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/* |
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* 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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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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package java.util.concurrent; |
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|
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import java.lang.invoke.MethodHandles; |
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import java.lang.invoke.VarHandle; |
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import java.util.AbstractQueue; |
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import java.util.Collection; |
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import java.util.Collections; |
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import java.util.Iterator; |
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import java.util.Objects; |
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import java.util.Spliterator; |
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import java.util.Spliterators; |
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import java.util.concurrent.locks.LockSupport; |
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import java.util.concurrent.locks.ReentrantLock; |
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|
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/** |
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* 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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* {@code peek} 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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* {@code poll()} will return {@code null}. For purposes of other |
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* {@code Collection} methods (for example {@code contains}), a |
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* {@code SynchronousQueue} acts as an empty collection. This queue |
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* does not permit {@code null} elements. |
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* |
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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 |
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* object running in one thread must sync up with an object running |
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* in another thread in order to hand it some information, event, or |
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* task. |
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* |
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* <p>This class supports an optional fairness policy for ordering |
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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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* to {@code true} grants threads access in FIFO order. |
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* |
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* <p>This class and its iterator implement all of the <em>optional</em> |
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* methods of the {@link Collection} and {@link Iterator} interfaces. |
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* |
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* <p>This class is a member of the |
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* <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework"> |
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* Java Collections Framework</a>. |
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* |
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* @since 1.5 |
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* @author Doug Lea and Bill Scherer and Michael Scott |
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* @param <E> the type of elements held in this queue |
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*/ |
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public class SynchronousQueue<E> extends AbstractQueue<E> |
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implements BlockingQueue<E>, java.io.Serializable { |
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private static final long serialVersionUID = -3223113410248163686L; |
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|
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/* |
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* 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 original 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. Support for cancellation via timeout and interrupts, |
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* including cleaning out cancelled nodes/threads |
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* from lists to 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 |
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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 |
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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 |
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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 |
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* issues that otherwise complicate nonblocking algorithms, care |
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* is taken to "forget" references to data, other nodes, and |
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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 |
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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 |
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* much for Stack nodes (because blocked threads do not hang on to |
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* old head pointers), but references in Queue nodes must be |
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* aggressively forgotten to avoid reachability of everything any |
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* node has ever referred to since arrival. |
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*/ |
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|
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/** |
