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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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|
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package java.util.concurrent.locks; |
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|
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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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|
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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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* <h3>Usage</h3> |
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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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* Acquire: |
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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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* Release: |
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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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* <h3>Usage Examples</h3> |
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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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* |
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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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|
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private static final long serialVersionUID = 7373984972572414691L; |
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|
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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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/** |
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* Wait queue node class. |
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* |
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* <p>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, but |
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* use the same basic tactic of holding some of the control |
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* information about a thread in the predecessor of its node. A |
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* "status" field in each node keeps track of whether a thread |
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* should block. A node is signalled when its predecessor |
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* releases. Each node of the queue otherwise serves as a |
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* specific-notification-style monitor holding a single waiting |
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* thread. The status field does NOT control whether threads are |
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* granted locks etc though. A thread may try to acquire if it is |
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* first in the queue. But being first does not guarantee success; |
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* it only gives the right to contend. So the currently released |
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* contender thread may need to rewait. |
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* |
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* <p>To enqueue into a CLH lock, you atomically splice it in as new |
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* tail. To dequeue, you just set the head field. |
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* <pre> |
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* +------+ prev +-----+ +-----+ |
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* head | | <---- | | <---- | | tail |
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* +------+ +-----+ +-----+ |
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* </pre> |
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* |
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* <p>Insertion into a CLH queue requires only a single atomic |
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* operation on "tail", so there is a simple atomic point of |
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* demarcation from unqueued to queued. Similarly, dequeuing |
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* involves only updating the "head". However, it takes a bit |
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* more work for nodes to determine who their successors are, |
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* in part to deal with possible cancellation due to timeouts |
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* and interrupts. |
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* |
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* <p>The "prev" links (not used in original CLH locks), are mainly |
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* needed to handle cancellation. If a node is cancelled, its |
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* successor is (normally) relinked to a non-cancelled |
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* predecessor. For explanation of similar mechanics in the case |
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* of spin locks, see the papers by Scott and Scherer at |
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* http://www.cs.rochester.edu/u/scott/synchronization/ |
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* |
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* <p>We also use "next" links to implement blocking mechanics. |
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* The thread id for each node is kept in its own node, so a |
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* predecessor signals the next node to wake up by traversing |
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* next link to determine which thread it is. Determination of |
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* successor must avoid races with newly queued nodes to set |
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* the "next" fields of their predecessors. This is solved |
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* when necessary by checking backwards from the atomically |
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* updated "tail" when a node's successor appears to be null. |
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* (Or, said differently, the next-links are an optimization |
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* so that we don't usually need a backward scan.) |
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* |
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* <p>Cancellation introduces some conservatism to the basic |
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* algorithms. Since we must poll for cancellation of other |
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* nodes, we can miss noticing whether a cancelled node is |
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* ahead or behind us. This is dealt with by always unparking |
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* successors upon cancellation, allowing them to stabilize on |
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* a new predecessor, unless we can identify an uncancelled |
