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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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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.base/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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1.59 |
* 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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* The above steps improve throughput when many threads produce |
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* and/or consume data. But they don't help much with |
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* single-source / single-sink usages in which one side or the |
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* other is always transiently blocked, and so throughput is |
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* mainly a function of thread scheduling. This is not usually |
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* noticeably improved with bounded short spin-waits. Instead both |
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* forms of transfer try Thread.yield if apparently the sole |
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* waiter. This works well when there are more tasks that cores, |
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* which is expected to be the main usage context of this mode. In |
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* other cases, waiters may help with some bookkeeping, then |
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* park/unpark. |
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1.55 |
*/ |
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1.2 |
|
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1.43 |
/** |
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1.55 |
* Shared internal API for dual stacks and queues. |
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1.43 |
*/ |
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1.82 |
abstract static class Transferer<E> { |
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1.55 |
/** |
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1.59 |
* 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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1.59 |
* @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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1.55 |
*/ |
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1.82 |
abstract E transfer(E e, boolean timed, long nanos); |
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} |
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/** |
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1.55 |
* 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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1.43 |
*/ |
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static final long SPIN_FOR_TIMEOUT_THRESHOLD = 1023L; |
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|
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1.60 |
/** Dual stack */ |
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jsr166 |
1.82 |
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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dl |
1.43 |
|
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dl |
1.55 |
/* 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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1.87 |
/** 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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/** Node class for TransferStacks. */ |
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1.128 |
static final class SNode implements ForkJoinPool.ManagedBlocker { |
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1.55 |
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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SNode(Object item) { |
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this.item = item; |
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} |
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boolean casNext(SNode cmp, SNode val) { |
217 |
dl |
1.69 |
return cmp == next && |
218 |
dl |
1.120 |
SNEXT.compareAndSet(this, cmp, val); |
219 |
dl |
1.55 |
} |
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/** |
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jsr166 |
1.63 |
* 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. |
225 |
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* |
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dl |
1.55 |
* @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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dl |
1.128 |
SNode m; Thread w; |
231 |
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if ((m = match) == null) { |
232 |
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if (SMATCH.compareAndSet(this, null, s)) { |
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if ((w = waiter) != null) |
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LockSupport.unpark(w); |
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return true; |
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dl |
1.55 |
} |
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dl |
1.128 |
else |
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m = match; |
239 |
dl |
1.47 |
} |
240 |
dl |
1.128 |
return m == s; |
241 |
dl |
1.55 |
} |
242 |
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/** |
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jsr166 |
1.59 |
* Tries to cancel a wait by matching node to itself. |
245 |
dl |
1.55 |
*/ |
246 |
dl |
1.128 |
boolean tryCancel() { |
247 |
