1169 lines
45 KiB
Java
1169 lines
45 KiB
Java
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/*
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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/*
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* This file is available under and governed by the GNU General Public
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* License version 2 only, as published by the Free Software Foundation.
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* However, the following notice accompanied the original version of this
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* file:
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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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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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* 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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* 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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* 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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*/
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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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* 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 = 1023L;
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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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/* 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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/** 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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static final class SNode implements ForkJoinPool.ManagedBlocker {
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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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SNode(Object item) {
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this.item = item;
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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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* 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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SNode m; Thread w;
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if ((m = match) == null) {
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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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}
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else
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m = match;
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}
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return m == s;
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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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boolean tryCancel() {
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return SMATCH.compareAndSet(this, null, this);
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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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public final boolean isReleasable() {
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return match != null || Thread.currentThread().isInterrupted();
|
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}
|
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public final boolean block() {
|
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|
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while (!isReleasable()) LockSupport.park();
|
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|
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return true;
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}
|
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|
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|
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void forgetWaiter() {
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SWAITER.setOpaque(this, null);
|
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}
|
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// VarHandle mechanics
|
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private static final VarHandle SMATCH;
|
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|
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private static final VarHandle SNEXT;
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private static final VarHandle SWAITER;
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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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SWAITER = l.findVarHandle(SNode.class, "waiter", Thread.class);
|
||
|
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} catch (ReflectiveOperationException e) {
|
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|
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throw new ExceptionInInitializerError(e);
|
||
|
|
}
|
||
|
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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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|
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volatile SNode head;
|
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|
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|
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|
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boolean casHead(SNode h, SNode nh) {
|
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|
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return h == head &&
|
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|
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SHEAD.compareAndSet(this, h, nh);
|
||
|
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}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Creates or resets fields of a node. Called only from transfer
|
||
|
|
* where the node to push on stack is lazily created and
|
||
|
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* reused when possible to help reduce intervals between reads
|
||
|
|
* 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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*/