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* Shared internal API for dual stacks and queues. |
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*/ |
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abstract static class Transferer<E> { |
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/** |
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* Performs a put or take. |
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* |
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* @param e if non-null, the item to be handed to a consumer; |
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* if null, requests that transfer return an item |
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* offered by producer. |
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* @param timed if this operation should timeout |
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* @param nanos the timeout, in nanoseconds |
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* @return if non-null, the item provided or received; if null, |
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* the operation failed due to timeout or interrupt -- |
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* the caller can distinguish which of these occurred |
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* by checking Thread.interrupted. |
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*/ |
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abstract E transfer(E e, boolean timed, long nanos); |
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} |
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|
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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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static final int MAX_TIMED_SPINS = |
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(Runtime.getRuntime().availableProcessors() < 2) ? 0 : 32; |
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|
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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 MAX_UNTIMED_SPINS = MAX_TIMED_SPINS * 16; |
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|
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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 SPIN_FOR_TIMEOUT_THRESHOLD = 1000L; |
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|
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/** Dual stack */ |
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static final class TransferStack<E> extends Transferer<E> { |
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/* |
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* This extends Scherer-Scott dual stack algorithm, differing, |
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* among other ways, by using "covering" nodes rather than |
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* bit-marked pointers: Fulfilling operations push on marker |
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* nodes (with FULFILLING bit set in mode) to reserve a spot |
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* to match a waiting node. |
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*/ |
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|
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/* Modes for SNodes, ORed together in node fields */ |
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/** Node represents an unfulfilled consumer */ |
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static final int REQUEST = 0; |
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/** Node represents an unfulfilled producer */ |
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static final int DATA = 1; |
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/** Node is fulfilling another unfulfilled DATA or REQUEST */ |
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static final int FULFILLING = 2; |
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|
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/** Returns true if m has fulfilling bit set. */ |
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static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; } |
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|
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/** Node class for TransferStacks. */ |
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static final class SNode { |
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volatile SNode next; // next node in stack |
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volatile SNode match; // the node matched to this |
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volatile Thread waiter; // to control park/unpark |
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Object item; // data; or null for REQUESTs |
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int mode; |
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// Note: item and mode fields don't need to be volatile |
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// since they are always written before, and read after, |
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// other volatile/atomic operations. |
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|
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SNode(Object item) { |
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this.item = item; |
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} |
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|
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boolean casNext(SNode cmp, SNode val) { |
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return cmp == next && |
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SNEXT.compareAndSet(this, cmp, val); |
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} |
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|
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/** |
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* Tries to match node s to this node, if so, waking up thread. |
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* Fulfillers call tryMatch to identify their waiters. |
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* Waiters block until they have been matched. |
