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1.2 |
/* |
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1.55 |
* Written by Doug Lea, Bill Scherer, and Michael Scott with |
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* assistance from members of JCP JSR-166 Expert Group and released to |
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* the public domain, as explained at |
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1.29 |
* http://creativecommons.org/licenses/publicdomain |
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dl |
1.2 |
*/ |
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1.1 |
package java.util.concurrent; |
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1.8 |
import java.util.concurrent.locks.*; |
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1.55 |
import java.util.concurrent.atomic.*; |
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1.1 |
import java.util.*; |
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/** |
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jsr166 |
1.52 |
* A {@linkplain BlockingQueue blocking queue} in which each insert |
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* operation must wait for a corresponding remove operation by another |
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* thread, and vice versa. A synchronous queue does not have any |
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* internal capacity, not even a capacity of one. You cannot |
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* <tt>peek</tt> at a synchronous queue because an element is only |
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* present when you try to remove it; you cannot insert an element |
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* (using any method) unless another thread is trying to remove it; |
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* you cannot iterate as there is nothing to iterate. The |
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* <em>head</em> of the queue is the element that the first queued |
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* inserting thread is trying to add to the queue; if there is no such |
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* queued thread then no element is available for removal and |
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* <tt>poll()</tt> will return <tt>null</tt>. For purposes of other |
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* <tt>Collection</tt> methods (for example <tt>contains</tt>), a |
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* <tt>SynchronousQueue</tt> acts as an empty collection. This queue |
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* does not permit <tt>null</tt> elements. |
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1.18 |
* |
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* <p>Synchronous queues are similar to rendezvous channels used in |
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* CSP and Ada. They are well suited for handoff designs, in which an |
32 |
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1.30 |
* object running in one thread must sync up with an object running |
33 |
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1.18 |
* in another thread in order to hand it some information, event, or |
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* task. |
35 |
dl |
1.43 |
* |
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* <p> This class supports an optional fairness policy for ordering |
37 |
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* waiting producer and consumer threads. By default, this ordering |
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* is not guaranteed. However, a queue constructed with fairness set |
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1.55 |
* to <tt>true</tt> grants threads access in FIFO order. |
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1.43 |
* |
41 |
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1.46 |
* <p>This class and its iterator implement all of the |
42 |
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* <em>optional</em> methods of the {@link Collection} and {@link |
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jsr166 |
1.48 |
* Iterator} interfaces. |
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dl |
1.42 |
* |
45 |
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* <p>This class is a member of the |
46 |
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* <a href="{@docRoot}/../guide/collections/index.html"> |
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* Java Collections Framework</a>. |
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* |
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dl |
1.6 |
* @since 1.5 |
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* @author Doug Lea |
51 |
dl |
1.24 |
* @param <E> the type of elements held in this collection |
52 |
dl |
1.23 |
*/ |
53 |
dl |
1.2 |
public class SynchronousQueue<E> extends AbstractQueue<E> |
54 |
dl |
1.55 |
implements BlockingQueue<E>, java.io.Serializable { |
55 |
dl |
1.15 |
private static final long serialVersionUID = -3223113410248163686L; |
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1.1 |
|
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1.2 |
/* |
58 |
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1.55 |
* This class implements extensions of the dual stack and dual |
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* queue algorithms described in "Nonblocking Concurrent Objects |
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* with Condition Synchronization", by W. N. Scherer III and |
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* M. L. Scott. 18th Annual Conf. on Distributed Computing, |
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* Oct. 2004 (see also |
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* http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/duals.html). |
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* The (Lifo) stack is used for non-fair mode, and the (Fifo) |
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* queue for fair mode. The performance of the two is generally |
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* similar. Fifo usually supports higher throughput under |
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* contention but Lifo maintains higher thread locality in common |
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* applications. |
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* |
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* A dual queue (and similarly stack) is one that at any given |
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* time either holds "data" -- items provided by put operations, |
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* or "requests" -- slots representing take operations, or is |
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* empty. A call to "fulfill" (i.e., a call requesting an item |
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* from a queue holding data or vice versa) dequeues a |
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* complementary node. The most interesting feature of these |
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* queues is that any operation can figure out which mode the |
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* queue is in, and act accordingly without needing locks. |
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* |
