1857 lines
76 KiB
Java
1857 lines
76 KiB
Java
/*
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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 with assistance from members of JCP JSR-166
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* Expert Group and released to the public domain, as explained at
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* http://creativecommons.org/publicdomain/zero/1.0/
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*/
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package java.util.concurrent.locks;
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import java.util.ArrayList;
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import java.util.Collection;
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import java.util.Date;
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import java.util.concurrent.TimeUnit;
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import java.util.concurrent.ForkJoinPool;
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import java.util.concurrent.RejectedExecutionException;
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import jdk.internal.misc.Unsafe;
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/**
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* Provides a framework for implementing blocking locks and related
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* synchronizers (semaphores, events, etc) that rely on
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* first-in-first-out (FIFO) wait queues. This class is designed to
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* be a useful basis for most kinds of synchronizers that rely on a
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* single atomic {@code int} value to represent state. Subclasses
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* must define the protected methods that change this state, and which
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* define what that state means in terms of this object being acquired
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* or released. Given these, the other methods in this class carry
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* out all queuing and blocking mechanics. Subclasses can maintain
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* other state fields, but only the atomically updated {@code int}
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* value manipulated using methods {@link #getState}, {@link
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* #setState} and {@link #compareAndSetState} is tracked with respect
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* to synchronization.
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*
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* <p>Subclasses should be defined as non-public internal helper
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* classes that are used to implement the synchronization properties
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* of their enclosing class. Class
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* {@code AbstractQueuedSynchronizer} does not implement any
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* synchronization interface. Instead it defines methods such as
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* {@link #acquireInterruptibly} that can be invoked as
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* appropriate by concrete locks and related synchronizers to
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* implement their public methods.
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*
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* <p>This class supports either or both a default <em>exclusive</em>
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* mode and a <em>shared</em> mode. When acquired in exclusive mode,
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* attempted acquires by other threads cannot succeed. Shared mode
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* acquires by multiple threads may (but need not) succeed. This class
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* does not "understand" these differences except in the
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* mechanical sense that when a shared mode acquire succeeds, the next
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* waiting thread (if one exists) must also determine whether it can
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* acquire as well. Threads waiting in the different modes share the
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* same FIFO queue. Usually, implementation subclasses support only
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* one of these modes, but both can come into play for example in a
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* {@link ReadWriteLock}. Subclasses that support only exclusive or
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* only shared modes need not define the methods supporting the unused mode.
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*
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* <p>This class defines a nested {@link ConditionObject} class that
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* can be used as a {@link Condition} implementation by subclasses
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* supporting exclusive mode for which method {@link
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* #isHeldExclusively} reports whether synchronization is exclusively
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* held with respect to the current thread, method {@link #release}
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* invoked with the current {@link #getState} value fully releases
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* this object, and {@link #acquire}, given this saved state value,
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* eventually restores this object to its previous acquired state. No
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* {@code AbstractQueuedSynchronizer} method otherwise creates such a
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* condition, so if this constraint cannot be met, do not use it. The
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* behavior of {@link ConditionObject} depends of course on the
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* semantics of its synchronizer implementation.
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*
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* <p>This class provides inspection, instrumentation, and monitoring
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* methods for the internal queue, as well as similar methods for
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* condition objects. These can be exported as desired into classes
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* using an {@code AbstractQueuedSynchronizer} for their
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* synchronization mechanics.
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*
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* <p>Serialization of this class stores only the underlying atomic
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* integer maintaining state, so deserialized objects have empty
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* thread queues. Typical subclasses requiring serializability will
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* define a {@code readObject} method that restores this to a known
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* initial state upon deserialization.
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*
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* <h2>Usage</h2>
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*
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* <p>To use this class as the basis of a synchronizer, redefine the
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* following methods, as applicable, by inspecting and/or modifying
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* the synchronization state using {@link #getState}, {@link
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* #setState} and/or {@link #compareAndSetState}:
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*
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* <ul>
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* <li>{@link #tryAcquire}
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* <li>{@link #tryRelease}
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* <li>{@link #tryAcquireShared}
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* <li>{@link #tryReleaseShared}
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* <li>{@link #isHeldExclusively}
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* </ul>
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*
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* Each of these methods by default throws {@link
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* UnsupportedOperationException}. Implementations of these methods
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* must be internally thread-safe, and should in general be short and
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* not block. Defining these methods is the <em>only</em> supported
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* means of using this class. All other methods are declared
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* {@code final} because they cannot be independently varied.
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*
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* <p>You may also find the inherited methods from {@link
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* AbstractOwnableSynchronizer} useful to keep track of the thread
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* owning an exclusive synchronizer. You are encouraged to use them
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* -- this enables monitoring and diagnostic tools to assist users in
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* determining which threads hold locks.
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*
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* <p>Even though this class is based on an internal FIFO queue, it
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* does not automatically enforce FIFO acquisition policies. The core
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* of exclusive synchronization takes the form:
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*
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* <pre>
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* <em>Acquire:</em>
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* while (!tryAcquire(arg)) {
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* <em>enqueue thread if it is not already queued</em>;
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* <em>possibly block current thread</em>;
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* }
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*
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* <em>Release:</em>
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* if (tryRelease(arg))
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* <em>unblock the first queued thread</em>;
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* </pre>
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*
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* (Shared mode is similar but may involve cascading signals.)
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*
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* <p id="barging">Because checks in acquire are invoked before
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* enqueuing, a newly acquiring thread may <em>barge</em> ahead of
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* others that are blocked and queued. However, you can, if desired,
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* define {@code tryAcquire} and/or {@code tryAcquireShared} to
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* disable barging by internally invoking one or more of the inspection
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* methods, thereby providing a <em>fair</em> FIFO acquisition order.
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* In particular, most fair synchronizers can define {@code tryAcquire}
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* to return {@code false} if {@link #hasQueuedPredecessors} (a method
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* specifically designed to be used by fair synchronizers) returns
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* {@code true}. Other variations are possible.
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*
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* <p>Throughput and scalability are generally highest for the
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* default barging (also known as <em>greedy</em>,
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* <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy.
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* While this is not guaranteed to be fair or starvation-free, earlier
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* queued threads are allowed to recontend before later queued
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* threads, and each recontention has an unbiased chance to succeed
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* against incoming threads. Also, while acquires do not
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* "spin" in the usual sense, they may perform multiple
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* invocations of {@code tryAcquire} interspersed with other
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* computations before blocking. This gives most of the benefits of
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* spins when exclusive synchronization is only briefly held, without
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* most of the liabilities when it isn't. If so desired, you can
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* augment this by preceding calls to acquire methods with
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* "fast-path" checks, possibly prechecking {@link #hasContended}
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* and/or {@link #hasQueuedThreads} to only do so if the synchronizer
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* is likely not to be contended.
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*
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* <p>This class provides an efficient and scalable basis for
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* synchronization in part by specializing its range of use to
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* synchronizers that can rely on {@code int} state, acquire, and
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* release parameters, and an internal FIFO wait queue. When this does
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* not suffice, you can build synchronizers from a lower level using
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* {@link java.util.concurrent.atomic atomic} classes, your own custom
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* {@link java.util.Queue} classes, and {@link LockSupport} blocking
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* support.
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*
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* <h2>Usage Examples</h2>
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*
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* <p>Here is a non-reentrant mutual exclusion lock class that uses
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* the value zero to represent the unlocked state, and one to
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* represent the locked state. While a non-reentrant lock
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* does not strictly require recording of the current owner
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* thread, this class does so anyway to make usage easier to monitor.
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* It also supports conditions and exposes some instrumentation methods:
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*
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* <pre> {@code
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* class Mutex implements Lock, java.io.Serializable {
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*
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* // Our internal helper class
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* private static class Sync extends AbstractQueuedSynchronizer {
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* // Acquires the lock if state is zero
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* public boolean tryAcquire(int acquires) {
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* assert acquires == 1; // Otherwise unused
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* if (compareAndSetState(0, 1)) {
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* setExclusiveOwnerThread(Thread.currentThread());
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* return true;
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* }
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* return false;
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* }
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*
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* // Releases the lock by setting state to zero
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* protected boolean tryRelease(int releases) {
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* assert releases == 1; // Otherwise unused
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* if (!isHeldExclusively())
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* throw new IllegalMonitorStateException();
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* setExclusiveOwnerThread(null);
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* setState(0);
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* return true;
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* }
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*
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* // Reports whether in locked state
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* public boolean isLocked() {
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* return getState() != 0;
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* }
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*
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* public boolean isHeldExclusively() {
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* // a data race, but safe due to out-of-thin-air guarantees
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* return getExclusiveOwnerThread() == Thread.currentThread();
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* }
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*
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* // Provides a Condition
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* public Condition newCondition() {
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* return new ConditionObject();
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* }
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*
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* // Deserializes properly
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* private void readObject(ObjectInputStream s)
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* throws IOException, ClassNotFoundException {
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* s.defaultReadObject();
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* setState(0); // reset to unlocked state
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* }
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* }
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*
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* // The sync object does all the hard work. We just forward to it.
