| /* |
| * 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/licenses/publicdomain |
| */ |
| |
| 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"> |
| * 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(); |
| } |
| } |
| } |
| } |