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return SMATCH.compareAndSet(this, null, this); |
248 |
dl |
1.55 |
} |
249 |
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boolean isCancelled() { |
251 |
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return match == this; |
252 |
dl |
1.47 |
} |
253 |
dl |
1.69 |
|
254 |
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1.128 |
public final boolean isReleasable() { |
255 |
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return match != null || Thread.currentThread().isInterrupted(); |
256 |
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} |
257 |
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258 |
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public final boolean block() { |
259 |
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while (!isReleasable()) LockSupport.park(); |
260 |
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return true; |
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} |
262 |
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263 |
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void forgetWaiter() { |
264 |
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SWAITER.setOpaque(this, null); |
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} |
266 |
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267 |
dl |
1.120 |
// VarHandle mechanics |
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private static final VarHandle SMATCH; |
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private static final VarHandle SNEXT; |
270 |
dl |
1.128 |
private static final VarHandle SWAITER; |
271 |
dl |
1.73 |
static { |
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try { |
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1.120 |
MethodHandles.Lookup l = MethodHandles.lookup(); |
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SMATCH = l.findVarHandle(SNode.class, "match", SNode.class); |
275 |
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SNEXT = l.findVarHandle(SNode.class, "next", SNode.class); |
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dl |
1.128 |
SWAITER = l.findVarHandle(SNode.class, "waiter", Thread.class); |
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jsr166 |
1.111 |
} catch (ReflectiveOperationException e) { |
278 |
jsr166 |
1.126 |
throw new ExceptionInInitializerError(e); |
279 |
dl |
1.73 |
} |
280 |
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} |
281 |
dl |
1.47 |
} |
282 |
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1.43 |
|
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1.55 |
/** The head (top) of the stack */ |
284 |
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volatile SNode head; |
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jsr166 |
1.70 |
|
286 |
dl |
1.55 |
boolean casHead(SNode h, SNode nh) { |
287 |
jsr166 |
1.70 |
return h == head && |
288 |
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1.120 |
SHEAD.compareAndSet(this, h, nh); |
289 |
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1.55 |
} |
290 |
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1.2 |
|
291 |
dl |
1.55 |
/** |
292 |
jsr166 |
1.57 |
* Creates or resets fields of a node. Called only from transfer |
293 |
dl |
1.55 |
* where the node to push on stack is lazily created and |
294 |
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* reused when possible to help reduce intervals between reads |
295 |
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* and CASes of head and to avoid surges of garbage when CASes |
296 |
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* to push nodes fail due to contention. |
297 |
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*/ |
298 |
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static SNode snode(SNode s, Object e, SNode next, int mode) { |
299 |
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if (s == null) s = new SNode(e); |
300 |
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s.mode = mode; |
301 |
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s.next = next; |
302 |
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return s; |
303 |
dl |
1.43 |
} |
304 |
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|
305 |
dl |
1.55 |
/** |
306 |
jsr166 |
1.57 |
* Puts or takes an item. |
307 |
dl |
1.55 |
*/ |
308 |
jsr166 |
1.83 |
@SuppressWarnings("unchecked") |
309 |
jsr166 |
1.82 |
E transfer(E e, boolean timed, long nanos) { |
310 |
dl |
1.55 |
/* |
311 |
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* Basic algorithm is to loop trying one of three actions: |
312 |
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* |
313 |
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* 1. If apparently empty or already containing nodes of same |
314 |
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* mode, try to push node on stack and wait for a match, |
315 |
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* returning it, or null if cancelled. |
316 |
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* |
317 |
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* 2. If apparently containing node of complementary mode, |
318 |
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* try to push a fulfilling node on to stack, match |
319 |
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* with corresponding waiting node, pop both from |
320 |
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* stack, and return matched item. The matching or |
321 |
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* unlinking might not actually be necessary because of |
322 |
dl |
1.62 |
* other threads performing action 3: |
323 |
dl |
1.55 |
* |