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* |
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* @param s the node to match |
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* @return true if successfully matched to s |
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*/ |
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boolean tryMatch(SNode s) { |
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if (match == null && |
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SMATCH.compareAndSet(this, null, s)) { |
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Thread w = waiter; |
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if (w != null) { // waiters need at most one unpark |
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waiter = null; |
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LockSupport.unpark(w); |
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} |
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return true; |
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} |
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return match == s; |
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} |
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|
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/** |
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* Tries to cancel a wait by matching node to itself. |
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*/ |
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void tryCancel() { |
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SMATCH.compareAndSet(this, null, this); |
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} |
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|
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boolean isCancelled() { |
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return match == this; |
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} |
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|
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// VarHandle mechanics |
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private static final VarHandle SMATCH; |
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private static final VarHandle SNEXT; |
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static { |
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try { |
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MethodHandles.Lookup l = MethodHandles.lookup(); |
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SMATCH = l.findVarHandle(SNode.class, "match", SNode.class); |
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SNEXT = l.findVarHandle(SNode.class, "next", SNode.class); |
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} catch (ReflectiveOperationException e) { |
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throw new ExceptionInInitializerError(e); |
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} |
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} |
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} |
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|
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/** The head (top) of the stack */ |
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volatile SNode head; |
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|
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boolean casHead(SNode h, SNode nh) { |
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return h == head && |
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SHEAD.compareAndSet(this, h, nh); |
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} |
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|
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/** |
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* Creates or resets fields of a node. Called only from transfer |
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* where the node to push on stack is lazily created and |
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* reused when possible to help reduce intervals between reads |
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* and CASes of head and to avoid surges of garbage when CASes |
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* to push nodes fail due to contention. |
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*/ |
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static SNode snode(SNode s, Object e, SNode next, int mode) { |
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if (s == null) s = new SNode(e); |
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s.mode = mode; |
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s.next = next; |
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return s; |
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} |
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|
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/** |
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* Puts or takes an item. |
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*/ |
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@SuppressWarnings("unchecked") |
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E transfer(E e, boolean timed, long nanos) { |
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/* |
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* Basic algorithm is to loop trying one of three actions: |
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* |
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* 1. If apparently empty or already containing nodes of same |
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* mode, try to push node on stack and wait for a match, |
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* returning it, or null if cancelled. |
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* |
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* 2. If apparently containing node of complementary mode, |
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* try to push a fulfilling node on to stack, match |
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* with corresponding waiting node, pop both from |
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* stack, and return matched item. The matching or |
