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dl |
1.1 |
/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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*/ |
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package jsr166y; |
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import java.util.concurrent.*; |
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import java.util.concurrent.locks.*; |
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import java.util.concurrent.atomic.*; |
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import java.util.*; |
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import java.io.*; |
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dl |
1.7 |
import sun.misc.Unsafe; |
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import java.lang.reflect.*; |
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dl |
1.1 |
|
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/** |
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* An unbounded {@linkplain TransferQueue} based on linked nodes. |
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* This queue orders elements FIFO (first-in-first-out) with respect |
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* to any given producer. The <em>head</em> of the queue is that |
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* element that has been on the queue the longest time for some |
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* producer. The <em>tail</em> of the queue is that element that has |
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* been on the queue the shortest time for some producer. |
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* |
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jsr166 |
1.11 |
* <p>Beware that, unlike in most collections, the {@code size} |
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dl |
1.1 |
* method is <em>NOT</em> a constant-time operation. Because of the |
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* asynchronous nature of these queues, determining the current number |
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* of elements requires a traversal of the elements. |
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* |
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* <p>This class and its iterator implement all of the |
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* <em>optional</em> methods of the {@link Collection} and {@link |
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* Iterator} interfaces. |
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* |
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* <p>Memory consistency effects: As with other concurrent |
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* collections, actions in a thread prior to placing an object into a |
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* {@code LinkedTransferQueue} |
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
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* actions subsequent to the access or removal of that element from |
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* the {@code LinkedTransferQueue} in another thread. |
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* |
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* <p>This class is a member of the |
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* <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
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* Java Collections Framework</a>. |
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* |
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dl |
1.3 |
* @since 1.7 |
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dl |
1.1 |
* @author Doug Lea |
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* @param <E> the type of elements held in this collection |
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*/ |
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public class LinkedTransferQueue<E> extends AbstractQueue<E> |
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implements TransferQueue<E>, java.io.Serializable { |
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private static final long serialVersionUID = -3223113410248163686L; |
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/* |
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* This class extends the approach used in FIFO-mode |
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* SynchronousQueues. See the internal documentation, as well as |
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* the PPoPP 2006 paper "Scalable Synchronous Queues" by Scherer, |
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* Lea & Scott |
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* (http://www.cs.rice.edu/~wns1/papers/2006-PPoPP-SQ.pdf) |
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* |
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dl |
1.9 |
* The main extension is to provide different Wait modes for the |
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* main "xfer" method that puts or takes items. These don't |
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* impact the basic dual-queue logic, but instead control whether |
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* or how threads block upon insertion of request or data nodes |
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* into the dual queue. It also uses slightly different |
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* conventions for tracking whether nodes are off-list or |