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* predecessor who will carry this responsibility. |
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* |
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* <p>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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* <p>Threads waiting on Conditions use the same nodes, but |
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* use an additional link. Conditions only need to link nodes |
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* in simple (non-concurrent) linked queues because they are |
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* only accessed when exclusively held. Upon await, a node is |
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* inserted into a condition queue. Upon signal, the node is |
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* transferred to the main queue. A special value of status |
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* field is used to mark which queue a node is on. |
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* |
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* <p>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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static final class Node { |
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/** Marker to indicate a node is waiting in shared mode */ |
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static final Node SHARED = new Node(); |
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/** Marker to indicate a node is waiting in exclusive mode */ |
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static final Node EXCLUSIVE = null; |
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|
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/** waitStatus value to indicate thread has cancelled. */ |
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static final int CANCELLED = 1; |
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/** waitStatus value to indicate successor's thread needs unparking. */ |
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static final int SIGNAL = -1; |
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/** waitStatus value to indicate thread is waiting on condition. */ |
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static final int CONDITION = -2; |
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/** |
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* waitStatus value to indicate the next acquireShared should |
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* unconditionally propagate. |
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*/ |
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static final int PROPAGATE = -3; |
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|
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/** |
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* Status field, taking on only the values: |
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* SIGNAL: The successor of this node is (or will soon be) |
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* blocked (via park), so the current node must |
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* unpark its successor when it releases or |
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* cancels. To avoid races, acquire methods must |
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* first indicate they need a signal, |
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* then retry the atomic acquire, and then, |
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* on failure, block. |
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* CANCELLED: This node is cancelled due to timeout or interrupt. |
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* Nodes never leave this state. In particular, |
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* a thread with cancelled node never again blocks. |
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* CONDITION: This node is currently on a condition queue. |
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* It will not be used as a sync queue node |
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* until transferred, at which time the status |
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* will be set to 0. (Use of this value here has |
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* nothing to do with the other uses of the |
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* field, but simplifies mechanics.) |
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* PROPAGATE: A releaseShared should be propagated to other |
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* nodes. This is set (for head node only) in |
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* doReleaseShared to ensure propagation |
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* continues, even if other operations have |
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* since intervened. |
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* 0: None of the above |
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* |
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* The values are arranged numerically to simplify use. |
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* Non-negative values mean that a node doesn't need to |
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* signal. So, most code doesn't need to check for particular |
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* values, just for sign. |
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* |
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* The field is initialized to 0 for normal sync nodes, and |
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* CONDITION for condition nodes. It is modified using CAS |
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* (or when possible, unconditional volatile writes). |
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*/ |
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volatile int waitStatus; |
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|
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/** |
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* Link to predecessor node that current node/thread relies on |
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* for checking waitStatus. Assigned during enqueuing, and nulled |
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* out (for sake of GC) only upon dequeuing. Also, upon |
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* cancellation of a predecessor, we short-circuit while |
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* finding a non-cancelled one, which will always exist |
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* because the head node is never cancelled: A node becomes |
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* head only as a result of successful acquire. A |
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* cancelled thread never succeeds in acquiring, and a thread only |
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* cancels itself, not any other node. |
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*/ |
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volatile Node prev; |
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|