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* private final Sync sync = new Sync();
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*
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* public void lock() { sync.acquire(1); }
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* public boolean tryLock() { return sync.tryAcquire(1); }
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* public void unlock() { sync.release(1); }
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* public Condition newCondition() { return sync.newCondition(); }
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* public boolean isLocked() { return sync.isLocked(); }
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* public boolean isHeldByCurrentThread() {
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* return sync.isHeldExclusively();
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* }
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* public boolean hasQueuedThreads() {
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* return sync.hasQueuedThreads();
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* }
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* public void lockInterruptibly() throws InterruptedException {
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* sync.acquireInterruptibly(1);
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* }
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* public boolean tryLock(long timeout, TimeUnit unit)
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* throws InterruptedException {
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* return sync.tryAcquireNanos(1, unit.toNanos(timeout));
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* }
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* }}</pre>
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*
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* <p>Here is a latch class that is like a
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* {@link java.util.concurrent.CountDownLatch CountDownLatch}
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* except that it only requires a single {@code signal} to
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* fire. Because a latch is non-exclusive, it uses the {@code shared}
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* acquire and release methods.
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*
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* <pre> {@code
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* class BooleanLatch {
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*
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* private static class Sync extends AbstractQueuedSynchronizer {
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* boolean isSignalled() { return getState() != 0; }
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*
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* protected int tryAcquireShared(int ignore) {
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* return isSignalled() ? 1 : -1;
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* }
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*
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* protected boolean tryReleaseShared(int ignore) {
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* setState(1);
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* return true;
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* }
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* }
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*
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* private final Sync sync = new Sync();
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* public boolean isSignalled() { return sync.isSignalled(); }
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* public void signal() { sync.releaseShared(1); }
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* public void await() throws InterruptedException {
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* sync.acquireSharedInterruptibly(1);
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* }
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* }}</pre>
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*
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* @since 1.5
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* @author Doug Lea
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*/
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public abstract class AbstractQueuedSynchronizer
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extends AbstractOwnableSynchronizer
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implements java.io.Serializable {
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private static final long serialVersionUID = 7373984972572414691L;
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/**
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* Creates a new {@code AbstractQueuedSynchronizer} instance
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* with initial synchronization state of zero.
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*/
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protected AbstractQueuedSynchronizer() { }
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/*
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* Overview.
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*
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* The wait queue is a variant of a "CLH" (Craig, Landin, and
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* Hagersten) lock queue. CLH locks are normally used for
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* spinlocks. We instead use them for blocking synchronizers by
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* including explicit ("prev" and "next") links plus a "status"
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* field that allow nodes to signal successors when releasing
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* locks, and handle cancellation due to interrupts and timeouts.
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* The status field includes bits that track whether a thread
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* needs a signal (using LockSupport.unpark). Despite these
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* additions, we maintain most CLH locality properties.
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*
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* To enqueue into a CLH lock, you atomically splice it in as new
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* tail. To dequeue, you set the head field, so the next eligible
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* waiter becomes first.
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*
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* +------+ prev +-------+ +------+
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* | head | <---- | first | <---- | tail |
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* +------+ +-------+ +------+
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*
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* Insertion into a CLH queue requires only a single atomic
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* operation on "tail", so there is a simple point of demarcation
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* from unqueued to queued. The "next" link of the predecessor is
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* set by the enqueuing thread after successful CAS. Even though
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* non-atomic, this suffices to ensure that any blocked thread is
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* signalled by a predecessor when eligible (although in the case
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* of cancellation, possibly with the assistance of a signal in
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* method cleanQueue). Signalling is based in part on a
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* Dekker-like scheme in which the to-be waiting thread indicates
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* WAITING status, then retries acquiring, and then rechecks
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* status before blocking. The signaller atomically clears WAITING
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* status when unparking.
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*
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* Dequeuing on acquire involves detaching (nulling) a node's
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* "prev" node and then updating the "head". Other threads check
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* if a node is or was dequeued by checking "prev" rather than
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* head. We enforce the nulling then setting order by spin-waiting
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* if necessary. Because of this, the lock algorithm is not itself
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* strictly "lock-free" because an acquiring thread may need to
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* wait for a previous acquire to make progress. When used with
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* exclusive locks, such progress is required anyway. However
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* Shared mode may (uncommonly) require a spin-wait before
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* setting head field to ensure proper propagation. (Historical
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* note: This allows some simplifications and efficiencies
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* compared to previous versions of this class.)
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*
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* A node's predecessor can change due to cancellation while it is
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* waiting, until the node is first in queue, at which point it
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* cannot change. The acquire methods cope with this by rechecking
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* "prev" before waiting. The prev and next fields are modified
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* only via CAS by cancelled nodes in method cleanQueue. The
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* unsplice strategy is reminiscent of Michael-Scott queues in
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* that after a successful CAS to prev field, other threads help
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* fix next fields. Because cancellation often occurs in bunches
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* that complicate decisions about necessary signals, each call to
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* cleanQueue traverses the queue until a clean sweep. Nodes that
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* become relinked as first are unconditionally unparked
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* (sometimes unnecessarily, but those cases are not worth
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* avoiding).
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*
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* A thread may try to acquire if it is first (frontmost) in the
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* queue, and sometimes before. Being first does not guarantee
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* success; it only gives the right to contend. We balance
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* throughput, overhead, and fairness by allowing incoming threads
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* to "barge" and acquire the synchronizer while in the process of
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* enqueuing, in which case an awakened first thread may need to
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* rewait. To counteract possible repeated unlucky rewaits, we
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* exponentially increase retries (up to 256) to acquire each time
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* a thread is unparked. Except in this case, AQS locks do not
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* spin; they instead interleave attempts to acquire with
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* bookkeeping steps. (Users who want spinlocks can use
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* tryAcquire.)
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*
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* To improve garbage collectibility, fields of nodes not yet on
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* list are null. (It is not rare to create and then throw away a
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* node without using it.) Fields of nodes coming off the list are
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* nulled out as soon as possible. This accentuates the challenge
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* of externally determining the first waiting thread (as in
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* method getFirstQueuedThread). This sometimes requires the
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* fallback of traversing backwards from the atomically updated
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* "tail" when fields appear null. (This is never needed in the
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* process of signalling though.)
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*
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* CLH queues need a dummy header node to get started. But
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* we don't create them on construction, because it would be wasted
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* effort if there is never contention. Instead, the node
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* is constructed and head and tail pointers are set upon first
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* contention.
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*
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* Shared mode operations differ from Exclusive in that an acquire
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* signals the next waiter to try to acquire if it is also
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* Shared. The tryAcquireShared API allows users to indicate the
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* degree of propagation, but in most applications, it is more
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* efficient to ignore this, allowing the successor to try
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* acquiring in any case.
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*
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* Threads waiting on Conditions use nodes with an additional
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* link to maintain the (FIFO) list of conditions. Conditions only
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* need to link nodes in simple (non-concurrent) linked queues
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* because they are only accessed when exclusively held. Upon
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* await, a node is inserted into a condition queue. Upon signal,
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* the node is enqueued on the main queue. A special status field
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* value is used to track and atomically trigger this.
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*
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* Accesses to fields head, tail, and state use full Volatile
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* mode, along with CAS. Node fields status, prev and next also do
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* so while threads may be signallable, but sometimes use weaker
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* modes otherwise. Accesses to field "waiter" (the thread to be
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* signalled) are always sandwiched between other atomic accesses
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* so are used in Plain mode. We use jdk.internal Unsafe versions
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* of atomic access methods rather than VarHandles to avoid
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* potential VM bootstrap issues.
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*
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* Most of the above is performed by primary internal method
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* acquire, that is invoked in some way by all exported acquire
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* methods. (It is usually easy for compilers to optimize
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* call-site specializations when heavily used.)
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*
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* There are several arbitrary decisions about when and how to
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* check interrupts in both acquire and await before and/or after
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* blocking. The decisions are less arbitrary in implementation
|
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* updates because some users appear to rely on original behaviors
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* in ways that are racy and so (rarely) wrong in general but hard
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* to justify changing.
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*
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* Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
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|
* Scherer and Michael Scott, along with members of JSR-166
|
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* expert group, for helpful ideas, discussions, and critiques
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* on the design of this class.