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* 3. If top of stack already holds another fulfilling node, |
325 |
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* help it out by doing its match and/or pop |
326 |
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* operations, and then continue. The code for helping |
327 |
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* is essentially the same as for fulfilling, except |
328 |
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* that it doesn't return the item. |
329 |
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*/ |
330 |
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|
331 |
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SNode s = null; // constructed/reused as needed |
332 |
jsr166 |
1.72 |
int mode = (e == null) ? REQUEST : DATA; |
333 |
dl |
1.55 |
|
334 |
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for (;;) { |
335 |
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SNode h = head; |
336 |
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if (h == null || h.mode == mode) { // empty or same-mode |
337 |
jsr166 |
1.118 |
if (timed && nanos <= 0L) { // can't wait |
338 |
jsr166 |
1.58 |
if (h != null && h.isCancelled()) |
339 |
dl |
1.55 |
casHead(h, h.next); // pop cancelled node |
340 |
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else |
341 |
jsr166 |
1.58 |
return null; |
342 |
dl |
1.55 |
} else if (casHead(h, s = snode(s, e, h, mode))) { |
343 |
dl |
1.128 |
long deadline = timed ? System.nanoTime() + nanos : 0L; |
344 |
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Thread w = Thread.currentThread(); |
345 |
|
|
int stat = -1; // -1: may yield, +1: park, else 0 |
346 |
|
|
SNode m; // await fulfill or cancel |
347 |
|
|
while ((m = s.match) == null) { |
348 |
|
|
if ((timed && |
349 |
|
|
(nanos = deadline - System.nanoTime()) <= 0) || |
350 |
|
|
w.isInterrupted()) { |
351 |
|
|
if (s.tryCancel()) { |
352 |
|
|
clean(s); // wait cancelled |
353 |
|
|
return null; |
354 |
|
|
} |
355 |
|
|
} else if ((m = s.match) != null) { |
356 |
|
|
break; // recheck |
357 |
|
|
} else if (stat <= 0) { |
358 |
|
|
if (stat < 0 && h == null && head == s) { |
359 |
|
|
stat = 0; // yield once if was empty |
360 |
|
|
Thread.yield(); |
361 |
|
|
} else { |
362 |
|
|
stat = 1; |
363 |
|
|
s.waiter = w; // enable signal |
364 |
|
|
} |
365 |
|
|
} else if (!timed) { |
366 |
|
|
LockSupport.setCurrentBlocker(this); |
367 |
|
|
try { |
368 |
|
|
ForkJoinPool.managedBlock(s); |
369 |
|
|
} catch (InterruptedException cannotHappen) { } |
370 |
|
|
LockSupport.setCurrentBlocker(null); |
371 |
|
|
} else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD) |
372 |
|
|
LockSupport.parkNanos(this, nanos); |
373 |
dl |
1.55 |
} |
374 |
dl |
1.128 |
if (stat == 1) |
375 |
|
|
s.forgetWaiter(); |
376 |
|
|
Object result = (mode == REQUEST) ? m.item : s.item; |
377 |
|
|
if (h != null && h.next == s) |
378 |
|
|
casHead(h, s.next); // help fulfiller |
379 |
|
|
return (E) result; |
380 |
dl |
1.55 |
} |
381 |
|
|
} else if (!isFulfilling(h.mode)) { // try to fulfill |
382 |
|
|
if (h.isCancelled()) // already cancelled |
383 |
|
|
casHead(h, h.next); // pop and retry |
384 |
|
|
else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) { |
385 |
|
|
for (;;) { // loop until matched or waiters disappear |
386 |
|
|
SNode m = s.next; // m is s's match |
387 |
|
|
if (m == null) { // all waiters are gone |
388 |
|
|
casHead(s, null); // pop fulfill node |
389 |
|
|
s = null; // use new node next time |
390 |
|
|
break; // restart main loop |
391 |
|
|
} |
392 |
|
|
SNode mn = m.next; |
393 |
|
|
if (m.tryMatch(s)) { |
394 |
|
|
casHead(s, mn); // pop both s and m |
395 |
jsr166 |
1.83 |
return (E) ((mode == REQUEST) ? m.item : s.item); |
396 |
dl |
1.55 |
} else // lost match |
397 |
|
|
s.casNext(m, mn); // help unlink |
398 |
|
|
} |
399 |
|
|
} |
400 |
|
|
} else { // help a fulfiller |
401 |
|
|
SNode m = h.next; // m is h's match |
402 |
|
|
if (m == null) // waiter is gone |
403 |
|
|
casHead(h, null); // pop fulfilling node |
404 |
|
|
else { |
405 |
|
|
SNode mn = m.next; |
406 |
|
|
if (m.tryMatch(h)) // help match |
407 |
|
|
casHead(h, mn); // pop both h and m |
408 |
|
|
else // lost match |
409 |
|
|
h.casNext(m, mn); // help unlink |
410 |
|
|
} |
411 |
dl |
1.47 |
} |
412 |
|
|
} |
413 |
|
|
} |
414 |
|
|
|
415 |
dl |
1.55 |
/** |
416 |
jsr166 |
1.57 |
* Unlinks s from the stack. |
417 |
dl |
1.55 |
*/ |
418 |
|
|
void clean(SNode s) { |
419 |
jsr166 |
1.58 |
s.item = null; // forget item |
420 |
dl |
1.128 |
s.forgetWaiter(); |
421 |
dl |
1.55 |
|
422 |
|
|
/* |
423 |
|
|
* At worst we may need to traverse entire stack to unlink |
424 |
|
|
* s. If there are multiple concurrent calls to clean, we |
425 |
|
|
* might not see s if another thread has already removed |
426 |
|
|
* it. But we can stop when we see any node known to |
427 |
|
|
* follow s. We use s.next unless it too is cancelled, in |
428 |
|
|
* which case we try the node one past. We don't check any |
429 |
jsr166 |
1.59 |
* further because we don't want to doubly traverse just to |
430 |
dl |
1.55 |
* find sentinel. |
431 |
|
|
*/ |
432 |
|
|
|
433 |
|
|
SNode past = s.next; |
434 |
|
|
if (past != null && past.isCancelled()) |
435 |
|
|
past = past.next; |
436 |
|
|
|
437 |
|
|
// Absorb cancelled nodes at head |
438 |
|
|
SNode p; |
439 |
|
|
while ((p = head) != null && p != past && p.isCancelled()) |
440 |
|
|
casHead(p, p.next); |
441 |
|
|
|
442 |
|
|
// Unsplice embedded nodes |
443 |
|
|
while (p != null && p != past) { |
444 |
|
|
SNode n = p.next; |
445 |
|
|
if (n != null && n.isCancelled()) |
446 |
|
|
p.casNext(n, n.next); |
447 |
|
|
else |
448 |