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* cancelled. |
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dl |
1.1 |
*/ |
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// Wait modes for xfer method |
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static final int NOWAIT = 0; |
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static final int TIMEOUT = 1; |
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static final int WAIT = 2; |
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/** The number of CPUs, for spin control */ |
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static final int NCPUS = Runtime.getRuntime().availableProcessors(); |
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/** |
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* The number of times to spin before blocking in timed waits. |
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* The value is empirically derived -- it works well across a |
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* variety of processors and OSes. Empirically, the best value |
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* seems not to vary with number of CPUs (beyond 2) so is just |
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* a constant. |
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*/ |
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jsr166 |
1.5 |
static final int maxTimedSpins = (NCPUS < 2)? 0 : 32; |
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dl |
1.1 |
|
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/** |
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* The number of times to spin before blocking in untimed waits. |
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* This is greater than timed value because untimed waits spin |
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* faster since they don't need to check times on each spin. |
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*/ |
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static final int maxUntimedSpins = maxTimedSpins * 16; |
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/** |
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* The number of nanoseconds for which it is faster to spin |
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* rather than to use timed park. A rough estimate suffices. |
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*/ |
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static final long spinForTimeoutThreshold = 1000L; |
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|
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jsr166 |
1.5 |
/** |
99 |
dl |
1.9 |
* Node class for LinkedTransferQueue. Opportunistically |
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* subclasses from AtomicReference to represent item. Uses Object, |
101 |
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* not E, to allow setting item to "this" after use, to avoid |
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* garbage retention. Similarly, setting the next field to this is |
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* used as sentinel that node is off list. |
104 |
dl |
1.1 |
*/ |
105 |
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static final class QNode extends AtomicReference<Object> { |
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volatile QNode next; |
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volatile Thread waiter; // to control park/unpark |
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final boolean isData; |
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QNode(Object item, boolean isData) { |
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super(item); |
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this.isData = isData; |
112 |
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} |
113 |
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114 |
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static final AtomicReferenceFieldUpdater<QNode, QNode> |
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nextUpdater = AtomicReferenceFieldUpdater.newUpdater |
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(QNode.class, QNode.class, "next"); |
117 |
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118 |
dl |
1.15 |
final boolean casNext(QNode cmp, QNode val) { |
119 |
dl |
1.1 |
return nextUpdater.compareAndSet(this, cmp, val); |
120 |
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} |
121 |
dl |
1.15 |
|
122 |
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final void clearNext() { |
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nextUpdater.lazySet(this, this); |
124 |
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} |
125 |
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126 |
dl |
1.1 |
} |
127 |
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128 |
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/** |
129 |
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* Padded version of AtomicReference used for head, tail and |
130 |
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* cleanMe, to alleviate contention across threads CASing one vs |
131 |
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* the other. |
132 |
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*/ |
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static final class PaddedAtomicReference<T> extends AtomicReference<T> { |
134 |