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*/
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// Node status bits, also used as argument and return values
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static final int WAITING = 1; // must be 1
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static final int CANCELLED = 0x80000000; // must be negative
|
|
static final int COND = 2; // in a condition wait
|
|
|
|
/** CLH Nodes */
|
|
abstract static class Node {
|
|
volatile Node prev; // initially attached via casTail
|
|
volatile Node next; // visibly nonnull when signallable
|
|
Thread waiter; // visibly nonnull when enqueued
|
|
volatile int status; // written by owner, atomic bit ops by others
|
|
|
|
// methods for atomic operations
|
|
final boolean casPrev(Node c, Node v) { // for cleanQueue
|
|
return U.weakCompareAndSetReference(this, PREV, c, v);
|
|
}
|
|
final boolean casNext(Node c, Node v) { // for cleanQueue
|
|
return U.weakCompareAndSetReference(this, NEXT, c, v);
|
|
}
|
|
final int getAndUnsetStatus(int v) { // for signalling
|
|
return U.getAndBitwiseAndInt(this, STATUS, ~v);
|
|
}
|
|
final void setPrevRelaxed(Node p) { // for off-queue assignment
|
|
U.putReference(this, PREV, p);
|
|
}
|
|
final void setStatusRelaxed(int s) { // for off-queue assignment
|
|
U.putInt(this, STATUS, s);
|
|
}
|
|
final void clearStatus() { // for reducing unneeded signals
|
|
U.putIntOpaque(this, STATUS, 0);
|
|
}
|
|
|
|
private static final long STATUS
|
|
= U.objectFieldOffset(Node.class, "status");
|
|
private static final long NEXT
|
|
= U.objectFieldOffset(Node.class, "next");
|
|
private static final long PREV
|
|
= U.objectFieldOffset(Node.class, "prev");
|
|
}
|
|
|
|
// Concrete classes tagged by type
|
|
static final class ExclusiveNode extends Node { }
|
|
static final class SharedNode extends Node { }
|
|
|
|
static final class ConditionNode extends Node
|
|
implements ForkJoinPool.ManagedBlocker {
|
|
ConditionNode nextWaiter; // link to next waiting node
|
|
|
|
/**
|
|
* Allows Conditions to be used in ForkJoinPools without
|
|
* risking fixed pool exhaustion. This is usable only for
|
|
* untimed Condition waits, not timed versions.
|
|
*/
|
|
public final boolean isReleasable() {
|
|
return status <= 1 || Thread.currentThread().isInterrupted();
|
|
}
|
|
|
|
public final boolean block() {
|
|
while (!isReleasable()) LockSupport.park();
|
|
return true;
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Head of the wait queue, lazily initialized.
|
|
*/
|
|
private transient volatile Node head;
|
|
|
|
/**
|
|
* Tail of the wait queue. After initialization, modified only via casTail.
|
|
*/
|
|
private transient volatile Node tail;
|
|
|
|
/**
|
|
* The synchronization state.
|
|
*/
|
|
private volatile int state;
|
|
|
|
/**
|
|
* Returns the current value of synchronization state.
|
|
* This operation has memory semantics of a {@code volatile} read.
|
|
* @return current state value
|
|
*/
|
|
protected final int getState() {
|
|
return state;
|
|
}
|
|
|
|
/**
|
|
* Sets the value of synchronization state.
|
|
* This operation has memory semantics of a {@code volatile} write.
|
|
* @param newState the new state value
|
|
*/
|
|
protected final void setState(int newState) {
|
|
state = newState;
|
|
}
|
|
|
|
/**
|
|
* Atomically sets synchronization state to the given updated
|
|
* value if the current state value equals the expected value.
|
|
* This operation has memory semantics of a {@code volatile} read
|
|
* and write.
|
|
*
|
|
* @param expect the expected value
|
|
* @param update the new value
|
|
* @return {@code true} if successful. False return indicates that the actual
|
|
* value was not equal to the expected value.
|
|
*/
|
|
protected final boolean compareAndSetState(int expect, int update) {
|
|
return U.compareAndSetInt(this, STATE, expect, update);
|
|
}
|
|
|
|
// Queuing utilities
|
|
|
|
private boolean casTail(Node c, Node v) {
|
|
return U.compareAndSetReference(this, TAIL, c, v);
|
|
}
|
|
|
|
/** tries once to CAS a new dummy node for head */
|
|
private void tryInitializeHead() {
|
|
Node h = new ExclusiveNode();
|
|
if (U.compareAndSetReference(this, HEAD, null, h))
|
|
tail = h;
|
|
}
|
|
|
|
/**
|
|
* Enqueues the node unless null. (Currently used only for
|
|
* ConditionNodes; other cases are interleaved with acquires.)
|
|
*/
|
|
final void enqueue(Node node) {
|
|
if (node != null) {
|
|
for (;;) {
|
|
Node t = tail;
|
|
node.setPrevRelaxed(t); // avoid unnecessary fence
|
|
if (t == null) // initialize
|
|
tryInitializeHead();
|
|
else if (casTail(t, node)) {
|
|
t.next = node;
|
|
if (t.status < 0) // wake up to clean link
|
|
LockSupport.unpark(node.waiter);
|
|
break;
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
/** Returns true if node is found in traversal from tail */
|
|
final boolean isEnqueued(Node node) {
|
|
for (Node t = tail; t != null; t = t.prev)
|
|
if (t == node)
|
|
return true;
|
|
return false;
|
|
}
|
|
|
|
/**
|
|
* Wakes up the successor of given node, if one exists, and unsets its
|
|
* WAITING status to avoid park race. This may fail to wake up an
|
|
* eligible thread when one or more have been cancelled, but
|
|
* cancelAcquire ensures liveness.
|
|
*/
|
|
private static void signalNext(Node h) {
|
|
Node s;
|
|
if (h != null && (s = h.next) != null && s.status != 0) {
|
|
s.getAndUnsetStatus(WAITING);
|
|
LockSupport.unpark(s.waiter);
|
|
}
|
|
}
|
|
|
|
/** Wakes up the given node if in shared mode */
|
|
private static void signalNextIfShared(Node h) {
|
|
Node s;
|
|
if (h != null && (s = h.next) != null &&
|
|
(s instanceof SharedNode) && s.status != 0) {
|
|
s.getAndUnsetStatus(WAITING);
|
|
LockSupport.unpark(s.waiter);
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Main acquire method, invoked by all exported acquire methods.
|
|
*
|
|
* @param node null unless a reacquiring Condition
|
|
* @param arg the acquire argument
|
|
* @param shared true if shared mode else exclusive
|
|
* @param interruptible if abort and return negative on interrupt
|
|
* @param timed if true use timed waits
|
|
* @param time if timed, the System.nanoTime value to timeout
|
|
* @return positive if acquired, 0 if timed out, negative if interrupted
|
|
*/
|
|
final int acquire(Node node, int arg, boolean shared,
|
|
boolean interruptible, boolean timed, long time) {
|
|
Thread current = Thread.currentThread();
|
|
byte spins = 0, postSpins = 0; // retries upon unpark of first thread
|
|
boolean interrupted = false, first = false;
|
|
Node pred = null; // predecessor of node when enqueued
|
|
|
|
/*
|
|
* Repeatedly:
|
|
* Check if node now first
|
|
* if so, ensure head stable, else ensure valid predecessor
|
|
* if node is first or not yet enqueued, try acquiring
|
|
* else if node not yet created, create it
|
|
* else if not yet enqueued, try once to enqueue
|
|
* else if woken from park, retry (up to postSpins times)
|
|
* else if WAITING status not set, set and retry
|
|
* else park and clear WAITING status, and check cancellation
|
|
*/
|
|
|
|
for (;;) {
|
|
if (!first && (pred = (node == null) ? null : node.prev) != null &&
|
|
!(first = (head == pred))) {
|
|
if (pred.status < 0) {
|
|
cleanQueue(); // predecessor cancelled
|
|
continue;
|
|
} else if (pred.prev == null) {
|
|
Thread.onSpinWait(); // ensure serialization
|
|
continue;
|
|
}
|
|
}
|
|
if (first || pred == null) {
|
|
boolean acquired;
|
|
try {
|
|
if (shared)
|
|
acquired = (tryAcquireShared(arg) >= 0);
|
|
else
|
|
acquired = tryAcquire(arg);
|
|
} catch (Throwable ex) {
|
|
cancelAcquire(node, interrupted, false);
|
|
throw ex;
|
|
}
|
|
if (acquired) {
|
|
if (first) {
|
|
node.prev = null;
|
|
head = node;
|
|
pred.next = null;
|
|
node.waiter = null;
|
|
if (shared)
|
|
signalNextIfShared(node);
|
|
if (interrupted)
|
|
current.interrupt();
|
|
}
|
|
return 1;
|
|
}
|
|
}
|
|
if (node == null) { // allocate; retry before enqueue
|
|
if (shared)
|
|
node = new SharedNode();
|
|
else
|
|
node = new ExclusiveNode();
|
|
} else if (pred == null) { // try to enqueue
|
|
node.waiter = current;
|
|
Node t = tail;
|
|
node.setPrevRelaxed(t); // avoid unnecessary fence
|
|
if (t == null)
|
|
tryInitializeHead();
|
|
else if (!casTail(t, node))
|
|
node.setPrevRelaxed(null); // back out
|
|
else
|
|
t.next = node;
|
|
} else if (first && spins != 0) {
|
|
--spins; // reduce unfairness on rewaits
|
|
Thread.onSpinWait();
|
|
} else if (node.status == 0) {
|
|
node.status = WAITING; // enable signal and recheck
|
|
} else {
|
|
long nanos;
|
|
spins = postSpins = (byte)((postSpins << 1) | 1);
|
|
if (!timed)
|
|
LockSupport.park(this);
|
|
else if ((nanos = time - System.nanoTime()) > 0L)
|
|
LockSupport.parkNanos(this, nanos);
|
|
else
|
|
break;
|
|
node.clearStatus();
|
|
if ((interrupted |= Thread.interrupted()) && interruptible)
|
|
break;
|
|
}
|
|
}
|
|
return cancelAcquire(node, interrupted, interruptible);
|
|
}
|
|
|
|
/**
|
|
* Possibly repeatedly traverses from tail, unsplicing cancelled
|
|
* nodes until none are found. Unparks nodes that may have been
|
|
* relinked to be next eligible acquirer.