|
||
|
|
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;
|
||
|
|
s.next = next;
|
||
|
|
return s;
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Puts or takes an item.
|
||
|
|
*/
|
||
|
|
@SuppressWarnings("unchecked")
|
||
|
|
E transfer(E e, boolean timed, long nanos) {
|
||
|
|
/*
|
||
|
|
* Basic algorithm is to loop trying one of three actions:
|
||
|
|
*
|
||
|
|
* 1. If apparently empty or already containing nodes of same
|
||
|
|
* mode, try to push node on stack and wait for a match,
|
||
|
|
* returning it, or null if cancelled.
|
||
|
|
*
|
||
|
|
* 2. If apparently containing node of complementary mode,
|
||
|
|
* try to push a fulfilling node on to stack, match
|
||
|
|
* with corresponding waiting node, pop both from
|
||
|
|
* stack, and return matched item. The matching or
|
||
|
|
* unlinking might not actually be necessary because of
|
||
|
|
* other threads performing action 3:
|
||
|
|
*
|
||
|
|
* 3. If top of stack already holds another fulfilling node,
|
||
|
|
* help it out by doing its match and/or pop
|
||
|
|
* operations, and then continue. The code for helping
|
||
|
|
* is essentially the same as for fulfilling, except
|
||
|
|
* that it doesn't return the item.
|
||
|
|
*/
|
||
|
|
|
||
|
|
SNode s = null; // constructed/reused as needed
|
||
|
|
int mode = (e == null) ? REQUEST : DATA;
|
||
|
|
|
||
|
|
for (;;) {
|
||
|
|
SNode h = head;
|
||
|
|
if (h == null || h.mode == mode) { // empty or same-mode
|
||
|
|
if (timed && nanos <= 0L) { // can't wait
|
||
|
|
if (h != null && h.isCancelled())
|
||
|
|
casHead(h, h.next); // pop cancelled node
|
||
|
|
else
|
||
|
|
return null;
|
||
|
|
} else if (casHead(h, s = snode(s, e, h, mode))) {
|
||
|
|
long deadline = timed ? System.nanoTime() + nanos : 0L;
|
||
|
|
Thread w = Thread.currentThread();
|
||
|
|
int stat = -1; // -1: may yield, +1: park, else 0
|
||
|
|
SNode m; // await fulfill or cancel
|
||
|
|
while ((m = s.match) == null) {
|
||
|
|
if ((timed &&
|
||
|
|
(nanos = deadline - System.nanoTime()) <= 0) ||
|
||
|
|
w.isInterrupted()) {
|
||
|
|
if (s.tryCancel()) {
|
||
|
|
clean(s); // wait cancelled
|
||
|
|
return null;
|
||
|
|
}
|
||
|
|
} else if ((m = s.match) != null) {
|
||
|
|
break; // recheck
|
||
|
|
} else if (stat <= 0) {
|
||
|
|
if (stat < 0 && h == null && head == s) {
|
||
|
|
stat = 0; // yield once if was empty
|
||
|
|
Thread.yield();
|
||
|
|
} else {
|
||
|
|
stat = 1;
|
||
|
|
s.waiter = w; // enable signal
|
||
|
|
}
|
||
|
|
} else if (!timed) {
|
||
|
|
LockSupport.setCurrentBlocker(this);
|
||
|
|
try {
|
||
|
|
ForkJoinPool.managedBlock(s);
|
||
|
|
} catch (InterruptedException cannotHappen) { }
|
||
|
|
LockSupport.setCurrentBlocker(null);
|
||
|
|
} else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD)
|
||
|
|
LockSupport.parkNanos(this, nanos);
|
||
|
|
}
|
||
|
|
if (stat == 1)
|
||
|
|
s.forgetWaiter();
|
||
|
|
Object result = (mode == REQUEST) ? m.item : s.item;
|
||
|
|
if (h != null && h.next == s)
|
||
|
|
casHead(h, s.next); // help fulfiller
|
||
|
|
return (E) result;
|
||
|
|
}
|
||
|
|
} else if (!isFulfilling(h.mode)) { // try to fulfill
|
||
|
|
if (h.isCancelled()) // already cancelled
|
||
|
|
casHead(h, h.next); // pop and retry
|
||
|
|
else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {
|
||
|
|
for (;;) { // loop until matched or waiters disappear
|
||
|
|
SNode m = s.next; // m is s's match
|
||
|
|
if (m == null) { // all waiters are gone
|
||
|
|
casHead(s, null); // pop fulfill node
|
||
|
|
s = null; // use new node next time
|
||
|
|
break; // restart main loop
|
||
|
|
}
|
||
|
|
SNode mn = m.next;
|
||
|
|
if (m.tryMatch(s)) {
|
||
|
|
casHead(s, mn); // pop both s and m
|
||
|
|
return (E) ((mode == REQUEST) ? m.item : s.item);
|
||
|
|
} else // lost match
|
||
|
|
s.casNext(m, mn); // help unlink
|
||
|
|
}
|
||
|
|
}
|
||
|
|
} else { // help a fulfiller
|
||
|
|
SNode m = h.next; // m is h's match
|
||
|
|
if (m == null) // waiter is gone
|
||
|
|
casHead(h, null); // pop fulfilling node
|
||
|
|
else {
|
||
|
|
SNode mn = m.next;
|
||
|
|
if (m.tryMatch(h)) // help match
|
||
|
|
casHead(h, mn); // pop both h and m
|
||
|
|
else // lost match
|
||
|
|
h.casNext(m, mn); // help unlink
|
||
|
|
}
|
||
|
|
}
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Unlinks s from the stack.
|
||
|
|
*/
|
||
|
|
void clean(SNode s) {
|
||
|
|
s.item = null; // forget item
|
||
|
|
s.forgetWaiter();
|
||
|
|
|
||
|
|
/*
|
||
|
|
* At worst we may need to traverse entire stack to unlink
|
||
|
|
* s. If there are multiple concurrent calls to clean, we
|
||
|
|
* might not see s if another thread has already removed
|
||
|
|
* it. But we can stop when we see any node known to
|
||
|
|
* follow s. We use s.next unless it too is cancelled, in
|
||
|
|
* which case we try the node one past. We don't check any
|
||
|
|
* further because we don't want to doubly traverse just to
|
||
|
|
* find sentinel.
|
||
|
|
*/
|
||
|
|
|
||
|
|
SNode past = s.next;
|
||
|
|
if (past != null && past.isCancelled())
|
||
|
|
past = past.next;
|
||
|
|
|
||
|
|
// Absorb cancelled nodes at head
|
||
|
|
SNode p;
|
||
|
|
while ((p = head) != null && p != past && p.isCancelled())
|
||
|
|
casHead(p, p.next);
|
||
|
|
|
||
|
|
// Unsplice embedded nodes
|
||
|
|
while (p != null && p != past) {
|
||
|
|
SNode n = p.next;
|
||
|
|
if (n != null && n.isCancelled())
|
||
|
|
p.casNext(n, n.next);
|
||
|
|
else
|
||
|
|
p = n;
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
// VarHandle mechanics
|
||
|
|
private static final VarHandle SHEAD;
|
||
|
|
static {
|
||
|
|
try {
|
||
|
|
MethodHandles.Lookup l = MethodHandles.lookup();
|
||
|
|
SHEAD = l.findVarHandle(TransferStack.class, "head", SNode.class);
|
||
|
|
} catch (ReflectiveOperationException e) {
|
||
|
|
throw new ExceptionInInitializerError(e);
|
||
|
|
}
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
/** Dual Queue */
|
||
|
|
static final class TransferQueue<E> extends Transferer<E> {
|
||
|
|
/*
|
||
|
|
* This extends Scherer-Scott dual queue algorithm, differing,
|
||
|
|
* among other ways, by using modes within nodes rather than
|
||
|
|
* marked pointers. The algorithm is a little simpler than
|
||
|
|
* that for stacks because fulfillers do not need explicit
|
||
|
|
* nodes, and matching is done by CAS'ing QNode.item field
|
||
|
|
* from non-null to null (for put) or vice versa (for take).