|
|
p = n; |
449 |
dl |
1.47 |
} |
450 |
|
|
} |
451 |
dl |
1.69 |
|
452 |
dl |
1.120 |
// VarHandle mechanics |
453 |
|
|
private static final VarHandle SHEAD; |
454 |
dl |
1.73 |
static { |
455 |
|
|
try { |
456 |
dl |
1.120 |
MethodHandles.Lookup l = MethodHandles.lookup(); |
457 |
|
|
SHEAD = l.findVarHandle(TransferStack.class, "head", SNode.class); |
458 |
jsr166 |
1.111 |
} catch (ReflectiveOperationException e) { |
459 |
jsr166 |
1.126 |
throw new ExceptionInInitializerError(e); |
460 |
dl |
1.73 |
} |
461 |
|
|
} |
462 |
dl |
1.47 |
} |
463 |
jsr166 |
1.48 |
|
464 |
jsr166 |
1.61 |
/** Dual Queue */ |
465 |
jsr166 |
1.82 |
static final class TransferQueue<E> extends Transferer<E> { |
466 |
dl |
1.55 |
/* |
467 |
|
|
* This extends Scherer-Scott dual queue algorithm, differing, |
468 |
|
|
* among other ways, by using modes within nodes rather than |
469 |
|
|
* marked pointers. The algorithm is a little simpler than |
470 |
|
|
* that for stacks because fulfillers do not need explicit |
471 |
|
|
* nodes, and matching is done by CAS'ing QNode.item field |
472 |
jsr166 |
1.59 |
* from non-null to null (for put) or vice versa (for take). |
473 |
dl |
1.55 |
*/ |
474 |
dl |
1.53 |
|
475 |
dl |
1.55 |
/** Node class for TransferQueue. */ |
476 |
dl |
1.128 |
static final class QNode implements ForkJoinPool.ManagedBlocker { |
477 |
dl |
1.55 |
volatile QNode next; // next node in queue |
478 |
|
|
volatile Object item; // CAS'ed to or from null |
479 |
|
|
volatile Thread waiter; // to control park/unpark |
480 |
jsr166 |
1.58 |
final boolean isData; |
481 |
dl |
1.35 |
|
482 |
dl |
1.55 |
QNode(Object item, boolean isData) { |
483 |
|
|
this.item = item; |
484 |
|
|
this.isData = isData; |
485 |
|
|
} |
486 |
dl |
1.35 |
|
487 |
dl |
1.55 |
boolean casNext(QNode cmp, QNode val) { |
488 |
dl |
1.69 |
return next == cmp && |
489 |
dl |
1.120 |
QNEXT.compareAndSet(this, cmp, val); |
490 |
dl |
1.55 |
} |
491 |
|
|
|
492 |
|
|
boolean casItem(Object cmp, Object val) { |
493 |
dl |
1.69 |
return item == cmp && |
494 |
dl |
1.120 |
QITEM.compareAndSet(this, cmp, val); |
495 |
dl |
1.55 |
} |
496 |
|
|
|
497 |
|
|
/** |
498 |
jsr166 |
1.59 |
* Tries to cancel by CAS'ing ref to this as item. |
499 |
dl |
1.55 |
*/ |
500 |
dl |
1.128 |
boolean tryCancel(Object cmp) { |
501 |
|
|
return QITEM.compareAndSet(this, cmp, this); |
502 |
dl |
1.55 |
} |
503 |
jsr166 |
1.70 |
|
504 |
dl |
1.55 |
boolean isCancelled() { |
505 |
|
|
return item == this; |
506 |
|
|
} |
507 |
dl |
1.56 |
|
508 |
jsr166 |
1.58 |
/** |
509 |
jsr166 |
1.57 |
* Returns true if this node is known to be off the queue |
510 |
dl |
1.56 |
* because its next pointer has been forgotten due to |
511 |
|
|
* an advanceHead operation. |
512 |
|
|
*/ |
513 |
|
|
boolean isOffList() { |
514 |
|
|
return next == this; |
515 |
|
|
} |
516 |
dl |
1.74 |
|
517 |
dl |
1.128 |
void forgetWaiter() { |
518 |
|
|
QWAITER.setOpaque(this, null); |
519 |
|
|
} |
520 |
|
|
|
521 |
|
|
boolean isFulfilled() { |
522 |
|
|
Object x; |
523 |
|
|
return isData == ((x = item) == null) || x == this; |
524 |
|
|
} |
525 |
|
|
|
526 |
|
|
public final boolean isReleasable() { |
527 |
|
|
Object x; |
528 |
|
|
return isData == ((x = item) == null) || x == this || |
529 |
|
|
Thread.currentThread().isInterrupted(); |
530 |
|
|
} |
531 |
|
|
|
532 |
|
|
public final boolean block() { |
533 |
|
|
while (!isReleasable()) LockSupport.park(); |
534 |
|
|
return true; |
535 |
|
|
} |
536 |
|
|
|
537 |
dl |
1.120 |
// VarHandle mechanics |
538 |
|
|
private static final VarHandle QITEM; |
539 |
|
|
private static final VarHandle QNEXT; |
540 |
dl |
1.128 |
private static final VarHandle QWAITER; |
541 |
dl |
1.73 |
static { |
542 |
|
|
try { |
543 |
dl |
1.120 |
MethodHandles.Lookup l = MethodHandles.lookup(); |
544 |
|
|
QITEM = l.findVarHandle(QNode.class, "item", Object.class); |
545 |
|
|
QNEXT = l.findVarHandle(QNode.class, "next", QNode.class); |
546 |
dl |
1.128 |
QWAITER = l.findVarHandle(QNode.class, "waiter", Thread.class); |
547 |
jsr166 |
1.112 |
} catch (ReflectiveOperationException e) { |
548 |
jsr166 |
1.126 |
throw new ExceptionInInitializerError(e); |
549 |
dl |
1.73 |
} |
550 |
|
|
} |
551 |
dl |
1.31 |
} |
552 |
|
|
|
553 |
dl |
1.55 |
/** Head of queue */ |
554 |
|
|
transient volatile QNode head; |
555 |
|
|
/** Tail of queue */ |
556 |
|
|
transient volatile QNode tail; |
557 |
dl |
1.31 |
/** |
558 |
dl |
1.55 |
* Reference to a cancelled node that might not yet have been |
559 |
|
|
* unlinked from queue because it was the last inserted node |
560 |
jsr166 |
1.91 |
* when it was cancelled. |
561 |
dl |
1.31 |
*/ |
562 |
dl |
1.55 |
transient volatile QNode cleanMe; |
563 |
|
|
|
564 |
|
|
TransferQueue() { |
565 |
|
|
QNode h = new QNode(null, false); // initialize to dummy node. |
566 |
|
|
head = h; |
567 |
|
|
tail = h; |
568 |
dl |
1.31 |
} |
569 |
|
|
|
570 |
|
|
/** |
571 |
jsr166 |
1.59 |
* Tries to cas nh as new head; if successful, unlink |
572 |
dl |
1.55 |
* old head's next node to avoid garbage retention. |
573 |
dl |
1.31 |
*/ |
574 |
dl |
1.55 |
void advanceHead(QNode h, QNode nh) { |
575 |
jsr166 |
1.70 |
if (h == head && |
576 |
dl |
1.120 |
QHEAD.compareAndSet(this, h, nh)) |
577 |
dl |
1.55 |
h.next = h; // forget old next |
578 |
dl |
1.31 |
} |
579 |
|
|
|
580 |
|
|
/** |
581 |
jsr166 |
1.57 |
* Tries to cas nt as new tail. |
582 |
dl |
1.31 |
*/ |
583 |
dl |
1.55 |
void advanceTail(QNode t, QNode nt) { |
584 |
|
|
if (tail == t) |
585 |
dl |
1.120 |
QTAIL.compareAndSet(this, t, nt); |
586 |
dl |
1.31 |
} |
587 |
dl |
1.2 |
|
588 |
|
|
/** |
589 |
jsr166 |
1.57 |
* Tries to CAS cleanMe slot. |
590 |
dl |
1.2 |
*/ |
591 |
dl |
1.55 |
boolean casCleanMe(QNode cmp, QNode val) { |