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* Both the queue and stack extend abstract class Transferer |
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* defining the single method transfer that does a put or a |
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* take. These are unified into a single method because in dual |
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* data structures, the put and take operations are symmetrical, |
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* so nearly all code can be combined. The resulting transfer |
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* methods are on the long side, but are easier to follow than |
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* they would be if broken up into nearly-duplicated parts. |
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* |
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* The queue and stack data structures share many conceptual |
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* similarities but very few concrete details. For simplicity, |
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* they are kept distinct so that they can later evolve |
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* separately. |
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* |
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* The algorithms here differ from the versions in the above paper |
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* in extending them for use in synchronous queues, as well as |
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* dealing with cancellation. The main differences include: |
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* |
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* 1. The orginal algorithms used bit-marked pointers, but |
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* the ones here use mode bits in nodes, leading to a number |
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* of further adaptations. |
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* 2. SynchronousQueues must block threads waiting to become |
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* fulfilled. |
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* 3. Nodes/threads that have been cancelled due to timeouts |
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* or interruptions are cleaned out of the lists to |
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* avoid garbage retention and memory depletion. |
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* |
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* Blocking is mainly accomplished using LockSupport park/unpark, |
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* except that nodes that appear to be the next ones to become |
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* fulfilled first spin a bit (on multiprocessors only). On very |
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* busy synchronous queues, spinning can dramatically improve |
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* throughput. And on less busy ones, the amount of spinning is |
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* small enough not to be noticeable. |
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* |
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* Cleaning is done in different ways in queues vs stacks. For |
113 |
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* queues, we can almost always remove a node immediately in O(1) |
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* time (modulo retries for consistency checks) when it is |
115 |
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* cancelled. But if it may be pinned as the current tail, it must |
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* wait until some subsequent cancellation. For stacks, we need a |
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* potentially O(n) traversal to be sure that we can remove the |
118 |
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* node, but this can run concurrently with other threads |
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* accessing the stack. |
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* |
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* While garbage collection takes care of most node reclamation |
122 |
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* issues that otherwise complicate nonblocking algorithms, care |
123 |
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* is made to "forget" references to data, other nodes, and |
124 |
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* threads that might be held on to long-term by blocked |
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* threads. In cases where setting to null would otherwise |
126 |
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* conflict with main algorithms, this is done by changing a |
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* node's link to now point to the node itself. This doesn't arise |
128 |
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* much for Stack nodes (because blocked threads do not hang on to |
129 |
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* old head pointers), but references in Queue nodes must be |
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* agressively forgotten to avoid reachability of everything any |
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* node has ever referred to since arrival. |
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*/ |
133 |
dl |
1.2 |
|
134 |
dl |
1.43 |
/** |
135 |
dl |
1.55 |
* Shared internal API for dual stacks and queues. |
136 |
dl |
1.43 |
*/ |
137 |
dl |
1.55 |
static abstract class Transferer { |
138 |
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/** |
139 |
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* Perform a put or take. |
140 |
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* @param e if non-null, the item to be handed to a consumer; |
141 |
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* if null, requests that transfer return an item offered by |
142 |
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* producer. |
143 |
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* @param timed if this operation should timeout |
144 |
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* @param nanos the timeout, in nanoseconds |
145 |
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* @return if nonnull, the item provided or received; if null, |
146 |
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* the operation failed due to timeout or interrupt -- the |
147 |
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* caller can distinguish which of these occurred by checking |
148 |
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* Thread.interrupted. |
149 |
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*/ |
150 |
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abstract Object transfer(Object e, boolean timed, long nanos); |
151 |
dl |
1.43 |
} |
152 |
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153 |
dl |
1.55 |
/** The number of CPUs, for spin control */ |
154 |
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static final int NCPUS = Runtime.getRuntime().availableProcessors(); |
155 |
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156 |
dl |
1.43 |
/** |
157 |
dl |
1.55 |
* The number of times to spin before blocking in timed waits. |
158 |
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* The value is empirically derived -- it works well across a |