|
|
*/
|
|
private void cleanQueue() {
|
|
for (;;) { // restart point
|
|
for (Node q = tail, s = null, p, n;;) { // (p, q, s) triples
|
|
if (q == null || (p = q.prev) == null)
|
|
return; // end of list
|
|
if (s == null ? tail != q : (s.prev != q || s.status < 0))
|
|
break; // inconsistent
|
|
if (q.status < 0) { // cancelled
|
|
if ((s == null ? casTail(q, p) : s.casPrev(q, p)) &&
|
|
q.prev == p) {
|
|
p.casNext(q, s); // OK if fails
|
|
if (p.prev == null)
|
|
signalNext(p);
|
|
}
|
|
break;
|
|
}
|
|
if ((n = p.next) != q) { // help finish
|
|
if (n != null && q.prev == p) {
|
|
p.casNext(n, q);
|
|
if (p.prev == null)
|
|
signalNext(p);
|
|
}
|
|
break;
|
|
}
|
|
s = q;
|
|
q = q.prev;
|
|
}
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Cancels an ongoing attempt to acquire.
|
|
*
|
|
* @param node the node (may be null if cancelled before enqueuing)
|
|
* @param interrupted true if thread interrupted
|
|
* @param interruptible if should report interruption vs reset
|
|
*/
|
|
private int cancelAcquire(Node node, boolean interrupted,
|
|
boolean interruptible) {
|
|
if (node != null) {
|
|
node.waiter = null;
|
|
node.status = CANCELLED;
|
|
if (node.prev != null)
|
|
cleanQueue();
|
|
}
|
|
if (interrupted) {
|
|
if (interruptible)
|
|
return CANCELLED;
|
|
else
|
|
Thread.currentThread().interrupt();
|
|
}
|
|
return 0;
|
|
}
|
|
|
|
// Main exported methods
|
|
|
|
/**
|
|
* Attempts to acquire in exclusive mode. This method should query
|
|
* if the state of the object permits it to be acquired in the
|
|
* exclusive mode, and if so to acquire it.
|
|
*
|
|
* <p>This method is always invoked by the thread performing
|
|
* acquire. If this method reports failure, the acquire method
|
|
* may queue the thread, if it is not already queued, until it is
|
|
* signalled by a release from some other thread. This can be used
|
|
* to implement method {@link Lock#tryLock()}.
|
|
*
|
|
* <p>The default
|
|
* implementation throws {@link UnsupportedOperationException}.
|
|
*
|
|
* @param arg the acquire argument. This value is always the one
|
|
* passed to an acquire method, or is the value saved on entry
|
|
* to a condition wait. The value is otherwise uninterpreted
|
|
* and can represent anything you like.
|
|
* @return {@code true} if successful. Upon success, this object has
|
|
* been acquired.
|
|
* @throws IllegalMonitorStateException if acquiring would place this
|
|
* synchronizer in an illegal state. This exception must be
|
|
* thrown in a consistent fashion for synchronization to work
|
|
* correctly.
|
|
* @throws UnsupportedOperationException if exclusive mode is not supported
|
|
*/
|
|
protected boolean tryAcquire(int arg) {
|
|
throw new UnsupportedOperationException();
|
|
}
|
|
|
|
/**
|
|
* Attempts to set the state to reflect a release in exclusive
|
|
* mode.
|
|
*
|
|
* <p>This method is always invoked by the thread performing release.
|
|
*
|
|
* <p>The default implementation throws
|
|
* {@link UnsupportedOperationException}.
|
|
*
|
|
* @param arg the release argument. This value is always the one
|
|
* passed to a release method, or the current state value upon
|
|
* entry to a condition wait. The value is otherwise
|
|
* uninterpreted and can represent anything you like.
|
|
* @return {@code true} if this object is now in a fully released
|
|
* state, so that any waiting threads may attempt to acquire;
|
|
* and {@code false} otherwise.
|
|
* @throws IllegalMonitorStateException if releasing would place this
|
|
* synchronizer in an illegal state. This exception must be
|
|
* thrown in a consistent fashion for synchronization to work
|
|
* correctly.
|
|
* @throws UnsupportedOperationException if exclusive mode is not supported
|
|
*/
|
|
protected boolean tryRelease(int arg) {
|
|
throw new UnsupportedOperationException();
|
|
}
|
|
|
|
/**
|
|
* Attempts to acquire in shared mode. This method should query if
|
|
* the state of the object permits it to be acquired in the shared
|
|
* mode, and if so to acquire it.
|
|
*
|
|
* <p>This method is always invoked by the thread performing
|
|
* acquire. If this method reports failure, the acquire method
|
|
* may queue the thread, if it is not already queued, until it is
|
|
* signalled by a release from some other thread.
|
|
*
|
|
* <p>The default implementation throws {@link
|
|
* UnsupportedOperationException}.
|
|
*
|
|
* @param arg the acquire argument. This value is always the one
|
|
* passed to an acquire method, or is the value saved on entry
|
|
* to a condition wait. The value is otherwise uninterpreted
|
|
* and can represent anything you like.
|
|
* @return a negative value on failure; zero if acquisition in shared
|
|
* mode succeeded but no subsequent shared-mode acquire can
|
|
* succeed; and a positive value if acquisition in shared
|
|
* mode succeeded and subsequent shared-mode acquires might
|
|
* also succeed, in which case a subsequent waiting thread
|
|
* must check availability. (Support for three different
|
|
* return values enables this method to be used in contexts
|
|
* where acquires only sometimes act exclusively.) Upon
|
|
* success, this object has been acquired.
|
|
* @throws IllegalMonitorStateException if acquiring would place this
|
|
* synchronizer in an illegal state. This exception must be
|
|
* thrown in a consistent fashion for synchronization to work
|
|
* correctly.
|
|
* @throws UnsupportedOperationException if shared mode is not supported
|
|
*/
|
|
protected int tryAcquireShared(int arg) {
|
|
throw new UnsupportedOperationException();
|
|
}
|
|
|
|
/**
|
|
* Attempts to set the state to reflect a release in shared mode.
|
|
*
|
|
* <p>This method is always invoked by the thread performing release.
|
|
*
|
|
* <p>The default implementation throws
|
|
* {@link UnsupportedOperationException}.
|
|
*
|
|
* @param arg the release argument. This value is always the one
|
|
* passed to a release method, or the current state value upon
|
|
* entry to a condition wait. The value is otherwise
|
|
* uninterpreted and can represent anything you like.
|
|
* @return {@code true} if this release of shared mode may permit a
|
|
* waiting acquire (shared or exclusive) to succeed; and
|
|
* {@code false} otherwise
|
|
* @throws IllegalMonitorStateException if releasing would place this
|
|
* synchronizer in an illegal state. This exception must be
|
|
* thrown in a consistent fashion for synchronization to work
|
|
* correctly.
|
|
* @throws UnsupportedOperationException if shared mode is not supported
|
|
*/
|
|
protected boolean tryReleaseShared(int arg) {
|
|
throw new UnsupportedOperationException();
|
|
}
|
|
|
|
/**
|
|
* Returns {@code true} if synchronization is held exclusively with
|
|
* respect to the current (calling) thread. This method is invoked
|
|
* upon each call to a {@link ConditionObject} method.
|
|
*
|
|
* <p>The default implementation throws {@link
|
|
* UnsupportedOperationException}. This method is invoked
|
|
* internally only within {@link ConditionObject} methods, so need
|
|
* not be defined if conditions are not used.
|
|
*
|
|
* @return {@code true} if synchronization is held exclusively;
|
|
* {@code false} otherwise
|
|
* @throws UnsupportedOperationException if conditions are not supported
|
|
*/
|
|
protected boolean isHeldExclusively() {
|
|
throw new UnsupportedOperationException();
|
|
}
|
|
|
|
/**
|
|
* Acquires in exclusive mode, ignoring interrupts. Implemented
|
|
* by invoking at least once {@link #tryAcquire},
|
|
* returning on success. Otherwise the thread is queued, possibly
|
|
* repeatedly blocking and unblocking, invoking {@link
|
|
* #tryAcquire} until success. This method can be used
|
|
* to implement method {@link Lock#lock}.
|
|
*
|
|
* @param arg the acquire argument. This value is conveyed to
|
|
* {@link #tryAcquire} but is otherwise uninterpreted and
|
|
* can represent anything you like.
|
|
*/
|
|
public final void acquire(int arg) {
|
|
if (!tryAcquire(arg))
|
|
acquire(null, arg, false, false, false, 0L);
|
|
}
|
|
|
|
/**
|
|
* Acquires in exclusive mode, aborting if interrupted.