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/** |
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* Link to the successor node that the current node/thread |
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* unparks upon release. Assigned during enqueuing, adjusted |
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* when bypassing cancelled predecessors, and nulled out (for |
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* sake of GC) when dequeued. The enq operation does not |
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* assign next field of a predecessor until after attachment, |
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* so seeing a null next field does not necessarily mean that |
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* node is at end of queue. However, if a next field appears |
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* to be null, we can scan prev's from the tail to |
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* double-check. The next field of cancelled nodes is set to |
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* point to the node itself instead of null, to make life |
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* easier for isOnSyncQueue. |
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*/ |
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volatile Node next; |
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|
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/** |
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* The thread that enqueued this node. Initialized on |
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* construction and nulled out after use. |
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*/ |
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volatile Thread thread; |
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|
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/** |
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* Link to next node waiting on condition, or the special |
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* value SHARED. Because condition queues are accessed only |
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* when holding in exclusive mode, we just need a simple |
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* linked queue to hold nodes while they are waiting on |
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* conditions. They are then transferred to the queue to |
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* re-acquire. And because conditions can only be exclusive, |
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* we save a field by using special value to indicate shared |
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* mode. |
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*/ |
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Node nextWaiter; |
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|
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/** |
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* Returns true if node is waiting in shared mode. |
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*/ |
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final boolean isShared() { |
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return nextWaiter == SHARED; |
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} |
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|
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/** |
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* Returns previous node, or throws NullPointerException if null. |
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* Use when predecessor cannot be null. The null check could |
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* be elided, but is present to help the VM. |
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* |
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* @return the predecessor of this node |
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*/ |
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final Node predecessor() { |
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Node p = prev; |
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if (p == null) |
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throw new NullPointerException(); |
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else |
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return p; |
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} |
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|
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/** Establishes initial head or SHARED marker. */ |
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Node() {} |
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|
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/** Constructor used by addWaiter. */ |
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Node(Node nextWaiter) { |
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this.nextWaiter = nextWaiter; |
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U.putObject(this, THREAD, Thread.currentThread()); |
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} |
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|
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/** Constructor used by addConditionWaiter. */ |
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Node(int waitStatus) { |
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U.putInt(this, WAITSTATUS, waitStatus); |
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U.putObject(this, THREAD, Thread.currentThread()); |
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} |
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|
501 |
/** CASes waitStatus field. */ |
502 |
final boolean compareAndSetWaitStatus(int expect, int update) { |
503 |
return U.compareAndSwapInt(this, WAITSTATUS, expect, update); |
504 |
} |
505 |
|
506 |
/** CASes next field. */ |
507 |
final boolean compareAndSetNext(Node expect, Node update) { |
508 |
return U.compareAndSwapObject(this, NEXT, expect, update); |
509 |
} |
510 |
|
511 |
private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); |
512 |
private static final long NEXT; |
513 |
static final long PREV; |
514 |
private static final long THREAD; |
515 |
private static final long WAITSTATUS; |
516 |
static { |
517 |
try { |
518 |
NEXT = U.objectFieldOffset |
519 |
(Node.class.getDeclaredField("next")); |
520 |
PREV = U.objectFieldOffset |
521 |
(Node.class.getDeclaredField("prev")); |
522 |
THREAD = U.objectFieldOffset |
523 |
(Node.class.getDeclaredField("thread")); |
524 |
WAITSTATUS = U.objectFieldOffset |
525 |
(Node.class.getDeclaredField("waitStatus")); |
526 |
} catch (ReflectiveOperationException e) { |
527 |
throw new Error(e); |
528 |
} |
529 |
} |
530 |
} |
531 |
|
532 |
/** |
533 |
* Head of the wait queue, lazily initialized. Except for |
534 |
* initialization, it is modified only via method setHead. Note: |
535 |
* If head exists, its waitStatus is guaranteed not to be |
536 |
* CANCELLED. |
537 |
*/ |
538 |
private transient volatile Node head; |
539 |
|
540 |
/** |
541 |
* Tail of the wait queue, lazily initialized. Modified only via |
542 |
* method enq to add new wait node. |
543 |
*/ |
544 |
private transient volatile Node tail; |
545 |
|
546 |
/** |
547 |
* The synchronization state. |
548 |
*/ |
549 |