159 |
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* variety of processors and OSes. Emprically, the best value |
160 |
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* seems not to vary with number of CPUs (beyond 2) so is just |
161 |
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* a constant. |
162 |
dl |
1.43 |
*/ |
163 |
dl |
1.55 |
static final int maxTimedSpins = (NCPUS < 2)? 0 : 32; |
164 |
dl |
1.43 |
|
165 |
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/** |
166 |
dl |
1.55 |
* The number of times to spin before blocking in untimed |
167 |
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* waits. This is greater than timed value because untimed |
168 |
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* waits spin faster since they don't need to check times on |
169 |
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* each spin. |
170 |
dl |
1.43 |
*/ |
171 |
dl |
1.55 |
static final int maxUntimedSpins = maxTimedSpins * 16; |
172 |
dl |
1.43 |
|
173 |
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/** |
174 |
dl |
1.55 |
* The number of nanoseconds for which it is faster to spin |
175 |
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* rather than to use timed park. A rough estimate suffices. |
176 |
dl |
1.43 |
*/ |
177 |
dl |
1.55 |
static final long spinForTimeoutThreshold = 1000L; |
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179 |
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/** Dual stack */ |
180 |
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static final class TransferStack extends Transferer { |
181 |
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/* |
182 |
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* This extends Scherer-Scott dual stack algorithm, differing, |
183 |
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* among other ways, by using "covering" nodes rather than |
184 |
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* bit-marked pointers: Fulfilling operations push on marker |
185 |
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* nodes (with FULFILLING bit set in mode) to reserve a spot |
186 |
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* to match a waiting node. |
187 |
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*/ |
188 |
dl |
1.43 |
|
189 |
dl |
1.55 |
/* Modes for SNodes, ORed together in node fields */ |
190 |
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/** Node represents an unfulfilled consumer */ |
191 |
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static final int REQUEST = 0; |
192 |
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/** Node represents an unfulfilled producer */ |
193 |
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static final int DATA = 1; |
194 |
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/** Node is fulfilling another unfulfilled DATA or REQUEST */ |
195 |
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static final int FULFILLING = 2; |
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197 |
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/** Return true if m has fulfilling bit set */ |
198 |
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static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; } |
199 |
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200 |
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/** Node class for TransferStacks. */ |
201 |
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static final class SNode { |
202 |
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volatile SNode next; // next node in stack |
203 |
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volatile SNode match; // the node matched to this |
204 |
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volatile Thread waiter; // to control park/unpark |
205 |
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Object item; // data; or null for REQUESTs |
206 |
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int mode; |
207 |
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// Note: item and mode fields don't need to be volatile |
208 |
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// since they are always written before, and read after, |
209 |
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// other volatile/atomic operations. |
210 |
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211 |
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SNode(Object item) { |
212 |
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this.item = item; |
213 |
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} |
214 |
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215 |
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static final AtomicReferenceFieldUpdater<SNode, SNode> |
216 |
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nextUpdater = AtomicReferenceFieldUpdater.newUpdater |
217 |
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(SNode.class, SNode.class, "next"); |
218 |
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219 |
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boolean casNext(SNode cmp, SNode val) { |
220 |
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return (cmp == next && |
221 |
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nextUpdater.compareAndSet(this, cmp, val)); |
222 |
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} |
223 |
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224 |
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static final AtomicReferenceFieldUpdater<SNode, SNode> |
225 |
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matchUpdater = AtomicReferenceFieldUpdater.newUpdater |
226 |
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(SNode.class, SNode.class, "match"); |
227 |
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228 |
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/** |
229 |
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* Try to match node s to this node, if so, waking up |
230 |
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* thread. Fulfillers call tryMatch to identify their |
231 |
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* waiters. Waiters block until they have been |
232 |
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* matched. |
233 |
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* @param s the node to match |
234 |
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* @return true if successfully matched to s |
235 |
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*/ |
236 |
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boolean tryMatch(SNode s) { |
237 |
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if (match == null && |
238 |
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matchUpdater.compareAndSet(this, null, s)) { |
239 |
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Thread w = waiter; |
240 |
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if (w != null) { // waiters need at most one unpark |
241 |
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waiter = null; |
242 |
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LockSupport.unpark(w); |
243 |
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} |
244 |
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return true; |
245 |
dl |
1.47 |
} |
246 |
dl |
1.55 |
return match == s; |
247 |
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} |
248 |
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249 |
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/** |
250 |