|
||
|
|
*/
|
||
|
|
|
||
|
|
/** Node class for TransferQueue. */
|
||
|
|
static final class QNode implements ForkJoinPool.ManagedBlocker {
|
||
|
|
volatile QNode next; // next node in queue
|
||
|
|
volatile Object item; // CAS'ed to or from null
|
||
|
|
volatile Thread waiter; // to control park/unpark
|
||
|
|
final boolean isData;
|
||
|
|
|
||
|
|
QNode(Object item, boolean isData) {
|
||
|
|
this.item = item;
|
||
|
|
this.isData = isData;
|
||
|
|
}
|
||
|
|
|
||
|
|
boolean casNext(QNode cmp, QNode val) {
|
||
|
|
return next == cmp &&
|
||
|
|
QNEXT.compareAndSet(this, cmp, val);
|
||
|
|
}
|
||
|
|
|
||
|
|
boolean casItem(Object cmp, Object val) {
|
||
|
|
return item == cmp &&
|
||
|
|
QITEM.compareAndSet(this, cmp, val);
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Tries to cancel by CAS'ing ref to this as item.
|
||
|
|
*/
|
||
|
|
boolean tryCancel(Object cmp) {
|
||
|
|
return QITEM.compareAndSet(this, cmp, this);
|
||
|
|
}
|
||
|
|
|
||
|
|
boolean isCancelled() {
|
||
|
|
return item == this;
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Returns true if this node is known to be off the queue
|
||
|
|
* because its next pointer has been forgotten due to
|
||
|
|
* an advanceHead operation.
|
||
|
|
*/
|
||
|
|
boolean isOffList() {
|
||
|
|
return next == this;
|
||
|
|
}
|
||
|
|
|
||
|
|
void forgetWaiter() {
|
||
|
|
QWAITER.setOpaque(this, null);
|
||
|
|
}
|
||
|
|
|
||
|
|
boolean isFulfilled() {
|
||
|
|
Object x;
|
||
|
|
return isData == ((x = item) == null) || x == this;
|
||
|
|
}
|
||
|
|
|
||
|
|
public final boolean isReleasable() {
|
||
|
|
Object x;
|
||
|
|
return isData == ((x = item) == null) || x == this ||
|
||
|
|
Thread.currentThread().isInterrupted();
|
||
|
|
}
|
||
|
|
|
||
|
|
public final boolean block() {
|
||
|
|
while (!isReleasable()) LockSupport.park();
|
||
|
|
return true;
|
||
|
|
}
|
||
|
|
|
||
|
|
// VarHandle mechanics
|
||
|
|
private static final VarHandle QITEM;
|
||
|
|
private static final VarHandle QNEXT;
|
||
|
|
private static final VarHandle QWAITER;
|
||
|
|
static {
|
||
|
|
try {
|
||
|
|
MethodHandles.Lookup l = MethodHandles.lookup();
|
||
|
|
QITEM = l.findVarHandle(QNode.class, "item", Object.class);
|
||
|
|
QNEXT = l.findVarHandle(QNode.class, "next", QNode.class);
|
||
|
|
QWAITER = l.findVarHandle(QNode.class, "waiter", Thread.class);
|
||
|
|
} catch (ReflectiveOperationException e) {
|
||
|
|
throw new ExceptionInInitializerError(e);
|
||
|
|
}
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
/** Head of queue */
|
||
|
|
transient volatile QNode head;
|
||
|
|
/** Tail of queue */
|
||
|
|
transient volatile QNode tail;
|
||
|
|
/**
|
||
|
|
* Reference to a cancelled node that might not yet have been
|
||
|
|
* unlinked from queue because it was the last inserted node
|
||
|
|
* when it was cancelled.
|
||
|
|
*/
|
||
|
|
transient volatile QNode cleanMe;
|
||
|
|
|
||
|
|
TransferQueue() {
|
||
|
|
QNode h = new QNode(null, false); // initialize to dummy node.