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// enough padding for 64bytes with 4byte refs |
135 |
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Object p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd, pe; |
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PaddedAtomicReference(T r) { super(r); } |
137 |
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} |
138 |
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139 |
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140 |
dl |
1.7 |
/** head of the queue */ |
141 |
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private transient final PaddedAtomicReference<QNode> head; |
142 |
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/** tail of the queue */ |
143 |
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private transient final PaddedAtomicReference<QNode> tail; |
144 |
dl |
1.1 |
|
145 |
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/** |
146 |
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* Reference to a cancelled node that might not yet have been |
147 |
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* unlinked from queue because it was the last inserted node |
148 |
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* when it cancelled. |
149 |
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*/ |
150 |
dl |
1.7 |
private transient final PaddedAtomicReference<QNode> cleanMe; |
151 |
dl |
1.1 |
|
152 |
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/** |
153 |
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* Tries to cas nh as new head; if successful, unlink |
154 |
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* old head's next node to avoid garbage retention. |
155 |
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*/ |
156 |
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private boolean advanceHead(QNode h, QNode nh) { |
157 |
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if (h == head.get() && head.compareAndSet(h, nh)) { |
158 |
dl |
1.15 |
h.clearNext(); // forget old next |
159 |
dl |
1.1 |
return true; |
160 |
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} |
161 |
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return false; |
162 |
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} |
163 |
jsr166 |
1.5 |
|
164 |
dl |
1.1 |
/** |
165 |
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* Puts or takes an item. Used for most queue operations (except |
166 |
dl |
1.9 |
* poll() and tryTransfer()). See the similar code in |
167 |
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* SynchronousQueue for detailed explanation. |
168 |
jsr166 |
1.17 |
* |
169 |
jsr166 |
1.4 |
* @param e the item or if null, signifies that this is a take |
170 |
dl |
1.1 |
* @param mode the wait mode: NOWAIT, TIMEOUT, WAIT |
171 |
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* @param nanos timeout in nanosecs, used only if mode is TIMEOUT |
172 |
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* @return an item, or null on failure |
173 |
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*/ |
174 |
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private Object xfer(Object e, int mode, long nanos) { |
175 |
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boolean isData = (e != null); |
176 |
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QNode s = null; |
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final PaddedAtomicReference<QNode> head = this.head; |
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final PaddedAtomicReference<QNode> tail = this.tail; |
179 |
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180 |
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for (;;) { |
181 |
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QNode t = tail.get(); |
182 |
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QNode h = head.get(); |
183 |
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184 |
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if (t != null && (t == h || t.isData == isData)) { |
185 |
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if (s == null) |
186 |
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s = new QNode(e, isData); |
187 |
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QNode last = t.next; |
188 |
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if (last != null) { |
189 |
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if (t == tail.get()) |
190 |
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tail.compareAndSet(t, last); |
191 |
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} |
192 |
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else if (t.casNext(null, s)) { |
193 |
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tail.compareAndSet(t, s); |
194 |
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return awaitFulfill(t, s, e, mode, nanos); |
195 |
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} |
196 |
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} |
197 |
jsr166 |
1.5 |
|
198 |
dl |
1.1 |
else if (h != null) { |
199 |
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QNode first = h.next; |
200 |
jsr166 |
1.5 |
if (t == tail.get() && first != null && |
201 |
dl |
1.1 |
advanceHead(h, first)) { |
202 |
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Object x = first.get(); |
203 |
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if (x != first && first.compareAndSet(x, e)) { |
204 |
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LockSupport.unpark(first.waiter); |
205 |
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return isData? e : x; |