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* Try to cancel a wait by matching node to itself. |
251 |
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*/ |
252 |
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void tryCancel() { |
253 |
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matchUpdater.compareAndSet(this, null, this); |
254 |
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} |
255 |
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256 |
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boolean isCancelled() { |
257 |
|
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return match == this; |
258 |
dl |
1.47 |
} |
259 |
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} |
260 |
dl |
1.43 |
|
261 |
dl |
1.55 |
/** The head (top) of the stack */ |
262 |
|
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volatile SNode head; |
263 |
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|
264 |
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static final AtomicReferenceFieldUpdater<TransferStack, SNode> |
265 |
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headUpdater = AtomicReferenceFieldUpdater.newUpdater |
266 |
|
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(TransferStack.class, SNode.class, "head"); |
267 |
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268 |
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boolean casHead(SNode h, SNode nh) { |
269 |
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return h == head && headUpdater.compareAndSet(this, h, nh); |
270 |
|
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} |
271 |
dl |
1.2 |
|
272 |
dl |
1.55 |
/** |
273 |
|
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* Create or reset fields of a node. Called only from transfer |
274 |
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* where the node to push on stack is lazily created and |
275 |
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* reused when possible to help reduce intervals between reads |
276 |
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* and CASes of head and to avoid surges of garbage when CASes |
277 |
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* to push nodes fail due to contention. |
278 |
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*/ |
279 |
|
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static SNode snode(SNode s, Object e, SNode next, int mode) { |
280 |
|
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if (s == null) s = new SNode(e); |
281 |
|
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s.mode = mode; |
282 |
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s.next = next; |
283 |
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return s; |
284 |
dl |
1.43 |
} |
285 |
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|
286 |
dl |
1.55 |
/** |
287 |
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* Put or take an item. |
288 |
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*/ |
289 |
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Object transfer(Object e, boolean timed, long nanos) { |
290 |
|
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/* |
291 |
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* Basic algorithm is to loop trying one of three actions: |
292 |
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* |
293 |
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* 1. If apparently empty or already containing nodes of same |
294 |
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* mode, try to push node on stack and wait for a match, |
295 |
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* returning it, or null if cancelled. |
296 |
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* |
297 |
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* 2. If apparently containing node of complementary mode, |
298 |
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* try to push a fulfilling node on to stack, match |
299 |
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* with corresponding waiting node, pop both from |
300 |
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* stack, and return matched item. The matching or |
301 |
|
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* unlinking might not actually be necessary because of |
302 |
|
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* another threads performing action 3: |
303 |
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* |
304 |
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* 3. If top of stack already holds another fulfilling node, |
305 |
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* help it out by doing its match and/or pop |
306 |
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* operations, and then continue. The code for helping |
307 |
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* is essentially the same as for fulfilling, except |
308 |
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* that it doesn't return the item. |
309 |
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*/ |
310 |
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|
311 |
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SNode s = null; // constructed/reused as needed |
312 |
|
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int mode = (e == null)? REQUEST : DATA; |
313 |
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|
314 |
|
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for (;;) { |
315 |
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SNode h = head; |
316 |
|
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if (h == null || h.mode == mode) { // empty or same-mode |
317 |
|
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if (timed && nanos <= 0) { // can't wait |
318 |
|
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if (h != null && h.isCancelled()) |
319 |
|
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casHead(h, h.next); // pop cancelled node |
320 |
|
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else |
321 |
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return null; |
322 |
|
|
} else if (casHead(h, s = snode(s, e, h, mode))) { |
323 |
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SNode m = awaitFulfill(s, timed, nanos); |
324 |
|
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if (m == s) { // wait was cancelled |
325 |
|
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clean(s); |
326 |
|
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return null; |
327 |
|
|
} |
328 |
|
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if ((h = head) != null && h.next == s) |
329 |
|
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casHead(h, s.next); // help s's fulfiller |
330 |
|
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return mode == REQUEST? m.item : s.item; |
331 |
|
|
} |
332 |
|
|
} else if (!isFulfilling(h.mode)) { // try to fulfill |
333 |
|
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if (h.isCancelled()) // already cancelled |
334 |
|
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casHead(h, h.next); // pop and retry |
335 |
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else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) { |
336 |
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for (;;) { // loop until matched or waiters disappear |
337 |
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SNode m = s.next; // m is s's match |
338 |
|
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if (m == null) { // all waiters are gone |
339 |