|
||
|
|
head = h;
|
||
|
|
tail = h;
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Tries to cas nh as new head; if successful, unlink
|
||
|
|
* old head's next node to avoid garbage retention.
|
||
|
|
*/
|
||
|
|
void advanceHead(QNode h, QNode nh) {
|
||
|
|
if (h == head &&
|
||
|
|
QHEAD.compareAndSet(this, h, nh))
|
||
|
|
h.next = h; // forget old next
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Tries to cas nt as new tail.
|
||
|
|
*/
|
||
|
|
void advanceTail(QNode t, QNode nt) {
|
||
|
|
if (tail == t)
|
||
|
|
QTAIL.compareAndSet(this, t, nt);
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Tries to CAS cleanMe slot.
|
||
|
|
*/
|
||
|
|
boolean casCleanMe(QNode cmp, QNode val) {
|
||
|
|
return cleanMe == cmp &&
|
||
|
|
QCLEANME.compareAndSet(this, cmp, val);
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Puts or takes an item.
|
||
|
|
*/
|
||
|
|
@SuppressWarnings("unchecked")
|
||
|
|
E transfer(E e, boolean timed, long nanos) {
|
||
|
|
/* Basic algorithm is to loop trying to take either of
|
||
|
|
* two actions:
|
||
|
|
*
|
||
|
|
* 1. If queue apparently empty or holding same-mode nodes,
|
||
|
|
* try to add node to queue of waiters, wait to be
|
||
|
|
* fulfilled (or cancelled) and return matching item.
|
||
|
|
*
|
||
|
|
* 2. If queue apparently contains waiting items, and this
|
||
|
|
* call is of complementary mode, try to fulfill by CAS'ing
|
||
|
|
* item field of waiting node and dequeuing it, and then
|
||
|
|
* returning matching item.
|
||
|
|
*
|
||
|
|
* In each case, along the way, check for and try to help
|
||
|
|
* advance head and tail on behalf of other stalled/slow
|
||
|
|
* threads.
|
||
|
|
*
|
||
|
|
* The loop starts off with a null check guarding against
|
||
|
|
* seeing uninitialized head or tail values. This never
|
||
|
|
* happens in current SynchronousQueue, but could if
|
||
|
|
* callers held non-volatile/final ref to the
|
||
|
|
* transferer. The check is here anyway because it places
|
||
|
|
* null checks at top of loop, which is usually faster
|
||
|
|
* than having them implicitly interspersed.
|
||
|
|
*/
|
||
|
|
|
||
|
|
QNode s = null; // constructed/reused as needed
|
||
|
|
boolean isData = (e != null);
|
||
|
|
for (;;) {
|
||
|
|
QNode t = tail, h = head, m, tn; // m is node to fulfill
|
||
|
|
if (t == null || h == null)
|
||
|
|
; // inconsistent
|
||
|
|
else if (h == t || t.isData == isData) { // empty or same-mode
|
||
|
|
if (t != tail) // inconsistent
|
||
|
|
;
|
||
|
|
else if ((tn = t.next) != null) // lagging tail
|
||
|
|
advanceTail(t, tn);
|
||
|
|
else if (timed && nanos <= 0L) // can't wait
|
||
|
|
return null;
|
||
|
|
else if (t.casNext(null, (s != null) ? s :
|
||
|
|
(s = new QNode(e, isData)))) {
|
||
|
|
advanceTail(t, s);
|
||
|
|
long deadline = timed ? System.nanoTime() + nanos : 0L;
|
||
|
|
Thread w = Thread.currentThread();
|
||
|
|
int stat = -1; // same idea as TransferStack
|
||
|
|
Object item;
|
||
|
|
while ((item = s.item) == e) {
|
||
|
|
if ((timed &&
|
||
|
|
(nanos = deadline - System.nanoTime()) <= 0) ||
|
||
|
|
w.isInterrupted()) {
|
||
|
|
if (s.tryCancel(e)) {
|
||
|
|
clean(t, s);
|
||
|
|
return null;
|
||
|
|
}
|
||
|
|
} else if ((item = s.item) != e) {
|
||
|
|
break; // recheck
|
||
|
|
} else if (stat <= 0) {
|
||
|
|
if (t.next == s) {
|
||
|
|
if (stat < 0 && t.isFulfilled()) {
|
||
|
|
stat = 0; // yield once if first
|
||
|
|
Thread.yield();
|
||
|
|
}
|
||
|
|
else {
|
||
|
|
stat = 1;
|
||
|
|
s.waiter = w;
|
||
|
|
}
|
||
|
|
}
|
||
|
|
} else if (!timed) {
|
||
|
|
LockSupport.setCurrentBlocker(this);
|
||
|
|
try {
|
||
|
|
ForkJoinPool.managedBlock(s);
|
||
|
|
} catch (InterruptedException cannotHappen) { }
|
||
|
|
LockSupport.setCurrentBlocker(null);
|
||
|
|
}
|
||
|
|
