util/concurrent/MonitorBasedArrayBlockingQueue.java

/*
 * Copyright (C) 2010 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.util.concurrent;

import com.google.common.collect.ObjectArrays;

import java.util.AbstractQueue;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.TimeUnit;

import javax.annotation.Nullable;

/**
 * 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 &quot;bounded buffer&quot;, 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.
 *
 * @author Doug Lea
 * @author Justin T. Sampson
 * @param <E> the type of elements held in this collection
 */
public class MonitorBasedArrayBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E> {

    // Based on revision 1.58 of ArrayBlockingQueue by Doug Lea, from
    // http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/main/java/util/concurrent/

    /** The queued items  */
    final E[] items;
    /** items index for next take, poll or remove */
    int takeIndex;
    /** items index for next put, offer, or add. */
    int putIndex;
    /** Number of items in the queue */
    private int count;

    /*
     * Concurrency control uses the classic two-condition algorithm
     * found in any textbook.
     */

    /** Monitor guarding all access */
    final Monitor monitor;

    /** Guard for waiting takes */
    private final Monitor.Guard notEmpty;

    /** Guard for waiting puts */
    private final Monitor.Guard 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 occupying monitor.
     */
    private void insert(E x) {
        items[putIndex] = x;
        putIndex = inc(putIndex);
        ++count;
    }

    /**
     * Extracts element at current take position, advances, and signals.
     * Call only when occupying monitor.
     */
    private E extract() {
        final E[] items = this.items;
        E x = items[takeIndex];
        items[takeIndex] = null;
        takeIndex = inc(takeIndex);
        --count;
        return x;
    }

    /**
     * Utility for remove and iterator.remove: Delete item at position i.
     * Call only when occupying monitor.
     */
    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;
    }

    /**
     * Creates an <tt>MonitorBasedArrayBlockingQueue</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 MonitorBasedArrayBlockingQueue(int capacity) {
        this(capacity, false);
    }

    /**
     * Creates an <tt>MonitorBasedArrayBlockingQueue</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 MonitorBasedArrayBlockingQueue(int capacity, boolean fair) {
        if (capacity <= 0)
            throw new IllegalArgumentException();
        this.items = newEArray(capacity);
        monitor = new Monitor(fair);
        notEmpty = new Monitor.Guard(monitor) {
            @Override public boolean isSatisfied() {
                return count > 0;
            }
        };
        notFull = new Monitor.Guard(monitor) {
            @Override public boolean isSatisfied() {
                return count < items.length;
            }
        };
    }

    @SuppressWarnings("unchecked") // please don't try this home, kids
    private static <E> E[] newEArray(int capacity) {
        return (E[]) new Object[capacity];
    }

    /**
     * Creates an <tt>MonitorBasedArrayBlockingQueue</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 MonitorBasedArrayBlockingQueue(int capacity, boolean fair,
                              Collection<? extends E> c) {
        this(capacity, fair);
        if (capacity < c.size())
            throw new IllegalArgumentException();

        for (E e : c)
            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 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
     */
    @Override 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
     */
    @Override
    public boolean offer(E e) {
        if (e == null) throw new NullPointerException();
        final Monitor monitor = this.monitor;
        if (monitor.enterIf(notFull)) {
            try {
                insert(e);
                return true;
            } finally {
                monitor.leave();
            }
        } else {
          return false;
        }
    }

    /**
     * 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}
     */
    @Override
    public void put(E e) throws InterruptedException {
        if (e == null) throw new NullPointerException();
        final Monitor monitor = this.monitor;
        monitor.enterWhen(notFull);
        try {
            insert(e);
        } finally {
            monitor.leave();
        }
    }

    /**
     * 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}
     */
    @Override
    public boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException {

        if (e == null) throw new NullPointerException();
        final Monitor monitor = this.monitor;
        if (monitor.enterWhen(notFull, timeout, unit)) {
            try {
                insert(e);
                return true;
            } finally {
                monitor.leave();
            }
        } else {
          return false;
        }
    }

    @Override
    public E poll() {
        final Monitor monitor = this.monitor;
        if (monitor.enterIf(notEmpty)) {
            try {
                return extract();
            } finally {
                monitor.leave();
            }
        } else {
          return null;
        }
    }

    @Override
    public E take() throws InterruptedException {
        final Monitor monitor = this.monitor;
        monitor.enterWhen(notEmpty);
        try {
            return extract();
        } finally {
            monitor.leave();
        }
    }