|
|
* Implemented by first checking interrupt status, then invoking
|
|
* at least once {@link #tryAcquire}, returning on
|
|
* success. Otherwise the thread is queued, possibly repeatedly
|
|
* blocking and unblocking, invoking {@link #tryAcquire}
|
|
* until success or the thread is interrupted. This method can be
|
|
* used to implement method {@link Lock#lockInterruptibly}.
|
|
*
|
|
* @param arg the acquire argument. This value is conveyed to
|
|
* {@link #tryAcquire} but is otherwise uninterpreted and
|
|
* can represent anything you like.
|
|
* @throws InterruptedException if the current thread is interrupted
|
|
*/
|
|
public final void acquireInterruptibly(int arg)
|
|
throws InterruptedException {
|
|
if (Thread.interrupted() ||
|
|
(!tryAcquire(arg) && acquire(null, arg, false, true, false, 0L) < 0))
|
|
throw new InterruptedException();
|
|
}
|
|
|
|
/**
|
|
* Attempts to acquire in exclusive mode, aborting if interrupted,
|
|
* and failing if the given timeout elapses. Implemented by first
|
|
* checking interrupt status, then invoking at least once {@link
|
|
* #tryAcquire}, returning on success. Otherwise, the thread is
|
|
* queued, possibly repeatedly blocking and unblocking, invoking
|
|
* {@link #tryAcquire} until success or the thread is interrupted
|
|
* or the timeout elapses. This method can be used to implement
|
|
* method {@link Lock#tryLock(long, TimeUnit)}.
|
|
*
|
|
* @param arg the acquire argument. This value is conveyed to
|
|
* {@link #tryAcquire} but is otherwise uninterpreted and
|
|
* can represent anything you like.
|
|
* @param nanosTimeout the maximum number of nanoseconds to wait
|
|
* @return {@code true} if acquired; {@code false} if timed out
|
|
* @throws InterruptedException if the current thread is interrupted
|
|
*/
|
|
public final boolean tryAcquireNanos(int arg, long nanosTimeout)
|
|
throws InterruptedException {
|
|
if (!Thread.interrupted()) {
|
|
if (tryAcquire(arg))
|
|
return true;
|
|
if (nanosTimeout <= 0L)
|
|
return false;
|
|
int stat = acquire(null, arg, false, true, true,
|
|
System.nanoTime() + nanosTimeout);
|
|
if (stat > 0)
|
|
return true;
|
|
if (stat == 0)
|
|
return false;
|
|
}
|
|
throw new InterruptedException();
|
|
}
|
|
|
|
/**
|
|
* Releases in exclusive mode. Implemented by unblocking one or
|
|
* more threads if {@link #tryRelease} returns true.
|
|
* This method can be used to implement method {@link Lock#unlock}.
|
|
*
|
|
* @param arg the release argument. This value is conveyed to
|
|
* {@link #tryRelease} but is otherwise uninterpreted and
|
|
* can represent anything you like.
|
|
* @return the value returned from {@link #tryRelease}
|
|
*/
|
|
public final boolean release(int arg) {
|
|
if (tryRelease(arg)) {
|
|
signalNext(head);
|
|
return true;
|
|
}
|
|
return false;
|
|
}
|
|
|
|
/**
|
|
* Acquires in shared mode, ignoring interrupts. Implemented by
|
|
* first invoking at least once {@link #tryAcquireShared},
|
|
* returning on success. Otherwise the thread is queued, possibly
|
|
* repeatedly blocking and unblocking, invoking {@link
|
|
* #tryAcquireShared} until success.
|
|
*
|
|
* @param arg the acquire argument. This value is conveyed to
|
|
* {@link #tryAcquireShared} but is otherwise uninterpreted
|
|
* and can represent anything you like.
|
|
*/
|
|
public final void acquireShared(int arg) {
|
|
if (tryAcquireShared(arg) < 0)
|
|
acquire(null, arg, true, false, false, 0L);
|
|
}
|
|
|
|
/**
|
|
* Acquires in shared mode, aborting if interrupted. Implemented
|
|
* by first checking interrupt status, then invoking at least once
|
|
* {@link #tryAcquireShared}, returning on success. Otherwise the
|
|
* thread is queued, possibly repeatedly blocking and unblocking,
|
|
* invoking {@link #tryAcquireShared} until success or the thread
|
|
* is interrupted.
|
|
* @param arg the acquire argument.
|
|
* This value is conveyed to {@link #tryAcquireShared} but is
|
|
* otherwise uninterpreted and can represent anything
|
|
* you like.
|
|
* @throws InterruptedException if the current thread is interrupted
|
|
*/
|
|
public final void acquireSharedInterruptibly(int arg)
|
|
throws InterruptedException {
|
|
if (Thread.interrupted() ||
|
|
(tryAcquireShared(arg) < 0 &&
|
|
acquire(null, arg, true, true, false, 0L) < 0))
|
|
throw new InterruptedException();
|
|
}
|
|
|
|
/**
|
|
* Attempts to acquire in shared mode, aborting if interrupted, and
|
|
* failing if the given timeout elapses. Implemented by first
|
|
* checking interrupt status, then invoking at least once {@link
|
|
* #tryAcquireShared}, returning on success. Otherwise, the
|
|
* thread is queued, possibly repeatedly blocking and unblocking,
|
|
* invoking {@link #tryAcquireShared} until success or the thread
|
|
* is interrupted or the timeout elapses.
|
|
*
|
|
* @param arg the acquire argument. This value is conveyed to
|
|
* {@link #tryAcquireShared} but is otherwise uninterpreted
|
|
* and can represent anything you like.
|
|
* @param nanosTimeout the maximum number of nanoseconds to wait
|
|
* @return {@code true} if acquired; {@code false} if timed out
|
|
* @throws InterruptedException if the current thread is interrupted
|
|
*/
|
|
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
|
|
throws InterruptedException {
|
|
if (!Thread.interrupted()) {
|
|
if (tryAcquireShared(arg) >= 0)
|
|
return true;
|
|
if (nanosTimeout <= 0L)
|
|
return false;
|
|
int stat = acquire(null, arg, true, true, true,
|
|
System.nanoTime() + nanosTimeout);
|
|
if (stat > 0)
|
|
return true;
|
|
if (stat == 0)
|
|
return false;
|
|
}
|
|
throw new InterruptedException();
|
|
}
|
|
|
|
/**
|
|
* Releases in shared mode. Implemented by unblocking one or more
|
|
* threads if {@link #tryReleaseShared} returns true.
|
|
*
|
|
* @param arg the release argument. This value is conveyed to
|
|
* {@link #tryReleaseShared} but is otherwise uninterpreted
|
|
* and can represent anything you like.
|
|
* @return the value returned from {@link #tryReleaseShared}
|
|
*/
|
|
public final boolean releaseShared(int arg) {
|
|
if (tryReleaseShared(arg)) {
|
|
signalNext(head);
|
|
return true;
|
|
}
|
|
return false;
|
|
}
|
|
|
|
// Queue inspection methods
|
|
|
|
/**
|
|
* Queries whether any threads are waiting to acquire. Note that
|
|
* because cancellations due to interrupts and timeouts may occur
|
|
* at any time, a {@code true} return does not guarantee that any
|
|
* other thread will ever acquire.
|
|
*
|
|
* @return {@code true} if there may be other threads waiting to acquire
|
|
*/
|
|
public final boolean hasQueuedThreads() {
|
|
for (Node p = tail, h = head; p != h && p != null; p = p.prev)
|
|
if (p.status >= 0)
|
|
return true;
|
|
return false;
|
|
}
|
|
|
|
/**
|
|
* Queries whether any threads have ever contended to acquire this
|
|
* synchronizer; that is, if an acquire method has ever blocked.
|
|
*
|
|
* <p>In this implementation, this operation returns in
|
|
* constant time.
|
|
*
|
|
* @return {@code true} if there has ever been contention
|
|
*/
|
|
public final boolean hasContended() {
|
|
return head != null;
|
|
}
|
|
|
|
/**
|
|
* Returns the first (longest-waiting) thread in the queue, or
|
|
* {@code null} if no threads are currently queued.
|
|
*
|
|
* <p>In this implementation, this operation normally returns in
|
|
* constant time, but may iterate upon contention if other threads are
|
|
* concurrently modifying the queue.
|
|
*
|
|
* @return the first (longest-waiting) thread in the queue, or
|
|
* {@code null} if no threads are currently queued
|
|
*/
|
|
public final Thread getFirstQueuedThread() {
|
|
Thread first = null, w; Node h, s;
|
|
if ((h = head) != null && ((s = h.next) == null ||
|
|
(first = s.waiter) == null ||
|
|
s.prev == null)) {
|
|
// traverse from tail on stale reads
|
|
for (Node p = tail, q; p != null && (q = p.prev) != null; p = q)
|
|
if ((w = p.waiter) != null)
|
|
first = w;
|
|
}
|
|
return first;
|
|
}
|
|
|
|
/**
|
|
* Returns true if the given thread is currently queued.
|
|
*
|
|
* <p>This implementation traverses the queue to determine
|
|
* presence of the given thread.