592 |
dl |
1.69 |
return cleanMe == cmp && |
593 |
dl |
1.120 |
QCLEANME.compareAndSet(this, cmp, val); |
594 |
dl |
1.35 |
} |
595 |
|
|
|
596 |
|
|
/** |
597 |
jsr166 |
1.57 |
* Puts or takes an item. |
598 |
dl |
1.35 |
*/ |
599 |
jsr166 |
1.83 |
@SuppressWarnings("unchecked") |
600 |
jsr166 |
1.82 |
E transfer(E e, boolean timed, long nanos) { |
601 |
jsr166 |
1.58 |
/* Basic algorithm is to loop trying to take either of |
602 |
dl |
1.55 |
* two actions: |
603 |
|
|
* |
604 |
jsr166 |
1.58 |
* 1. If queue apparently empty or holding same-mode nodes, |
605 |
dl |
1.55 |
* try to add node to queue of waiters, wait to be |
606 |
|
|
* fulfilled (or cancelled) and return matching item. |
607 |
|
|
* |
608 |
|
|
* 2. If queue apparently contains waiting items, and this |
609 |
|
|
* call is of complementary mode, try to fulfill by CAS'ing |
610 |
|
|
* item field of waiting node and dequeuing it, and then |
611 |
|
|
* returning matching item. |
612 |
|
|
* |
613 |
|
|
* In each case, along the way, check for and try to help |
614 |
|
|
* advance head and tail on behalf of other stalled/slow |
615 |
|
|
* threads. |
616 |
|
|
* |
617 |
|
|
* The loop starts off with a null check guarding against |
618 |
|
|
* seeing uninitialized head or tail values. This never |
619 |
|
|
* happens in current SynchronousQueue, but could if |
620 |
|
|
* callers held non-volatile/final ref to the |
621 |
|
|
* transferer. The check is here anyway because it places |
622 |
|
|
* null checks at top of loop, which is usually faster |
623 |
|
|
* than having them implicitly interspersed. |
624 |
|
|
*/ |
625 |
|
|
|
626 |
dl |
1.128 |
QNode s = null; // constructed/reused as needed |
627 |
dl |
1.55 |
boolean isData = (e != null); |
628 |
|
|
for (;;) { |
629 |
dl |
1.128 |
QNode t = tail, h = head, m, tn; // m is node to fulfill |
630 |
|
|
if (t == null || h == null) |
631 |
|
|
; // inconsistent |
632 |
|
|
else if (h == t || t.isData == isData) { // empty or same-mode |
633 |
|
|
if (t != tail) // inconsistent |
634 |
|
|
; |
635 |
|
|
else if ((tn = t.next) != null) // lagging tail |
636 |
dl |
1.55 |
advanceTail(t, tn); |
637 |
dl |
1.128 |
else if (timed && nanos <= 0L) // can't wait |
638 |
dl |
1.55 |
return null; |
639 |
dl |
1.128 |
else if (t.casNext(null, (s != null) ? s : |
640 |
|
|
(s = new QNode(e, isData)))) { |
641 |
|
|
advanceTail(t, s); |
642 |
|
|
long deadline = timed ? System.nanoTime() + nanos : 0L; |
643 |
|
|
Thread w = Thread.currentThread(); |
644 |
|
|
int stat = -1; // same idea as TransferStack |
645 |
|
|
Object item; |
646 |
|
|
while ((item = s.item) == e) { |
647 |
|
|
if ((timed && |
648 |
|
|
(nanos = deadline - System.nanoTime()) <= 0) || |
649 |
|
|
w.isInterrupted()) { |
650 |
|
|
if (s.tryCancel(e)) { |
651 |
|
|
clean(t, s); |
652 |
|
|
return null; |
653 |
|
|
} |
654 |
|
|
} else if ((item = s.item) != e) { |
655 |
|
|
break; // recheck |
656 |
|
|
} else if (stat <= 0) { |
657 |
|
|
if (t.next == s) { |
658 |
|
|
if (stat < 0 && t.isFulfilled()) { |
659 |
|
|
stat = 0; // yield once if first |
660 |
|
|
Thread.yield(); |
661 |
|
|
} |
662 |
|
|
else { |
663 |
|
|
stat = 1; |
664 |
|
|
s.waiter = w; |
665 |
|
|
} |
666 |
|
|
} |
667 |
|
|
} else if (!timed) { |
668 |
|
|
LockSupport.setCurrentBlocker(this); |
669 |
|
|
try { |
670 |
|
|
ForkJoinPool.managedBlock(s); |
671 |
|
|
} catch (InterruptedException cannotHappen) { } |
672 |
|
|
LockSupport.setCurrentBlocker(null); |
673 |
|
|
} |
674 |
|
|
else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD) |
675 |
|
|
LockSupport.parkNanos(this, nanos); |
676 |
|
|
} |
677 |
|
|
if (stat == 1) |
678 |
|
|
s.forgetWaiter(); |
679 |
|
|
if (!s.isOffList()) { // not already unlinked |
680 |
|
|
advanceHead(t, s); // unlink if head |
681 |
|
|
if (item != null) // and forget fields |
682 |
|
|
s.item = s; |
683 |
|
|
} |
684 |
|
|
return (item != null) ? (E)item : e; |
685 |
dl |
1.55 |
} |
686 |
|
|
|
687 |
dl |
1.128 |
} else if ((m = h.next) != null && t == tail && h == head) { |
688 |
|
|
Thread waiter; |
689 |
dl |
1.55 |
Object x = m.item; |
690 |
dl |
1.128 |
boolean fulfilled = ((isData == (x == null)) && |
691 |
|
|
x != m && m.casItem(x, e)); |
692 |
|
|
advanceHead(h, m); // (help) dequeue |
693 |
|
|
if (fulfilled) { |
694 |
|
|
if ((waiter = m.waiter) != null) |
695 |
|
|
LockSupport.unpark(waiter); |
696 |
|
|
return (x != null) ? (E)x : e; |
697 |
dl |
1.55 |
} |
698 |
dl |
1.120 |
} |
699 |
dl |
1.35 |
} |
700 |
dl |
1.31 |
} |
701 |
|
|
|
702 |
|
|
/** |
703 |
jsr166 |
1.57 |
* Gets rid of cancelled node s with original predecessor pred. |
704 |
dl |
1.31 |
*/ |
705 |
dl |
1.55 |
void clean(QNode pred, QNode s) { |
706 |
dl |
1.128 |
s.forgetWaiter(); |
707 |
dl |
1.55 |
/* |
708 |
|
|
* At any given time, exactly one node on list cannot be |
709 |
|
|
* deleted -- the last inserted node. To accommodate this, |
710 |
|
|
* if we cannot delete s, we save its predecessor as |
711 |
|
|
* "cleanMe", deleting the previously saved version |
712 |
|
|
* first. At least one of node s or the node previously |
713 |
|
|
* saved can always be deleted, so this always terminates. |
714 |
|
|
*/ |
715 |
|
|
while (pred.next == s) { // Return early if already unlinked |
716 |
|
|
QNode h = head; |
717 |
|
|
QNode hn = h.next; // Absorb cancelled first node as head |
718 |
|
|
if (hn != null && hn.isCancelled()) { |
719 |
|
|
advanceHead(h, hn); |
720 |
|
|
continue; |
721 |
|
|
} |
722 |
jsr166 |
1.68 |
QNode t = tail; // Ensure consistent read for tail |
723 |
dl |
1.55 |
if (t == h) |
724 |
|
|
return; |
725 |
jsr166 |
1.68 |
QNode tn = t.next; |
726 |
|
|
if (t != tail) |
727 |
dl |
1.55 |
continue; |
728 |
|
|
if (tn != null) { |
729 |
|
|
advanceTail(t, tn); |