206 |
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} |
207 |
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} |
208 |
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} |
209 |
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} |
210 |
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} |
211 |
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212 |
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213 |
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/** |
214 |
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* Version of xfer for poll() and tryTransfer, which |
215 |
jsr166 |
1.17 |
* simplifies control paths both here and in xfer. |
216 |
dl |
1.1 |
*/ |
217 |
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private Object fulfill(Object e) { |
218 |
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boolean isData = (e != null); |
219 |
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final PaddedAtomicReference<QNode> head = this.head; |
220 |
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final PaddedAtomicReference<QNode> tail = this.tail; |
221 |
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222 |
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for (;;) { |
223 |
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QNode t = tail.get(); |
224 |
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QNode h = head.get(); |
225 |
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226 |
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if (t != null && (t == h || t.isData == isData)) { |
227 |
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QNode last = t.next; |
228 |
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if (t == tail.get()) { |
229 |
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if (last != null) |
230 |
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tail.compareAndSet(t, last); |
231 |
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else |
232 |
|
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return null; |
233 |
|
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} |
234 |
|
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} |
235 |
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else if (h != null) { |
236 |
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QNode first = h.next; |
237 |
jsr166 |
1.5 |
if (t == tail.get() && |
238 |
dl |
1.1 |
first != null && |
239 |
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advanceHead(h, first)) { |
240 |
|
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Object x = first.get(); |
241 |
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if (x != first && first.compareAndSet(x, e)) { |
242 |
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LockSupport.unpark(first.waiter); |
243 |
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return isData? e : x; |
244 |
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} |
245 |
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} |
246 |
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} |
247 |
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} |
248 |
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} |
249 |
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250 |
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/** |
251 |
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* Spins/blocks until node s is fulfilled or caller gives up, |
252 |
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* depending on wait mode. |
253 |
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* |
254 |
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* @param pred the predecessor of waiting node |
255 |
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* @param s the waiting node |
256 |
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* @param e the comparison value for checking match |
257 |
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* @param mode mode |
258 |
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* @param nanos timeout value |
259 |
|
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* @return matched item, or s if cancelled |
260 |
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*/ |
261 |
jsr166 |
1.5 |
private Object awaitFulfill(QNode pred, QNode s, Object e, |
262 |
dl |
1.1 |
int mode, long nanos) { |
263 |
|
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if (mode == NOWAIT) |
264 |
|
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return null; |
265 |
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|
266 |
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long lastTime = (mode == TIMEOUT)? System.nanoTime() : 0; |
267 |
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Thread w = Thread.currentThread(); |
268 |
|
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int spins = -1; // set to desired spin count below |
269 |
|
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for (;;) { |
270 |
|
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if (w.isInterrupted()) |
271 |
|
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s.compareAndSet(e, s); |
272 |
|
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Object x = s.get(); |
273 |
|
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if (x != e) { // Node was matched or cancelled |
274 |
|
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advanceHead(pred, s); // unlink if head |
275 |
jsr166 |
1.17 |
if (x == s) { // was cancelled |
276 |
dl |
1.9 |
clean(pred, s); |
277 |
|
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return null; |