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casHead(s, null); // pop fulfill node |
340 |
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s = null; // use new node next time |
341 |
|
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break; // restart main loop |
342 |
|
|
} |
343 |
|
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SNode mn = m.next; |
344 |
|
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if (m.tryMatch(s)) { |
345 |
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casHead(s, mn); // pop both s and m |
346 |
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|
return (mode == REQUEST)? m.item : s.item; |
347 |
|
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} else // lost match |
348 |
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s.casNext(m, mn); // help unlink |
349 |
|
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} |
350 |
|
|
} |
351 |
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|
} else { // help a fulfiller |
352 |
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SNode m = h.next; // m is h's match |
353 |
|
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if (m == null) // waiter is gone |
354 |
|
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casHead(h, null); // pop fulfilling node |
355 |
|
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else { |
356 |
|
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SNode mn = m.next; |
357 |
|
|
if (m.tryMatch(h)) // help match |
358 |
|
|
casHead(h, mn); // pop both h and m |
359 |
|
|
else // lost match |
360 |
|
|
h.casNext(m, mn); // help unlink |
361 |
|
|
} |
362 |
dl |
1.47 |
} |
363 |
|
|
} |
364 |
|
|
} |
365 |
|
|
|
366 |
dl |
1.55 |
/** |
367 |
|
|
* Spin/block until node s is matched by a fulfill operation. |
368 |
|
|
* @param s the waiting node |
369 |
|
|
* @param timed true if timed wait |
370 |
|
|
* @param nanos timeout value |
371 |
|
|
* @return matched node, or s if cancelled |
372 |
|
|
*/ |
373 |
|
|
SNode awaitFulfill(SNode s, boolean timed, long nanos) { |
374 |
|
|
/* |
375 |
|
|
* When a node/thread is about to block, it sets its waiter |
376 |
|
|
* field and then rechecks state at least one more time |
377 |
|
|
* before actually parking, thus covering race vs |
378 |
|
|
* fulfiller noticing that waiter is nonnull so should be |
379 |
|
|
* woken. |
380 |
|
|
* |
381 |
|
|
* When invoked by nodes that appear at the point of call |
382 |
|
|
* to be at the head of the stack, calls to park are |
383 |
|
|
* preceded by spins to avoid blocking when producers and |
384 |
|
|
* consumers are arriving very close in time. This can |
385 |
|
|
* happen enough to bother only on multiprocessors. |
386 |
|
|
* |
387 |
|
|
* The order of checks for returning out of main loop |
388 |
|
|
* reflects fact that interrupts have precedence over |
389 |
|
|
* normal returns, which have precedence over |
390 |
|
|
* timeouts. (So, on timeout, one last check for match is |
391 |
|
|
* done before giving up.) Except that calls from untimed |
392 |
|
|
* SynchronousQueue.{poll/offer} don't check interrupts |
393 |
|
|
* and don't wait at all, so are trapped in transfer |
394 |
|
|
* method rather than calling awaitFulfill. |
395 |
|
|
*/ |
396 |
|
|
long lastTime = (timed)? System.nanoTime() : 0; |
397 |
|
|
Thread w = Thread.currentThread(); |
398 |
|
|
SNode h = head; |
399 |
|
|
int spins = (shouldSpin(s)? |
400 |
|
|
(timed? maxTimedSpins : maxUntimedSpins) : 0); |
401 |
|
|
for (;;) { |
402 |
|
|
if (w.isInterrupted()) |
403 |
|
|
s.tryCancel(); |
404 |
|
|
SNode m = s.match; |
405 |
|
|
if (m != null) |
406 |
|
|
return m; |
407 |
|
|
if (timed) { |
408 |
|
|
long now = System.nanoTime(); |
409 |
|
|
nanos -= now - lastTime; |
410 |
|
|
lastTime = now; |
411 |
|
|
if (nanos <= 0) { |
412 |
|
|
s.tryCancel(); |
413 |
|
|
continue; |
414 |
|
|
} |
415 |
|
|
} |
416 |
|
|
if (spins > 0) |
417 |
|
|
spins = shouldSpin(s)? (spins-1) : 0; |
418 |
|
|
else if (s.waiter == null) |
419 |
|
|
s.waiter = w; // establish waiter so can park next iter |
420 |
|
|
else if (!timed) |
421 |
|
|
LockSupport.park(this); |
422 |
|
|
else if (nanos > spinForTimeoutThreshold) |
423 |
|
|
LockSupport.parkNanos(this, nanos); |
424 |
dl |
1.47 |
} |
425 |
|
|
} |
426 |
dl |
1.2 |
|
427 |
dl |
1.55 |
/** |
428 |
|
|
* Return true if node s is at head or there is an active |
429 |
|
|
* fulfiller. |
430 |
|
|
*/ |
431 |
|
|
boolean shouldSpin(SNode s) { |
432 |
|
|
SNode h = head; |
433 |
|
|
return (h == null || h == s || isFulfilling(h.mode)); |
434 |
|
|
} |
435 |
|
|
|
436 |
|
|
/** |
437 |
|
|
* Unlink s from the stack |
438 |
|
|
*/ |
439 |
|
|
void clean(SNode s) { |
440 |
|
|
s.item = null; // forget item |
441 |
|
|
s.waiter = null; // forget thread |
442 |
|
|
|
443 |
|
|
/* |
444 |
|
|
* At worst we may need to traverse entire stack to unlink |
445 |
|
|
* s. If there are multiple concurrent calls to clean, we |
446 |
|
|
* might not see s if another thread has already removed |
447 |
|
|
* it. But we can stop when we see any node known to |
448 |
|
|
* follow s. We use s.next unless it too is cancelled, in |
449 |
|
|
* which case we try the node one past. We don't check any |
450 |
|
|
* futher because we don't want to doubly traverse just to |
451 |
|
|
* find sentinel. |
452 |
|
|
*/ |
453 |
|
|
|
454 |
|
|
SNode past = s.next; |
455 |
|
|
if (past != null && past.isCancelled()) |
456 |
|
|
past = past.next; |
457 |
|
|
|
458 |
|
|
// Absorb cancelled nodes at head |
459 |
|
|
SNode p; |
460 |
|
|
while ((p = head) != null && p != past && p.isCancelled()) |
461 |
|
|
casHead(p, p.next); |
462 |
|
|
|
463 |
|
|
// Unsplice embedded nodes |
464 |
|
|
while (p != null && p != past) { |
465 |
|
|
SNode n = p.next; |
466 |
|
|
if (n != null && n.isCancelled()) |
467 |
|
|
p.casNext(n, n.next); |
468 |
|
|
else |
469 |
|
|
p = n; |
470 |
dl |
1.47 |
} |
471 |
|
|
} |
472 |
|
|
} |
473 |
jsr166 |
1.48 |
|
474 |
dl |
1.55 |
/** Dual Queue. */ |
475 |
|
|
static final class TransferQueue extends Transferer { |
476 |
|
|
/* |
477 |
|
|
* This extends Scherer-Scott dual queue algorithm, differing, |
478 |
|
|
* among other ways, by using modes within nodes rather than |
479 |
|
|
* marked pointers. The algorithm is a little simpler than |
480 |
|
|
* that for stacks because fulfillers do not need explicit |
481 |
|
|
* nodes, and matching is done by CAS'ing QNode.item field |
482 |
|
|
* from nonnull to null (for put) or vice versa (for take). |
483 |
|
|
*/ |
484 |
dl |
1.53 |
|
485 |
dl |
1.55 |
/** Node class for TransferQueue. */ |
486 |
|
|
static final class QNode { |
487 |
|
|
volatile QNode next; // next node in queue |
488 |
|
|
volatile Object item; // CAS'ed to or from null |
489 |
|
|
volatile Thread waiter; // to control park/unpark |
490 |
|
|
final boolean isData; |
491 |
dl |
1.35 |
|
492 |
dl |
1.55 |
QNode(Object item, boolean isData) { |
493 |
|
|
this.item = item; |
494 |
|
|
this.isData = isData; |
495 |
|
|
} |
496 |
dl |
1.35 |
|
497 |
dl |
1.55 |
static final AtomicReferenceFieldUpdater<QNode, QNode> |
498 |
|
|
nextUpdater = AtomicReferenceFieldUpdater.newUpdater |
499 |
|
|
(QNode.class, QNode.class, "next"); |
500 |
dl |
1.31 |
|
501 |
dl |
1.55 |
boolean casNext(QNode cmp, QNode val) { |
502 |
|
|
return (next == cmp && |
503 |
|
|
nextUpdater.compareAndSet(this, cmp, val)); |
504 |
|
|
} |
505 |
|
|
|
506 |
|
|
static final AtomicReferenceFieldUpdater<QNode, Object> |
507 |
|
|
itemUpdater = AtomicReferenceFieldUpdater.newUpdater |
508 |
|
|
(QNode.class, Object.class, "item"); |
509 |
dl |
1.43 |
|
510 |
dl |
1.55 |
boolean casItem(Object cmp, Object val) { |
511 |
|
|
return (item == cmp && |
512 |
|
|