730 |
|
|
continue; |
731 |
|
|
} |
732 |
|
|
if (s != t) { // If not tail, try to unsplice |
733 |
|
|
QNode sn = s.next; |
734 |
|
|
if (sn == s || pred.casNext(s, sn)) |
735 |
|
|
return; |
736 |
|
|
} |
737 |
|
|
QNode dp = cleanMe; |
738 |
|
|
if (dp != null) { // Try unlinking previous cancelled node |
739 |
|
|
QNode d = dp.next; |
740 |
|
|
QNode dn; |
741 |
|
|
if (d == null || // d is gone or |
742 |
|
|
d == dp || // d is off list or |
743 |
|
|
!d.isCancelled() || // d not cancelled or |
744 |
|
|
(d != t && // d not tail and |
745 |
|
|
(dn = d.next) != null && // has successor |
746 |
|
|
dn != d && // that is on list |
747 |
|
|
dp.casNext(d, dn))) // d unspliced |
748 |
jsr166 |
1.58 |
casCleanMe(dp, null); |
749 |
|
|
if (dp == pred) |
750 |
dl |
1.55 |
return; // s is already saved node |
751 |
jsr166 |
1.58 |
} else if (casCleanMe(null, pred)) |
752 |
dl |
1.55 |
return; // Postpone cleaning s |
753 |
dl |
1.2 |
} |
754 |
|
|
} |
755 |
dl |
1.69 |
|
756 |
dl |
1.120 |
// VarHandle mechanics |
757 |
|
|
private static final VarHandle QHEAD; |
758 |
|
|
private static final VarHandle QTAIL; |
759 |
|
|
private static final VarHandle QCLEANME; |
760 |
dl |
1.73 |
static { |
761 |
|
|
try { |
762 |
dl |
1.120 |
MethodHandles.Lookup l = MethodHandles.lookup(); |
763 |
|
|
QHEAD = l.findVarHandle(TransferQueue.class, "head", |
764 |
|
|
QNode.class); |
765 |
|
|
QTAIL = l.findVarHandle(TransferQueue.class, "tail", |
766 |
|
|
QNode.class); |
767 |
|
|
QCLEANME = l.findVarHandle(TransferQueue.class, "cleanMe", |
768 |
|
|
QNode.class); |
769 |
jsr166 |
1.111 |
} catch (ReflectiveOperationException e) { |
770 |
jsr166 |
1.126 |
throw new ExceptionInInitializerError(e); |
771 |
dl |
1.73 |
} |
772 |
|
|
} |
773 |
dl |
1.55 |
} |
774 |
|
|
|
775 |
|
|
/** |
776 |
|
|
* The transferer. Set only in constructor, but cannot be declared |
777 |
|
|
* as final without further complicating serialization. Since |
778 |
dl |
1.56 |
* this is accessed only at most once per public method, there |
779 |
|
|
* isn't a noticeable performance penalty for using volatile |
780 |
|
|
* instead of final here. |
781 |
dl |
1.55 |
*/ |
782 |
jsr166 |
1.82 |
private transient volatile Transferer<E> transferer; |
783 |
dl |
1.55 |
|
784 |
|
|
/** |
785 |
jsr166 |
1.90 |
* Creates a {@code SynchronousQueue} with nonfair access policy. |
786 |
dl |
1.55 |
*/ |
787 |
|
|
public SynchronousQueue() { |
788 |
|
|
this(false); |
789 |
|
|
} |
790 |
dl |
1.2 |
|
791 |
dl |
1.55 |
/** |
792 |
jsr166 |
1.90 |
* Creates a {@code SynchronousQueue} with the specified fairness policy. |
793 |
jsr166 |
1.63 |
* |
794 |
|
|
* @param fair if true, waiting threads contend in FIFO order for |
795 |
|
|
* access; otherwise the order is unspecified. |
796 |
dl |
1.55 |
*/ |
797 |
|
|
public SynchronousQueue(boolean fair) { |
798 |
jsr166 |
1.82 |
transferer = fair ? new TransferQueue<E>() : new TransferStack<E>(); |
799 |
dl |
1.2 |
} |
800 |
|
|
|
801 |
|
|
/** |
802 |
dl |
1.35 |
* Adds the specified element to this queue, waiting if necessary for |
803 |
|
|
* another thread to receive it. |
804 |
jsr166 |
1.50 |
* |
805 |
|
|
* @throws InterruptedException {@inheritDoc} |
806 |
|
|
* @throws NullPointerException {@inheritDoc} |
807 |
tim |
1.10 |
*/ |
808 |
jsr166 |
1.82 |
public void put(E e) throws InterruptedException { |
809 |
|
|
if (e == null) throw new NullPointerException(); |
810 |
|
|
if (transferer.transfer(e, false, 0) == null) { |
811 |
jsr166 |
1.68 |
Thread.interrupted(); |
812 |
dl |
1.55 |
throw new InterruptedException(); |
813 |
jsr166 |
1.68 |
} |
814 |
tim |
1.1 |
} |
815 |
|
|
|
816 |
dholmes |
1.11 |
/** |
817 |
dl |
1.20 |
* Inserts the specified element into this queue, waiting if necessary |
818 |
dl |
1.18 |
* up to the specified wait time for another thread to receive it. |
819 |
jsr166 |
1.50 |
* |
820 |
jsr166 |
1.90 |
* @return {@code true} if successful, or {@code false} if the |
821 |
jsr166 |
1.92 |
* specified waiting time elapses before a consumer appears |
822 |
jsr166 |
1.50 |
* @throws InterruptedException {@inheritDoc} |
823 |
|
|
* @throws NullPointerException {@inheritDoc} |
824 |
dholmes |
1.11 |
*/ |
825 |
jsr166 |
1.82 |
public boolean offer(E e, long timeout, TimeUnit unit) |
826 |
dl |
1.55 |
throws InterruptedException { |
827 |
jsr166 |
1.82 |
if (e == null) throw new NullPointerException(); |
828 |
|
|
if (transferer.transfer(e, true, unit.toNanos(timeout)) != null) |
829 |
dl |
1.55 |
return true; |
830 |
|
|
if (!Thread.interrupted()) |
831 |
|
|
return false; |
832 |
|
|
throw new InterruptedException(); |
833 |
|
|
} |
834 |
|
|
|
835 |
|
|
/** |
836 |
|
|
* Inserts the specified element into this queue, if another thread is |
837 |
|
|
* waiting to receive it. |
838 |
|
|
* |
839 |
|
|
* @param e the element to add |
840 |
jsr166 |
1.90 |
* @return {@code true} if the element was added to this queue, else |
841 |
|
|
* {@code false} |
842 |
dl |
1.55 |
* @throws NullPointerException if the specified element is null |
843 |
|
|
*/ |
844 |
|
|
public boolean offer(E e) { |
845 |
jsr166 |
1.49 |
if (e == null) throw new NullPointerException(); |
846 |
dl |
1.55 |
return transferer.transfer(e, true, 0) != null; |
847 |
tim |
1.1 |
} |
848 |
|
|
|
849 |
dholmes |
1.11 |
/** |
850 |
|
|
* Retrieves and removes the head of this queue, waiting if necessary |
851 |
|
|
* for another thread to insert it. |
852 |
jsr166 |
1.50 |
* |
853 |
dholmes |
1.11 |
* @return the head of this queue |
854 |
jsr166 |
1.50 |
* @throws InterruptedException {@inheritDoc} |
855 |
dholmes |
1.11 |
*/ |
856 |
dl |
1.2 |
public E take() throws InterruptedException { |
857 |
jsr166 |
1.82 |
E e = transferer.transfer(null, false, 0); |