else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD)
|
||
|
|
LockSupport.parkNanos(this, nanos);
|
||
|
|
}
|
||
|
|
if (stat == 1)
|
||
|
|
s.forgetWaiter();
|
||
|
|
if (!s.isOffList()) { // not already unlinked
|
||
|
|
advanceHead(t, s); // unlink if head
|
||
|
|
if (item != null) // and forget fields
|
||
|
|
s.item = s;
|
||
|
|
}
|
||
|
|
return (item != null) ? (E)item : e;
|
||
|
|
}
|
||
|
|
|
||
|
|
} else if ((m = h.next) != null && t == tail && h == head) {
|
||
|
|
Thread waiter;
|
||
|
|
Object x = m.item;
|
||
|
|
boolean fulfilled = ((isData == (x == null)) &&
|
||
|
|
x != m && m.casItem(x, e));
|
||
|
|
advanceHead(h, m); // (help) dequeue
|
||
|
|
if (fulfilled) {
|
||
|
|
if ((waiter = m.waiter) != null)
|
||
|
|
LockSupport.unpark(waiter);
|
||
|
|
return (x != null) ? (E)x : e;
|
||
|
|
}
|
||
|
|
}
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Gets rid of cancelled node s with original predecessor pred.
|
||
|
|
*/
|
||
|
|
void clean(QNode pred, QNode s) {
|
||
|
|
s.forgetWaiter();
|
||
|
|
/*
|
||
|
|
* At any given time, exactly one node on list cannot be
|
||
|
|
* deleted -- the last inserted node. To accommodate this,
|
||
|
|
* if we cannot delete s, we save its predecessor as
|
||
|
|
* "cleanMe", deleting the previously saved version
|
||
|
|
* first. At least one of node s or the node previously
|
||
|
|
* saved can always be deleted, so this always terminates.
|
||
|
|
*/
|
||
|
|
while (pred.next == s) { // Return early if already unlinked
|
||
|
|
QNode h = head;
|
||
|
|
QNode hn = h.next; // Absorb cancelled first node as head
|
||
|
|
if (hn != null && hn.isCancelled()) {
|
||
|
|
advanceHead(h, hn);
|
||
|
|
continue;
|
||
|
|
}
|
||
|
|
QNode t = tail; // Ensure consistent read for tail
|
||
|
|
if (t == h)
|
||
|
|
return;
|
||
|
|
QNode tn = t.next;
|
||
|
|
if (t != tail)
|
||
|
|
continue;
|
||
|
|
if (tn != null) {
|
||
|
|
advanceTail(t, tn);
|
||
|
|
continue;
|
||
|
|
}
|
||
|
|
if (s != t) { // If not tail, try to unsplice
|
||
|
|
QNode sn = s.next;
|
||
|
|
if (sn == s || pred.casNext(s, sn))
|
||
|
|
return;
|
||
|
|
}
|
||
|
|
QNode dp = cleanMe;
|
||
|
|
if (dp != null) { // Try unlinking previous cancelled node
|
||
|
|
QNode d = dp.next;
|
||
|
|
QNode dn;
|
||
|
|
if (d == null || // d is gone or
|
||
|
|
d == dp || // d is off list or
|
||
|
|
!d.isCancelled() || // d not cancelled or
|
||
|
|
(d != t && // d not tail and
|
||
|
|
(dn = d.next) != null && // has successor
|
||
|
|
dn != d && // that is on list
|
||
|
|
dp.casNext(d, dn))) // d unspliced
|
||
|
|
casCleanMe(dp, null);
|
||
|
|
if (dp == pred)
|
||
|
|
return; // s is already saved node
|
||
|
|
} else if (casCleanMe(null, pred))
|
||
|
|
return; // Postpone cleaning s
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
// VarHandle mechanics
|
||
|
|
private static final VarHandle QHEAD;
|
||
|
|
private static final VarHandle QTAIL;
|
||
|
|
private static final VarHandle QCLEANME;
|
||
|
|
static {
|
||
|
|
try {
|
||
|
|
MethodHandles.Lookup l = MethodHandles.lookup();
|
||
|
|
QHEAD = l.findVarHandle(TransferQueue.class, "head",
|
||
|
|
QNode.class);
|
||
|
|
QTAIL = l.findVarHandle(TransferQueue.class, "tail",
|
||
|
|
QNode.class);
|
||
|
|
QCLEANME = l.findVarHandle(TransferQueue.class, "cleanMe",
|
||
|
|
QNode.class);
|
||
|
|
} catch (ReflectiveOperationException e) {
|
||
|
|
throw new ExceptionInInitializerError(e);
|
||
|
|
}
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* The transferer. Set only in constructor, but cannot be declared
|
||
|
|
* as final without further complicating serialization. Since
|
||
|
|
* this is accessed only at most once per public method, there
|
||
|
|
* isn't a noticeable performance penalty for using volatile
|
||
|
|
* instead of final here.