278 |
|
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} |
279 |
jsr166 |
1.5 |
else if (x != null) { |
280 |
dl |
1.1 |
s.set(s); // avoid garbage retention |
281 |
|
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return x; |
282 |
|
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} |
283 |
|
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else |
284 |
|
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return e; |
285 |
|
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} |
286 |
|
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if (mode == TIMEOUT) { |
287 |
|
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long now = System.nanoTime(); |
288 |
|
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nanos -= now - lastTime; |
289 |
|
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lastTime = now; |
290 |
|
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if (nanos <= 0) { |
291 |
|
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s.compareAndSet(e, s); // try to cancel |
292 |
|
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continue; |
293 |
|
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} |
294 |
|
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} |
295 |
|
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if (spins < 0) { |
296 |
|
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QNode h = head.get(); // only spin if at head |
297 |
|
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spins = ((h != null && h.next == s) ? |
298 |
jsr166 |
1.5 |
(mode == TIMEOUT? |
299 |
dl |
1.1 |
maxTimedSpins : maxUntimedSpins) : 0); |
300 |
|
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} |
301 |
|
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if (spins > 0) |
302 |
|
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--spins; |
303 |
|
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else if (s.waiter == null) |
304 |
|
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s.waiter = w; |
305 |
|
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else if (mode != TIMEOUT) { |
306 |
dl |
1.12 |
LockSupport.park(this); |
307 |
dl |
1.1 |
s.waiter = null; |
308 |
|
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spins = -1; |
309 |
|
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} |
310 |
|
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else if (nanos > spinForTimeoutThreshold) { |
311 |
dl |
1.12 |
LockSupport.parkNanos(this, nanos); |
312 |
dl |
1.1 |
s.waiter = null; |
313 |
|
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spins = -1; |
314 |
|
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} |
315 |
|
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} |
316 |
|
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} |
317 |
|
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|
318 |
|
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/** |
319 |
jsr166 |
1.17 |
* Returns validated tail for use in cleaning methods. |
320 |
dl |
1.9 |
*/ |
321 |
|
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private QNode getValidatedTail() { |
322 |
|
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for (;;) { |
323 |
|
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QNode h = head.get(); |
324 |
|
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QNode first = h.next; |
325 |
|
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if (first != null && first.next == first) { // help advance |
326 |
|
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advanceHead(h, first); |
327 |
|
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continue; |
328 |
|
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} |
329 |
|
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QNode t = tail.get(); |
330 |
|
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QNode last = t.next; |
331 |
|
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if (t == tail.get()) { |
332 |
|
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if (last != null) |
333 |
|
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tail.compareAndSet(t, last); // help advance |
334 |
|
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else |
335 |
|
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return t; |
336 |
|
|
} |
337 |
|
|
} |
338 |
jsr166 |
1.10 |
} |
339 |
dl |
1.9 |
|
340 |
|
|
/** |
341 |
dl |
1.1 |
* Gets rid of cancelled node s with original predecessor pred. |
342 |
jsr166 |
1.17 |
* |
343 |
dl |
1.9 |
* @param pred predecessor of cancelled node |
344 |
|
|
* @param s the cancelled node |
345 |
dl |
1.1 |
*/ |
346 |
dl |
1.9 |
private void clean(QNode pred, QNode s) { |
347 |
dl |
1.1 |
Thread w = s.waiter; |
348 |
|
|
if (w != null) { // Wake up thread |
349 |
|
|
s.waiter = null; |
350 |
|
|
if (w != Thread.currentThread()) |
351 |
|
|
LockSupport.unpark(w); |
352 |
|
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} |
353 |
dl |
1.15 |
|
354 |
|
|
if (pred == null) |
355 |
|
|
return; |
356 |
|
|
|
357 |
dl |
1.9 |
/* |
358 |
|
|
* At any given time, exactly one node on list cannot be |
359 |
|
|
* deleted -- the last inserted node. To accommodate this, if |
360 |
|
|
* we cannot delete s, we save its predecessor as "cleanMe", |
361 |
|
|
* processing the previously saved version first. At least one |
362 |
|
|
* of node s or the node previously saved can always be |
363 |
|
|
* processed, so this always terminates. |
364 |
|
|
*/ |
365 |
|
|
while (pred.next == s) { |
366 |
|
|
QNode oldpred = reclean(); // First, help get rid of cleanMe |
367 |
|
|
QNode t = getValidatedTail(); |
368 |
|