itemUpdater.compareAndSet(this, cmp, val)); |
513 |
|
|
} |
514 |
|
|
|
515 |
|
|
/** |
516 |
|
|
* Try to cancel by CAS'ing ref to this as item. |
517 |
|
|
*/ |
518 |
|
|
void tryCancel(Object cmp) { |
519 |
|
|
itemUpdater.compareAndSet(this, cmp, this); |
520 |
|
|
} |
521 |
|
|
|
522 |
|
|
boolean isCancelled() { |
523 |
|
|
return item == this; |
524 |
|
|
} |
525 |
dl |
1.31 |
} |
526 |
|
|
|
527 |
dl |
1.55 |
/** Head of queue */ |
528 |
|
|
transient volatile QNode head; |
529 |
|
|
/** Tail of queue */ |
530 |
|
|
transient volatile QNode tail; |
531 |
dl |
1.31 |
/** |
532 |
dl |
1.55 |
* Reference to a cancelled node that might not yet have been |
533 |
|
|
* unlinked from queue because it was the last inserted node |
534 |
|
|
* when it cancelled. |
535 |
dl |
1.31 |
*/ |
536 |
dl |
1.55 |
transient volatile QNode cleanMe; |
537 |
|
|
|
538 |
|
|
TransferQueue() { |
539 |
|
|
QNode h = new QNode(null, false); // initialize to dummy node. |
540 |
|
|
head = h; |
541 |
|
|
tail = h; |
542 |
dl |
1.31 |
} |
543 |
|
|
|
544 |
dl |
1.55 |
static final AtomicReferenceFieldUpdater<TransferQueue, QNode> |
545 |
|
|
headUpdater = AtomicReferenceFieldUpdater.newUpdater |
546 |
|
|
(TransferQueue.class, QNode.class, "head"); |
547 |
|
|
|
548 |
dl |
1.31 |
/** |
549 |
dl |
1.55 |
* Try to cas nh as new head; if successful unlink |
550 |
|
|
* old head's next node to avoid garbage retention. |
551 |
dl |
1.31 |
*/ |
552 |
dl |
1.55 |
void advanceHead(QNode h, QNode nh) { |
553 |
|
|
if (h == head && headUpdater.compareAndSet(this, h, nh)) |
554 |
|
|
h.next = h; // forget old next |
555 |
dl |
1.31 |
} |
556 |
|
|
|
557 |
dl |
1.55 |
static final AtomicReferenceFieldUpdater<TransferQueue, QNode> |
558 |
|
|
tailUpdater = AtomicReferenceFieldUpdater.newUpdater |
559 |
|
|
(TransferQueue.class, QNode.class, "tail"); |
560 |
|
|
|
561 |
dl |
1.31 |
/** |
562 |
dl |
1.55 |
* Try to cas nt as new tail. |
563 |
dl |
1.31 |
*/ |
564 |
dl |
1.55 |
void advanceTail(QNode t, QNode nt) { |
565 |
|
|
if (tail == t) |
566 |
|
|
tailUpdater.compareAndSet(this, t, nt); |
567 |
dl |
1.31 |
} |
568 |
dl |
1.2 |
|
569 |
dl |
1.55 |
static final AtomicReferenceFieldUpdater<TransferQueue, QNode> |
570 |
|
|
cleanMeUpdater = AtomicReferenceFieldUpdater.newUpdater |
571 |
|
|
(TransferQueue.class, QNode.class, "cleanMe"); |
572 |
dl |
1.2 |
|
573 |
|
|
/** |
574 |
dl |
1.55 |
* Try to CAS cleanMe slot |
575 |
dl |
1.2 |
*/ |
576 |
dl |
1.55 |
boolean casCleanMe(QNode cmp, QNode val) { |
577 |
|
|
return (cleanMe == cmp && |
578 |
|
|
cleanMeUpdater.compareAndSet(this, cmp, val)); |
579 |
dl |
1.35 |
} |
580 |
|
|
|
581 |
|
|
/** |
582 |
dl |
1.55 |
* Put or take an item. |
583 |
dl |
1.35 |
*/ |
584 |
dl |
1.55 |
Object transfer(Object e, boolean timed, long nanos) { |
585 |
|
|
/* Basic algorithm is to loop trying to take either of |
586 |
|
|
* two actions: |
587 |
|
|
* |
588 |
|
|
* 1. If queue apparently empty or holding same-mode nodes, |
589 |
|
|
* try to add node to queue of waiters, wait to be |
590 |
|
|
* fulfilled (or cancelled) and return matching item. |
591 |
|
|
* |
592 |
|
|
* 2. If queue apparently contains waiting items, and this |
593 |
|
|
* call is of complementary mode, try to fulfill by CAS'ing |
594 |
|
|
* item field of waiting node and dequeuing it, and then |
595 |
|
|
* returning matching item. |
596 |
|
|
* |
597 |
|
|
* In each case, along the way, check for and try to help |
598 |
|
|
* advance head and tail on behalf of other stalled/slow |
599 |
|
|
* threads. |
600 |
|
|
* |
601 |
|
|
* The loop starts off with a null check guarding against |
602 |
|
|
* seeing uninitialized head or tail values. This never |
603 |
|
|
* happens in current SynchronousQueue, but could if |
604 |
|
|
* callers held non-volatile/final ref to the |
605 |
|
|
* transferer. The check is here anyway because it places |
606 |
|
|
* null checks at top of loop, which is usually faster |
607 |
|
|
* than having them implicitly interspersed. |
608 |
|
|
*/ |
609 |
|
|
|
610 |
|
|
QNode s = null; // constructed/reused as needed |
611 |
|
|
boolean isData = (e != null); |
612 |
|
|
|
613 |
|
|
for (;;) { |
614 |
|
|
QNode t = tail; |
615 |
|
|
QNode h = head; |
616 |
|
|
if (t == null || h == null) // saw unitialized values |
617 |
|
|
continue; // spin |
618 |
|
|
|
619 |
|
|
if (h == t || t.isData == isData) { // empty or same-mode |
620 |
|
|
QNode tn = t.next; |
621 |
|
|
if (t != tail) // inconsistent read |
622 |
|
|
continue; |
623 |
|
|
if (tn != null) { // lagging tail |
624 |
|
|
advanceTail(t, tn); |
625 |
|
|
continue; |
626 |
|
|
} |
627 |
|
|
if (timed && nanos <= 0) // can't wait |
628 |
|
|
return null; |
629 |
|
|
if (s == null) |
630 |
|
|
s = new QNode(e, isData); |
631 |
|
|
if (!t.casNext(null, s)) // failed to link in |
632 |
|
|
continue; |
633 |
|
|
|
634 |
|
|
advanceTail(t, s); // swing tail and wait |
635 |
|
|
Object x = awaitFulfill(s, e, timed, nanos); |
636 |
|
|
if (x == s) { // wait was cancelled |
637 |
|
|
clean(t, s); |
638 |
|
|
return null; |
639 |
|
|
} |
640 |
|
|
|
641 |
|
|
if (s.next != s) { // not already unlinked |
642 |
|
|
advanceHead(t, s); // unlink |
643 |
|
|
if (x != null) // and forget fields |
644 |
|
|
s.item = s; |
645 |
|
|
s.waiter = null; |
646 |
|
|
} |
647 |
|
|
return (x != null)? x : e; |
648 |
|
|
|
649 |
|
|
} else { // complementary-mode |
650 |
|
|
QNode m = h.next; // node to fulfill |
651 |
|
|
if (t != tail || m == null || h != head) |
652 |
|
|
continue; // inconsistent read |
653 |
|
|
|
654 |
|
|
Object x = m.item; |
655 |
|
|
if (isData == (x != null) || // m already fulfilled |
656 |
|
|
x == m || // m cancelled |
657 |
|
|
!m.casItem(x, e)) { // lost CAS |
658 |
|
|
advanceHead(h, m); // dequeue and retry |
659 |
|
|
continue; |
660 |
|
|
} |
661 |
|
|
|
662 |
|
|
advanceHead(h, m); // successfully fulfilled |
663 |
|
|
LockSupport.unpark(m.waiter); |
664 |
|
|
return (x != null)? x : e; |
665 |
|
|
} |
666 |
dl |
1.35 |
} |
667 |
|
|
} |
668 |
|
|
|
669 |
|
|
/** |
670 |
dl |
1.55 |
* Spin/block until node s is fulfilled. |
671 |
|
|
* @param s the waiting node |
672 |
|
|
* @param e the comparison value for checking match |
673 |
|
|
* @param timed true if timed wait |
674 |
|
|
* @param nanos timeout value |
675 |
|
|
* @return matched item, or s if cancelled |
676 |
dl |
1.35 |
*/ |
677 |
dl |
1.55 |
Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) { |
678 |
|
|
/* Same idea as TransferStack.awaitFulfill */ |
679 |
|
|
long lastTime = (timed)? System.nanoTime() : 0; |
680 |
|
|
Thread w = Thread.currentThread(); |
681 |
|
|
int spins = ((head.next == s) ? |
682 |
|
|
(timed? maxTimedSpins : maxUntimedSpins) : 0); |
683 |
|
|
for (;;) { |
684 |
|
|
if (w.isInterrupted()) |
685 |
|
|
s.tryCancel(e); |
686 |
|
|
Object x = s.item; |
687 |
|
|
if (x != e) |
688 |
|
|
return x; |
689 |
|
|
if (timed) { |
690 |
|
|
long now = System.nanoTime(); |
691 |
|
|
nanos -= now - lastTime; |
692 |
|
|
lastTime = now; |
693 |
|
|
if (nanos <= 0) { |
694 |
|
|
s.tryCancel(e); |
695 |
|
|
continue; |
696 |
|
|
} |
697 |
|
|
} |
698 |
|
|
if (spins > 0) |
699 |
|
|
--spins; |
700 |
|
|
else if (s.waiter == null) |
701 |
|
|
s.waiter = w; |
702 |
|
|
else if (!timed) |
703 |
|
|
LockSupport.park(this); |
704 |
|
|
else if (nanos > spinForTimeoutThreshold) |
705 |
|
|
LockSupport.parkNanos(this, nanos); |
706 |
dl |
1.35 |
} |
707 |
dl |
1.31 |
} |
708 |
|
|
|
709 |
|
|
/** |
710 |
dl |
1.55 |
* Get rid of cancelled node s with original predecessor pred. |
711 |
dl |
1.31 |
*/ |
712 |
dl |
1.55 |
void clean(QNode pred, QNode s) { |
713 |
|
|
s.waiter = null; // forget thread |
714 |
|
|
/* |
715 |
|
|
