--- a/jdk/src/share/classes/java/util/concurrent/ArrayBlockingQueue.java Tue Jan 11 13:42:34 2011 -0800
+++ b/jdk/src/share/classes/java/util/concurrent/ArrayBlockingQueue.java Wed Jan 12 14:40:36 2011 +0000
@@ -49,14 +49,14 @@
* <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
+ * changed. Attempts to {@code put} an element into a full queue
+ * will result in the operation blocking; attempts to {@code take} an
* element from an empty queue will similarly block.
*
- * <p> This class supports an optional fairness policy for ordering
+ * <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
+ * to {@code true} grants threads access in FIFO order. Fairness
* generally decreases throughput but reduces variability and avoids
* starvation.
*
@@ -83,14 +83,17 @@
*/
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;
+ /** The queued items */
+ final Object[] items;
+
+ /** items index for next take, poll, peek or remove */
+ int takeIndex;
+
+ /** items index for next put, offer, or add */
+ int putIndex;
+
+ /** Number of elements in the queue */
+ int count;
/*
* Concurrency control uses the classic two-condition algorithm
@@ -98,7 +101,7 @@
*/
/** Main lock guarding all access */
- private final ReentrantLock lock;
+ final ReentrantLock lock;
/** Condition for waiting takes */
private final Condition notEmpty;
/** Condition for waiting puts */
@@ -110,7 +113,36 @@
* Circularly increment i.
*/
final int inc(int i) {
- return (++i == items.length)? 0 : i;
+ return (++i == items.length) ? 0 : i;
+ }
+
+ /**
+ * Circularly decrement i.
+ */
+ final int dec(int i) {
+ return ((i == 0) ? items.length : i) - 1;
+ }
+
+ @SuppressWarnings("unchecked")
+ static <E> E cast(Object item) {
+ return (E) item;
+ }
+
+ /**
+ * Returns item at index i.
+ */
+ final E itemAt(int i) {
+ return this.<E>cast(items[i]);
+ }
+
+ /**
+ * Throws NullPointerException if argument is null.
+ *
+ * @param v the element
+ */
+ private static void checkNotNull(Object v) {
+ if (v == null)
+ throw new NullPointerException();
}
/**
@@ -129,8 +161,8 @@
* Call only when holding lock.
*/
private E extract() {
- final E[] items = this.items;
- E x = items[takeIndex];
+ final Object[] items = this.items;
+ E x = this.<E>cast(items[takeIndex]);
items[takeIndex] = null;
takeIndex = inc(takeIndex);
--count;
@@ -139,11 +171,12 @@
}
/**
- * Utility for remove and iterator.remove: Delete item at position i.
+ * Deletes item at position i.
+ * Utility for remove and iterator.remove.
* Call only when holding lock.
*/
void removeAt(int i) {
- final E[] items = this.items;
+ final Object[] items = this.items;
// if removing front item, just advance
if (i == takeIndex) {
items[takeIndex] = null;
@@ -167,69 +200,82 @@
}
/**
- * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
+ * Creates an {@code ArrayBlockingQueue} 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
+ * @throws IllegalArgumentException if {@code capacity < 1}
*/
public ArrayBlockingQueue(int capacity) {
this(capacity, false);
}
/**
- * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
+ * Creates an {@code ArrayBlockingQueue} 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
+ * @param fair if {@code true} 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
+ * if {@code false} the access order is unspecified.
+ * @throws IllegalArgumentException if {@code capacity < 1}
*/
public ArrayBlockingQueue(int capacity, boolean fair) {
if (capacity <= 0)
throw new IllegalArgumentException();
- this.items = (E[]) new Object[capacity];
+ this.items = new Object[capacity];
lock = new ReentrantLock(fair);
notEmpty = lock.newCondition();
notFull = lock.newCondition();
}
/**
- * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
+ * Creates an {@code 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 <tt>true</tt> then queue accesses for threads blocked
+ * @param fair if {@code true} then queue accesses for threads blocked
* on insertion or removal, are processed in FIFO order;
- * if <tt>false</tt> the access order is unspecified.
+ * if {@code false} 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 IllegalArgumentException if {@code capacity} is less than
+ * {@code 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);
+ final ReentrantLock lock = this.lock;
+ lock.lock(); // Lock only for visibility, not mutual exclusion
+ try {
+ int i = 0;
+ try {
+ for (E e : c) {
+ checkNotNull(e);
+ items[i++] = e;
+ }
+ } catch (ArrayIndexOutOfBoundsException ex) {
+ throw new IllegalArgumentException();
+ }
+ count = i;
+ putIndex = (i == capacity) ? 0 : i;
+ } finally {
+ lock.unlock();
+ }
}
/**
* 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.
+ * returning {@code true} upon success and throwing an
+ * {@code IllegalStateException} if this queue is full.