|
|
*
|
|
* @param thread the thread
|
|
* @return {@code true} if the given thread is on the queue
|
|
* @throws NullPointerException if the thread is null
|
|
*/
|
|
public final boolean isQueued(Thread thread) {
|
|
if (thread == null)
|
|
throw new NullPointerException();
|
|
for (Node p = tail; p != null; p = p.prev)
|
|
if (p.waiter == thread)
|
|
return true;
|
|
return false;
|
|
}
|
|
|
|
/**
|
|
* Returns {@code true} if the apparent first queued thread, if one
|
|
* exists, is waiting in exclusive mode. If this method returns
|
|
* {@code true}, and the current thread is attempting to acquire in
|
|
* shared mode (that is, this method is invoked from {@link
|
|
* #tryAcquireShared}) then it is guaranteed that the current thread
|
|
* is not the first queued thread. Used only as a heuristic in
|
|
* ReentrantReadWriteLock.
|
|
*/
|
|
final boolean apparentlyFirstQueuedIsExclusive() {
|
|
Node h, s;
|
|
return (h = head) != null && (s = h.next) != null &&
|
|
!(s instanceof SharedNode) && s.waiter != null;
|
|
}
|
|
|
|
/**
|
|
* Queries whether any threads have been waiting to acquire longer
|
|
* than the current thread.
|
|
*
|
|
* <p>An invocation of this method is equivalent to (but may be
|
|
* more efficient than):
|
|
* <pre> {@code
|
|
* getFirstQueuedThread() != Thread.currentThread()
|
|
* && hasQueuedThreads()}</pre>
|
|
*
|
|
* <p>Note that because cancellations due to interrupts and
|
|
* timeouts may occur at any time, a {@code true} return does not
|
|
* guarantee that some other thread will acquire before the current
|
|
* thread. Likewise, it is possible for another thread to win a
|
|
* race to enqueue after this method has returned {@code false},
|
|
* due to the queue being empty.
|
|
*
|
|
* <p>This method is designed to be used by a fair synchronizer to
|
|
* avoid <a href="AbstractQueuedSynchronizer.html#barging">barging</a>.
|
|
* Such a synchronizer's {@link #tryAcquire} method should return
|
|
* {@code false}, and its {@link #tryAcquireShared} method should
|
|
* return a negative value, if this method returns {@code true}
|
|
* (unless this is a reentrant acquire). For example, the {@code
|
|
* tryAcquire} method for a fair, reentrant, exclusive mode
|
|
* synchronizer might look like this:
|
|
*
|
|
* <pre> {@code
|
|
* protected boolean tryAcquire(int arg) {
|
|
* if (isHeldExclusively()) {
|
|
* // A reentrant acquire; increment hold count
|
|
* return true;
|
|
* } else if (hasQueuedPredecessors()) {
|
|
* return false;
|
|
* } else {
|
|
* // try to acquire normally
|
|
* }
|
|
* }}</pre>
|
|
*
|
|
* @return {@code true} if there is a queued thread preceding the
|
|
* current thread, and {@code false} if the current thread
|
|
* is at the head of the queue or the queue is empty
|
|
* @since 1.7
|
|
*/
|
|
public final boolean hasQueuedPredecessors() {
|
|
Thread first = null; Node h, s;
|
|
if ((h = head) != null && ((s = h.next) == null ||
|
|
(first = s.waiter) == null ||
|
|
s.prev == null))
|
|
first = getFirstQueuedThread(); // retry via getFirstQueuedThread
|
|
return first != null && first != Thread.currentThread();
|
|
}
|
|
|
|
// Instrumentation and monitoring methods
|
|
|
|
/**
|
|
* Returns an estimate of the number of threads waiting to
|
|
* acquire. The value is only an estimate because the number of
|
|
* threads may change dynamically while this method traverses
|
|
* internal data structures. This method is designed for use in
|
|
* monitoring system state, not for synchronization control.
|
|
*
|
|
* @return the estimated number of threads waiting to acquire
|
|
*/
|
|
public final int getQueueLength() {
|
|
int n = 0;
|
|
for (Node p = tail; p != null; p = p.prev) {
|
|
if (p.waiter != null)
|
|
++n;
|
|
}
|
|
return n;
|
|
}
|
|
|
|
/**
|
|
* Returns a collection containing threads that may be waiting to
|
|
* acquire. Because the actual set of threads may change
|
|
* dynamically while constructing this result, the returned
|
|
* collection is only a best-effort estimate. The elements of the
|
|
* returned collection are in no particular order. This method is
|
|
* designed to facilitate construction of subclasses that provide
|
|
* more extensive monitoring facilities.
|
|
*
|
|
* @return the collection of threads
|
|
*/
|
|
public final Collection<Thread> getQueuedThreads() {
|
|
ArrayList<Thread> list = new ArrayList<>();
|
|
for (Node p = tail; p != null; p = p.prev) {
|
|
Thread t = p.waiter;
|
|
if (t != null)
|
|
list.add(t);
|
|
}
|
|
return list;
|
|
}
|
|
|
|
/**
|
|
* Returns a collection containing threads that may be waiting to
|
|
* acquire in exclusive mode. This has the same properties
|
|
* as {@link #getQueuedThreads} except that it only returns
|
|
* those threads waiting due to an exclusive acquire.
|
|
*
|
|
* @return the collection of threads
|
|
*/
|
|
public final Collection<Thread> getExclusiveQueuedThreads() {
|
|
ArrayList<Thread> list = new ArrayList<>();
|
|
for (Node p = tail; p != null; p = p.prev) {
|
|
if (!(p instanceof SharedNode)) {
|
|
Thread t = p.waiter;
|
|
if (t != null)
|
|
list.add(t);
|
|
}
|
|
}
|
|
return list;
|
|
}
|
|
|
|
/**
|
|
* Returns a collection containing threads that may be waiting to
|
|
* acquire in shared mode. This has the same properties
|
|
* as {@link #getQueuedThreads} except that it only returns
|
|
* those threads waiting due to a shared acquire.
|
|
*
|
|
* @return the collection of threads
|
|
*/
|
|
public final Collection<Thread> getSharedQueuedThreads() {
|
|
ArrayList<Thread> list = new ArrayList<>();
|
|
for (Node p = tail; p != null; p = p.prev) {
|
|
if (p instanceof SharedNode) {
|
|
Thread t = p.waiter;
|
|
if (t != null)
|
|
list.add(t);
|
|
}
|
|
}
|
|
return list;
|
|
}
|
|
|
|
/**
|
|
* Returns a string identifying this synchronizer, as well as its state.
|
|
* The state, in brackets, includes the String {@code "State ="}
|
|
* followed by the current value of {@link #getState}, and either
|
|
* {@code "nonempty"} or {@code "empty"} depending on whether the
|
|
* queue is empty.
|
|
*
|
|
* @return a string identifying this synchronizer, as well as its state
|
|
*/
|
|
public String toString() {
|
|
return super.toString()
|
|
+ "[State = " + getState() + ", "
|
|
+ (hasQueuedThreads() ? "non" : "") + "empty queue]";
|
|
}
|
|
|
|
// Instrumentation methods for conditions
|
|
|
|
/**
|
|
* Queries whether the given ConditionObject
|
|
* uses this synchronizer as its lock.
|
|
*
|
|
* @param condition the condition
|
|
* @return {@code true} if owned
|
|
* @throws NullPointerException if the condition is null
|
|
*/
|
|
public final boolean owns(ConditionObject condition) {
|
|
return condition.isOwnedBy(this);
|
|
}
|
|
|
|
/**
|
|
* Queries whether any threads are waiting on the given condition
|
|
* associated with this synchronizer. Note that because timeouts
|
|
* and interrupts may occur at any time, a {@code true} return
|
|
* does not guarantee that a future {@code signal} will awaken
|
|
* any threads. This method is designed primarily for use in
|
|
* monitoring of the system state.
|
|
*
|
|
* @param condition the condition
|
|
* @return {@code true} if there are any waiting threads
|
|
* @throws IllegalMonitorStateException if exclusive synchronization
|
|
* is not held
|
|
* @throws IllegalArgumentException if the given condition is
|
|
* not associated with this synchronizer
|
|
* @throws NullPointerException if the condition is null
|
|
*/
|
|
public final boolean hasWaiters(ConditionObject condition) {
|
|
if (!owns(condition))
|
|
throw new IllegalArgumentException("Not owner");
|
|
return condition.hasWaiters();
|
|
}
|
|
|
|
/**
|
|
* Returns an estimate of the number of threads waiting on the
|
|
* given condition associated with this synchronizer. Note that
|
|
* because timeouts and interrupts may occur at any time, the
|
|
* estimate serves only as an upper bound on the actual number of
|
|
* waiters. This method is designed for use in monitoring system
|
|
* state, not for synchronization control.