858 |
dl |
1.55 |
if (e != null) |
859 |
jsr166 |
1.82 |
return e; |
860 |
jsr166 |
1.68 |
Thread.interrupted(); |
861 |
dl |
1.55 |
throw new InterruptedException(); |
862 |
tim |
1.1 |
} |
863 |
dl |
1.2 |
|
864 |
dholmes |
1.11 |
/** |
865 |
|
|
* Retrieves and removes the head of this queue, waiting |
866 |
|
|
* if necessary up to the specified wait time, for another thread |
867 |
|
|
* to insert it. |
868 |
jsr166 |
1.50 |
* |
869 |
jsr166 |
1.90 |
* @return the head of this queue, or {@code null} if the |
870 |
jsr166 |
1.92 |
* specified waiting time elapses before an element is present |
871 |
jsr166 |
1.50 |
* @throws InterruptedException {@inheritDoc} |
872 |
dholmes |
1.11 |
*/ |
873 |
dl |
1.2 |
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
874 |
jsr166 |
1.82 |
E e = transferer.transfer(null, true, unit.toNanos(timeout)); |
875 |
dl |
1.55 |
if (e != null || !Thread.interrupted()) |
876 |
jsr166 |
1.82 |
return e; |
877 |
dl |
1.55 |
throw new InterruptedException(); |
878 |
tim |
1.1 |
} |
879 |
dl |
1.2 |
|
880 |
dl |
1.18 |
/** |
881 |
|
|
* Retrieves and removes the head of this queue, if another thread |
882 |
|
|
* is currently making an element available. |
883 |
|
|
* |
884 |
jsr166 |
1.90 |
* @return the head of this queue, or {@code null} if no |
885 |
jsr166 |
1.92 |
* element is available |
886 |
dl |
1.18 |
*/ |
887 |
dl |
1.2 |
public E poll() { |
888 |
jsr166 |
1.82 |
return transferer.transfer(null, true, 0); |
889 |
tim |
1.1 |
} |
890 |
dl |
1.2 |
|
891 |
dl |
1.5 |
/** |
892 |
jsr166 |
1.90 |
* Always returns {@code true}. |
893 |
|
|
* A {@code SynchronousQueue} has no internal capacity. |
894 |
jsr166 |
1.63 |
* |
895 |
jsr166 |
1.90 |
* @return {@code true} |
896 |
dl |
1.5 |
*/ |
897 |
|
|
public boolean isEmpty() { |
898 |
|
|
return true; |
899 |
|
|
} |
900 |
|
|
|
901 |
|
|
/** |
902 |
dholmes |
1.11 |
* Always returns zero. |
903 |
jsr166 |
1.90 |
* A {@code SynchronousQueue} has no internal capacity. |
904 |
jsr166 |
1.63 |
* |
905 |
jsr166 |
1.89 |
* @return zero |
906 |
dl |
1.5 |
*/ |
907 |
|
|
public int size() { |
908 |
|
|
return 0; |
909 |
tim |
1.1 |
} |
910 |
dl |
1.2 |
|
911 |
dl |
1.5 |
/** |
912 |
dholmes |
1.11 |
* Always returns zero. |
913 |
jsr166 |
1.90 |
* A {@code SynchronousQueue} has no internal capacity. |
914 |
jsr166 |
1.63 |
* |
915 |
jsr166 |
1.89 |
* @return zero |
916 |
dl |
1.5 |
*/ |
917 |
|
|
public int remainingCapacity() { |
918 |
|
|
return 0; |
919 |
|
|
} |
920 |
|
|
|
921 |
|
|
/** |
922 |
dholmes |
1.11 |
* Does nothing. |
923 |
jsr166 |
1.90 |
* A {@code SynchronousQueue} has no internal capacity. |
924 |
dholmes |
1.11 |
*/ |
925 |
dl |
1.55 |
public void clear() { |
926 |
|
|
} |
927 |
dholmes |
1.11 |
|
928 |
|
|
/** |
929 |
jsr166 |
1.90 |
* Always returns {@code false}. |
930 |
|
|
* A {@code SynchronousQueue} has no internal capacity. |
931 |
jsr166 |
1.63 |
* |
932 |
dl |
1.55 |
* @param o the element |
933 |
jsr166 |
1.90 |
* @return {@code false} |
934 |
dholmes |
1.11 |
*/ |
935 |
|
|
public boolean contains(Object o) { |
936 |
|
|
return false; |
937 |
|
|
} |
938 |
|
|
|
939 |
|
|
/** |
940 |
jsr166 |
1.90 |
* Always returns {@code false}. |
941 |
|
|
* A {@code SynchronousQueue} has no internal capacity. |
942 |
dl |
1.18 |
* |
943 |
|
|
* @param o the element to remove |
944 |
jsr166 |
1.90 |
* @return {@code false} |
945 |
dl |
1.18 |
*/ |
946 |
|
|
public boolean remove(Object o) { |
947 |
|
|
return false; |
948 |
|
|
} |
949 |
|
|
|
950 |
|
|
/** |
951 |
jsr166 |
1.90 |
* Returns {@code false} unless the given collection is empty. |
952 |
|
|
* A {@code SynchronousQueue} has no internal capacity. |
953 |
jsr166 |
1.63 |
* |
954 |
dl |
1.18 |
* @param c the collection |
955 |
jsr166 |
1.90 |
* @return {@code false} unless given collection is empty |
956 |
dholmes |
1.11 |
*/ |
957 |
dl |
1.12 |
public boolean containsAll(Collection<?> c) { |
958 |
dl |
1.16 |
return c.isEmpty(); |
959 |
dholmes |
1.11 |
} |
960 |
|
|
|
961 |
|
|
/** |
962 |
jsr166 |
1.90 |
* Always returns {@code false}. |
963 |
|
|
* A {@code SynchronousQueue} has no internal capacity. |
964 |
jsr166 |
1.63 |
* |
965 |
dl |
1.18 |
* @param c the collection |
966 |
jsr166 |
1.90 |
* @return {@code false} |
967 |
dholmes |
1.11 |
*/ |
968 |
dl |
1.12 |
public boolean removeAll(Collection<?> c) { |
969 |
dholmes |
1.11 |
return false; |
970 |
|
|
} |
971 |
|
|
|
972 |
|
|
/** |
973 |
jsr166 |
1.90 |
* Always returns {@code false}. |
974 |
|
|
* A {@code SynchronousQueue} has no internal capacity. |
975 |
jsr166 |
1.63 |
* |
976 |
dl |
1.18 |
* @param c the collection |
977 |
jsr166 |
1.90 |
* @return {@code false} |
978 |
dholmes |
1.11 |
*/ |
979 |
dl |
1.12 |
public boolean retainAll(Collection<?> c) { |
980 |
dholmes |
1.11 |
return false; |
981 |
|
|
} |
982 |
|
|
|
983 |
|
|
/** |
984 |
jsr166 |
1.90 |
* Always returns {@code null}. |
985 |
|
|
* A {@code SynchronousQueue} does not return elements |
986 |
dl |
1.5 |
* unless actively waited on. |
987 |
jsr166 |
1.63 |
* |
988 |
jsr166 |
1.90 |
* @return {@code null} |
989 |
dl |
1.5 |
*/ |
990 |
|
|
public E peek() { |
991 |
|
|
return null; |
992 |
|
|
} |
993 |
|
|
|
994 |
|
|
/** |
995 |
jsr166 |
1.90 |
* Returns an empty iterator in which {@code hasNext} always returns |
996 |
|
|
* {@code false}. |
997 |
tim |
1.13 |
* |
998 |
dholmes |
1.11 |
* @return an empty iterator |
999 |
dl |
1.5 |
*/ |
1000 |
dl |
1.2 |
public Iterator<E> iterator() { |
1001 |
jsr166 |
1.102 |
return Collections.emptyIterator(); |
1002 |
tim |
1.1 |
} |
1003 |
|
|
|
1004 |
jsr166 |
1.101 |
/** |
1005 |
|
|
* Returns an empty spliterator in which calls to |
1006 |
jsr166 |
1.124 |
* {@link Spliterator#trySplit() trySplit} always return {@code null}. |