*
* @param e the element to add
- * @return <tt>true</tt> (as specified by {@link Collection#add})
+ * @return {@code true} (as specified by {@link Collection#add})
* @throws IllegalStateException if this queue is full
* @throws NullPointerException if the specified element is null
*/
@@ -240,14 +286,14 @@
/**
* 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
+ * returning {@code true} upon success and {@code 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();
+ checkNotNull(e);
final ReentrantLock lock = this.lock;
lock.lock();
try {
@@ -270,18 +316,12 @@
* @throws NullPointerException {@inheritDoc}
*/
public void put(E e) throws InterruptedException {
- if (e == null) throw new NullPointerException();
- final E[] items = this.items;
+ checkNotNull(e);
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;
- }
+ while (count == items.length)
+ notFull.await();
insert(e);
} finally {
lock.unlock();
@@ -299,25 +339,18 @@
public boolean offer(E e, long timeout, TimeUnit unit)
throws InterruptedException {
- if (e == null) throw new NullPointerException();
+ checkNotNull(e);
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
- for (;;) {
- if (count != items.length) {
- insert(e);
- return true;
- }
+ while (count == items.length) {
if (nanos <= 0)
return false;
- try {
- nanos = notFull.awaitNanos(nanos);
- } catch (InterruptedException ie) {
- notFull.signal(); // propagate to non-interrupted thread
- throw ie;
- }
+ nanos = notFull.awaitNanos(nanos);
}
+ insert(e);
+ return true;
} finally {
lock.unlock();
}
@@ -327,10 +360,7 @@
final ReentrantLock lock = this.lock;
lock.lock();
try {
- if (count == 0)
- return null;
- E x = extract();
- return x;
+ return (count == 0) ? null : extract();
} finally {
lock.unlock();
}
@@ -340,15 +370,9 @@
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;
+ while (count == 0)
+ notEmpty.await();
+ return extract();
} finally {
lock.unlock();
}
@@ -359,21 +383,12 @@
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
- for (;;) {
- if (count != 0) {
- E x = extract();
- return x;
- }
+ while (count == 0) {
if (nanos <= 0)
return null;
- try {
- nanos = notEmpty.awaitNanos(nanos);
- } catch (InterruptedException ie) {
- notEmpty.signal(); // propagate to non-interrupted thread
- throw ie;
- }
-
+ nanos = notEmpty.awaitNanos(nanos);
}
+ return extract();
} finally {
lock.unlock();
}
@@ -383,7 +398,7 @@
final ReentrantLock lock = this.lock;
lock.lock();
try {
- return (count == 0) ? null : items[takeIndex];
+ return (count == 0) ? null : itemAt(takeIndex);
} finally {
lock.unlock();
}
@@ -412,10 +427,10 @@
* 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.
+ * less the current {@code size} 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>
+ * an element will succeed by inspecting {@code remainingCapacity}
* because it may be the case that another thread is about to
* insert or remove an element.
*/
@@ -431,59 +446,56 @@
/**
* 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
+ * if it is present. More formally, removes an element {@code e} such
+ * that {@code o.equals(e)}, if this queue contains one or more such
* elements.
- * Returns <tt>true</tt> if this queue contained the specified element
+ * Returns {@code true} if this queue contained the specified element
* (or equivalently, if this queue changed as a result of the call).
*
+ * <p>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 <tt>true</tt> if this queue changed as a result of the call
+ * @return {@code 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 Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
- int i = takeIndex;
- int k = 0;
- for (;;) {
- if (k++ >= count)
- return false;
+ for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) {
if (o.equals(items[i])) {
removeAt(i);
return true;
}
- i = inc(i);
}
-
+ return false;
} 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>.
+ * Returns {@code true} if this queue contains the specified element.
+ * More formally, returns {@code true} if and only if this queue contains
+ * at least one element {@code e} such that {@code o.equals(e)}.
*
* @param o object to be checked for containment in this queue
- * @return <tt>true</tt> if this queue contains the specified element
+ * @return {@code true} if this queue contains the specified element
*/
public boolean contains(Object o) {
if (o == null) return false;
- final E[] items = this.items;
+ final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
- int i = takeIndex;
- int k = 0;
- while (k++ < count) {
+ for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
if (o.equals(items[i]))
return true;
- i = inc(i);
- }
return false;
} finally {
lock.unlock();
@@ -504,17 +516,14 @@
* @return an array containing all of the elements in this queue
*/
public Object[] toArray() {
- final E[] items = this.items;
+ final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
+ final int count = this.count;
Object[] a = new Object[count];
- int k = 0;
- int i = takeIndex;
- while (k < count) {
- a[k++] = items[i];
- i = inc(i);
- }
+ for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)
+ a[k] = items[i];
return a;
} finally {
lock.unlock();
@@ -531,22 +540,22 @@
* <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>.
+ * {@code null}.
*
* <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.