|
||
|
|
*/
|
||
|
|
private transient volatile Transferer<E> transferer;
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Creates a {@code SynchronousQueue} with nonfair access policy.
|
||
|
|
*/
|
||
|
|
public SynchronousQueue() {
|
||
|
|
this(false);
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Creates a {@code SynchronousQueue} with the specified fairness policy.
|
||
|
|
*
|
||
|
|
* @param fair if true, waiting threads contend in FIFO order for
|
||
|
|
* access; otherwise the order is unspecified.
|
||
|
|
*/
|
||
|
|
public SynchronousQueue(boolean fair) {
|
||
|
|
transferer = fair ? new TransferQueue<E>() : new TransferStack<E>();
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Adds the specified element to this queue, waiting if necessary for
|
||
|
|
* another thread to receive it.
|
||
|
|
*
|
||
|
|
* @throws InterruptedException {@inheritDoc}
|
||
|
|
* @throws NullPointerException {@inheritDoc}
|
||
|
|
*/
|
||
|
|
public void put(E e) throws InterruptedException {
|
||
|
|
if (e == null) throw new NullPointerException();
|
||
|
|
if (transferer.transfer(e, false, 0) == null) {
|
||
|
|
Thread.interrupted();
|
||
|
|
throw new InterruptedException();
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Inserts the specified element into this queue, waiting if necessary
|
||
|
|
* up to the specified wait time for another thread to receive it.
|
||
|
|
*
|
||
|
|
* @return {@code true} if successful, or {@code false} if the
|
||
|
|
* specified waiting time elapses before a consumer appears
|
||
|
|
* @throws InterruptedException {@inheritDoc}
|
||
|
|
* @throws NullPointerException {@inheritDoc}
|
||
|
|
*/
|
||
|
|
public boolean offer(E e, long timeout, TimeUnit unit)
|
||
|
|
throws InterruptedException {
|
||
|
|
if (e == null) throw new NullPointerException();
|
||
|
|
if (transferer.transfer(e, true, unit.toNanos(timeout)) != null)
|
||
|
|
return true;
|
||
|
|
if (!Thread.interrupted())
|
||
|
|
return false;
|
||
|
|
throw new InterruptedException();
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Inserts the specified element into this queue, if another thread is
|
||
|
|
* waiting to receive it.
|
||
|
|
*
|
||
|
|
* @param e the element to add
|
||
|
|
* @return {@code true} if the element was added to this queue, else
|
||
|
|
* {@code false}
|
||
|
|
* @throws NullPointerException if the specified element is null
|
||
|
|
*/
|
||
|
|
public boolean offer(E e) {
|
||
|
|
if (e == null) throw new NullPointerException();
|
||
|
|
return transferer.transfer(e, true, 0) != null;
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Retrieves and removes the head of this queue, waiting if necessary
|
||
|
|
* for another thread to insert it.
|
||
|
|
*
|
||
|
|
* @return the head of this queue
|
||
|
|
* @throws InterruptedException {@inheritDoc}
|
||
|
|
*/
|
||
|
|
public E take() throws InterruptedException {
|
||
|
|
E e = transferer.transfer(null, false, 0);
|
||
|
|
if (e != null)
|
||
|
|
return e;
|
||
|
|
Thread.interrupted();
|
||
|
|
throw new InterruptedException();
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Retrieves and removes the head of this queue, waiting
|
||
|
|
* if necessary up to the specified wait time, for another thread
|
||
|
|
* to insert it.
|
||
|
|
*
|
||
|
|
* @return the head of this queue, or {@code null} if the
|
||
|
|
* specified waiting time elapses before an element is present
|
||
|
|
* @throws InterruptedException {@inheritDoc}
|
||
|
|
*/
|
||
|
|
public E poll(long timeout, TimeUnit unit) throws InterruptedException {
|
||
|
|
E e = transferer.transfer(null, true, unit.toNanos(timeout));
|
||
|
|
if (e != null || !Thread.interrupted())
|
||
|
|
return e;
|
||
|
|
throw new InterruptedException();
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Retrieves and removes the head of this queue, if another thread
|
||
|
|
* is currently making an element available.