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* unlinking might not actually be necessary because of |
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* other threads performing action 3: |
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* |
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* 3. If top of stack already holds another fulfilling node, |
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* help it out by doing its match and/or pop |
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* operations, and then continue. The code for helping |
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* is essentially the same as for fulfilling, except |
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* that it doesn't return the item. |
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*/ |
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|
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SNode s = null; // constructed/reused as needed |
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int mode = (e == null) ? REQUEST : DATA; |
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|
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for (;;) { |
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SNode h = head; |
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if (h == null || h.mode == mode) { // empty or same-mode |
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if (timed && nanos <= 0L) { // can't wait |
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if (h != null && h.isCancelled()) |
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casHead(h, h.next); // pop cancelled node |
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else |
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return null; |
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} else if (casHead(h, s = snode(s, e, h, mode))) { |
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SNode m = awaitFulfill(s, timed, nanos); |
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if (m == s) { // wait was cancelled |
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clean(s); |
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return null; |
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} |
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if ((h = head) != null && h.next == s) |
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casHead(h, s.next); // help s's fulfiller |
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return (E) ((mode == REQUEST) ? m.item : s.item); |
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} |
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} else if (!isFulfilling(h.mode)) { // try to fulfill |
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if (h.isCancelled()) // already cancelled |
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casHead(h, h.next); // pop and retry |
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else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) { |
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for (;;) { // loop until matched or waiters disappear |
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SNode m = s.next; // m is s's match |
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if (m == null) { // all waiters are gone |
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casHead(s, null); // pop fulfill node |
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s = null; // use new node next time |
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break; // restart main loop |
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} |
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SNode mn = m.next; |
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if (m.tryMatch(s)) { |
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casHead(s, mn); // pop both s and m |
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return (E) ((mode == REQUEST) ? m.item : s.item); |
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} else // lost match |
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s.casNext(m, mn); // help unlink |
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} |
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} |
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} else { // help a fulfiller |
361 |
SNode m = h.next; // m is h's match |
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if (m == null) // waiter is gone |
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casHead(h, null); // pop fulfilling node |
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else { |
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SNode mn = m.next; |
366 |
if (m.tryMatch(h)) // help match |
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casHead(h, mn); // pop both h and m |
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else // lost match |
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h.casNext(m, mn); // help unlink |
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} |
371 |
} |
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} |
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} |
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|
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/** |
376 |
* Spins/blocks until node s is matched by a fulfill operation. |
377 |
* |
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* @param s the waiting node |
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* @param timed true if timed wait |
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* @param nanos timeout value |
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* @return matched node, or s if cancelled |
382 |
*/ |
383 |
SNode awaitFulfill(SNode s, boolean timed, long nanos) { |
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/* |
385 |
* When a node/thread is about to block, it sets its waiter |
386 |
* field and then rechecks state at least one more time |
387 |
* before actually parking, thus covering race vs |
388 |
* fulfiller noticing that waiter is non-null so should be |
389 |
* woken. |
390 |
* |
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* When invoked by nodes that appear at the point of call |
392 |
* to be at the head of the stack, calls to park are |
393 |
* preceded by spins to avoid blocking when producers and |
394 |
* consumers are arriving very close in time. This can |