    @Override
    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        final Monitor monitor = this.monitor;
        if (monitor.enterWhen(notEmpty, timeout, unit)) {
            try {
                return extract();
            } finally {
                monitor.leave();
            }
        } else {
          return null;
        }
    }

    @Override
    public E peek() {
        final Monitor monitor = this.monitor;
        if (monitor.enterIf(notEmpty)) {
            try {
                return items[takeIndex];
            } finally {
                monitor.leave();
            }
        } else {
            return null;
        }
    }

    // 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
     */
    @Override public int size() {
        final Monitor monitor = this.monitor;
        monitor.enter();
        try {
            return count;
        } finally {
            monitor.leave();
        }
    }

    // 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.
     */
    @Override
    public int remainingCapacity() {
        final Monitor monitor = this.monitor;
        monitor.enter();
        try {
            return items.length - count;
        } finally {
            monitor.leave();
        }
    }

    /**
     * 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
     */
    @Override public boolean remove(@Nullable Object o) {
        if (o == null) return false;
        final E[] items = this.items;
        final Monitor monitor = this.monitor;
        monitor.enter();
        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 {
            monitor.leave();
        }
    }

    /**
     * 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
     */
    @Override public boolean contains(@Nullable Object o) {
        if (o == null) return false;
        final E[] items = this.items;
        final Monitor monitor = this.monitor;
        monitor.enter();
        try {
            int i = takeIndex;
            int k = 0;
            while (k++ < count) {
                if (o.equals(items[i]))
                    return true;
                i = inc(i);
            }
            return false;
        } finally {
            monitor.leave();
        }
    }

    /**
     * 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
     */
    @Override public Object[] toArray() {
        final E[] items = this.items;
        final Monitor monitor = this.monitor;
        monitor.enter();
        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 {
            monitor.leave();
        }
    }

    /**
     * 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>
     *
     * <p>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
     */
    @Override public <T> T[] toArray(T[] a) {
        final E[] items = this.items;
        final Monitor monitor = this.monitor;
        monitor.enter();
        try {
            if (a.length < count)
                a = ObjectArrays.newArray(a, count);

            int k = 0;
            int i = takeIndex;
            while (k < count) {
                // This cast is not itself safe, but the following statement
                // will fail if the runtime type of items[i] is not assignable
                // to the runtime type of a[k++], which is all that the method
                // contract requires (see @throws ArrayStoreException above).
                @SuppressWarnings("unchecked")
                T t = (T) items[i];
                a[k++] = t;
                i = inc(i);
            }
            if (a.length > count)
                a[count] = null;
            return a;
        } finally {
            monitor.leave();
        }
    }

    @Override public String toString() {
        final Monitor monitor = this.monitor;
        monitor.enter();
        try {
            return super.toString();
        } finally {
            monitor.leave();
        }
    }

    /**
     * Atomically removes all of the elements from this queue.
     * The queue will be empty after this call returns.
     */
    @Override public void clear() {
        final E[] items = this.items;
        final Monitor monitor = this.monitor;
        monitor.enter();
        try {
            int i = takeIndex;
            int k = count;
            while (k-- > 0) {
                items[i] = null;
                i = inc(i);
            }
            count = 0;
            putIndex = 0;
            takeIndex = 0;
        } finally {
            monitor.leave();
        }
    }

    /**
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    @Override
    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 Monitor monitor = this.monitor;
        monitor.enter();
        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;
            }
            return n;
        } finally {
            monitor.leave();
        }
    }

    /**
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    @Override
    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 Monitor monitor = this.monitor;
        monitor.enter();
        try {
            int i = takeIndex;
            int n = 0;
            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;
            }
            return n;
        } finally {
            monitor.leave();
        }
    }

    /**
     * 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
     */
    @Override public Iterator<E> iterator() {
        final Monitor monitor = this.monitor;
        monitor.enter();
        try {
            return new Itr();
        } finally {
            monitor.leave();
        }
    }

    /**
     * Iterator for MonitorBasedArrayBlockingQueue
     */
    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];
            }
        }

        @Override
        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;
            }
        }

        @Override
        public E next() {
            final Monitor monitor = MonitorBasedArrayBlockingQueue.this.monitor;
            monitor.enter();
            try {
                if (nextIndex < 0)
                    throw new NoSuchElementException();
                lastRet = nextIndex;
                E x = nextItem;
                nextIndex = inc(nextIndex);
                checkNext();
                return x;
            } finally {
                monitor.leave();
            }
        }

        @Override
        public void remove() {
            final Monitor monitor = MonitorBasedArrayBlockingQueue.this.monitor;
            monitor.enter();
            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 {
                monitor.leave();
            }
        }
    }
}