+ * <p>Suppose {@code 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 <tt>String</tt>:
+ * allocated array of {@code String}:
*
* <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>.
+ * Note that {@code toArray(new Object[0])} is identical in function to
+ * {@code 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
@@ -557,24 +566,20 @@
* this queue
* @throws NullPointerException if the specified array is null
*/
+ @SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
- final E[] items = this.items;
+ final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
- if (a.length < count)
+ final int count = this.count;
+ final int len = a.length;
+ if (len < 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.getClass().getComponentType(), count);
+ for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)
+ a[k] = (T) items[i];
+ if (len > count)
a[count] = null;
return a;
} finally {
@@ -586,7 +591,19 @@
final ReentrantLock lock = this.lock;
lock.lock();
try {
- return super.toString();
+ int k = count;
+ if (k == 0)
+ return "[]";
+
+ StringBuilder sb = new StringBuilder();
+ sb.append('[');
+ for (int i = takeIndex; ; i = inc(i)) {
+ Object e = items[i];
+ sb.append(e == this ? "(this Collection)" : e);
+ if (--k == 0)
+ return sb.append(']').toString();
+ sb.append(',').append(' ');
+ }
} finally {
lock.unlock();
}
@@ -597,16 +614,12 @@
* The queue will be empty after this call returns.
*/
public void clear() {
- final E[] items = this.items;
+ final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
- int i = takeIndex;
- int k = count;
- while (k-- > 0) {
+ for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
items[i] = null;
- i = inc(i);
- }
count = 0;
putIndex = 0;
takeIndex = 0;
@@ -623,11 +636,10 @@
* @throws IllegalArgumentException {@inheritDoc}
*/
public int drainTo(Collection<? super E> c) {
- if (c == null)
- throw new NullPointerException();
+ checkNotNull(c);
if (c == this)
throw new IllegalArgumentException();
- final E[] items = this.items;
+ final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
@@ -635,7 +647,7 @@
int n = 0;
int max = count;
while (n < max) {
- c.add(items[i]);
+ c.add(this.<E>cast(items[i]));
items[i] = null;
i = inc(i);
++n;
@@ -659,22 +671,20 @@
* @throws IllegalArgumentException {@inheritDoc}
*/
public int drainTo(Collection<? super E> c, int maxElements) {
- if (c == null)
- throw new NullPointerException();
+ checkNotNull(c);
if (c == this)
throw new IllegalArgumentException();
if (maxElements <= 0)
return 0;
- final E[] items = this.items;
+ final Object[] 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;
+ int max = (maxElements < count) ? maxElements : count;
while (n < max) {
- c.add(items[i]);
+ c.add(this.<E>cast(items[i]));
items[i] = null;
i = inc(i);
++n;
@@ -690,11 +700,13 @@
}
}
-
/**
* 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},
+ * The elements will be returned in order from first (head) to last (tail).
+ *
+ * <p>The returned {@code Iterator} is a "weakly consistent" iterator that
+ * will never throw {@link java.util.ConcurrentModificationException
+ * 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.
@@ -702,83 +714,65 @@
* @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();
- }
+ return new Itr();
}
/**
- * Iterator for ArrayBlockingQueue
+ * 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 -- however we later recheck this slot to use the most
+ * current value; (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<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;
+ 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 (count == 0)
- nextIndex = -1;
- else {
- nextIndex = takeIndex;
- nextItem = items[takeIndex];
+ final ReentrantLock lock = ArrayBlockingQueue.this.lock;
+ lock.lock();
+ try {
+ lastRet = -1;
+ if ((remaining = count) > 0)
+ nextItem = itemAt(nextIndex = takeIndex);
+ } finally {
+ lock.unlock();
}
}
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;
- }
+ return remaining > 0;
}
public E next() {
final ReentrantLock lock = ArrayBlockingQueue.this.lock;
lock.lock();
try {
- if (nextIndex < 0)
+ if (remaining <= 0)
throw new NoSuchElementException();
lastRet = nextIndex;
- E x = nextItem;
- nextIndex = inc(nextIndex);
- checkNext();
+ E x = itemAt(nextIndex); // check for fresher value
+ if (x == null) {
+ x = nextItem; // we are forced to report old value
+ lastItem = null; // but ensure remove fails
+ }
+ else
+ lastItem = x;
+ while (--remaining > 0 && // skip over nulls
+ (nextItem = itemAt(nextIndex = inc(nextIndex))) == null)
+ ;
return x;
} finally {
lock.unlock();
@@ -793,15 +787,19 @@
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();
+ E x = lastItem;
+ lastItem = null;
+ // only remove if item still at index
+ if (x != null && x == items[i]) {
+ boolean removingHead = (i == takeIndex);
+ removeAt(i);
+ if (!removingHead)
+ nextIndex = dec(nextIndex);
+ }
} finally {
lock.unlock();
}
}
}
+
}