private volatile int state; |
550 |
|
551 |
/** |
552 |
* Returns the current value of synchronization state. |
553 |
* This operation has memory semantics of a {@code volatile} read. |
554 |
* @return current state value |
555 |
*/ |
556 |
protected final int getState() { |
557 |
return state; |
558 |
} |
559 |
|
560 |
/** |
561 |
* Sets the value of synchronization state. |
562 |
* This operation has memory semantics of a {@code volatile} write. |
563 |
* @param newState the new state value |
564 |
*/ |
565 |
protected final void setState(int newState) { |
566 |
state = newState; |
567 |
} |
568 |
|
569 |
/** |
570 |
* Atomically sets synchronization state to the given updated |
571 |
* value if the current state value equals the expected value. |
572 |
* This operation has memory semantics of a {@code volatile} read |
573 |
* and write. |
574 |
* |
575 |
* @param expect the expected value |
576 |
* @param update the new value |
577 |
* @return {@code true} if successful. False return indicates that the actual |
578 |
* value was not equal to the expected value. |
579 |
*/ |
580 |
protected final boolean compareAndSetState(int expect, int update) { |
581 |
return U.compareAndSwapInt(this, STATE, expect, update); |
582 |
} |
583 |
|
584 |
// Queuing utilities |
585 |
|
586 |
/** |
587 |
* The number of nanoseconds for which it is faster to spin |
588 |
* rather than to use timed park. A rough estimate suffices |
589 |
* to improve responsiveness with very short timeouts. |
590 |
*/ |
591 |
static final long SPIN_FOR_TIMEOUT_THRESHOLD = 1000L; |
592 |
|
593 |
/** |
594 |
* Inserts node into queue, initializing if necessary. See picture above. |
595 |
* @param node the node to insert |
596 |
* @return node's predecessor |
597 |
*/ |
598 |
private Node enq(Node node) { |
599 |
for (;;) { |
600 |
Node oldTail = tail; |
601 |
if (oldTail != null) { |
602 |
U.putObject(node, Node.PREV, oldTail); |
603 |
if (compareAndSetTail(oldTail, node)) { |
604 |
oldTail.next = node; |
605 |
return oldTail; |
606 |
} |
607 |
} else { |
608 |
initializeSyncQueue(); |
609 |
} |
610 |
} |
611 |
} |
612 |
|
613 |
/** |
614 |
* Creates and enqueues node for current thread and given mode. |
615 |
* |
616 |
* @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared |
617 |
* @return the new node |
618 |
*/ |
619 |
private Node addWaiter(Node mode) { |
620 |
Node node = new Node(mode); |
621 |
|
622 |
for (;;) { |
623 |
Node oldTail = tail; |
624 |
if (oldTail != null) { |
625 |
U.putObject(node, Node.PREV, oldTail); |
626 |
if (compareAndSetTail(oldTail, node)) { |
627 |
oldTail.next = node; |
628 |
return node; |
629 |
} |
630 |
} else { |
631 |
initializeSyncQueue(); |
632 |
} |
633 |
} |
634 |
} |
635 |
|
636 |
/** |
637 |
* Sets head of queue to be node, thus dequeuing. Called only by |
638 |
* acquire methods. Also nulls out unused fields for sake of GC |
639 |
* and to suppress unnecessary signals and traversals. |
640 |
* |
641 |
* @param node the node |
642 |
*/ |
643 |
private void setHead(Node node) { |
644 |
head = node; |
645 |
node.thread = null; |
646 |
node.prev = null; |
647 |
} |
648 |
|
649 |
/** |
650 |
* Wakes up node's successor, if one exists. |
651 |
* |
652 |
* @param node the node |
653 |
*/ |
654 |
private void unparkSuccessor(Node node) { |
655 |
/* |
656 |
* If status is negative (i.e., possibly needing signal) try |
657 |
* to clear in anticipation of signalling. It is OK if this |
658 |
* fails or if status is changed by waiting thread. |
659 |
*/ |
660 |
int ws = node.waitStatus; |
661 |
if (ws < 0) |
662 |
node.compareAndSetWaitStatus(ws, 0); |
663 |
|
664 |
/* |
665 |
* Thread to unpark is held in successor, which is normally |
666 |
* just the next node. But if cancelled or apparently null, |
667 |
* traverse backwards from tail to find the actual |
668 |
* non-cancelled successor. |
669 |
*/ |
670 |
Node s = node.next; |
671 |
if (s == null || s.waitStatus > 0) { |
672 |
s = null; |
673 |
for (Node p = tail; p != node && p != null; p = p.prev) |
674 |
if (p.waitStatus <= 0) |
675 |
s = p; |
676 |
} |
677 |
if (s != null) |
678 |
LockSupport.unpark(s.thread); |
679 |
} |
680 |
|
681 |
/** |
682 |
* Release action for shared mode -- signals successor and ensures |
683 |
* propagation. (Note: For exclusive mode, release just amounts |
684 |
* to calling unparkSuccessor of head if it needs signal.) |
685 |
*/ |
686 |
private void doReleaseShared() { |
687 |
/* |
688 |
* Ensure that a release propagates, even if there are other |
689 |
* in-progress acquires/releases. This proceeds in the usual |
690 |
* way of trying to unparkSuccessor of head if it needs |
691 |
* signal. But if it does not, status is set to PROPAGATE to |
692 |
* ensure that upon release, propagation continues. |
693 |
* Additionally, we must loop in case a new node is added |
694 |
* while we are doing this. Also, unlike other uses of |
695 |
* unparkSuccessor, we need to know if CAS to reset status |
696 |
* fails, if so rechecking. |
697 |
*/ |
698 |
for (;;) { |
699 |
Node h = head; |
700 |
if (h != null && h != tail) { |
701 |
int ws = h.waitStatus; |
702 |
if (ws == Node.SIGNAL) { |
703 |
if (!h.compareAndSetWaitStatus(Node.SIGNAL, 0)) |
704 |
continue; // loop to recheck cases |
705 |
unparkSuccessor(h); |
706 |
} |
707 |
else if (ws == 0 && |
708 |
!h.compareAndSetWaitStatus(0, Node.PROPAGATE)) |
709 |
continue; // loop on failed CAS |
710 |
} |
711 |
if (h == head) // loop if head changed |
712 |
break; |
713 |
} |
714 |
} |
715 |
|
716 |
/** |
717 |
* Sets head of queue, and checks if successor may be waiting |
718 |
* in shared mode, if so propagating if either propagate > 0 or |
719 |
* PROPAGATE status was set. |
720 |
* |
721 |
* @param node the node |
722 |
* @param propagate the return value from a tryAcquireShared |
723 |
*/ |
724 |
private void setHeadAndPropagate(Node node, int propagate) { |
725 |
Node h = head; // Record old head for check below |
726 |
setHead(node); |
727 |
/* |
728 |
* Try to signal next queued node if: |
729 |
* Propagation was indicated by caller, |
730 |
* or was recorded (as h.waitStatus either before |
731 |
* or after setHead) by a previous operation |
732 |
* (note: this uses sign-check of waitStatus because |
733 |
* PROPAGATE status may transition to SIGNAL.) |
734 |
* and |
735 |
* The next node is waiting in shared mode, |
736 |
* or we don't know, because it appears null |
737 |
* |
738 |
* The conservatism in both of these checks may cause |
739 |
* unnecessary wake-ups, but only when there are multiple |
740 |
* racing acquires/releases, so most need signals now or soon |
741 |
* anyway. |
742 |
*/ |
743 |
if (propagate > 0 || h == null || h.waitStatus < 0 || |
744 |
(h = head) == null || h.waitStatus < 0) { |
745 |
Node s = node.next; |
746 |
if (s == null || s.isShared()) |
747 |
doReleaseShared(); |
748 |
} |
749 |
} |
750 |
|
751 |
// Utilities for various versions of acquire |
752 |
|
753 |
/** |
754 |
* Cancels an ongoing attempt to acquire. |
755 |
* |
756 |
* @param node the node |
757 |
*/ |
758 |
private void cancelAcquire(Node node) { |
759 |
// Ignore if node doesn't exist |
760 |
if (node == null) |
761 |
return; |
762 |
|
763 |
node.thread = null; |
764 |
|
765 |
// Skip cancelled predecessors |
766 |
Node pred = node.prev; |
767 |
while (pred.waitStatus > 0) |
768 |
node.prev = pred = pred.prev; |
769 |
|
770 |
// predNext is the apparent node to unsplice. CASes below will |
771 |
// fail if not, in which case, we lost race vs another cancel |