395 |
* happen enough to bother only on multiprocessors. |
396 |
* |
397 |
* The order of checks for returning out of main loop |
398 |
* reflects fact that interrupts have precedence over |
399 |
* normal returns, which have precedence over |
400 |
* timeouts. (So, on timeout, one last check for match is |
401 |
* done before giving up.) Except that calls from untimed |
402 |
* SynchronousQueue.{poll/offer} don't check interrupts |
403 |
* and don't wait at all, so are trapped in transfer |
404 |
* method rather than calling awaitFulfill. |
405 |
*/ |
406 |
final long deadline = timed ? System.nanoTime() + nanos : 0L; |
407 |
Thread w = Thread.currentThread(); |
408 |
int spins = shouldSpin(s) |
409 |
? (timed ? MAX_TIMED_SPINS : MAX_UNTIMED_SPINS) |
410 |
: 0; |
411 |
for (;;) { |
412 |
if (w.isInterrupted()) |
413 |
s.tryCancel(); |
414 |
SNode m = s.match; |
415 |
if (m != null) |
416 |
return m; |
417 |
if (timed) { |
418 |
nanos = deadline - System.nanoTime(); |
419 |
if (nanos <= 0L) { |
420 |
s.tryCancel(); |
421 |
continue; |
422 |
} |
423 |
} |
424 |
if (spins > 0) { |
425 |
Thread.onSpinWait(); |
426 |
spins = shouldSpin(s) ? (spins - 1) : 0; |
427 |
} |
428 |
else if (s.waiter == null) |
429 |
s.waiter = w; // establish waiter so can park next iter |
430 |
else if (!timed) |
431 |
LockSupport.park(this); |
432 |
else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD) |
433 |
LockSupport.parkNanos(this, nanos); |
434 |
} |
435 |
} |
436 |
|
437 |
/** |
438 |
* Returns true if node s is at head or there is an active |
439 |
* fulfiller. |
440 |
*/ |
441 |
boolean shouldSpin(SNode s) { |
442 |
SNode h = head; |
443 |
return (h == s || h == null || isFulfilling(h.mode)); |
444 |
} |
445 |
|
446 |
/** |
447 |
* Unlinks s from the stack. |
448 |
*/ |
449 |
void clean(SNode s) { |
450 |
s.item = null; // forget item |
451 |
s.waiter = null; // forget thread |
452 |
|
453 |
/* |
454 |
* At worst we may need to traverse entire stack to unlink |
455 |
* s. If there are multiple concurrent calls to clean, we |
456 |
* might not see s if another thread has already removed |
457 |
* it. But we can stop when we see any node known to |
458 |
* follow s. We use s.next unless it too is cancelled, in |
459 |
* which case we try the node one past. We don't check any |
460 |
* further because we don't want to doubly traverse just to |
461 |
* find sentinel. |
462 |
*/ |
463 |
|
464 |
SNode past = s.next; |
465 |
if (past != null && past.isCancelled()) |
466 |
past = past.next; |
467 |
|
468 |
// Absorb cancelled nodes at head |
469 |
SNode p; |
470 |
while ((p = head) != null && p != past && p.isCancelled()) |
471 |
casHead(p, p.next); |
472 |
|
473 |
// Unsplice embedded nodes |
474 |
while (p != null && p != past) { |
475 |
SNode n = p.next; |
476 |
if (n != null && n.isCancelled()) |
477 |
p.casNext(n, n.next); |
478 |
else |
479 |
p = n; |
480 |
} |
481 |
} |
482 |
|
483 |
// VarHandle mechanics |
484 |
private static final VarHandle SHEAD; |
485 |
static { |
486 |
try { |
487 |
MethodHandles.Lookup l = MethodHandles.lookup(); |
488 |
SHEAD = l.findVarHandle(TransferStack.class, "head", SNode.class); |
489 |
} catch (ReflectiveOperationException e) { |
490 |
throw new ExceptionInInitializerError(e); |
491 |
} |
492 |
} |
493 |
} |
494 |
|
495 |
/** Dual Queue */ |
496 |
static final class TransferQueue<E> extends Transferer<E> { |
497 |
/* |
498 |
* This extends Scherer-Scott dual queue algorithm, differing, |
499 |
* among other ways, by using modes within nodes rather than |
500 |
* marked pointers. The algorithm is a little simpler than |
501 |
* that for stacks because fulfillers do not need explicit |
502 |
* nodes, and matching is done by CAS'ing QNode.item field |
503 |
* from non-null to null (for put) or vice versa (for take). |
504 |
*/ |
505 |
|
506 |
/** Node class for TransferQueue. */ |
507 |
static final class QNode { |
508 |
volatile QNode next; // next node in queue |
509 |
volatile Object item; // CAS'ed to or from null |
510 |
volatile Thread waiter; // to control park/unpark |
511 |
final boolean isData; |
512 |
|
513 |
QNode(Object item, boolean isData) { |
514 |
this.item = item; |
515 |
this.isData = isData; |
516 |
} |
517 |
|
518 |
boolean casNext(QNode cmp, QNode val) { |
519 |
return next == cmp && |
520 |
QNEXT.compareAndSet(this, cmp, val); |
521 |
} |
522 |
|
523 |
boolean casItem(Object cmp, Object val) { |
524 |
return item == cmp && |
525 |
QITEM.compareAndSet(this, cmp, val); |
526 |
} |
527 |
|
528 |
/** |
529 |
* Tries to cancel by CAS'ing ref to this as item. |
530 |
*/ |
531 |
void tryCancel(Object cmp) { |
532 |
QITEM.compareAndSet(this, cmp, this); |
533 |
} |
534 |
|
535 |
boolean isCancelled() { |
536 |
return item == this; |
537 |
} |
538 |
|
539 |
/** |
540 |
* Returns true if this node is known to be off the queue |
541 |
* because its next pointer has been forgotten due to |
542 |
* an advanceHead operation. |
543 |
*/ |
544 |
boolean isOffList() { |
545 |
return next == this; |
546 |
} |
547 |
|
548 |
// VarHandle mechanics |
549 |
private static final VarHandle QITEM; |
550 |
private static final VarHandle QNEXT; |
551 |
static { |
552 |
try { |
553 |
MethodHandles.Lookup l = MethodHandles.lookup(); |
554 |
QITEM = l.findVarHandle(QNode.class, "item", Object.class); |
555 |
QNEXT = l.findVarHandle(QNode.class, "next", QNode.class); |
556 |
} catch (ReflectiveOperationException e) { |
557 |
throw new ExceptionInInitializerError(e); |
558 |
} |
559 |
} |
560 |
} |
561 |
|
562 |
/** Head of queue */ |
563 |
transient volatile QNode head; |
564 |
/** Tail of queue */ |
565 |
transient volatile QNode tail; |
566 |
/** |
567 |
* Reference to a cancelled node that might not yet have been |
568 |
* unlinked from queue because it was the last inserted node |
569 |
* when it was cancelled. |
570 |
*/ |
571 |
transient volatile QNode cleanMe; |
572 |
|
573 |
TransferQueue() { |
574 |
QNode h = new QNode(null, false); // initialize to dummy node. |
575 |
head = h; |
576 |
tail = h; |
577 |
} |
578 |
|
579 |
/** |
580 |
* Tries to cas nh as new head; if successful, unlink |
581 |
* old head's next node to avoid garbage retention. |
582 |
*/ |
583 |
void advanceHead(QNode h, QNode nh) { |
584 |
if (h == head && |
585 |
QHEAD.compareAndSet(this, h, nh)) |
586 |
h.next = h; // forget old next |
587 |
} |
588 |
|
589 |
/** |
590 |