* At any given time, exactly one node on list cannot be |
716 |
|
|
* deleted -- the last inserted node. To accommodate this, |
717 |
|
|
* if we cannot delete s, we save its predecessor as |
718 |
|
|
* "cleanMe", deleting the previously saved version |
719 |
|
|
* first. At least one of node s or the node previously |
720 |
|
|
* saved can always be deleted, so this always terminates. |
721 |
|
|
*/ |
722 |
|
|
while (pred.next == s) { // Return early if already unlinked |
723 |
|
|
QNode h = head; |
724 |
|
|
QNode hn = h.next; // Absorb cancelled first node as head |
725 |
|
|
if (hn != null && hn.isCancelled()) { |
726 |
|
|
advanceHead(h, hn); |
727 |
|
|
continue; |
728 |
|
|
} |
729 |
|
|
QNode t = tail; // Ensure consistent read for tail |
730 |
|
|
if (t == h) |
731 |
|
|
return; |
732 |
|
|
QNode tn = t.next; |
733 |
|
|
if (t != tail) |
734 |
|
|
continue; |
735 |
|
|
if (tn != null) { |
736 |
|
|
advanceTail(t, tn); |
737 |
|
|
continue; |
738 |
|
|
} |
739 |
|
|
if (s != t) { // If not tail, try to unsplice |
740 |
|
|
QNode sn = s.next; |
741 |
|
|
if (sn == s || pred.casNext(s, sn)) |
742 |
|
|
return; |
743 |
|
|
} |
744 |
|
|
QNode dp = cleanMe; |
745 |
|
|
if (dp != null) { // Try unlinking previous cancelled node |
746 |
|
|
QNode d = dp.next; |
747 |
|
|
QNode dn; |
748 |
|
|
if (d == null || // d is gone or |
749 |
|
|
d == dp || // d is off list or |
750 |
|
|
!d.isCancelled() || // d not cancelled or |
751 |
|
|
(d != t && // d not tail and |
752 |
|
|
(dn = d.next) != null && // has successor |
753 |
|
|
dn != d && // that is on list |
754 |
|
|
dp.casNext(d, dn))) // d unspliced |
755 |
|
|
casCleanMe(dp, null); |
756 |
|
|
if (dp == pred) |
757 |
|
|
return; // s is already saved node |
758 |
|
|
} else if (casCleanMe(null, pred)) |
759 |
|
|
return; // Postpone cleaning s |
760 |
dl |
1.2 |
} |
761 |
|
|
} |
762 |
dl |
1.55 |
} |
763 |
|
|
|
764 |
|
|
/** |
765 |
|
|
* The transferer. Set only in constructor, but cannot be declared |
766 |
|
|
* as final without further complicating serialization. Since |
767 |
|
|
* this is accessed only once per public method, there isn't a |
768 |
|
|
* noticeable performance penalty for using volatile instead of |
769 |
|
|
* final here. |
770 |
|
|
*/ |
771 |
|
|
private transient volatile Transferer transferer; |
772 |
|
|
|
773 |
|
|
/** |
774 |
|
|
* Creates a <tt>SynchronousQueue</tt> with nonfair access policy. |
775 |
|
|
*/ |
776 |
|
|
public SynchronousQueue() { |
777 |
|
|
this(false); |
778 |
|
|
} |
779 |
dl |
1.2 |
|
780 |
dl |
1.55 |
/** |
781 |
|
|
* Creates a <tt>SynchronousQueue</tt> with specified fairness policy. |
782 |
|
|
* @param fair if true, waiting threads contend in FIFO order for access; |
783 |
|
|
* otherwise the order is unspecified. |
784 |
|
|
*/ |
785 |
|
|
public SynchronousQueue(boolean fair) { |
786 |
|
|
transferer = (fair)? new TransferQueue() : new TransferStack(); |
787 |
dl |
1.2 |
} |
788 |
|
|
|
789 |
|
|
/** |
790 |
dl |
1.35 |
* Adds the specified element to this queue, waiting if necessary for |
791 |
|
|
* another thread to receive it. |
792 |
jsr166 |
1.50 |
* |
793 |
|
|
* @throws InterruptedException {@inheritDoc} |
794 |
|
|
* @throws NullPointerException {@inheritDoc} |
795 |
tim |
1.10 |
*/ |
796 |
dl |
1.55 |
public void put(E o) throws InterruptedException { |
797 |
|
|
if (o == null) throw new NullPointerException(); |
798 |
|
|
if (transferer.transfer(o, false, 0) == null) |
799 |
|
|
throw new InterruptedException(); |
800 |
tim |
1.1 |
} |
801 |
|
|
|
802 |
dholmes |
1.11 |
/** |
803 |
dl |
1.20 |
* Inserts the specified element into this queue, waiting if necessary |
804 |
dl |
1.18 |
* up to the specified wait time for another thread to receive it. |
805 |
jsr166 |
1.50 |
* |
806 |
|
|
* @return <tt>true</tt> if successful, or <tt>false</tt> if the |
807 |
|
|
* specified waiting time elapses before a consumer appears. |
808 |
|
|
* @throws InterruptedException {@inheritDoc} |
809 |
|
|
* @throws NullPointerException {@inheritDoc} |
810 |
dholmes |
1.11 |
*/ |
811 |
dl |
1.55 |
public boolean offer(E o, long timeout, TimeUnit unit) |
812 |
|
|
throws InterruptedException { |
813 |
|
|
if (o == null) throw new NullPointerException(); |
814 |
|
|
if (transferer.transfer(o, true, unit.toNanos(timeout)) != null) |
815 |
|
|
return true; |
816 |
|
|
if (!Thread.interrupted()) |
817 |
|
|
return false; |
818 |
|
|
throw new InterruptedException(); |
819 |
|
|
} |
820 |
|
|
|
821 |
|
|
/** |
822 |
|
|
* Inserts the specified element into this queue, if another thread is |
823 |
|
|
* waiting to receive it. |
824 |
|
|
* |
825 |
|
|
* @param e the element to add |
826 |
|
|
* @return <tt>true</tt> if the element was added to this queue, else |
827 |
|
|
* <tt>false</tt> |
828 |
|
|
* @throws NullPointerException if the specified element is null |
829 |
|
|
*/ |
830 |
|
|
public boolean offer(E e) { |
831 |
jsr166 |
1.49 |
if (e == null) throw new NullPointerException(); |
832 |
dl |
1.55 |
return transferer.transfer(e, true, 0) != null; |
833 |
tim |
1.1 |
} |
834 |
|
|
|
835 |
dholmes |
1.11 |
/** |
836 |
|
|
* Retrieves and removes the head of this queue, waiting if necessary |
837 |
|
|
* for another thread to insert it. |
838 |
jsr166 |
1.50 |
* |
839 |
dholmes |
1.11 |
* @return the head of this queue |
840 |
jsr166 |
1.50 |
* @throws InterruptedException {@inheritDoc} |
841 |
dholmes |
1.11 |
*/ |
842 |
dl |
1.2 |
public E take() throws InterruptedException { |
843 |
dl |
1.55 |
Object e = transferer.transfer(null, false, 0); |
844 |
|
|
if (e != null) |
845 |
|
|
return (E)e; |
846 |
|
|
throw new InterruptedException(); |
847 |
tim |
1.1 |
} |
848 |
dl |
1.2 |
|
849 |
dholmes |
1.11 |
/** |
850 |
|
|
* Retrieves and removes the head of this queue, waiting |
851 |
|
|
* if necessary up to the specified wait time, for another thread |
852 |
|
|
* to insert it. |
853 |
jsr166 |
1.50 |
* |
854 |
dl |
1.18 |
* @return the head of this queue, or <tt>null</tt> if the |
855 |
jsr166 |
1.50 |
* specified waiting time elapses before an element is present. |
856 |
|
|
* @throws InterruptedException {@inheritDoc} |
857 |
dholmes |
1.11 |
*/ |
858 |
dl |
1.2 |
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
859 |
dl |
1.55 |
Object e = transferer.transfer(null, true, unit.toNanos(timeout)); |
860 |
|
|
if (e != null || !Thread.interrupted()) |
861 |
|
|
return (E)e; |
862 |
|
|
throw new InterruptedException(); |
863 |
tim |
1.1 |
} |
864 |
dl |
1.2 |
|
865 |
dl |
1.18 |
/** |
866 |
|
|
* Retrieves and removes the head of this queue, if another thread |
867 |
|
|
* is currently making an element available. |
868 |
|
|
* |
869 |
|
|
* @return the head of this queue, or <tt>null</tt> if no |
870 |
|
|
* element is available. |
871 |
|
|
*/ |
872 |
dl |
1.2 |
public E poll() { |
873 |
dl |
1.55 |
return (E)transferer.transfer(null, true, 0); |
874 |
tim |
1.1 |
} |
875 |
dl |
1.2 |
|
876 |
dl |
1.5 |
/** |
877 |
jsr166 |
1.48 |
* Always returns <tt>true</tt>. |
878 |
dholmes |
1.11 |
* A <tt>SynchronousQueue</tt> has no internal capacity. |
879 |
|
|
* @return <tt>true</tt> |
880 |
dl |
1.5 |
*/ |
881 |
|
|
public boolean isEmpty() { |
882 |
|
|
return true; |
883 |
|
|
} |
884 |
|
|
|
885 |
|
|
/** |
886 |
dholmes |
1.11 |
* Always returns zero. |
887 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
888 |
dl |
1.55 |
* @return zero. |
889 |
dl |
1.5 |
*/ |
890 |
|
|
public int size() { |
891 |
|
|
return 0; |
892 |
tim |
1.1 |
} |
893 |
dl |
1.2 |
|
894 |
dl |
1.5 |
/** |
895 |
dholmes |
1.11 |
* Always returns zero. |
896 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
897 |
dl |
1.55 |
* @return zero. |
898 |
dl |
1.5 |