772 |
// or signal, so no further action is necessary, although with |
773 |
// a possibility that a cancelled node may transiently remain |
774 |
// reachable. |
775 |
Node predNext = pred.next; |
776 |
|
777 |
// Can use unconditional write instead of CAS here. |
778 |
// After this atomic step, other Nodes can skip past us. |
779 |
// Before, we are free of interference from other threads. |
780 |
node.waitStatus = Node.CANCELLED; |
781 |
|
782 |
// If we are the tail, remove ourselves. |
783 |
if (node == tail && compareAndSetTail(node, pred)) { |
784 |
pred.compareAndSetNext(predNext, null); |
785 |
} else { |
786 |
// If successor needs signal, try to set pred's next-link |
787 |
// so it will get one. Otherwise wake it up to propagate. |
788 |
int ws; |
789 |
if (pred != head && |
790 |
((ws = pred.waitStatus) == Node.SIGNAL || |
791 |
(ws <= 0 && pred.compareAndSetWaitStatus(ws, Node.SIGNAL))) && |
792 |
pred.thread != null) { |
793 |
Node next = node.next; |
794 |
if (next != null && next.waitStatus <= 0) |
795 |
pred.compareAndSetNext(predNext, next); |
796 |
} else { |
797 |
unparkSuccessor(node); |
798 |
} |
799 |
|
800 |
node.next = node; // help GC |
801 |
} |
802 |
} |
803 |
|
804 |
/** |
805 |
* Checks and updates status for a node that failed to acquire. |
806 |
* Returns true if thread should block. This is the main signal |
807 |
* control in all acquire loops. Requires that pred == node.prev. |
808 |
* |
809 |
* @param pred node's predecessor holding status |
810 |
* @param node the node |
811 |
* @return {@code true} if thread should block |
812 |
*/ |
813 |
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) { |
814 |
int ws = pred.waitStatus; |
815 |
if (ws == Node.SIGNAL) |
816 |
/* |
817 |
* This node has already set status asking a release |
818 |
* to signal it, so it can safely park. |
819 |
*/ |
820 |
return true; |
821 |
if (ws > 0) { |
822 |
/* |
823 |
* Predecessor was cancelled. Skip over predecessors and |
824 |
* indicate retry. |
825 |
*/ |
826 |
do { |
827 |
node.prev = pred = pred.prev; |
828 |
} while (pred.waitStatus > 0); |
829 |
pred.next = node; |
830 |
} else { |
831 |
/* |
832 |
* waitStatus must be 0 or PROPAGATE. Indicate that we |
833 |
* need a signal, but don't park yet. Caller will need to |
834 |
* retry to make sure it cannot acquire before parking. |
835 |
*/ |
836 |
pred.compareAndSetWaitStatus(ws, Node.SIGNAL); |
837 |
} |
838 |
return false; |
839 |
} |
840 |
|
841 |
/** |
842 |
* Convenience method to interrupt current thread. |
843 |
*/ |
844 |
static void selfInterrupt() { |
845 |
Thread.currentThread().interrupt(); |
846 |
} |
847 |
|
848 |
/** |
849 |
* Convenience method to park and then check if interrupted. |
850 |
* |
851 |
* @return {@code true} if interrupted |
852 |
*/ |
853 |
private final boolean parkAndCheckInterrupt() { |
854 |
LockSupport.park(this); |
855 |
return Thread.interrupted(); |
856 |
} |
857 |
|
858 |
/* |
859 |
* Various flavors of acquire, varying in exclusive/shared and |
860 |
* control modes. Each is mostly the same, but annoyingly |
861 |
* different. Only a little bit of factoring is possible due to |
862 |
* interactions of exception mechanics (including ensuring that we |
863 |
* cancel if tryAcquire throws exception) and other control, at |
864 |
* least not without hurting performance too much. |
865 |
*/ |
866 |
|
867 |
/** |
868 |
* Acquires in exclusive uninterruptible mode for thread already in |
869 |
* queue. Used by condition wait methods as well as acquire. |
870 |
* |
871 |
* @param node the node |
872 |
* @param arg the acquire argument |
873 |
* @return {@code true} if interrupted while waiting |
874 |
*/ |
875 |
final boolean acquireQueued(final Node node, int arg) { |
876 |
boolean interrupted = false; |
877 |
try { |
878 |
for (;;) { |
879 |
final Node p = node.predecessor(); |
880 |
if (p == head && tryAcquire(arg)) { |
881 |
setHead(node); |
882 |
p.next = null; // help GC |
883 |
return interrupted; |
884 |
} |
885 |
if (shouldParkAfterFailedAcquire(p, node)) |
886 |
interrupted |= parkAndCheckInterrupt(); |
887 |
} |
888 |
} catch (Throwable t) { |
889 |
cancelAcquire(node); |
890 |
if (interrupted) |
891 |
selfInterrupt(); |
892 |
throw t; |
893 |
} |
894 |
} |
895 |
|
896 |
/** |
897 |
* Acquires in exclusive interruptible mode. |
898 |
* @param arg the acquire argument |
899 |
*/ |
900 |
private void doAcquireInterruptibly(int arg) |
901 |
throws InterruptedException { |
902 |
final Node node = addWaiter(Node.EXCLUSIVE); |
903 |
try { |
904 |
for (;;) { |
905 |
final Node p = node.predecessor(); |
906 |
if (p == head && tryAcquire(arg)) { |
907 |
setHead(node); |
908 |
p.next = null; // help GC |
909 |
return; |
910 |
} |
911 |
if (shouldParkAfterFailedAcquire(p, node) && |
912 |
parkAndCheckInterrupt()) |
913 |
throw new InterruptedException(); |
914 |
} |
915 |
} catch (Throwable t) { |
916 |
cancelAcquire(node); |
917 |
throw t; |
918 |
} |
919 |
} |
920 |
|
921 |
/** |
922 |
* Acquires in exclusive timed mode. |
923 |
* |
924 |
* @param arg the acquire argument |
925 |
* @param nanosTimeout max wait time |
926 |
* @return {@code true} if acquired |
927 |
*/ |
928 |
private boolean doAcquireNanos(int arg, long nanosTimeout) |
929 |
throws InterruptedException { |
930 |
if (nanosTimeout <= 0L) |
931 |
return false; |
932 |
final long deadline = System.nanoTime() + nanosTimeout; |
933 |
final Node node = addWaiter(Node.EXCLUSIVE); |
934 |
try { |
935 |
for (;;) { |
936 |
final Node p = node.predecessor(); |
937 |
if (p == head && tryAcquire(arg)) { |
938 |
setHead(node); |
939 |
p.next = null; // help GC |
940 |
return true; |
941 |
} |
942 |
nanosTimeout = deadline - System.nanoTime(); |
943 |
if (nanosTimeout <= 0L) { |
944 |
cancelAcquire(node); |
945 |
return false; |
946 |
} |
947 |
if (shouldParkAfterFailedAcquire(p, node) && |
948 |
nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD) |
949 |
LockSupport.parkNanos(this, nanosTimeout); |
950 |
if (Thread.interrupted()) |
951 |
throw new InterruptedException(); |
952 |
} |
953 |
} catch (Throwable t) { |
954 |
cancelAcquire(node); |
955 |
throw t; |
956 |
} |
957 |
} |
958 |
|
959 |
/** |
960 |
* Acquires in shared uninterruptible mode. |
961 |
* @param arg the acquire argument |
962 |
*/ |
963 |
private void doAcquireShared(int arg) { |
964 |
final Node node = addWaiter(Node.SHARED); |
965 |
boolean interrupted = false; |
966 |
try { |
967 |
for (;;) { |
968 |
final Node p = node.predecessor(); |
969 |
if (p == head) { |
970 |
int r = tryAcquireShared(arg); |
971 |
if (r >= 0) { |
972 |
setHeadAndPropagate(node, r); |
973 |
p.next = null; // help GC |
974 |
return; |
975 |
} |
976 |
} |
977 |
if (shouldParkAfterFailedAcquire(p, node)) |
978 |
interrupted |= parkAndCheckInterrupt(); |
979 |
} |
980 |
} catch (Throwable t) { |
981 |
cancelAcquire(node); |
982 |
throw t; |
983 |
} finally { |
984 |
if (interrupted) |
985 |
selfInterrupt(); |
986 |
} |
987 |
} |
988 |
|
989 |
/** |
990 |
* Acquires in shared interruptible mode. |
991 |
* @param arg the acquire argument |
992 |
*/ |
993 |
private void doAcquireSharedInterruptibly(int arg) |
994 |
throws InterruptedException { |
995 |
final Node node = addWaiter(Node.SHARED); |
996 |
try { |
997 |
for (;;) { |
998 |
final Node p = node.predecessor(); |
999 |
if (p == head) { |
1000 |
int r = tryAcquireShared(arg); |
1001 |
if (r >= 0) { |
1002 |
setHeadAndPropagate(node, r); |
1003 |
p.next = null; // help GC |
1004 |
return; |
1005 |
} |
1006 |
} |
1007 |