|
if (s != t) { // If not tail, try to unsplice |
369 |
|
|
QNode sn = s.next; // s.next == s means s already off list |
370 |
jsr166 |
1.10 |
if (sn == s || pred.casNext(s, sn)) |
371 |
dl |
1.9 |
break; |
372 |
|
|
} |
373 |
|
|
else if (oldpred == pred || // Already saved |
374 |
|
|
(oldpred == null && cleanMe.compareAndSet(null, pred))) |
375 |
|
|
break; // Postpone cleaning |
376 |
|
|
} |
377 |
|
|
} |
378 |
jsr166 |
1.5 |
|
379 |
dl |
1.9 |
/** |
380 |
|
|
* Tries to unsplice the cancelled node held in cleanMe that was |
381 |
|
|
* previously uncleanable because it was at tail. |
382 |
jsr166 |
1.17 |
* |
383 |
dl |
1.9 |
* @return current cleanMe node (or null) |
384 |
|
|
*/ |
385 |
|
|
private QNode reclean() { |
386 |
jsr166 |
1.10 |
/* |
387 |
dl |
1.9 |
* cleanMe is, or at one time was, predecessor of cancelled |
388 |
|
|
* node s that was the tail so could not be unspliced. If s |
389 |
|
|
* is no longer the tail, try to unsplice if necessary and |
390 |
|
|
* make cleanMe slot available. This differs from similar |
391 |
|
|
* code in clean() because we must check that pred still |
392 |
|
|
* points to a cancelled node that must be unspliced -- if |
393 |
|
|
* not, we can (must) clear cleanMe without unsplicing. |
394 |
|
|
* This can loop only due to contention on casNext or |
395 |
jsr166 |
1.10 |
* clearing cleanMe. |
396 |
dl |
1.9 |
*/ |
397 |
|
|
QNode pred; |
398 |
|
|
while ((pred = cleanMe.get()) != null) { |
399 |
|
|
QNode t = getValidatedTail(); |
400 |
|
|
QNode s = pred.next; |
401 |
jsr166 |
1.10 |
if (s != t) { |
402 |
dl |
1.9 |
QNode sn; |
403 |
|
|
if (s == null || s == pred || s.get() != s || |
404 |
|
|
(sn = s.next) == s || pred.casNext(s, sn)) |
405 |
|
|
cleanMe.compareAndSet(pred, null); |
406 |
dl |
1.1 |
} |
407 |
dl |
1.9 |
else // s is still tail; cannot clean |
408 |
|
|
break; |
409 |
dl |
1.1 |
} |
410 |
dl |
1.9 |
return pred; |
411 |
dl |
1.1 |
} |
412 |
jsr166 |
1.5 |
|
413 |
dl |
1.1 |
/** |
414 |
jsr166 |
1.11 |
* Creates an initially empty {@code LinkedTransferQueue}. |
415 |
dl |
1.1 |
*/ |
416 |
|
|
public LinkedTransferQueue() { |
417 |
dl |
1.7 |
QNode dummy = new QNode(null, false); |
418 |
|
|
head = new PaddedAtomicReference<QNode>(dummy); |
419 |
|
|
tail = new PaddedAtomicReference<QNode>(dummy); |
420 |
|
|
cleanMe = new PaddedAtomicReference<QNode>(null); |
421 |
dl |
1.1 |
} |
422 |
|
|
|
423 |
|
|
/** |
424 |
jsr166 |
1.11 |
* Creates a {@code LinkedTransferQueue} |
425 |
dl |
1.1 |
* initially containing the elements of the given collection, |
426 |
|
|
* added in traversal order of the collection's iterator. |
427 |
jsr166 |
1.17 |
* |
428 |
dl |
1.1 |
* @param c the collection of elements to initially contain |
429 |
|
|
* @throws NullPointerException if the specified collection or any |
430 |
|
|
* of its elements are null |
431 |
|
|
*/ |
432 |
|
|
public LinkedTransferQueue(Collection<? extends E> c) { |
433 |
dl |
1.7 |
this(); |
434 |
dl |
1.1 |
addAll(c); |
435 |
|
|
} |
436 |
|
|
|
437 |
|
|
public void put(E e) throws InterruptedException { |
438 |
|
|
if (e == null) throw new NullPointerException(); |
439 |
|
|
if (Thread.interrupted()) throw new InterruptedException(); |
440 |
|
|
xfer(e, NOWAIT, 0); |
441 |
|
|
} |
442 |
|
|
|
443 |
jsr166 |
1.5 |
public boolean offer(E e, long timeout, TimeUnit unit) |
444 |
dl |
1.1 |
throws InterruptedException { |
445 |
|
|
if (e == null) throw new NullPointerException(); |
446 |
|
|
if (Thread.interrupted()) throw new InterruptedException(); |
447 |
|
|
xfer(e, NOWAIT, 0); |
448 |
|
|
return true; |
449 |
|
|
} |
450 |
|
|
|
451 |
|
|
public boolean offer(E e) { |
452 |
|
|
if (e == null) throw new NullPointerException(); |
453 |
|
|
xfer(e, NOWAIT, 0); |
454 |
|
|
return true; |
455 |
|
|
} |
456 |
|
|
|
457 |
dl |
1.15 |
public boolean add(E e) { |
458 |
|
|
if (e == null) throw new NullPointerException(); |
459 |
|
|
xfer(e, NOWAIT, 0); |
460 |
|
|
return true; |
461 |
|
|
} |
462 |
|
|
|
463 |
dl |
1.1 |
public void transfer(E e) throws InterruptedException { |
464 |
|
|
if (e == null) throw new NullPointerException(); |
465 |
|
|
if (xfer(e, WAIT, 0) == null) { |
466 |
jsr166 |
1.6 |
Thread.interrupted(); |
467 |
dl |
1.1 |
throw new InterruptedException(); |
468 |
jsr166 |
1.6 |
} |
469 |
dl |
1.1 |
} |
470 |
|
|
|
471 |
|
|
public boolean tryTransfer(E e, long timeout, TimeUnit unit) |
472 |
|
|
throws InterruptedException { |
473 |
|
|
if (e == null) throw new NullPointerException(); |
474 |
|
|
if (xfer(e, TIMEOUT, unit.toNanos(timeout)) != null) |
475 |
|
|
return true; |
476 |
|
|
if (!Thread.interrupted()) |
477 |
|
|
return false; |
478 |
|
|
throw new InterruptedException(); |
479 |
|
|
} |
480 |
|
|
|
481 |
|
|
public boolean tryTransfer(E e) { |
482 |
|
|
if (e == null) throw new NullPointerException(); |
483 |
|
|
return fulfill(e) != null; |
484 |
|
|
} |
485 |
|
|
|
486 |
|
|
public E take() throws InterruptedException { |
487 |
|
|
Object e = xfer(null, WAIT, 0); |
488 |
|
|
if (e != null) |
489 |
|
|
return (E)e; |
490 |
jsr166 |
1.6 |
Thread.interrupted(); |
491 |
dl |
1.1 |
throw new InterruptedException(); |
492 |
|
|
} |
493 |
|
|
|
494 |
|
|
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
495 |
|
|
Object e = xfer(null, TIMEOUT, unit.toNanos(timeout)); |
496 |
|
|
if (e != null || !Thread.interrupted()) |
497 |
|
|
return (E)e; |
498 |
|
|
throw new InterruptedException(); |
499 |
|
|
} |
500 |
|
|
|
501 |
|
|
public E poll() { |
502 |
|
|
return (E)fulfill(null); |
503 |
|
|
} |
504 |
|
|
|
505 |
|
|
public int drainTo(Collection<? super E> c) { |
506 |
|
|
if (c == null) |
507 |
