. * * @param capacity the capacity of this queue * @throws IllegalArgumentException if <tt>capacity</tt> is less than 1 */ public ArrayBlockingQueue(int capacity) { this(capacity, false); } /** * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed) * capacity and the specified access policy. * * @param capacity the capacity of this queue * @param fair if <tt>true</tt> then queue accesses for threads blocked * on insertion or removal, are processed in FIFO order; * if <tt>false</tt> the access order is unspecified. * @throws IllegalArgumentException if <tt>capacity</tt> is less than 1 */ public ArrayBlockingQueue(int capacity, boolean fair) { if (capacity <= 0) throw new IllegalArgumentException(); this.items = (E[]) new Object[capacity]; lock = new ReentrantLock(fair); notEmpty = lock.newCondition(); notFull = lock.newCondition(); } /** * Creates an <tt>ArrayBlockingQueue</tt> 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 <tt>true</tt> then queue accesses for threads blocked * on insertion or removal, are processed in FIFO order; * if <tt>false</tt> the access order is unspecified. * @param c the collection of elements to initially contain * @throws IllegalArgumentException if <tt>capacity</tt> is less than * <tt>c.size()</tt>, 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); if (capacity < c.size()) throw new IllegalArgumentException(); for (Iterator<? extends E> it = c.iterator(); it.hasNext();) add(it.next()); } /** * 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 <tt>true</tt> upon success and throwing an * <tt>IllegalStateException</tt> if this queue is full. * * @param e the element to add * @return <tt>true</tt> (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 <tt>true</tt> upon success and <tt>false</tt> 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) { if (e == null) throw new NullPointerException(); final ReentrantLock lock = this.lock; lock.lock(); try { if (count == items.length) return false; else { insert(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 { if (e == null) throw new NullPointerException(); final E[] items = this.items; final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { try { while (count == items.length) notFull.await(); } catch (InterruptedException ie) { notFull.signal(); // propagate to non-interrupted thread throw ie; } insert(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 { if (e == null) throw new NullPointerException();long nanos = unit.toNanos(timeout); final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { for (;;) { if (count != items.length) { insert(e); return true; } if (nanos <= 0) return false; try { nanos = notFull.awaitNanos(nanos); } catch (InterruptedException ie) { notFull.signal(); // propagate to non-interrupted thread throw ie; } } } finally { lock.unlock(); } } public E poll() { final ReentrantLock lock = this.lock; lock.lock(); try { if (count == 0) return null; E x = extract(); return x; } finally { lock.unlock(); } } public E take() throws InterruptedException { final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { try { while (count == 0) notEmpty.await(); } catch (InterruptedException ie) { notEmpty.signal(); // propagate to non-interrupted thread throw ie; } E x = extract(); return x; } 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 { for (;;) { if (count != 0) { E x = extract(); return x; } if (nanos <= 0) return null; try { nanos = notEmpty.awaitNanos(nanos); } catch (InterruptedException ie) { notEmpty.signal(); // propagate to non-interrupted thread throw ie; } } } finally { lock.unlock(); } } public E peek() { final ReentrantLock lock = this.lock; lock.lock(); try { return (count == 0) ? null : items[takeIndex]; } 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 <tt>size</tt> of this queue. * * <p>Note that you <em>cannot</em> always tell if an attempt to insert * an element will succeed by inspecting <tt>remainingCapacity</tt> * 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 <tt>e</tt> such * that <tt>o.equals(e)</tt>, if this queue contains one or more such * elements. * Returns <tt>true</tt> if this queue contained the specified element * (or equivalently, if this queue changed as a result of the call). * * @param o element to be removed from this queue, if present * @return <tt>true</tt> if this queue changed as a result of the call */ public boolean remove(Object o) { if (o == null) return false; final E[] items = this.items; final ReentrantLock lock = this.lock; lock.lock(); try { int i = takeIndex; int k = 0; for (;;) { if (k++ >= count) return false; if (o.equals(items[i])) { removeAt(i); return true; } i = inc(i); } } finally { lock.unlock(); } } /** * Returns <tt>true</tt> if this queue contains the specified element. * More formally, returns <tt>true</tt> if and only if this queue contains * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>. * * @param o object to be checked for containment in this queue * @return <tt>true</tt> if this queue contains the specified element */ public boolean contains(Object o) { if (o == null) return false; final E[] items = this.items; final ReentrantLock lock = this.lock; lock.lock(); try { int i = takeIndex; int k = 0; while (k++ < count) { if (o.equals(items[i])) return true; i = inc(i); } 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 E[] items = this.items; final ReentrantLock lock = this.lock; lock.lock(); try { Object[] a = new Object[count]; int k = 0; int i = takeIndex; while (k < count) { a[k++] = items[i]; i = inc(i); } 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 * <tt>null</tt>. * * <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 <tt>x</tt> is a queue known to contain only strings. * The following code can be used to dump the queue into a newly * allocated array of <tt>String</tt>: * * <pre> * String[] y = x.toArray(new String[0]);</pre> * * Note that <tt>toArray(new Object[0])</tt> is identical in function to * <tt>toArray()</tt>. * * @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 */ public <T> T[] toArray(T[] a) { final E[] items = this.items; final ReentrantLock lock = this.lock; lock.lock(); try { if (a.length < count) a = (T[])java.lang.reflect.Array.newInstance( a.getClass().getComponentType(), count ); int k = 0; int i = takeIndex; while (k < count) { a[k++] = (T)items[i]; i = inc(i); } if (a.length > count) a[count] = null; return a; } finally { lock.unlock(); } } public String toString() { final ReentrantLock lock = this.lock; lock.lock(); try { return super.toString(); } finally { lock.unlock(); } } /** * Atomically removes all of the elements from this queue. * The queue will be empty after this call returns. */ public void clear() { final E[] items = this.items; final ReentrantLock lock = this.lock; lock.lock(); try { int i = takeIndex; int k = count; while (k-- > 0) { items[i] = null; i = inc(i); } count = 0; putIndex = 0; takeIndex = 0; notFull.signalAll(); } finally { lock.unlock(); } } /** * @throws UnsupportedOperationException {@inheritDoc} * @throws ClassCastException {@inheritDoc} * @throws NullPointerException {@inheritDoc} * @throws IllegalArgumentException {@inheritDoc} */ public int drainTo(Collection<? super E> c) { if (c == null) throw new NullPointerException(); if (c == this) throw new IllegalArgumentException(); final E[] items = this.items; final ReentrantLock lock = this.lock; lock.lock(); try { int i = takeIndex; int n = 0; int max = count; while (n < max) { c.add(items[i]); items[i] = null; i = inc(i); ++n; } if (n > 0) { count = 0; putIndex = 0; takeIndex = 0; notFull.signalAll(); } return n; } finally { lock.unlock(); } } /** * @throws UnsupportedOperationException {@inheritDoc} * @throws ClassCastException {@inheritDoc} * @throws NullPointerException {@inheritDoc} * @throws IllegalArgumentException {@inheritDoc} */ public int drainTo(Collection<? super E> c, int maxElements) { if (c == null) throw new NullPointerException(); if (c == this) throw new IllegalArgumentException(); if (maxElements <= 0) return 0; final E[] items = this.items; final ReentrantLock lock = this.lock; lock.lock(); try { int i = takeIndex; int n = 0; int sz = count; int max = (maxElements < count)? maxElements : count; while (n < max) { c.add(items[i]); items[i] = null; i = inc(i); ++n; } if (n > 0) { count -= n; takeIndex = i; notFull.signalAll(); } return n; } finally { lock.unlock(); } } /** * Returns an iterator over the elements in this queue in proper sequence. * The returned <tt>Iterator</tt> is a "weakly consistent" iterator that * will never throw {@link ConcurrentModificationException}, * and guarantees to traverse elements as they existed upon * construction of the iterator, and may (but is not guaranteed to) * reflect any modifications subsequent to construction. * * @return an iterator over the elements in this queue in proper sequence */ public Iterator<E> iterator() { final ReentrantLock lock = this.lock; lock.lock(); try { return new Itr(); } finally { lock.unlock(); } } /** * Iterator for ArrayBlockingQueue */ private class Itr implements Iterator<E> { /** * Index of element to be returned by next, * or a negative number if no such. */ private int nextIndex; /** * nextItem holds on to item fields because once we claim * that an element exists in hasNext(), we must return it in * the following next() call even if it was in the process of * being removed when hasNext() was called. */ private E nextItem; /** * Index of element returned by most recent call to next. * Reset to -1 if this element is deleted by a call to remove. */ private int lastRet; Itr() { lastRet = -1; if (count == 0) nextIndex = -1; else { nextIndex = takeIndex; nextItem = items[takeIndex]; } } public boolean hasNext() { /* * No sync. We can return true by mistake here * only if this iterator passed across threads, * which we don't support anyway. */ return nextIndex >= 0; } /** * Checks whether nextIndex is valid; if so setting nextItem. * Stops iterator when either hits putIndex or sees null item. */ private void checkNext() { if (nextIndex == putIndex) { nextIndex = -1; nextItem = null; } else { nextItem = items[nextIndex]; if (nextItem == null) nextIndex = -1; } } public E next() { final ReentrantLock lock = ArrayBlockingQueue.this.lock; lock.lock(); try { if (nextIndex < 0) throw new NoSuchElementException(); lastRet = nextIndex; E x = nextItem; nextIndex = inc(nextIndex); checkNext(); return x; } finally { lock.unlock(); } } public void remove() { final ReentrantLock lock = ArrayBlockingQueue.this.lock; lock.lock(); try { int i = lastRet; if (i == -1) throw new IllegalStateException(); lastRet = -1; int ti = takeIndex; removeAt(i); // back up cursor (reset to front if was first element) nextIndex = (i == ti) ? takeIndex : i; checkNext(); } finally { lock.unlock(); } } }}