if (shouldParkAfterFailedAcquire(p, node) && |
1008 |
parkAndCheckInterrupt()) |
1009 |
throw new InterruptedException(); |
1010 |
} |
1011 |
} catch (Throwable t) { |
1012 |
cancelAcquire(node); |
1013 |
throw t; |
1014 |
} |
1015 |
} |
1016 |
|
1017 |
/** |
1018 |
* Acquires in shared timed mode. |
1019 |
* |
1020 |
* @param arg the acquire argument |
1021 |
* @param nanosTimeout max wait time |
1022 |
* @return {@code true} if acquired |
1023 |
*/ |
1024 |
private boolean doAcquireSharedNanos(int arg, long nanosTimeout) |
1025 |
throws InterruptedException { |
1026 |
if (nanosTimeout <= 0L) |
1027 |
return false; |
1028 |
final long deadline = System.nanoTime() + nanosTimeout; |
1029 |
final Node node = addWaiter(Node.SHARED); |
1030 |
try { |
1031 |
for (;;) { |
1032 |
final Node p = node.predecessor(); |
1033 |
if (p == head) { |
1034 |
int r = tryAcquireShared(arg); |
1035 |
if (r >= 0) { |
1036 |
setHeadAndPropagate(node, r); |
1037 |
p.next = null; // help GC |
1038 |
return true; |
1039 |
} |
1040 |
} |
1041 |
nanosTimeout = deadline - System.nanoTime(); |
1042 |
if (nanosTimeout <= 0L) { |
1043 |
cancelAcquire(node); |
1044 |
return false; |
1045 |
} |
1046 |
if (shouldParkAfterFailedAcquire(p, node) && |
1047 |
nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD) |
1048 |
LockSupport.parkNanos(this, nanosTimeout); |
1049 |
if (Thread.interrupted()) |
1050 |
throw new InterruptedException(); |
1051 |
} |
1052 |
} catch (Throwable t) { |
1053 |
cancelAcquire(node); |
1054 |
throw t; |
1055 |
} |
1056 |
} |
1057 |
|
1058 |
// Main exported methods |
1059 |
|
1060 |
/** |
1061 |
* Attempts to acquire in exclusive mode. This method should query |
1062 |
* if the state of the object permits it to be acquired in the |
1063 |
* exclusive mode, and if so to acquire it. |
1064 |
* |
1065 |
* <p>This method is always invoked by the thread performing |
1066 |
* acquire. If this method reports failure, the acquire method |
1067 |
* may queue the thread, if it is not already queued, until it is |
1068 |
* signalled by a release from some other thread. This can be used |
1069 |
* to implement method {@link Lock#tryLock()}. |
1070 |
* |
1071 |
* <p>The default |
1072 |
* implementation throws {@link UnsupportedOperationException}. |
1073 |
* |
1074 |
* @param arg the acquire argument. This value is always the one |
1075 |
* passed to an acquire method, or is the value saved on entry |
1076 |
* to a condition wait. The value is otherwise uninterpreted |
1077 |
* and can represent anything you like. |
1078 |
* @return {@code true} if successful. Upon success, this object has |
1079 |
* been acquired. |
1080 |
* @throws IllegalMonitorStateException if acquiring would place this |
1081 |
* synchronizer in an illegal state. This exception must be |
1082 |
* thrown in a consistent fashion for synchronization to work |
1083 |
* correctly. |
1084 |
* @throws UnsupportedOperationException if exclusive mode is not supported |
1085 |
*/ |
1086 |
protected boolean tryAcquire(int arg) { |
1087 |
throw new UnsupportedOperationException(); |
1088 |
} |
1089 |
|
1090 |
/** |
1091 |
* Attempts to set the state to reflect a release in exclusive |
1092 |
* mode. |
1093 |
* |
1094 |
* <p>This method is always invoked by the thread performing release. |
1095 |
* |
1096 |
* <p>The default implementation throws |
1097 |
* {@link UnsupportedOperationException}. |
1098 |
* |
1099 |
* @param arg the release argument. This value is always the one |
1100 |
* passed to a release method, or the current state value upon |
1101 |
* entry to a condition wait. The value is otherwise |
1102 |
* uninterpreted and can represent anything you like. |
1103 |
* @return {@code true} if this object is now in a fully released |
1104 |
* state, so that any waiting threads may attempt to acquire; |
1105 |
* and {@code false} otherwise. |
1106 |
* @throws IllegalMonitorStateException if releasing would place this |
1107 |
* synchronizer in an illegal state. This exception must be |
1108 |
* thrown in a consistent fashion for synchronization to work |
1109 |
* correctly. |
1110 |
* @throws UnsupportedOperationException if exclusive mode is not supported |
1111 |
*/ |
1112 |
protected boolean tryRelease(int arg) { |
1113 |
throw new UnsupportedOperationException(); |
1114 |
} |
1115 |
|
1116 |
/** |
1117 |
* Attempts to acquire in shared mode. This method should query if |
1118 |
* the state of the object permits it to be acquired in the shared |
1119 |
* mode, and if so to acquire it. |
1120 |
* |
1121 |
* <p>This method is always invoked by the thread performing |
1122 |
* acquire. If this method reports failure, the acquire method |
1123 |
* may queue the thread, if it is not already queued, until it is |
1124 |
* signalled by a release from some other thread. |
1125 |
* |
1126 |
* <p>The default implementation throws {@link |
1127 |
* UnsupportedOperationException}. |
1128 |
* |
1129 |
* @param arg the acquire argument. This value is always the one |
1130 |
* passed to an acquire method, or is the value saved on entry |
1131 |
* to a condition wait. The value is otherwise uninterpreted |
1132 |
* and can represent anything you like. |
1133 |
* @return a negative value on failure; zero if acquisition in shared |
1134 |
* mode succeeded but no subsequent shared-mode acquire can |
1135 |
* succeed; and a positive value if acquisition in shared |
1136 |
* mode succeeded and subsequent shared-mode acquires might |
1137 |
* also succeed, in which case a subsequent waiting thread |
1138 |
* must check availability. (Support for three different |
1139 |
* return values enables this method to be used in contexts |
1140 |
* where acquires only sometimes act exclusively.) Upon |
1141 |
* success, this object has been acquired. |
1142 |
* @throws IllegalMonitorStateException if acquiring would place this |
1143 |
* synchronizer in an illegal state. This exception must be |
1144 |
* thrown in a consistent fashion for synchronization to work |
1145 |
* correctly. |
1146 |
* @throws UnsupportedOperationException if shared mode is not supported |
1147 |
*/ |
1148 |
protected int tryAcquireShared(int arg) { |
1149 |
throw new UnsupportedOperationException(); |
1150 |
} |
1151 |
|
1152 |
/** |
1153 |
* Attempts to set the state to reflect a release in shared mode. |
1154 |
* |
1155 |
* <p>This method is always invoked by the thread performing release. |
1156 |
* |
1157 |
* <p>The default implementation throws |
1158 |
* {@link UnsupportedOperationException}. |
1159 |
* |
1160 |
* @param arg the release argument. This value is always the one |
1161 |
* passed to a release method, or the current state value upon |
1162 |
* entry to a condition wait. The value is otherwise |
1163 |
* uninterpreted and can represent anything you like. |
1164 |
* @return {@code true} if this release of shared mode may permit a |
1165 |
* waiting acquire (shared or exclusive) to succeed; and |
1166 |
* {@code false} otherwise |
1167 |
* @throws IllegalMonitorStateException if releasing would place this |
1168 |
* synchronizer in an illegal state. This exception must be |
1169 |
* thrown in a consistent fashion for synchronization to work |
1170 |
* correctly. |
1171 |
* @throws UnsupportedOperationException if shared mode is not supported |
1172 |
*/ |
1173 |
protected boolean tryReleaseShared(int arg) { |
1174 |
throw new UnsupportedOperationException(); |
1175 |
} |
1176 |
|
1177 |
/** |
1178 |
* Returns {@code true} if synchronization is held exclusively with |
1179 |