* Tries to cas nt as new tail. |
591 |
*/ |
592 |
void advanceTail(QNode t, QNode nt) { |
593 |
if (tail == t) |
594 |
QTAIL.compareAndSet(this, t, nt); |
595 |
} |
596 |
|
597 |
/** |
598 |
* Tries to CAS cleanMe slot. |
599 |
*/ |
600 |
boolean casCleanMe(QNode cmp, QNode val) { |
601 |
return cleanMe == cmp && |
602 |
QCLEANME.compareAndSet(this, cmp, val); |
603 |
} |
604 |
|
605 |
/** |
606 |
* Puts or takes an item. |
607 |
*/ |
608 |
@SuppressWarnings("unchecked") |
609 |
E transfer(E e, boolean timed, long nanos) { |
610 |
/* Basic algorithm is to loop trying to take either of |
611 |
* two actions: |
612 |
* |
613 |
* 1. If queue apparently empty or holding same-mode nodes, |
614 |
* try to add node to queue of waiters, wait to be |
615 |
* fulfilled (or cancelled) and return matching item. |
616 |
* |
617 |
* 2. If queue apparently contains waiting items, and this |
618 |
* call is of complementary mode, try to fulfill by CAS'ing |
619 |
* item field of waiting node and dequeuing it, and then |
620 |
* returning matching item. |
621 |
* |
622 |
* In each case, along the way, check for and try to help |
623 |
* advance head and tail on behalf of other stalled/slow |
624 |
* threads. |
625 |
* |
626 |
* The loop starts off with a null check guarding against |
627 |
* seeing uninitialized head or tail values. This never |
628 |
* happens in current SynchronousQueue, but could if |
629 |
* callers held non-volatile/final ref to the |
630 |
* transferer. The check is here anyway because it places |
631 |
* null checks at top of loop, which is usually faster |
632 |
* than having them implicitly interspersed. |
633 |
*/ |
634 |
|
635 |
QNode s = null; // constructed/reused as needed |
636 |
boolean isData = (e != null); |
637 |
|
638 |
for (;;) { |
639 |
QNode t = tail; |
640 |
QNode h = head; |
641 |
if (t == null || h == null) // saw uninitialized value |
642 |
continue; // spin |
643 |
|
644 |
if (h == t || t.isData == isData) { // empty or same-mode |
645 |
QNode tn = t.next; |
646 |
if (t != tail) // inconsistent read |
647 |
continue; |
648 |
if (tn != null) { // lagging tail |
649 |
advanceTail(t, tn); |
650 |
continue; |
651 |
} |
652 |
if (timed && nanos <= 0L) // can't wait |
653 |
return null; |
654 |
if (s == null) |
655 |
s = new QNode(e, isData); |
656 |
if (!t.casNext(null, s)) // failed to link in |
657 |
continue; |
658 |
|
659 |
advanceTail(t, s); // swing tail and wait |
660 |
Object x = awaitFulfill(s, e, timed, nanos); |
661 |
if (x == s) { // wait was cancelled |
662 |
clean(t, s); |
663 |
return null; |
664 |
} |
665 |
|
666 |
if (!s.isOffList()) { // not already unlinked |
667 |
advanceHead(t, s); // unlink if head |
668 |
if (x != null) // and forget fields |
669 |
s.item = s; |
670 |
s.waiter = null; |
671 |
} |
672 |
return (x != null) ? (E)x : e; |
673 |
|
674 |
} else { // complementary-mode |
675 |
QNode m = h.next; // node to fulfill |
676 |
if (t != tail || m == null || h != head) |
677 |
continue; // inconsistent read |
678 |
|
679 |
Object x = m.item; |
680 |
if (isData == (x != null) || // m already fulfilled |
681 |
x == m || // m cancelled |
682 |
!m.casItem(x, e)) { // lost CAS |
683 |
advanceHead(h, m); // dequeue and retry |
684 |
continue; |
685 |
} |
686 |
|
687 |
advanceHead(h, m); // successfully fulfilled |
688 |
LockSupport.unpark(m.waiter); |
689 |
return (x != null) ? (E)x : e; |
690 |
} |
691 |
} |
692 |
} |
693 |
|
694 |
/** |
695 |
* Spins/blocks until node s is fulfilled. |
696 |
* |
697 |
* @param s the waiting node |
698 |
* @param e the comparison value for checking match |
699 |
* @param timed true if timed wait |
700 |
* @param nanos timeout value |
701 |
* @return matched item, or s if cancelled |
702 |
*/ |
703 |
Object awaitFulfill(QNode s, E e, boolean timed, long nanos) { |
704 |
/* Same idea as TransferStack.awaitFulfill */ |
705 |
final long deadline = timed ? System.nanoTime() + nanos : 0L; |
706 |
Thread w = Thread.currentThread(); |
707 |
int spins = (head.next == s) |
708 |
? (timed ? MAX_TIMED_SPINS : MAX_UNTIMED_SPINS) |
709 |
: 0; |
710 |
for (;;) { |
711 |
if (w.isInterrupted()) |
712 |
s.tryCancel(e); |
713 |
Object x = s.item; |
714 |
if (x != e) |
715 |
return x; |
716 |
if (timed) { |
717 |
nanos = deadline - System.nanoTime(); |
718 |
if (nanos <= 0L) { |
719 |
s.tryCancel(e); |
720 |
continue; |
721 |
} |
722 |
} |
723 |
if (spins > 0) { |
724 |
--spins; |
725 |
Thread.onSpinWait(); |
726 |
} |
727 |
else if (s.waiter == null) |
728 |
s.waiter = w; |
729 |
else if (!timed) |
730 |
LockSupport.park(this); |
731 |
else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD) |
732 |
LockSupport.parkNanos(this, nanos); |
733 |
} |
734 |
} |
735 |
|
736 |
/** |
737 |
* Gets rid of cancelled node s with original predecessor pred. |
738 |
*/ |
739 |
void clean(QNode pred, QNode s) { |
740 |
s.waiter = null; // forget thread |
741 |
/* |
742 |
* At any given time, exactly one node on list cannot be |
743 |
* deleted -- the last inserted node. To accommodate this, |
744 |
* if we cannot delete s, we save its predecessor as |
745 |
* "cleanMe", deleting the previously saved version |
746 |
* first. At least one of node s or the node previously |
747 |
* saved can always be deleted, so this always terminates. |
748 |
*/ |
749 |
while (pred.next == s) { // Return early if already unlinked |
750 |
QNode h = head; |
751 |
QNode hn = h.next; // Absorb cancelled first node as head |
752 |
if (hn != null && hn.isCancelled()) { |
753 |
advanceHead(h, hn); |
754 |
continue; |
755 |
} |
756 |
QNode t = tail; // Ensure consistent read for tail |
757 |
if (t == h) |
758 |
return; |
759 |
QNode tn = t.next; |
760 |
if (t != tail) |
761 |
continue; |
762 |
if (tn != null) { |
763 |
advanceTail(t, tn); |
764 |
continue; |
765 |
} |
766 |
if (s != t) { // If not tail, try to unsplice |
767 |
QNode sn = s.next; |
768 |
if (sn == s || pred.casNext(s, sn)) |
769 |
return; |
770 |
} |
771 |
QNode dp = cleanMe; |
772 |
if (dp != null) { // Try unlinking previous cancelled node |
773 |
QNode d = dp.next; |
774 |
QNode dn; |
775 |
if (d == null || // d is gone or |
776 |
d == dp || // d is off list or |
777 |
!d.isCancelled() || // d not cancelled or |
778 |
(d != t && // d not tail and |
779 |
(dn = d.next) != null && // has successor |
780 |
dn != d && // that is on list |
781 |
dp.casNext(d, dn))) // d unspliced |
782 |
casCleanMe(dp, null); |
783 |
