/*
* 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
* head of the queue is that element that has been on the
* queue the longest time. The tail 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.
*
*
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 put an element into a full queue
* will result in the operation blocking; attempts to take an
* element from an empty queue will similarly block.
*
*
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 true grants threads access in FIFO order. Fairness
* generally decreases throughput but reduces variability and avoids
* starvation.
*
*
This class and its iterator implement all of the
* optional methods of the {@link Collection} and {@link
* Iterator} interfaces.
*
*
This class is a member of the
*
* Java Collections Framework.
*
* @since 1.5
* @author Doug Lea
* @param the type of elements held in this collection
*/
public class ArrayBlockingQueue extends AbstractQueue
implements BlockingQueue, 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 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 */
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;
// 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 ArrayBlockingQueue with the given (fixed)
* capacity and default access policy.
*
* @param capacity the capacity of this queue
* @throws IllegalArgumentException if capacity is less than 1
*/
public ArrayBlockingQueue(int capacity) {
this(capacity, false);
}
/**
* Creates an ArrayBlockingQueue with the given (fixed)
* capacity and the specified access policy.
*
* @param capacity the capacity of this queue
* @param fair if true then queue accesses for threads blocked
* on insertion or removal, are processed in FIFO order;
* if false the access order is unspecified.
* @throws IllegalArgumentException if capacity 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 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 true then queue accesses for threads blocked
* on insertion or removal, are processed in FIFO order;
* if false the access order is unspecified.
* @param c the collection of elements to initially contain
* @throws IllegalArgumentException if capacity is less than
* 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);
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 true upon success and throwing an
* IllegalStateException if this queue is full.
*
* @param e the element to add
* @return 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 true upon success and 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) {
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 ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == items.length)
notFull.await();
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 {
while (count == items.length) {
if (nanos <= 0)
return false;
nanos = notFull.awaitNanos(nanos);
}
insert(e);
return true;
} finally {
lock.unlock();
}
}
public E poll() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return (count == 0) ? null : extract();
} finally {
lock.unlock();
}
}
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == 0)
notEmpty.await();
return extract();
} 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 <= 0)
return null;
nanos = notEmpty.awaitNanos(nanos);
}
return extract();
} 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 size of this queue.
*
* Note that you cannot always tell if an attempt to insert
* an element will succeed by inspecting 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 e such
* that o.equals(e), if this queue contains one or more such
* elements.
* Returns true if this queue contained the specified element
* (or equivalently, if this queue changed as a result of the call).
*
* 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 true 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 true if this queue contains the specified element.
* More formally, returns true if and only if this queue contains
* at least one element e such that o.equals(e).
*
* @param o object to be checked for containment in this queue
* @return true 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.
*
*
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.
*
*
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.
*
*
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
* null.
*
*
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.
*
*
Suppose 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 String:
*
*
* String[] y = x.toArray(new String[0]);
*
* Note that toArray(new Object[0]) is identical in function to
* 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
*/
public 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 Iterator is "weakly
* consistent" with respect to operations at the head and tail of
* the queue, and will never throw {@link
* ConcurrentModificationException}. It might return elements
* that existed upon construction of the iterator but have since
* been polled or taken, and might not return elements that have
* since been added. Further, no consistency guarantees are made
* with respect to "interior" removals occuring in concurrent
* invocations of {@link Collection#remove(Object)} or {@link
* Iterator#remove} occurring in other threads.
*
* The returned iterator supports the optional {@link Iterator#remove}
* operation. However, 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.
*
* @return an iterator over the elements in this queue in proper sequence
*/
public Iterator iterator() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return new Itr();
} finally {
lock.unlock();
}
}
/**
* Iterator for ArrayBlockingQueue. To maintain weak consistency
* with respect to puts and takes, we (1) read ahead one slot, so
* as to not report hasNext true but then not have an element to
* return (2) ensure that each array slot is traversed at most
* once (by tracking "remaining" elements); (3) skip over null
* slots, which can occur if takes race ahead of iterators.
* However, for circular array-based queues, we cannot rely on any
* well established definition of what it means to be weakly
* consistent with respect to interior removes since these may
* require slot overwrites in the process of sliding elements to
* cover gaps. So we settle for resiliency, operating on
* established apparent nexts, which may miss some elements that
* have moved between calls to next.
*/
private class Itr implements Iterator {
private int remaining; // Number of elements yet to be returned
private int nextIndex; // Index of element to be returned by next
private E nextItem; // Element to be returned by next call to next
private E lastItem; // Element returned by last call to next
private int lastRet; // Index of last element returned, or -1 if none
Itr() {
lastRet = -1;
if ((remaining = count) > 0)
nextItem = items[nextIndex = takeIndex];
}
public boolean hasNext() {
return remaining > 0;
}
public E next() {
if (remaining <= 0)
throw new NoSuchElementException();
final ReentrantLock lock = ArrayBlockingQueue.this.lock;
lock.lock();
try {
lastRet = nextIndex;
E x = lastItem = nextItem;
while (--remaining > 0) {
if ((nextItem = items[nextIndex = inc(nextIndex)]) != null)
break;
}
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;
E x = lastItem;
lastItem = null;
if (x == items[i])
removeAt(i); // only remove if item still at index
} finally {
lock.unlock();
}
}
}
}