* respect to the current (calling) thread. This method is invoked |
1180 |
* upon each call to a {@link ConditionObject} method. |
1181 |
* |
1182 |
* <p>The default implementation throws {@link |
1183 |
* UnsupportedOperationException}. This method is invoked |
1184 |
* internally only within {@link ConditionObject} methods, so need |
1185 |
* not be defined if conditions are not used. |
1186 |
* |
1187 |
* @return {@code true} if synchronization is held exclusively; |
1188 |
* {@code false} otherwise |
1189 |
* @throws UnsupportedOperationException if conditions are not supported |
1190 |
*/ |
1191 |
protected boolean isHeldExclusively() { |
1192 |
throw new UnsupportedOperationException(); |
1193 |
} |
1194 |
|
1195 |
/** |
1196 |
* Acquires in exclusive mode, ignoring interrupts. Implemented |
1197 |
* by invoking at least once {@link #tryAcquire}, |
1198 |
* returning on success. Otherwise the thread is queued, possibly |
1199 |
* repeatedly blocking and unblocking, invoking {@link |
1200 |
* #tryAcquire} until success. This method can be used |
1201 |
* to implement method {@link Lock#lock}. |
1202 |
* |
1203 |
* @param arg the acquire argument. This value is conveyed to |
1204 |
* {@link #tryAcquire} but is otherwise uninterpreted and |
1205 |
* can represent anything you like. |
1206 |
*/ |
1207 |
public final void acquire(int arg) { |
1208 |
if (!tryAcquire(arg) && |
1209 |
acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) |
1210 |
selfInterrupt(); |
1211 |
} |
1212 |
|
1213 |
/** |
1214 |
* Acquires in exclusive mode, aborting if interrupted. |
1215 |
* Implemented by first checking interrupt status, then invoking |
1216 |
* at least once {@link #tryAcquire}, returning on |
1217 |
* success. Otherwise the thread is queued, possibly repeatedly |
1218 |
* blocking and unblocking, invoking {@link #tryAcquire} |
1219 |
* until success or the thread is interrupted. This method can be |
1220 |
* used to implement method {@link Lock#lockInterruptibly}. |
1221 |
* |
1222 |
* @param arg the acquire argument. This value is conveyed to |
1223 |
* {@link #tryAcquire} but is otherwise uninterpreted and |
1224 |
* can represent anything you like. |
1225 |
* @throws InterruptedException if the current thread is interrupted |
1226 |
*/ |
1227 |
public final void acquireInterruptibly(int arg) |
1228 |
throws InterruptedException { |
1229 |
if (Thread.interrupted()) |
1230 |
throw new InterruptedException(); |
1231 |
if (!tryAcquire(arg)) |
1232 |
doAcquireInterruptibly(arg); |
1233 |
} |
1234 |
|
1235 |
/** |
1236 |
* Attempts to acquire in exclusive mode, aborting if interrupted, |
1237 |
* and failing if the given timeout elapses. Implemented by first |
1238 |
* checking interrupt status, then invoking at least once {@link |
1239 |
* #tryAcquire}, returning on success. Otherwise, the thread is |
1240 |
* queued, possibly repeatedly blocking and unblocking, invoking |
1241 |
* {@link #tryAcquire} until success or the thread is interrupted |
1242 |
* or the timeout elapses. This method can be used to implement |
1243 |
* method {@link Lock#tryLock(long, TimeUnit)}. |
1244 |
* |
1245 |
* @param arg the acquire argument. This value is conveyed to |
1246 |
* {@link #tryAcquire} but is otherwise uninterpreted and |
1247 |
* can represent anything you like. |
1248 |
* @param nanosTimeout the maximum number of nanoseconds to wait |
1249 |
* @return {@code true} if acquired; {@code false} if timed out |
1250 |
* @throws InterruptedException if the current thread is interrupted |
1251 |
*/ |
1252 |
public final boolean tryAcquireNanos(int arg, long nanosTimeout) |
1253 |
throws InterruptedException { |
1254 |
if (Thread.interrupted()) |
1255 |
throw new InterruptedException(); |
1256 |
return tryAcquire(arg) || |
1257 |
doAcquireNanos(arg, nanosTimeout); |
1258 |
} |
1259 |
|
1260 |
/** |
1261 |
* Releases in exclusive mode. Implemented by unblocking one or |
1262 |
* more threads if {@link #tryRelease} returns true. |
1263 |
* This method can be used to implement method {@link Lock#unlock}. |
1264 |
* |
1265 |
* @param arg the release argument. This value is conveyed to |
1266 |
* {@link #tryRelease} but is otherwise uninterpreted and |
1267 |
* can represent anything you like. |
1268 |
* @return the value returned from {@link #tryRelease} |
1269 |
*/ |
1270 |
public final boolean release(int arg) { |
1271 |
if (tryRelease(arg)) { |
1272 |
Node h = head; |
1273 |
if (h != null && h.waitStatus != 0) |
1274 |
unparkSuccessor(h); |
1275 |
return true; |
1276 |
} |
1277 |
return false; |
1278 |
} |
1279 |
|
1280 |
/** |
1281 |
* Acquires in shared mode, ignoring interrupts. Implemented by |
1282 |
* first invoking at least once {@link #tryAcquireShared}, |
1283 |
* returning on success. Otherwise the thread is queued, possibly |
1284 |
* repeatedly blocking and unblocking, invoking {@link |
1285 |
* #tryAcquireShared} until success. |
1286 |
* |
1287 |
* @param arg the acquire argument. This value is conveyed to |
1288 |
* {@link #tryAcquireShared} but is otherwise uninterpreted |
1289 |
* and can represent anything you like. |
1290 |
*/ |
1291 |
public final void acquireShared(int arg) { |
1292 |
if (tryAcquireShared(arg) < 0) |
1293 |
doAcquireShared(arg); |
1294 |
} |
1295 |
|
1296 |
/** |
1297 |
* Acquires in shared mode, aborting if interrupted. Implemented |
1298 |
* by first checking interrupt status, then invoking at least once |
1299 |
* {@link #tryAcquireShared}, returning on success. Otherwise the |
1300 |
* thread is queued, possibly repeatedly blocking and unblocking, |
1301 |
* invoking {@link #tryAcquireShared} until success or the thread |
1302 |
* is interrupted. |
1303 |
* @param arg the acquire argument. |
1304 |
* This value is conveyed to {@link #tryAcquireShared} but is |
1305 |
* otherwise uninterpreted and can represent anything |
1306 |
* you like. |
1307 |
* @throws InterruptedException if the current thread is interrupted |
1308 |
*/ |
1309 |
public final void acquireSharedInterruptibly(int arg) |
1310 |
throws InterruptedException { |
1311 |
if (Thread.interrupted()) |
1312 |
throw new InterruptedException(); |
1313 |
if (tryAcquireShared(arg) < 0) |
1314 |
doAcquireSharedInterruptibly(arg); |
1315 |
} |
1316 |
|
1317 |
/** |
1318 |
* Attempts to acquire in shared mode, aborting if interrupted, and |
1319 |
* failing if the given timeout elapses. Implemented by first |
1320 |
* checking interrupt status, then invoking at least once {@link |
1321 |
* #tryAcquireShared}, returning on success. Otherwise, the |
1322 |
* thread is queued, possibly repeatedly blocking and unblocking, |
1323 |
* invoking {@link #tryAcquireShared} until success or the thread |
1324 |
* is interrupted or the timeout elapses. |
1325 |
* |
1326 |
* @param arg the acquire argument. This value is conveyed to |
1327 |
* {@link #tryAcquireShared} but is otherwise uninterpreted |
1328 |
* and can represent anything you like. |
1329 |
* @param nanosTimeout the maximum number of nanoseconds to wait |
1330 |
* @return {@code true} if acquired; {@code false} if timed out |
1331 |
* @throws InterruptedException if the current thread is interrupted |
1332 |
*/ |
1333 |
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) |
1334 |
throws InterruptedException { |
1335 |
if (Thread.interrupted()) |
1336 |
throw new InterruptedException(); |
1337 |
return tryAcquireShared(arg) >= 0 || |
1338 |
doAcquireSharedNanos(arg, nanosTimeout); |
1339 |
} |
1340 |
|
1341 |
/** |
1342 |
* Releases in shared mode. Implemented by unblocking one or more |