if (dp == pred) |
784 |
return; // s is already saved node |
785 |
} else if (casCleanMe(null, pred)) |
786 |
return; // Postpone cleaning s |
787 |
} |
788 |
} |
789 |
|
790 |
// VarHandle mechanics |
791 |
private static final VarHandle QHEAD; |
792 |
private static final VarHandle QTAIL; |
793 |
private static final VarHandle QCLEANME; |
794 |
static { |
795 |
try { |
796 |
MethodHandles.Lookup l = MethodHandles.lookup(); |
797 |
QHEAD = l.findVarHandle(TransferQueue.class, "head", |
798 |
QNode.class); |
799 |
QTAIL = l.findVarHandle(TransferQueue.class, "tail", |
800 |
QNode.class); |
801 |
QCLEANME = l.findVarHandle(TransferQueue.class, "cleanMe", |
802 |
QNode.class); |
803 |
} catch (ReflectiveOperationException e) { |
804 |
throw new ExceptionInInitializerError(e); |
805 |
} |
806 |
} |
807 |
} |
808 |
|
809 |
/** |
810 |
* The transferer. Set only in constructor, but cannot be declared |
811 |
* as final without further complicating serialization. Since |
812 |
* this is accessed only at most once per public method, there |
813 |
* isn't a noticeable performance penalty for using volatile |
814 |
* instead of final here. |
815 |
*/ |
816 |
private transient volatile Transferer<E> transferer; |
817 |
|
818 |
/** |
819 |
* Creates a {@code SynchronousQueue} with nonfair access policy. |
820 |
*/ |
821 |
public SynchronousQueue() { |
822 |
this(false); |
823 |
} |
824 |
|
825 |
/** |
826 |
* Creates a {@code SynchronousQueue} with the specified fairness policy. |
827 |
* |
828 |
* @param fair if true, waiting threads contend in FIFO order for |
829 |
* access; otherwise the order is unspecified. |
830 |
*/ |
831 |
public SynchronousQueue(boolean fair) { |
832 |
transferer = fair ? new TransferQueue<E>() : new TransferStack<E>(); |
833 |
} |
834 |
|
835 |
/** |
836 |
* Adds the specified element to this queue, waiting if necessary for |
837 |
* another thread to receive it. |
838 |
* |
839 |
* @throws InterruptedException {@inheritDoc} |
840 |
* @throws NullPointerException {@inheritDoc} |
841 |
*/ |
842 |
public void put(E e) throws InterruptedException { |
843 |
if (e == null) throw new NullPointerException(); |
844 |
if (transferer.transfer(e, false, 0) == null) { |
845 |
Thread.interrupted(); |
846 |
throw new InterruptedException(); |
847 |
} |
848 |
} |
849 |
|
850 |
/** |
851 |
* Inserts the specified element into this queue, waiting if necessary |
852 |
* up to the specified wait time for another thread to receive it. |
853 |
* |
854 |
* @return {@code true} if successful, or {@code false} if the |
855 |
* specified waiting time elapses before a consumer appears |
856 |
* @throws InterruptedException {@inheritDoc} |
857 |
* @throws NullPointerException {@inheritDoc} |
858 |
*/ |
859 |
public boolean offer(E e, long timeout, TimeUnit unit) |
860 |
throws InterruptedException { |
861 |
if (e == null) throw new NullPointerException(); |
862 |
if (transferer.transfer(e, true, unit.toNanos(timeout)) != null) |
863 |
return true; |
864 |
if (!Thread.interrupted()) |
865 |
return false; |
866 |
throw new InterruptedException(); |
867 |
} |
868 |
|
869 |
/** |
870 |
* Inserts the specified element into this queue, if another thread is |
871 |
* waiting to receive it. |
872 |
* |
873 |
* @param e the element to add |
874 |
* @return {@code true} if the element was added to this queue, else |
875 |
* {@code false} |
876 |
* @throws NullPointerException if the specified element is null |
877 |
*/ |
878 |
public boolean offer(E e) { |
879 |
if (e == null) throw new NullPointerException(); |
880 |
return transferer.transfer(e, true, 0) != null; |
881 |
} |
882 |
|
883 |
/** |
884 |
* Retrieves and removes the head of this queue, waiting if necessary |
885 |
* for another thread to insert it. |
886 |
* |
887 |
* @return the head of this queue |
888 |
* @throws InterruptedException {@inheritDoc} |
889 |
*/ |
890 |
public E take() throws InterruptedException { |
891 |
E e = transferer.transfer(null, false, 0); |
892 |
if (e != null) |
893 |
return e; |
894 |
Thread.interrupted(); |
895 |
throw new InterruptedException(); |
896 |
} |
897 |
|
898 |
/** |
899 |
* Retrieves and removes the head of this queue, waiting |
900 |
* if necessary up to the specified wait time, for another thread |
901 |
* to insert it. |
902 |
* |
903 |
* @return the head of this queue, or {@code null} if the |
904 |
* specified waiting time elapses before an element is present |
905 |
* @throws InterruptedException {@inheritDoc} |
906 |
*/ |
907 |
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
908 |
E e = transferer.transfer(null, true, unit.toNanos(timeout)); |
909 |
if (e != null || !Thread.interrupted()) |
910 |
return e; |
911 |
throw new InterruptedException(); |
912 |
} |
913 |
|
914 |
/** |
915 |
* Retrieves and removes the head of this queue, if another thread |
916 |
* is currently making an element available. |
917 |
* |
918 |
* @return the head of this queue, or {@code null} if no |
919 |
* element is available |
920 |
*/ |
921 |
public E poll() { |
922 |
return transferer.transfer(null, true, 0); |
923 |
} |
924 |
|
925 |
/** |
926 |
* Always returns {@code true}. |
927 |
* A {@code SynchronousQueue} has no internal capacity. |
928 |
* |
929 |
* @return {@code true} |
930 |
*/ |
931 |
public boolean isEmpty() { |
932 |
return true; |
933 |
} |
934 |
|
935 |
/** |
936 |
* Always returns zero. |
937 |
* A {@code SynchronousQueue} has no internal capacity. |
938 |
* |
939 |
* @return zero |
940 |
*/ |
941 |
public int size() { |
942 |
return 0; |
943 |
} |
944 |
|
945 |
/** |
946 |
* Always returns zero. |
947 |
* A {@code SynchronousQueue} has no internal capacity. |
948 |
* |
949 |
* @return zero |
950 |
*/ |
951 |
public int remainingCapacity() { |
952 |
return 0; |
953 |
} |
954 |
|
955 |
/** |
956 |
* Does nothing. |
957 |
* A {@code SynchronousQueue} has no internal capacity. |
958 |
*/ |
959 |
public void clear() { |
960 |
} |
961 |
|
962 |
/** |
963 |
* Always returns {@code false}. |
964 |
* A {@code SynchronousQueue} has no internal capacity. |
965 |
* |
966 |
* @param o the element |
967 |
* @return {@code false} |
968 |
*/ |
969 |
public boolean contains(Object o) { |
970 |
return false; |
971 |
} |
972 |
|
973 |
/** |
974 |
* Always returns {@code false}. |
975 |
* A {@code SynchronousQueue} has no internal capacity. |
976 |
* |
977 |
* @param o the element to remove |
978 |
* @return {@code false} |