|
||
|
|
*
|
||
|
|
* @return the head of this queue, or {@code null} if no
|
||
|
|
* element is available
|
||
|
|
*/
|
||
|
|
public E poll() {
|
||
|
|
return transferer.transfer(null, true, 0);
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Always returns {@code true}.
|
||
|
|
* A {@code SynchronousQueue} has no internal capacity.
|
||
|
|
*
|
||
|
|
* @return {@code true}
|
||
|
|
*/
|
||
|
|
public boolean isEmpty() {
|
||
|
|
return true;
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Always returns zero.
|
||
|
|
* A {@code SynchronousQueue} has no internal capacity.
|
||
|
|
*
|
||
|
|
* @return zero
|
||
|
|
*/
|
||
|
|
public int size() {
|
||
|
|
return 0;
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Always returns zero.
|
||
|
|
* A {@code SynchronousQueue} has no internal capacity.
|
||
|
|
*
|
||
|
|
* @return zero
|
||
|
|
*/
|
||
|
|
public int remainingCapacity() {
|
||
|
|
return 0;
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Does nothing.
|
||
|
|
* A {@code SynchronousQueue} has no internal capacity.
|
||
|
|
*/
|
||
|
|
public void clear() {
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Always returns {@code false}.
|
||
|
|
* A {@code SynchronousQueue} has no internal capacity.
|
||
|
|
*
|
||
|
|
* @param o the element
|
||
|
|
* @return {@code false}
|
||
|
|
*/
|
||
|
|
public boolean contains(Object o) {
|
||
|
|
return false;
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Always returns {@code false}.
|
||
|
|
* A {@code SynchronousQueue} has no internal capacity.
|
||
|
|
*
|
||
|
|
* @param o the element to remove
|
||
|
|
* @return {@code false}
|
||
|
|
*/
|
||
|
|
public boolean remove(Object o) {
|
||
|
|
return false;
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Returns {@code false} unless the given collection is empty.
|
||
|
|
* A {@code SynchronousQueue} has no internal capacity.
|
||
|
|
*
|
||
|
|
* @param c the collection
|
||
|
|
* @return {@code false} unless given collection is empty
|
||
|
|
*/
|
||
|
|
public boolean containsAll(Collection<?> c) {
|
||
|
|
return c.isEmpty();
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Always returns {@code false}.
|
||
|
|
* A {@code SynchronousQueue} has no internal capacity.
|
||
|
|
*
|
||
|
|
* @param c the collection
|
||
|
|
* @return {@code false}
|
||
|
|
*/
|
||
|
|
public boolean removeAll(Collection<?> c) {
|
||
|
|
return false;
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Always returns {@code false}.
|
||
|
|
* A {@code SynchronousQueue} has no internal capacity.
|
||
|
|
*
|
||
|
|
* @param c the collection
|
||
|
|
* @return {@code false}
|
||
|
|
*/
|
||
|
|
public boolean retainAll(Collection<?> c) {
|
||
|
|
return false;
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Always returns {@code null}.
|
||
|
|
* A {@code SynchronousQueue} does not return elements
|
||
|
|
* unless actively waited on.
|
||
|
|
*
|
||
|
|
* @return {@code null}
|
||
|
|
*/
|
||
|
|
public E peek() {
|
||
|
|
return null;
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Returns an empty iterator in which {@code hasNext} always returns
|
||
|
|
* {@code false}.
|
||
|
|
*
|
||
|
|
* @return an empty iterator
|
||
|
|
*/
|
||
|
|
public Iterator<E> iterator() {
|
||
|
|
return Collections.emptyIterator();
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Returns an empty spliterator in which calls to
|
||
|
|
* {@link Spliterator#trySplit() trySplit} always return {@code null}.
|
||
|
|
*
|
||
|
|
* @return an empty spliterator
|
||
|
|
* @since 1.8
|
||
|
|
*/
|
||
|
|
public Spliterator<E> spliterator() {
|
||
|
|
return Spliterators.emptySpliterator();
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Returns a zero-length array.
|
||
|
|
* @return a zero-length array
|
||
|
|
*/
|
||
|
|
public Object[] toArray() {
|
||
|
|
return new Object[0];
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Sets the zeroth element of the specified array to {@code null}
|
||
|
|
* (if the array has non-zero length) and returns it.
|
||
|
|
*
|
||
|
|
* @param a the array
|
||
|
|
* @return the specified array
|
||
|
|
* @throws NullPointerException if the specified array is null
|
||
|
|
*/
|
||
|
|
public <T> T[] toArray(T[] a) {
|
||
|
|
if (a.length > 0)
|
||
|
|
a[0] = null;
|
||
|
|
return a;
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Always returns {@code "[]"}.