1007 |
jsr166 |
1.101 |
* |
1008 |
|
|
* @return an empty spliterator |
1009 |
|
|
* @since 1.8 |
1010 |
|
|
*/ |
1011 |
dl |
1.96 |
public Spliterator<E> spliterator() { |
1012 |
dl |
1.95 |
return Spliterators.emptySpliterator(); |
1013 |
dl |
1.93 |
} |
1014 |
jsr166 |
1.94 |
|
1015 |
dl |
1.5 |
/** |
1016 |
dholmes |
1.11 |
* Returns a zero-length array. |
1017 |
|
|
* @return a zero-length array |
1018 |
dl |
1.5 |
*/ |
1019 |
dl |
1.3 |
public Object[] toArray() { |
1020 |
dl |
1.25 |
return new Object[0]; |
1021 |
tim |
1.1 |
} |
1022 |
|
|
|
1023 |
dholmes |
1.11 |
/** |
1024 |
jsr166 |
1.103 |
* Sets the zeroth element of the specified array to {@code null} |
1025 |
dholmes |
1.11 |
* (if the array has non-zero length) and returns it. |
1026 |
jsr166 |
1.50 |
* |
1027 |
dl |
1.40 |
* @param a the array |
1028 |
dholmes |
1.11 |
* @return the specified array |
1029 |
jsr166 |
1.50 |
* @throws NullPointerException if the specified array is null |
1030 |
dholmes |
1.11 |
*/ |
1031 |
dl |
1.2 |
public <T> T[] toArray(T[] a) { |
1032 |
|
|
if (a.length > 0) |
1033 |
|
|
a[0] = null; |
1034 |
|
|
return a; |
1035 |
|
|
} |
1036 |
dl |
1.21 |
|
1037 |
jsr166 |
1.50 |
/** |
1038 |
jsr166 |
1.113 |
* Always returns {@code "[]"}. |
1039 |
|
|
* @return {@code "[]"} |
1040 |
|
|
*/ |
1041 |
|
|
public String toString() { |
1042 |
|
|
return "[]"; |
1043 |
|
|
} |
1044 |
|
|
|
1045 |
|
|
/** |
1046 |
jsr166 |
1.50 |
* @throws UnsupportedOperationException {@inheritDoc} |
1047 |
|
|
* @throws ClassCastException {@inheritDoc} |
1048 |
|
|
* @throws NullPointerException {@inheritDoc} |
1049 |
|
|
* @throws IllegalArgumentException {@inheritDoc} |
1050 |
|
|
*/ |
1051 |
dl |
1.21 |
public int drainTo(Collection<? super E> c) { |
1052 |
jsr166 |
1.121 |
Objects.requireNonNull(c); |
1053 |
dl |
1.21 |
if (c == this) |
1054 |
|
|
throw new IllegalArgumentException(); |
1055 |
|
|
int n = 0; |
1056 |
jsr166 |
1.122 |
for (E e; (e = poll()) != null; n++) |
1057 |
dl |
1.21 |
c.add(e); |
1058 |
|
|
return n; |
1059 |
|
|
} |
1060 |
|
|
|
1061 |
jsr166 |
1.50 |
/** |
1062 |
|
|
* @throws UnsupportedOperationException {@inheritDoc} |
1063 |
|
|
* @throws ClassCastException {@inheritDoc} |
1064 |
|
|
* @throws NullPointerException {@inheritDoc} |
1065 |
|
|
* @throws IllegalArgumentException {@inheritDoc} |
1066 |
|
|
*/ |
1067 |
dl |
1.21 |
public int drainTo(Collection<? super E> c, int maxElements) { |
1068 |
jsr166 |
1.121 |
Objects.requireNonNull(c); |
1069 |
dl |
1.21 |
if (c == this) |
1070 |
|
|
throw new IllegalArgumentException(); |
1071 |
|
|
int n = 0; |
1072 |
jsr166 |
1.122 |
for (E e; n < maxElements && (e = poll()) != null; n++) |
1073 |
dl |
1.21 |
c.add(e); |
1074 |
|
|
return n; |
1075 |
|
|
} |
1076 |
dl |
1.55 |
|
1077 |
|
|
/* |
1078 |
|
|
* To cope with serialization strategy in the 1.5 version of |
1079 |
|
|
* SynchronousQueue, we declare some unused classes and fields |
1080 |
|
|
* that exist solely to enable serializability across versions. |
1081 |
|
|
* These fields are never used, so are initialized only if this |
1082 |
|
|
* object is ever serialized or deserialized. |
1083 |
|
|
*/ |
1084 |
|
|
|
1085 |
jsr166 |
1.82 |
@SuppressWarnings("serial") |
1086 |
dl |
1.55 |
static class WaitQueue implements java.io.Serializable { } |
1087 |
|
|
static class LifoWaitQueue extends WaitQueue { |
1088 |
|
|
private static final long serialVersionUID = -3633113410248163686L; |
1089 |
|
|
} |
1090 |
|
|
static class FifoWaitQueue extends WaitQueue { |
1091 |
|
|
private static final long serialVersionUID = -3623113410248163686L; |
1092 |
|
|
} |
1093 |
|
|
private ReentrantLock qlock; |
1094 |
|
|
private WaitQueue waitingProducers; |
1095 |
|
|
private WaitQueue waitingConsumers; |
1096 |
|
|
|
1097 |
|
|
/** |
1098 |
jsr166 |
1.84 |
* Saves this queue to a stream (that is, serializes it). |
1099 |
jsr166 |
1.99 |
* @param s the stream |
1100 |
jsr166 |
1.100 |
* @throws java.io.IOException if an I/O error occurs |
1101 |
dl |
1.55 |
*/ |
1102 |
|
|
private void writeObject(java.io.ObjectOutputStream s) |
1103 |
|
|
throws java.io.IOException { |
1104 |
|
|
boolean fair = transferer instanceof TransferQueue; |
1105 |
|
|
if (fair) { |
1106 |
|
|
qlock = new ReentrantLock(true); |
1107 |
|
|
waitingProducers = new FifoWaitQueue(); |
1108 |
|
|
waitingConsumers = new FifoWaitQueue(); |
1109 |
|
|
} |
1110 |
|
|
else { |
1111 |
|
|
qlock = new ReentrantLock(); |
1112 |
|
|
waitingProducers = new LifoWaitQueue(); |
1113 |
|
|
waitingConsumers = new LifoWaitQueue(); |
1114 |
|
|
} |
1115 |
|
|
s.defaultWriteObject(); |
1116 |
|
|
} |
1117 |
|
|
|
1118 |
jsr166 |
1.84 |
/** |
1119 |
|
|
* Reconstitutes this queue from a stream (that is, deserializes it). |
1120 |
jsr166 |
1.99 |
* @param s the stream |
1121 |
jsr166 |
1.100 |
* @throws ClassNotFoundException if the class of a serialized object |
1122 |
|
|
* could not be found |
1123 |
|
|
* @throws java.io.IOException if an I/O error occurs |
1124 |
jsr166 |
1.84 |
*/ |
1125 |
jsr166 |
1.98 |
private void readObject(java.io.ObjectInputStream s) |
1126 |
dl |
1.55 |
throws java.io.IOException, ClassNotFoundException { |
1127 |
|
|
s.defaultReadObject(); |
1128 |
|
|
if (waitingProducers instanceof FifoWaitQueue) |
1129 |
jsr166 |
1.82 |
transferer = new TransferQueue<E>(); |
1130 |
dl |
1.55 |
else |
1131 |
jsr166 |
1.82 |
transferer = new TransferStack<E>(); |
1132 |
dl |
1.55 |
} |
1133 |
|
|
|
1134 |
jsr166 |
1.114 |
static { |
1135 |
|
|
// Reduce the risk of rare disastrous classloading in first call to |
1136 |
|
|
// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773 |
1137 |
|
|
Class<?> ensureLoaded = LockSupport.class; |
1138 |
|
|
} |
1139 |
tim |
1.1 |
} |