|
|
*
|
|
* @param condition the condition
|
|
* @return the estimated number of waiting threads
|
|
* @throws IllegalMonitorStateException if exclusive synchronization
|
|
* is not held
|
|
* @throws IllegalArgumentException if the given condition is
|
|
* not associated with this synchronizer
|
|
* @throws NullPointerException if the condition is null
|
|
*/
|
|
public final int getWaitQueueLength(ConditionObject condition) {
|
|
if (!owns(condition))
|
|
throw new IllegalArgumentException("Not owner");
|
|
return condition.getWaitQueueLength();
|
|
}
|
|
|
|
/**
|
|
* Returns a collection containing those threads that may be
|
|
* waiting on the given condition associated with this
|
|
* synchronizer. Because the actual set of threads may change
|
|
* dynamically while constructing this result, the returned
|
|
* collection is only a best-effort estimate. The elements of the
|
|
* returned collection are in no particular order.
|
|
*
|
|
* @param condition the condition
|
|
* @return the collection of threads
|
|
* @throws IllegalMonitorStateException if exclusive synchronization
|
|
* is not held
|
|
* @throws IllegalArgumentException if the given condition is
|
|
* not associated with this synchronizer
|
|
* @throws NullPointerException if the condition is null
|
|
*/
|
|
public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
|
|
if (!owns(condition))
|
|
throw new IllegalArgumentException("Not owner");
|
|
return condition.getWaitingThreads();
|
|
}
|
|
|
|
/**
|
|
* Condition implementation for a {@link AbstractQueuedSynchronizer}
|
|
* serving as the basis of a {@link Lock} implementation.
|
|
*
|
|
* <p>Method documentation for this class describes mechanics,
|
|
* not behavioral specifications from the point of view of Lock
|
|
* and Condition users. Exported versions of this class will in
|
|
* general need to be accompanied by documentation describing
|
|
* condition semantics that rely on those of the associated
|
|
* {@code AbstractQueuedSynchronizer}.
|
|
*
|
|
* <p>This class is Serializable, but all fields are transient,
|
|
* so deserialized conditions have no waiters.
|
|
*/
|
|
public class ConditionObject implements Condition, java.io.Serializable {
|
|
private static final long serialVersionUID = 1173984872572414699L;
|
|
/** First node of condition queue. */
|
|
private transient ConditionNode firstWaiter;
|
|
/** Last node of condition queue. */
|
|
private transient ConditionNode lastWaiter;
|
|
|
|
/**
|
|
* Creates a new {@code ConditionObject} instance.
|
|
*/
|
|
public ConditionObject() { }
|
|
|
|
// Signalling methods
|
|
|
|
/**
|
|
* Removes and transfers one or all waiters to sync queue.
|
|
*/
|
|
private void doSignal(ConditionNode first, boolean all) {
|
|
while (first != null) {
|
|
ConditionNode next = first.nextWaiter;
|
|
if ((firstWaiter = next) == null)
|
|
lastWaiter = null;
|
|
if ((first.getAndUnsetStatus(COND) & COND) != 0) {
|
|
enqueue(first);
|
|
if (!all)
|
|
break;
|
|
}
|
|
first = next;
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Moves the longest-waiting thread, if one exists, from the
|
|
* wait queue for this condition to the wait queue for the
|
|
* owning lock.
|
|
*
|
|
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
|
|
* returns {@code false}
|
|
*/
|
|
public final void signal() {
|
|
ConditionNode first = firstWaiter;
|
|
if (!isHeldExclusively())
|
|
throw new IllegalMonitorStateException();
|
|
if (first != null)
|
|
doSignal(first, false);
|
|
}
|
|
|
|
/**
|
|
* Moves all threads from the wait queue for this condition to
|
|
* the wait queue for the owning lock.
|
|
*
|
|
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
|
|
* returns {@code false}
|
|
*/
|
|
public final void signalAll() {
|
|
ConditionNode first = firstWaiter;
|
|
if (!isHeldExclusively())
|
|
throw new IllegalMonitorStateException();
|
|
if (first != null)
|
|
doSignal(first, true);
|
|
}
|
|
|
|
// Waiting methods
|
|
|
|
/**
|
|
* Adds node to condition list and releases lock.
|
|
*
|
|
* @param node the node
|
|
* @return savedState to reacquire after wait
|
|
*/
|
|
private int enableWait(ConditionNode node) {
|
|
if (isHeldExclusively()) {
|
|
node.waiter = Thread.currentThread();
|
|
node.setStatusRelaxed(COND | WAITING);
|
|
ConditionNode last = lastWaiter;
|
|
if (last == null)
|
|
firstWaiter = node;
|
|
else
|
|
last.nextWaiter = node;
|
|
lastWaiter = node;
|
|
int savedState = getState();
|
|
if (release(savedState))
|
|
return savedState;
|
|
}
|
|
node.status = CANCELLED; // lock not held or inconsistent
|
|
throw new IllegalMonitorStateException();
|
|
}
|
|
|
|
/**
|
|
* Returns true if a node that was initially placed on a condition
|
|
* queue is now ready to reacquire on sync queue.
|
|
* @param node the node
|
|
* @return true if is reacquiring
|
|
*/
|
|
private boolean canReacquire(ConditionNode node) {
|
|
// check links, not status to avoid enqueue race
|
|
return node != null && node.prev != null && isEnqueued(node);
|
|
}
|
|
|
|
/**
|
|
* Unlinks the given node and other non-waiting nodes from
|
|
* condition queue unless already unlinked.
|
|
*/
|
|
private void unlinkCancelledWaiters(ConditionNode node) {
|
|
if (node == null || node.nextWaiter != null || node == lastWaiter) {
|
|
ConditionNode w = firstWaiter, trail = null;
|
|
while (w != null) {
|
|
ConditionNode next = w.nextWaiter;
|
|
if ((w.status & COND) == 0) {
|
|
w.nextWaiter = null;
|
|
if (trail == null)
|
|
firstWaiter = next;
|
|
else
|
|
trail.nextWaiter = next;
|
|
if (next == null)
|
|
lastWaiter = trail;
|
|
} else
|
|
trail = w;
|
|
w = next;
|
|
}
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Implements uninterruptible condition wait.
|
|
* <ol>
|
|
* <li>Save lock state returned by {@link #getState}.
|
|
* <li>Invoke {@link #release} with saved state as argument,
|
|
* throwing IllegalMonitorStateException if it fails.
|
|
* <li>Block until signalled.
|
|
* <li>Reacquire by invoking specialized version of
|
|
* {@link #acquire} with saved state as argument.
|
|
* </ol>
|
|
*/
|
|
public final void awaitUninterruptibly() {
|
|
ConditionNode node = new ConditionNode();
|
|
int savedState = enableWait(node);
|
|
LockSupport.setCurrentBlocker(this); // for back-compatibility
|
|
boolean interrupted = false, rejected = false;
|
|
while (!canReacquire(node)) {
|
|
if (Thread.interrupted())
|
|
interrupted = true;
|
|
else if ((node.status & COND) != 0) {
|
|
try {
|
|
if (rejected)
|
|
node.block();
|
|
else
|
|
ForkJoinPool.managedBlock(node);
|
|
} catch (RejectedExecutionException ex) {
|
|
rejected = true;
|
|
} catch (InterruptedException ie) {
|
|
interrupted = true;
|
|
}
|
|
} else
|
|
Thread.onSpinWait(); // awoke while enqueuing
|
|
}
|
|
LockSupport.setCurrentBlocker(null);
|
|
node.clearStatus();
|
|
acquire(node, savedState, false, false, false, 0L);
|
|
if (interrupted)
|
|
Thread.currentThread().interrupt();
|
|
}
|
|
|
|
/**
|
|
* Implements interruptible condition wait.
|
|
* <ol>
|
|
* <li>If current thread is interrupted, throw InterruptedException.
|
|
* <li>Save lock state returned by {@link #getState}.
|
|
* <li>Invoke {@link #release} with saved state as argument,
|
|
* throwing IllegalMonitorStateException if it fails.
|
|
* <li>Block until signalled or interrupted.
|
|
* <li>Reacquire by invoking specialized version of
|
|
* {@link #acquire} with saved state as argument.
|
|
* <li>If interrupted while blocked in step 4, throw InterruptedException.
|
|
* </ol>
|
|
*/
|
|
public final void await() throws InterruptedException {
|
|
if (Thread.interrupted())
|
|
throw new InterruptedException();
|
|
ConditionNode node = new ConditionNode();
|
|
int savedState = enableWait(node);
|
|
LockSupport.setCurrentBlocker(this); // for back-compatibility
|
|
boolean interrupted = false, cancelled = false, rejected = false;
|
|
while (!canReacquire(node)) {
|
|
if (interrupted |= Thread.interrupted()) {
|
|
if (cancelled = (node.getAndUnsetStatus(COND) & COND) != 0)
|
|
break; // else interrupted after signal
|
|
} else if ((node.status & COND) != 0) {
|
|
try {
|
|
if (rejected)
|
|
node.block();
|
|
else
|
|
ForkJoinPool.managedBlock(node);
|
|
} catch (RejectedExecutionException ex) {
|
|
rejected = true;
|
|
} catch (InterruptedException ie) {
|
|
interrupted = true;
|
|
}
|
|
} else
|
|
Thread.onSpinWait(); // awoke while enqueuing
|
|
}
|
|
LockSupport.setCurrentBlocker(null);
|
|
node.clearStatus();
|
|
acquire(node, savedState, false, false, false, 0L);
|
|
if (interrupted) {
|
|
if (cancelled) {
|
|
unlinkCancelledWaiters(node);
|
|
throw new InterruptedException();
|
|
}
|
|
Thread.currentThread().interrupt();
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Implements timed condition wait.