|
|
throw new NullPointerException(); |
508 |
|
|
if (c == this) |
509 |
|
|
throw new IllegalArgumentException(); |
510 |
|
|
int n = 0; |
511 |
|
|
E e; |
512 |
|
|
while ( (e = poll()) != null) { |
513 |
|
|
c.add(e); |
514 |
|
|
++n; |
515 |
|
|
} |
516 |
|
|
return n; |
517 |
|
|
} |
518 |
|
|
|
519 |
|
|
public int drainTo(Collection<? super E> c, int maxElements) { |
520 |
|
|
if (c == null) |
521 |
|
|
throw new NullPointerException(); |
522 |
|
|
if (c == this) |
523 |
|
|
throw new IllegalArgumentException(); |
524 |
|
|
int n = 0; |
525 |
|
|
E e; |
526 |
|
|
while (n < maxElements && (e = poll()) != null) { |
527 |
|
|
c.add(e); |
528 |
|
|
++n; |
529 |
|
|
} |
530 |
|
|
return n; |
531 |
|
|
} |
532 |
|
|
|
533 |
|
|
// Traversal-based methods |
534 |
|
|
|
535 |
|
|
/** |
536 |
jsr166 |
1.17 |
* Returns head after performing any outstanding helping steps. |
537 |
dl |
1.1 |
*/ |
538 |
|
|
private QNode traversalHead() { |
539 |
|
|
for (;;) { |
540 |
|
|
QNode t = tail.get(); |
541 |
|
|
QNode h = head.get(); |
542 |
|
|
if (h != null && t != null) { |
543 |
|
|
QNode last = t.next; |
544 |
|
|
QNode first = h.next; |
545 |
|
|
if (t == tail.get()) { |
546 |
jsr166 |
1.5 |
if (last != null) |
547 |
dl |
1.1 |
tail.compareAndSet(t, last); |
548 |
|
|
else if (first != null) { |
549 |
|
|
Object x = first.get(); |
550 |
jsr166 |
1.5 |
if (x == first) |
551 |
|
|
advanceHead(h, first); |
552 |
dl |
1.1 |
else |
553 |
|
|
return h; |
554 |
|
|
} |
555 |
|
|
else |
556 |
|
|
return h; |
557 |
|
|
} |
558 |
|
|
} |
559 |
dl |
1.15 |
reclean(); |
560 |
dl |
1.1 |
} |
561 |
|
|
} |
562 |
|
|
|
563 |
|
|
|
564 |
|
|
public Iterator<E> iterator() { |
565 |
|
|
return new Itr(); |
566 |
|
|
} |
567 |
|
|
|
568 |
|
|
/** |
569 |
jsr166 |
1.4 |
* Iterators. Basic strategy is to traverse list, treating |
570 |
dl |
1.1 |
* non-data (i.e., request) nodes as terminating list. |
571 |
|
|
* Once a valid data node is found, the item is cached |
572 |
|
|
* so that the next call to next() will return it even |
573 |
|
|
* if subsequently removed. |
574 |
|
|
*/ |
575 |
|
|
class Itr implements Iterator<E> { |
576 |
dl |
1.15 |
QNode next; // node to return next |
577 |
|
|
QNode pnext; // predecessor of next |
578 |
|
|
QNode snext; // successor of next |
579 |
|
|
QNode curr; // last returned node, for remove() |
580 |
|
|
QNode pcurr; // predecessor of curr, for remove() |
581 |
jsr166 |
1.18 |
E nextItem; // Cache of next item, once committed to in next |
582 |
jsr166 |
1.5 |
|
583 |
dl |
1.1 |
Itr() { |
584 |
dl |
1.15 |
findNext(); |
585 |
dl |
1.1 |
} |
586 |
jsr166 |
1.5 |
|
587 |
dl |
1.15 |
/** |
588 |
jsr166 |
1.17 |
* Ensures next points to next valid node, or null if none. |
589 |
dl |
1.15 |
*/ |
590 |
|
|
void findNext() { |
591 |
dl |
1.1 |
for (;;) { |
592 |
dl |
1.15 |
QNode pred = pnext; |
593 |
|
|
QNode q = next; |
594 |
|
|
if (pred == null || pred == q) { |
595 |
|
|
pred = traversalHead(); |
596 |
|
|
q = pred.next; |
597 |
|
|
} |
598 |
|
|
if (q == null || !q.isData) { |
599 |
|
|
next = null; |
600 |
|
|
return; |
601 |
|
|
} |
602 |
|
|
Object x = q.get(); |
603 |
|
|
QNode s = q.next; |
604 |
|
|
if (x != null && q != x && q != s) { |
605 |
|
|
nextItem = (E)x; |
606 |
|
|
snext = s; |
607 |
|
|
pnext = pred; |
608 |
|
|
next = q; |
609 |
|
|
return; |
610 |
dl |
1.1 |
} |
611 |
dl |
1.15 |
pnext = q; |
612 |
|
|
next = s; |
613 |
dl |
1.1 |
} |
614 |
|
|
} |
615 |
jsr166 |
1.5 |
|
616 |
dl |
1.1 |
public boolean hasNext() { |
617 |
dl |
1.15 |
return next != null; |
618 |
dl |
1.1 |
} |
619 |
jsr166 |
1.5 |
|
620 |
dl |
1.1 |
public E next() { |
621 |
dl |
1.15 |
if (next == null) throw new NoSuchElementException(); |
622 |
|
|
pcurr = pnext; |
623 |
|
|
curr = next; |
624 |
|
|
pnext = next; |
625 |
|
|
next = snext; |
626 |
|
|
E x = nextItem; |
627 |
|
|
findNext(); |
628 |
|
|
return x; |
629 |
dl |
1.1 |
} |
630 |
jsr166 |
1.5 |
|
631 |
dl |
1.1 |
public void remove() { |
632 |
dl |
1.15 |
QNode p = curr; |
633 |
|
|
if (p == null) |
634 |
dl |
1.1 |
throw new IllegalStateException(); |
635 |
|
|
Object x = p.get(); |
636 |
|
|
if (x != null && x != p && p.compareAndSet(x, p)) |
637 |
dl |
1.15 |
clean(pcurr, p); |
638 |
dl |
1.1 |
} |
639 |
|
|
} |
640 |
|
|
|
641 |
|
|
public E peek() { |
642 |
|
|
for (;;) { |
643 |
|
|
QNode h = traversalHead(); |
644 |
|
|
QNode p = h.next; |
645 |
|
|
if (p == null) |
646 |
|
|
return null; |
647 |
|
|
Object x = p.get(); |
648 |
|
|
if (p != x) { |
649 |
|
|
if (!p.isData) |
650 |
|
|
return null; |
651 |
|
|
if (x != null) |
652 |
|
|
return (E)x; |
653 |
|
|
} |
654 |
|
|
} |
655 |
|
|
} |
656 |
|
|
|
657 |
dl |
1.2 |
public boolean isEmpty() { |
658 |
|
|
for (;;) { |
659 |
|
|
QNode h = traversalHead(); |
660 |
|
|
QNode p = h.next; |
661 |
|
|
if (p == null) |
662 |
|
|
return true; |
663 |
|
|
Object x = p.get(); |
664 |
|
|
if (p != x) { |
665 |
|
|
if (!p.isData) |
666 |
|
|
return true; |
667 |
|
|
if (x != null) |
668 |
|
|
return false; |
669 |
|
|
} |
670 |
|
|
} |
671 |
|
|
} |
672 |
|
|
|
673 |
dl |
1.1 |
public boolean hasWaitingConsumer() { |
674 |
|
|
for (;;) { |
675 |
|
|
QNode h = traversalHead(); |
676 |
|
|
QNode p = h.next; |
677 |
|
|
if (p == null) |
678 |
|
|
return false; |
679 |
|
|
Object x = p.get(); |
680 |
jsr166 |
1.5 |
if (p != x) |
681 |
dl |
1.1 |
return !p.isData; |
682 |
|
|
} |
683 |
|
|
} |
684 |
jsr166 |
1.5 |
|
685 |
dl |
1.1 |
/** |
686 |
|
|
* Returns the number of elements in this queue. If this queue |
687 |
jsr166 |
1.11 |
* contains more than {@code Integer.MAX_VALUE} elements, returns |
688 |
|
|
* {@code Integer.MAX_VALUE}. |
