/*
* 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 com.google.errorprone.annotations.CanIgnoreReturnValue;
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 "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.
*
* @author Doug Lea
* @author Justin T. Sampson
* @param <E> the type of elements held in this collection
*/
@CanIgnoreReturnValue
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();
}
}
}
}