*/ |
899 |
|
|
public int remainingCapacity() { |
900 |
|
|
return 0; |
901 |
|
|
} |
902 |
|
|
|
903 |
|
|
/** |
904 |
dholmes |
1.11 |
* Does nothing. |
905 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
906 |
|
|
*/ |
907 |
dl |
1.55 |
public void clear() { |
908 |
|
|
} |
909 |
dholmes |
1.11 |
|
910 |
|
|
/** |
911 |
|
|
* Always returns <tt>false</tt>. |
912 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
913 |
dl |
1.55 |
* @param o the element |
914 |
dholmes |
1.11 |
* @return <tt>false</tt> |
915 |
|
|
*/ |
916 |
|
|
public boolean contains(Object o) { |
917 |
|
|
return false; |
918 |
|
|
} |
919 |
|
|
|
920 |
|
|
/** |
921 |
dl |
1.18 |
* Always returns <tt>false</tt>. |
922 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
923 |
|
|
* |
924 |
|
|
* @param o the element to remove |
925 |
|
|
* @return <tt>false</tt> |
926 |
|
|
*/ |
927 |
|
|
public boolean remove(Object o) { |
928 |
|
|
return false; |
929 |
|
|
} |
930 |
|
|
|
931 |
|
|
/** |
932 |
dl |
1.55 |
* Returns <tt>false</tt> unless given collection is empty. |
933 |
dholmes |
1.11 |
* A <tt>SynchronousQueue</tt> has no internal capacity. |
934 |
dl |
1.18 |
* @param c the collection |
935 |
dl |
1.55 |
* @return <tt>false</tt> unless given collection is empty |
936 |
dholmes |
1.11 |
*/ |
937 |
dl |
1.12 |
public boolean containsAll(Collection<?> c) { |
938 |
dl |
1.16 |
return c.isEmpty(); |
939 |
dholmes |
1.11 |
} |
940 |
|
|
|
941 |
|
|
/** |
942 |
|
|
* Always returns <tt>false</tt>. |
943 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
944 |
dl |
1.18 |
* @param c the collection |
945 |
dholmes |
1.11 |
* @return <tt>false</tt> |
946 |
|
|
*/ |
947 |
dl |
1.12 |
public boolean removeAll(Collection<?> c) { |
948 |
dholmes |
1.11 |
return false; |
949 |
|
|
} |
950 |
|
|
|
951 |
|
|
/** |
952 |
|
|
* Always returns <tt>false</tt>. |
953 |
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity. |
954 |
dl |
1.18 |
* @param c the collection |
955 |
dholmes |
1.11 |
* @return <tt>false</tt> |
956 |
|
|
*/ |
957 |
dl |
1.12 |
public boolean retainAll(Collection<?> c) { |
958 |
dholmes |
1.11 |
return false; |
959 |
|
|
} |
960 |
|
|
|
961 |
|
|
/** |
962 |
jsr166 |
1.48 |
* Always returns <tt>null</tt>. |
963 |
dholmes |
1.11 |
* A <tt>SynchronousQueue</tt> does not return elements |
964 |
dl |
1.5 |
* unless actively waited on. |
965 |
dholmes |
1.11 |
* @return <tt>null</tt> |
966 |
dl |
1.5 |
*/ |
967 |
|
|
public E peek() { |
968 |
|
|
return null; |
969 |
|
|
} |
970 |
|
|
|
971 |
|
|
static class EmptyIterator<E> implements Iterator<E> { |
972 |
dl |
1.2 |
public boolean hasNext() { |
973 |
|
|
return false; |
974 |
|
|
} |
975 |
|
|
public E next() { |
976 |
|
|
throw new NoSuchElementException(); |
977 |
|
|
} |
978 |
|
|
public void remove() { |
979 |
dl |
1.17 |
throw new IllegalStateException(); |
980 |
dl |
1.2 |
} |
981 |
tim |
1.1 |
} |
982 |
dl |
1.2 |
|
983 |
dl |
1.5 |
/** |
984 |
dl |
1.18 |
* Returns an empty iterator in which <tt>hasNext</tt> always returns |
985 |
tim |
1.13 |
* <tt>false</tt>. |
986 |
|
|
* |
987 |
dholmes |
1.11 |
* @return an empty iterator |
988 |
dl |
1.5 |
*/ |
989 |
dl |
1.2 |
public Iterator<E> iterator() { |
990 |
dl |
1.5 |
return new EmptyIterator<E>(); |
991 |
tim |
1.1 |
} |
992 |
|
|
|
993 |
dl |
1.5 |
/** |
994 |
dholmes |
1.11 |
* Returns a zero-length array. |
995 |
|
|
* @return a zero-length array |
996 |
dl |
1.5 |
*/ |
997 |
dl |
1.3 |
public Object[] toArray() { |
998 |
dl |
1.25 |
return new Object[0]; |
999 |
tim |
1.1 |
} |
1000 |
|
|
|
1001 |
dholmes |
1.11 |
/** |
1002 |
|
|
* Sets the zeroeth element of the specified array to <tt>null</tt> |
1003 |
|
|
* (if the array has non-zero length) and returns it. |
1004 |
jsr166 |
1.50 |
* |
1005 |
dl |
1.40 |
* @param a the array |
1006 |
dholmes |
1.11 |
* @return the specified array |
1007 |
jsr166 |
1.50 |
* @throws NullPointerException if the specified array is null |
1008 |
dholmes |
1.11 |
*/ |
1009 |
dl |
1.2 |
public <T> T[] toArray(T[] a) { |
1010 |
|
|
if (a.length > 0) |
1011 |
|
|
a[0] = null; |
1012 |
|
|
return a; |
1013 |
|
|
} |
1014 |
dl |
1.21 |
|
1015 |
jsr166 |
1.50 |
/** |
1016 |
|
|
* @throws UnsupportedOperationException {@inheritDoc} |
1017 |
|
|
* @throws ClassCastException {@inheritDoc} |
1018 |
|
|
* @throws NullPointerException {@inheritDoc} |
1019 |
|
|
* @throws IllegalArgumentException {@inheritDoc} |
1020 |
|
|
*/ |
1021 |
dl |
1.21 |
public int drainTo(Collection<? super E> c) { |
1022 |
|
|
if (c == null) |
1023 |
|
|
throw new NullPointerException(); |
1024 |
|
|
if (c == this) |
1025 |
|
|
throw new IllegalArgumentException(); |
1026 |
|
|
int n = 0; |
1027 |
|
|
E e; |
1028 |
|
|
while ( (e = poll()) != null) { |
1029 |
|
|
c.add(e); |
1030 |
|
|
++n; |
1031 |
|
|
} |
1032 |
|
|
return n; |
1033 |
|
|
} |
1034 |
|
|
|
1035 |
jsr166 |
1.50 |
/** |
1036 |
|
|
* @throws UnsupportedOperationException {@inheritDoc} |
1037 |
|
|
* @throws ClassCastException {@inheritDoc} |
1038 |
|
|
* @throws NullPointerException {@inheritDoc} |
1039 |
|
|
* @throws IllegalArgumentException {@inheritDoc} |
1040 |
|
|
*/ |
1041 |
dl |
1.21 |
public int drainTo(Collection<? super E> c, int maxElements) { |
1042 |
|
|
if (c == null) |
1043 |
|
|
throw new NullPointerException(); |
1044 |
|
|
if (c == this) |
1045 |
|
|
throw new IllegalArgumentException(); |
1046 |
|
|
int n = 0; |
1047 |
|
|
E e; |
1048 |
|
|
while (n < maxElements && (e = poll()) != null) { |
1049 |
|
|
c.add(e); |
1050 |
|
|
++n; |
1051 |
|
|
} |
1052 |
|
|
return n; |
1053 |
|
|
} |
1054 |
dl |
1.55 |
|
1055 |
|
|
/* |
1056 |
|
|
* To cope with serialization strategy in the 1.5 version of |
1057 |
|
|
* SynchronousQueue, we declare some unused classes and fields |
1058 |
|
|
* that exist solely to enable serializability across versions. |
1059 |
|
|
* These fields are never used, so are initialized only if this |
1060 |
|
|
* object is ever serialized or deserialized. |
1061 |
|
|
*/ |
1062 |
|
|
|
1063 |
|
|
static class WaitQueue implements java.io.Serializable { } |
1064 |
|
|
static class LifoWaitQueue extends WaitQueue { |
1065 |
|
|
private static final long serialVersionUID = -3633113410248163686L; |
1066 |
|
|
} |
1067 |
|
|
static class FifoWaitQueue extends WaitQueue { |
1068 |
|
|
private static final long serialVersionUID = -3623113410248163686L; |
1069 |
|
|
} |
1070 |
|
|
private ReentrantLock qlock; |
1071 |
|
|
private WaitQueue waitingProducers; |
1072 |
|
|
private WaitQueue waitingConsumers; |
1073 |
|
|
|
1074 |
|
|
/** |
1075 |
|
|
* Save the state to a stream (that is, serialize it). |
1076 |
|
|
* |
1077 |
|
|
* @param s the stream |
1078 |
|
|
*/ |
1079 |
|
|
private void writeObject(java.io.ObjectOutputStream s) |
1080 |
|
|
throws java.io.IOException { |
1081 |
|
|
boolean fair = transferer instanceof TransferQueue; |
1082 |
|
|
if (fair) { |
1083 |
|
|
qlock = new ReentrantLock(true); |
1084 |
|
|
waitingProducers = new FifoWaitQueue(); |
1085 |
|
|
waitingConsumers = new FifoWaitQueue(); |
1086 |
|
|
} |
1087 |
|
|
else { |
1088 |
|
|
qlock = new ReentrantLock(); |
1089 |
|
|
waitingProducers = new LifoWaitQueue(); |
1090 |
|
|
waitingConsumers = new LifoWaitQueue(); |
1091 |
|
|
} |
1092 |
|
|
s.defaultWriteObject(); |
1093 |
|
|
} |
1094 |
|
|
|
1095 |
|
|
private void readObject(final java.io.ObjectInputStream s) |
1096 |
|
|
throws java.io.IOException, ClassNotFoundException { |
1097 |
|
|
s.defaultReadObject(); |
1098 |
|
|
if (waitingProducers instanceof FifoWaitQueue) |
1099 |
|
|
transferer = new TransferQueue(); |
1100 |
|
|
else |
1101 |
|
|
transferer = new TransferStack(); |
1102 |
|
|
} |
1103 |
|
|
|
1104 |
tim |
1.1 |
} |