979 |
*/ |
980 |
public boolean remove(Object o) { |
981 |
return false; |
982 |
} |
983 |
|
984 |
/** |
985 |
* Returns {@code false} unless the given collection is empty. |
986 |
* A {@code SynchronousQueue} has no internal capacity. |
987 |
* |
988 |
* @param c the collection |
989 |
* @return {@code false} unless given collection is empty |
990 |
*/ |
991 |
public boolean containsAll(Collection<?> c) { |
992 |
return c.isEmpty(); |
993 |
} |
994 |
|
995 |
/** |
996 |
* Always returns {@code false}. |
997 |
* A {@code SynchronousQueue} has no internal capacity. |
998 |
* |
999 |
* @param c the collection |
1000 |
* @return {@code false} |
1001 |
*/ |
1002 |
public boolean removeAll(Collection<?> c) { |
1003 |
return false; |
1004 |
} |
1005 |
|
1006 |
/** |
1007 |
* Always returns {@code false}. |
1008 |
* A {@code SynchronousQueue} has no internal capacity. |
1009 |
* |
1010 |
* @param c the collection |
1011 |
* @return {@code false} |
1012 |
*/ |
1013 |
public boolean retainAll(Collection<?> c) { |
1014 |
return false; |
1015 |
} |
1016 |
|
1017 |
/** |
1018 |
* Always returns {@code null}. |
1019 |
* A {@code SynchronousQueue} does not return elements |
1020 |
* unless actively waited on. |
1021 |
* |
1022 |
* @return {@code null} |
1023 |
*/ |
1024 |
public E peek() { |
1025 |
return null; |
1026 |
} |
1027 |
|
1028 |
/** |
1029 |
* Returns an empty iterator in which {@code hasNext} always returns |
1030 |
* {@code false}. |
1031 |
* |
1032 |
* @return an empty iterator |
1033 |
*/ |
1034 |
public Iterator<E> iterator() { |
1035 |
return Collections.emptyIterator(); |
1036 |
} |
1037 |
|
1038 |
/** |
1039 |
* Returns an empty spliterator in which calls to |
1040 |
* {@link Spliterator#trySplit() trySplit} always return {@code null}. |
1041 |
* |
1042 |
* @return an empty spliterator |
1043 |
* @since 1.8 |
1044 |
*/ |
1045 |
public Spliterator<E> spliterator() { |
1046 |
return Spliterators.emptySpliterator(); |
1047 |
} |
1048 |
|
1049 |
/** |
1050 |
* Returns a zero-length array. |
1051 |
* @return a zero-length array |
1052 |
*/ |
1053 |
public Object[] toArray() { |
1054 |
return new Object[0]; |
1055 |
} |
1056 |
|
1057 |
/** |
1058 |
* Sets the zeroth element of the specified array to {@code null} |
1059 |
* (if the array has non-zero length) and returns it. |
1060 |
* |
1061 |
* @param a the array |
1062 |
* @return the specified array |
1063 |
* @throws NullPointerException if the specified array is null |
1064 |
*/ |
1065 |
public <T> T[] toArray(T[] a) { |
1066 |
if (a.length > 0) |
1067 |
a[0] = null; |
1068 |
return a; |
1069 |
} |
1070 |
|
1071 |
/** |
1072 |
* Always returns {@code "[]"}. |
1073 |
* @return {@code "[]"} |
1074 |
*/ |
1075 |
public String toString() { |
1076 |
return "[]"; |
1077 |
} |
1078 |
|
1079 |
/** |
1080 |
* @throws UnsupportedOperationException {@inheritDoc} |
1081 |
* @throws ClassCastException {@inheritDoc} |
1082 |
* @throws NullPointerException {@inheritDoc} |
1083 |
* @throws IllegalArgumentException {@inheritDoc} |
1084 |
*/ |
1085 |
public int drainTo(Collection<? super E> c) { |
1086 |
Objects.requireNonNull(c); |
1087 |
if (c == this) |
1088 |
throw new IllegalArgumentException(); |
1089 |
int n = 0; |
1090 |
for (E e; (e = poll()) != null; n++) |
1091 |
c.add(e); |
1092 |
return n; |
1093 |
} |
1094 |
|
1095 |
/** |
1096 |
* @throws UnsupportedOperationException {@inheritDoc} |
1097 |
* @throws ClassCastException {@inheritDoc} |
1098 |
* @throws NullPointerException {@inheritDoc} |
1099 |
* @throws IllegalArgumentException {@inheritDoc} |
1100 |
*/ |
1101 |
public int drainTo(Collection<? super E> c, int maxElements) { |
1102 |
Objects.requireNonNull(c); |
1103 |
if (c == this) |
1104 |
throw new IllegalArgumentException(); |
1105 |
int n = 0; |
1106 |
for (E e; n < maxElements && (e = poll()) != null; n++) |
1107 |
c.add(e); |
1108 |
return n; |
1109 |
} |
1110 |
|
1111 |
/* |
1112 |
* To cope with serialization strategy in the 1.5 version of |
1113 |
* SynchronousQueue, we declare some unused classes and fields |
1114 |
* that exist solely to enable serializability across versions. |
1115 |
* These fields are never used, so are initialized only if this |
1116 |
* object is ever serialized or deserialized. |
1117 |
*/ |
1118 |
|
1119 |
@SuppressWarnings("serial") |
1120 |
static class WaitQueue implements java.io.Serializable { } |
1121 |
static class LifoWaitQueue extends WaitQueue { |
1122 |
private static final long serialVersionUID = -3633113410248163686L; |
1123 |
} |
1124 |
static class FifoWaitQueue extends WaitQueue { |
1125 |
private static final long serialVersionUID = -3623113410248163686L; |
1126 |
} |
1127 |
private ReentrantLock qlock; |
1128 |
private WaitQueue waitingProducers; |
1129 |
private WaitQueue waitingConsumers; |
1130 |
|
1131 |
/** |
1132 |
* Saves this queue to a stream (that is, serializes it). |
1133 |
* @param s the stream |
1134 |
* @throws java.io.IOException if an I/O error occurs |
1135 |
*/ |
1136 |
private void writeObject(java.io.ObjectOutputStream s) |
1137 |
throws java.io.IOException { |
1138 |
boolean fair = transferer instanceof TransferQueue; |
1139 |
if (fair) { |
1140 |
qlock = new ReentrantLock(true); |
1141 |
waitingProducers = new FifoWaitQueue(); |
1142 |
waitingConsumers = new FifoWaitQueue(); |
1143 |
} |
1144 |
else { |
1145 |
qlock = new ReentrantLock(); |
1146 |
waitingProducers = new LifoWaitQueue(); |
1147 |
waitingConsumers = new LifoWaitQueue(); |
1148 |
} |
1149 |
s.defaultWriteObject(); |
1150 |
} |
1151 |
|
1152 |
/** |
1153 |
* Reconstitutes this queue from a stream (that is, deserializes it). |
1154 |
* @param s the stream |
1155 |
* @throws ClassNotFoundException if the class of a serialized object |
1156 |
* could not be found |
1157 |
* @throws java.io.IOException if an I/O error occurs |
1158 |
*/ |
1159 |
private void readObject(java.io.ObjectInputStream s) |
1160 |
throws java.io.IOException, ClassNotFoundException { |
1161 |
s.defaultReadObject(); |
1162 |
if (waitingProducers instanceof FifoWaitQueue) |
1163 |
transferer = new TransferQueue<E>(); |
1164 |
else |
1165 |
transferer = new TransferStack<E>(); |
1166 |
} |
1167 |
|
1168 |
static { |
1169 |
// Reduce the risk of rare disastrous classloading in first call to |
1170 |
// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773 |
1171 |
Class<?> ensureLoaded = LockSupport.class; |
1172 |
} |
1173 |
} |