|
|
* <ol>
|
|
* <li>If current thread is interrupted, throw InterruptedException.
|
|
* <li>Save lock state returned by {@link #getState}.
|
|
* <li>Invoke {@link #release} with saved state as argument,
|
|
* throwing IllegalMonitorStateException if it fails.
|
|
* <li>Block until signalled, interrupted, or timed out.
|
|
* <li>Reacquire by invoking specialized version of
|
|
* {@link #acquire} with saved state as argument.
|
|
* <li>If interrupted while blocked in step 4, throw InterruptedException.
|
|
* </ol>
|
|
*/
|
|
public final long awaitNanos(long nanosTimeout)
|
|
throws InterruptedException {
|
|
if (Thread.interrupted())
|
|
throw new InterruptedException();
|
|
ConditionNode node = new ConditionNode();
|
|
int savedState = enableWait(node);
|
|
long nanos = (nanosTimeout < 0L) ? 0L : nanosTimeout;
|
|
long deadline = System.nanoTime() + nanos;
|
|
boolean cancelled = false, interrupted = false;
|
|
while (!canReacquire(node)) {
|
|
if ((interrupted |= Thread.interrupted()) ||
|
|
(nanos = deadline - System.nanoTime()) <= 0L) {
|
|
if (cancelled = (node.getAndUnsetStatus(COND) & COND) != 0)
|
|
break;
|
|
} else
|
|
LockSupport.parkNanos(this, nanos);
|
|
}
|
|
node.clearStatus();
|
|
acquire(node, savedState, false, false, false, 0L);
|
|
if (cancelled) {
|
|
unlinkCancelledWaiters(node);
|
|
if (interrupted)
|
|
throw new InterruptedException();
|
|
} else if (interrupted)
|
|
Thread.currentThread().interrupt();
|
|
long remaining = deadline - System.nanoTime(); // avoid overflow
|
|
return (remaining <= nanosTimeout) ? remaining : Long.MIN_VALUE;
|
|
}
|
|
|
|
/**
|
|
* Implements absolute timed condition wait.
|
|
* <ol>
|
|
* <li>If current thread is interrupted, throw InterruptedException.
|
|
* <li>Save lock state returned by {@link #getState}.
|
|
* <li>Invoke {@link #release} with saved state as argument,
|
|
* throwing IllegalMonitorStateException if it fails.
|
|
* <li>Block until signalled, interrupted, or timed out.
|
|
* <li>Reacquire by invoking specialized version of
|
|
* {@link #acquire} with saved state as argument.
|
|
* <li>If interrupted while blocked in step 4, throw InterruptedException.
|
|
* <li>If timed out while blocked in step 4, return false, else true.
|
|
* </ol>
|
|
*/
|
|
public final boolean awaitUntil(Date deadline)
|
|
throws InterruptedException {
|
|
long abstime = deadline.getTime();
|
|
if (Thread.interrupted())
|
|
throw new InterruptedException();
|
|
ConditionNode node = new ConditionNode();
|
|
int savedState = enableWait(node);
|
|
boolean cancelled = false, interrupted = false;
|
|
while (!canReacquire(node)) {
|
|
if ((interrupted |= Thread.interrupted()) ||
|
|
System.currentTimeMillis() >= abstime) {
|
|
if (cancelled = (node.getAndUnsetStatus(COND) & COND) != 0)
|
|
break;
|
|
} else
|
|
LockSupport.parkUntil(this, abstime);
|
|
}
|
|
node.clearStatus();
|
|
acquire(node, savedState, false, false, false, 0L);
|
|
if (cancelled) {
|
|
unlinkCancelledWaiters(node);
|
|
if (interrupted)
|
|
throw new InterruptedException();
|
|
} else if (interrupted)
|
|
Thread.currentThread().interrupt();
|
|
return !cancelled;
|
|
}
|
|
|
|
/**
|
|
* Implements timed condition wait.
|
|
* <ol>
|
|
* <li>If current thread is interrupted, throw InterruptedException.
|
|
* <li>Save lock state returned by {@link #getState}.
|
|
* <li>Invoke {@link #release} with saved state as argument,
|
|
* throwing IllegalMonitorStateException if it fails.
|
|
* <li>Block until signalled, interrupted, or timed out.
|
|
* <li>Reacquire by invoking specialized version of
|
|
* {@link #acquire} with saved state as argument.
|
|
* <li>If interrupted while blocked in step 4, throw InterruptedException.
|
|
* <li>If timed out while blocked in step 4, return false, else true.
|
|
* </ol>
|
|
*/
|
|
public final boolean await(long time, TimeUnit unit)
|
|
throws InterruptedException {
|
|
long nanosTimeout = unit.toNanos(time);
|
|
if (Thread.interrupted())
|
|
throw new InterruptedException();
|
|
ConditionNode node = new ConditionNode();
|
|
int savedState = enableWait(node);
|
|
long nanos = (nanosTimeout < 0L) ? 0L : nanosTimeout;
|
|
long deadline = System.nanoTime() + nanos;
|
|
boolean cancelled = false, interrupted = false;
|
|
while (!canReacquire(node)) {
|
|
if ((interrupted |= Thread.interrupted()) ||
|
|
(nanos = deadline - System.nanoTime()) <= 0L) {
|
|
if (cancelled = (node.getAndUnsetStatus(COND) & COND) != 0)
|
|
break;
|
|
} else
|
|
LockSupport.parkNanos(this, nanos);
|
|
}
|
|
node.clearStatus();
|
|
acquire(node, savedState, false, false, false, 0L);
|
|
if (cancelled) {
|
|
unlinkCancelledWaiters(node);
|
|
if (interrupted)
|
|
throw new InterruptedException();
|
|
} else if (interrupted)
|
|
Thread.currentThread().interrupt();
|
|
return !cancelled;
|
|
}
|
|
|
|
// support for instrumentation
|
|
|
|
/**
|
|
* Returns true if this condition was created by the given
|
|
* synchronization object.
|
|
*
|
|
* @return {@code true} if owned
|
|
*/
|
|
final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
|
|
return sync == AbstractQueuedSynchronizer.this;
|
|
}
|
|
|
|
/**
|
|
* Queries whether any threads are waiting on this condition.
|
|
* Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
|
|
*
|
|
* @return {@code true} if there are any waiting threads
|
|
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
|
|
* returns {@code false}
|
|
*/
|
|
protected final boolean hasWaiters() {
|
|
if (!isHeldExclusively())
|
|
throw new IllegalMonitorStateException();
|
|
for (ConditionNode w = firstWaiter; w != null; w = w.nextWaiter) {
|
|
if ((w.status & COND) != 0)
|
|
return true;
|
|
}
|
|
return false;
|
|
}
|
|
|
|
/**
|
|
* Returns an estimate of the number of threads waiting on
|
|
* this condition.
|
|
* Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
|
|
*
|
|
* @return the estimated number of waiting threads
|
|
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
|
|
* returns {@code false}
|
|
*/
|
|
protected final int getWaitQueueLength() {
|
|
if (!isHeldExclusively())
|
|
throw new IllegalMonitorStateException();
|
|
int n = 0;
|
|
for (ConditionNode w = firstWaiter; w != null; w = w.nextWaiter) {
|
|
if ((w.status & COND) != 0)
|
|
++n;
|
|
}
|
|
return n;
|
|
}
|
|
|
|
/**
|
|
* Returns a collection containing those threads that may be
|
|
* waiting on this Condition.
|
|
* Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
|
|
*
|
|
* @return the collection of threads
|
|
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
|
|
* returns {@code false}
|
|
*/
|
|
protected final Collection<Thread> getWaitingThreads() {
|
|
if (!isHeldExclusively())
|
|
throw new IllegalMonitorStateException();
|
|
ArrayList<Thread> list = new ArrayList<>();
|
|
for (ConditionNode w = firstWaiter; w != null; w = w.nextWaiter) {
|
|
if ((w.status & COND) != 0) {
|
|
Thread t = w.waiter;
|
|
if (t != null)
|
|
list.add(t);
|
|
}
|
|
}
|
|
return list;
|
|
}
|
|
}
|
|
|
|
// Unsafe
|
|
private static final Unsafe U = Unsafe.getUnsafe();
|
|
private static final long STATE
|
|
= U.objectFieldOffset(AbstractQueuedSynchronizer.class, "state");
|
|
private static final long HEAD
|
|
= U.objectFieldOffset(AbstractQueuedSynchronizer.class, "head");
|
|
private static final long TAIL
|
|
= U.objectFieldOffset(AbstractQueuedSynchronizer.class, "tail");
|
|
|
|
static {
|
|
Class<?> ensureLoaded = LockSupport.class;
|
|
}
|
|
}
|