|
||
|
|
* @return {@code "[]"}
|
||
|
|
*/
|
||
|
|
public String toString() {
|
||
|
|
return "[]";
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* @throws UnsupportedOperationException {@inheritDoc}
|
||
|
|
* @throws ClassCastException {@inheritDoc}
|
||
|
|
* @throws NullPointerException {@inheritDoc}
|
||
|
|
* @throws IllegalArgumentException {@inheritDoc}
|
||
|
|
*/
|
||
|
|
public int drainTo(Collection<? super E> c) {
|
||
|
|
Objects.requireNonNull(c);
|
||
|
|
if (c == this)
|
||
|
|
throw new IllegalArgumentException();
|
||
|
|
int n = 0;
|
||
|
|
for (E e; (e = poll()) != null; n++)
|
||
|
|
c.add(e);
|
||
|
|
return n;
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* @throws UnsupportedOperationException {@inheritDoc}
|
||
|
|
* @throws ClassCastException {@inheritDoc}
|
||
|
|
* @throws NullPointerException {@inheritDoc}
|
||
|
|
* @throws IllegalArgumentException {@inheritDoc}
|
||
|
|
*/
|
||
|
|
public int drainTo(Collection<? super E> c, int maxElements) {
|
||
|
|
Objects.requireNonNull(c);
|
||
|
|
if (c == this)
|
||
|
|
throw new IllegalArgumentException();
|
||
|
|
int n = 0;
|
||
|
|
for (E e; n < maxElements && (e = poll()) != null; n++)
|
||
|
|
c.add(e);
|
||
|
|
return n;
|
||
|
|
}
|
||
|
|
|
||
|
|
/*
|
||
|
|
* To cope with serialization strategy in the 1.5 version of
|
||
|
|
* SynchronousQueue, we declare some unused classes and fields
|
||
|
|
* that exist solely to enable serializability across versions.
|
||
|
|
* These fields are never used, so are initialized only if this
|
||
|
|
* object is ever serialized or deserialized.
|
||
|
|
*/
|
||
|
|
|
||
|
|
@SuppressWarnings("serial")
|
||
|
|
static class WaitQueue implements java.io.Serializable { }
|
||
|
|
static class LifoWaitQueue extends WaitQueue {
|
||
|
|
private static final long serialVersionUID = -3633113410248163686L;
|
||
|
|
}
|
||
|
|
static class FifoWaitQueue extends WaitQueue {
|
||
|
|
private static final long serialVersionUID = -3623113410248163686L;
|
||
|
|
}
|
||
|
|
private ReentrantLock qlock;
|
||
|
|
private WaitQueue waitingProducers;
|
||
|
|
private WaitQueue waitingConsumers;
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Saves this queue to a stream (that is, serializes it).
|
||
|
|
* @param s the stream
|
||
|
|
* @throws java.io.IOException if an I/O error occurs
|
||
|
|
*/
|
||
|
|
private void writeObject(java.io.ObjectOutputStream s)
|
||
|
|
throws java.io.IOException {
|
||
|
|
boolean fair = transferer instanceof TransferQueue;
|
||
|
|
if (fair) {
|
||
|
|
qlock = new ReentrantLock(true);
|
||
|
|
waitingProducers = new FifoWaitQueue();
|
||
|
|
waitingConsumers = new FifoWaitQueue();
|
||
|
|
}
|
||
|
|
else {
|
||
|
|
qlock = new ReentrantLock();
|
||
|
|
waitingProducers = new LifoWaitQueue();
|
||
|
|
waitingConsumers = new LifoWaitQueue();
|
||
|
|
}
|
||
|
|
s.defaultWriteObject();
|
||
|
|
}
|
||
|
|
|
||
|
|
/**
|
||
|
|
* Reconstitutes this queue from a stream (that is, deserializes it).
|
||
|
|
* @param s the stream
|
||
|
|
* @throws ClassNotFoundException if the class of a serialized object
|
||
|
|
* could not be found
|
||
|
|
* @throws java.io.IOException if an I/O error occurs
|
||
|
|
*/
|
||
|
|
private void readObject(java.io.ObjectInputStream s)
|
||
|
|
throws java.io.IOException, ClassNotFoundException {
|
||
|
|
s.defaultReadObject();
|
||
|
|
if (waitingProducers instanceof FifoWaitQueue)
|
||
|
|
transferer = new TransferQueue<E>();
|
||
|
|
else
|
||
|
|
transferer = new TransferStack<E>();
|
||
|
|
}
|
||
|
|
|
||
|
|
static {
|
||
|
|
// Reduce the risk of rare disastrous classloading in first call to
|
||
|
|
// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
|
||
|
|
Class<?> ensureLoaded = LockSupport.class;
|
||
|
|
}
|
||
|
|
}
|