689 |
dl |
1.1 |
* |
690 |
|
|
* <p>Beware that, unlike in most collections, this method is |
691 |
|
|
* <em>NOT</em> a constant-time operation. Because of the |
692 |
|
|
* asynchronous nature of these queues, determining the current |
693 |
|
|
* number of elements requires an O(n) traversal. |
694 |
|
|
* |
695 |
|
|
* @return the number of elements in this queue |
696 |
|
|
*/ |
697 |
|
|
public int size() { |
698 |
|
|
int count = 0; |
699 |
|
|
QNode h = traversalHead(); |
700 |
|
|
for (QNode p = h.next; p != null && p.isData; p = p.next) { |
701 |
|
|
Object x = p.get(); |
702 |
jsr166 |
1.5 |
if (x != null && x != p) { |
703 |
dl |
1.1 |
if (++count == Integer.MAX_VALUE) // saturated |
704 |
|
|
break; |
705 |
|
|
} |
706 |
|
|
} |
707 |
|
|
return count; |
708 |
|
|
} |
709 |
|
|
|
710 |
|
|
public int getWaitingConsumerCount() { |
711 |
|
|
int count = 0; |
712 |
|
|
QNode h = traversalHead(); |
713 |
|
|
for (QNode p = h.next; p != null && !p.isData; p = p.next) { |
714 |
|
|
if (p.get() == null) { |
715 |
|
|
if (++count == Integer.MAX_VALUE) |
716 |
|
|
break; |
717 |
|
|
} |
718 |
|
|
} |
719 |
|
|
return count; |
720 |
|
|
} |
721 |
|
|
|
722 |
|
|
public int remainingCapacity() { |
723 |
|
|
return Integer.MAX_VALUE; |
724 |
|
|
} |
725 |
|
|
|
726 |
dl |
1.15 |
public boolean remove(Object o) { |
727 |
|
|
if (o == null) |
728 |
|
|
return false; |
729 |
|
|
for (;;) { |
730 |
|
|
QNode pred = traversalHead(); |
731 |
|
|
for (;;) { |
732 |
|
|
QNode q = pred.next; |
733 |
|
|
if (q == null || !q.isData) |
734 |
|
|
return false; |
735 |
|
|
if (q == pred) // restart |
736 |
|
|
break; |
737 |
|
|
Object x = q.get(); |
738 |
jsr166 |
1.17 |
if (x != null && x != q && o.equals(x) && |
739 |
dl |
1.15 |
q.compareAndSet(x, q)) { |
740 |
|
|
clean(pred, q); |
741 |
|
|
return true; |
742 |
|
|
} |
743 |
|
|
pred = q; |
744 |
|
|
} |
745 |
|
|
} |
746 |
|
|
} |
747 |
|
|
|
748 |
dl |
1.1 |
/** |
749 |
|
|
* Save the state to a stream (that is, serialize it). |
750 |
|
|
* |
751 |
jsr166 |
1.11 |
* @serialData All of the elements (each an {@code E}) in |
752 |
dl |
1.1 |
* the proper order, followed by a null |
753 |
|
|
* @param s the stream |
754 |
|
|
*/ |
755 |
|
|
private void writeObject(java.io.ObjectOutputStream s) |
756 |
|
|
throws java.io.IOException { |
757 |
|
|
s.defaultWriteObject(); |
758 |
jsr166 |
1.16 |
for (E e : this) |
759 |
|
|
s.writeObject(e); |
760 |
dl |
1.1 |
// Use trailing null as sentinel |
761 |
|
|
s.writeObject(null); |
762 |
|
|
} |
763 |
|
|
|
764 |
|
|
/** |
765 |
|
|
* Reconstitute the Queue instance from a stream (that is, |
766 |
|
|
* deserialize it). |
767 |
jsr166 |
1.19 |
* |
768 |
dl |
1.1 |
* @param s the stream |
769 |
|
|
*/ |
770 |
|
|
private void readObject(java.io.ObjectInputStream s) |
771 |
|
|
throws java.io.IOException, ClassNotFoundException { |
772 |
|
|
s.defaultReadObject(); |
773 |
dl |
1.7 |
resetHeadAndTail(); |
774 |
dl |
1.1 |
for (;;) { |
775 |
|
|
E item = (E)s.readObject(); |
776 |
|
|
if (item == null) |
777 |
|
|
break; |
778 |
|
|
else |
779 |
|
|
offer(item); |
780 |
|
|
} |
781 |
|
|
} |
782 |
dl |
1.7 |
|
783 |
|
|
|
784 |
|
|
// Support for resetting head/tail while deserializing |
785 |
dl |
1.12 |
private void resetHeadAndTail() { |
786 |
|
|
QNode dummy = new QNode(null, false); |
787 |
jsr166 |
1.20 |
UNSAFE.putObjectVolatile(this, headOffset, |
788 |
dl |
1.12 |
new PaddedAtomicReference<QNode>(dummy)); |
789 |
jsr166 |
1.20 |
UNSAFE.putObjectVolatile(this, tailOffset, |
790 |
dl |
1.12 |
new PaddedAtomicReference<QNode>(dummy)); |
791 |
jsr166 |
1.20 |
UNSAFE.putObjectVolatile(this, cleanMeOffset, |
792 |
dl |
1.12 |
new PaddedAtomicReference<QNode>(null)); |
793 |
|
|
} |
794 |
dl |
1.7 |
|
795 |
|
|
// Temporary Unsafe mechanics for preliminary release |
796 |
jsr166 |
1.13 |
private static Unsafe getUnsafe() throws Throwable { |
797 |
|
|
try { |
798 |
|
|
return Unsafe.getUnsafe(); |
799 |
|
|
} catch (SecurityException se) { |
800 |
|
|
try { |
801 |
|
|
return java.security.AccessController.doPrivileged |
802 |
|
|
(new java.security.PrivilegedExceptionAction<Unsafe>() { |
803 |
|
|
public Unsafe run() throws Exception { |
804 |
|
|
return getUnsafePrivileged(); |
805 |
|
|
}}); |
806 |
|
|
} catch (java.security.PrivilegedActionException e) { |
807 |
|
|
throw e.getCause(); |
808 |
|
|
} |
809 |
|
|
} |
810 |
|
|
} |
811 |
|
|
|
812 |
|
|
private static Unsafe getUnsafePrivileged() |
813 |
|
|
throws NoSuchFieldException, IllegalAccessException { |
814 |
|
|
Field f = Unsafe.class.getDeclaredField("theUnsafe"); |
815 |
|
|
f.setAccessible(true); |
816 |
jsr166 |
1.14 |
return (Unsafe) f.get(null); |
817 |
jsr166 |
1.13 |
} |
818 |
|
|
|
819 |
|
|
private static long fieldOffset(String fieldName) |
820 |
|
|
throws NoSuchFieldException { |
821 |
jsr166 |
1.20 |
return UNSAFE.objectFieldOffset |
822 |
jsr166 |
1.13 |
(LinkedTransferQueue.class.getDeclaredField(fieldName)); |
823 |
|
|
} |
824 |
|
|
|
825 |
jsr166 |
1.20 |
private static final Unsafe UNSAFE; |
826 |
dl |
1.7 |
private static final long headOffset; |
827 |
|
|
private static final long tailOffset; |
828 |
|
|
private static final long cleanMeOffset; |
829 |
|
|
static { |
830 |
|
|
try { |
831 |
jsr166 |
1.20 |
UNSAFE = getUnsafe(); |
832 |
jsr166 |
1.13 |
headOffset = fieldOffset("head"); |
833 |
|
|
tailOffset = fieldOffset("tail"); |
834 |
|
|
cleanMeOffset = fieldOffset("cleanMe"); |
835 |
|
|
} catch (Throwable e) { |
836 |
dl |
1.7 |
throw new RuntimeException("Could not initialize intrinsics", e); |
837 |
|
|
} |
838 |
|
|
} |
839 |
|
|
|
840 |
dl |
1.1 |
} |