1343 |
* threads if {@link #tryReleaseShared} returns true. |
1344 |
* |
1345 |
* @param arg the release argument. This value is conveyed to |
1346 |
* {@link #tryReleaseShared} but is otherwise uninterpreted |
1347 |
* and can represent anything you like. |
1348 |
* @return the value returned from {@link #tryReleaseShared} |
1349 |
*/ |
1350 |
public final boolean releaseShared(int arg) { |
1351 |
if (tryReleaseShared(arg)) { |
1352 |
doReleaseShared(); |
1353 |
return true; |
1354 |
} |
1355 |
return false; |
1356 |
} |
1357 |
|
1358 |
// Queue inspection methods |
1359 |
|
1360 |
/** |
1361 |
* Queries whether any threads are waiting to acquire. Note that |
1362 |
* because cancellations due to interrupts and timeouts may occur |
1363 |
* at any time, a {@code true} return does not guarantee that any |
1364 |
* other thread will ever acquire. |
1365 |
* |
1366 |
* @return {@code true} if there may be other threads waiting to acquire |
1367 |
*/ |
1368 |
public final boolean hasQueuedThreads() { |
1369 |
for (Node p = tail, h = head; p != h && p != null; p = p.prev) |
1370 |
if (p.waitStatus <= 0) |
1371 |
return true; |
1372 |
return false; |
1373 |
} |
1374 |
|
1375 |
/** |
1376 |
* Queries whether any threads have ever contended to acquire this |
1377 |
* synchronizer; that is, if an acquire method has ever blocked. |
1378 |
* |
1379 |
* <p>In this implementation, this operation returns in |
1380 |
* constant time. |
1381 |
* |
1382 |
* @return {@code true} if there has ever been contention |
1383 |
*/ |
1384 |
public final boolean hasContended() { |
1385 |
return head != null; |
1386 |
} |
1387 |
|
1388 |
/** |
1389 |
* Returns the first (longest-waiting) thread in the queue, or |
1390 |
* {@code null} if no threads are currently queued. |
1391 |
* |
1392 |
* <p>In this implementation, this operation normally returns in |
1393 |
* constant time, but may iterate upon contention if other threads are |
1394 |
* concurrently modifying the queue. |
1395 |
* |
1396 |
* @return the first (longest-waiting) thread in the queue, or |
1397 |
* {@code null} if no threads are currently queued |
1398 |
*/ |
1399 |
public final Thread getFirstQueuedThread() { |
1400 |
// handle only fast path, else relay |
1401 |
return (head == tail) ? null : fullGetFirstQueuedThread(); |
1402 |
} |
1403 |
|
1404 |
/** |
1405 |
* Version of getFirstQueuedThread called when fastpath fails. |
1406 |
*/ |
1407 |
private Thread fullGetFirstQueuedThread() { |
1408 |
/* |
1409 |
* The first node is normally head.next. Try to get its |
1410 |
* thread field, ensuring consistent reads: If thread |
1411 |
* field is nulled out or s.prev is no longer head, then |
1412 |
* some other thread(s) concurrently performed setHead in |
1413 |
* between some of our reads. We try this twice before |
1414 |
* resorting to traversal. |
1415 |
*/ |
1416 |
Node h, s; |
1417 |
Thread st; |
1418 |
if (((h = head) != null && (s = h.next) != null && |
1419 |
s.prev == head && (st = s.thread) != null) || |
1420 |
((h = head) != null && (s = h.next) != null && |
1421 |
s.prev == head && (st = s.thread) != null)) |
1422 |
return st; |
1423 |
|
1424 |
/* |
1425 |
* Head's next field might not have been set yet, or may have |
1426 |
* been unset after setHead. So we must check to see if tail |
1427 |
* is actually first node. If not, we continue on, safely |
1428 |
* traversing from tail back to head to find first, |
1429 |
* guaranteeing termination. |
1430 |
*/ |
1431 |
|
1432 |
Thread firstThread = null; |
1433 |
for (Node p = tail; p != null && p != head; p = p.prev) { |
1434 |
Thread t = p.thread; |
1435 |
if (t != null) |
1436 |
firstThread = t; |
1437 |
} |
1438 |
return firstThread; |
1439 |
} |
1440 |
|
1441 |
/** |
1442 |
* Returns true if the given thread is currently queued. |
1443 |
* |
1444 |
* <p>This implementation traverses the queue to determine |
1445 |
* presence of the given thread. |
1446 |
* |
1447 |
* @param thread the thread |
1448 |
* @return {@code true} if the given thread is on the queue |
1449 |
* @throws NullPointerException if the thread is null |
1450 |
*/ |
1451 |
public final boolean isQueued(Thread thread) { |
1452 |
if (thread == null) |
1453 |
throw new NullPointerException(); |
1454 |
for (Node p = tail; p != null; p = p.prev) |
1455 |
if (p.thread == thread) |
1456 |
return true; |
1457 |
return false; |
1458 |
} |
1459 |
|
1460 |
/** |
1461 |
* Returns {@code true} if the apparent first queued thread, if one |
1462 |
* exists, is waiting in exclusive mode. If this method returns |
1463 |
* {@code true}, and the current thread is attempting to acquire in |
1464 |
* shared mode (that is, this method is invoked from {@link |
1465 |
* #tryAcquireShared}) then it is guaranteed that the current thread |
1466 |
* is not the first queued thread. Used only as a heuristic in |
1467 |
* ReentrantReadWriteLock. |
1468 |
*/ |
1469 |
final boolean apparentlyFirstQueuedIsExclusive() { |
1470 |
Node h, s; |
1471 |
return (h = head) != null && |
1472 |
(s = h.next) != null && |
1473 |
!s.isShared() && |
1474 |
s.thread != null; |
1475 |
} |
1476 |
|
1477 |
/** |
1478 |
* Queries whether any threads have been waiting to acquire longer |
1479 |
* than the current thread. |
1480 |
* |
1481 |
* <p>An invocation of this method is equivalent to (but may be |
1482 |
* more efficient than): |
1483 |
* <pre> {@code |
1484 |
* getFirstQueuedThread() != Thread.currentThread() |
1485 |
* && hasQueuedThreads()}</pre> |
1486 |
* |
1487 |
* <p>Note that because cancellations due to interrupts and |
1488 |
* timeouts may occur at any time, a {@code true} return does not |
1489 |
* guarantee that some other thread will acquire before the current |
1490 |
* thread. Likewise, it is possible for another thread to win a |
1491 |
* race to enqueue after this method has returned {@code false}, |
1492 |
* due to the queue being empty. |
1493 |
* |
1494 |
* <p>This method is designed to be used by a fair synchronizer to |
1495 |
* avoid <a href="AbstractQueuedSynchronizer.html#barging">barging</a>. |
1496 |
* Such a synchronizer's {@link #tryAcquire} method should return |
1497 |
* {@code false}, and its {@link #tryAcquireShared} method should |
1498 |
* return a negative value, if this method returns {@code true} |
1499 |
* (unless this is a reentrant acquire). For example, the {@code |
1500 |
* tryAcquire} method for a fair, reentrant, exclusive mode |
1501 |
* synchronizer might look like this: |
1502 |
* |
1503 |
* <pre> {@code |
1504 |
* protected boolean tryAcquire(int arg) { |
1505 |
* if (isHeldExclusively()) { |
1506 |
* // A reentrant acquire; increment hold count |
1507 |
* return true; |
1508 |
* } else if (hasQueuedPredecessors()) { |
1509 |
* return false; |
1510 |
* } else { |
1511 |
* // try to acquire normally |
1512 |
* } |
1513 |
* }}</pre> |
1514 |
* |
1515 |
* @return {@code true} if there is a queued thread preceding the |
1516 |
* current thread, and {@code false} if the current thread |
1517 |
* is at the head of the queue or the queue is empty |
1518 |
* @since 1.7 |
1519 |
*/ |
1520 |
public final boolean hasQueuedPredecessors() { |
1521 |
Node h, s; |
1522 |
if ((h = head) != null) { |
1523 |
if ((s = h.next) == null || s.waitStatus > 0) { |
1524 |
s = null; // traverse in case of concurrent cancellation |
1525 |
for (Node p = tail; p != h && p != null; p = p.prev) { |
1526 |
if (p.waitStatus <= 0) |
1527 |
s = p; |
1528 |
} |
1529 |
} |
1530 |
if (s != null && s.thread != Thread.currentThread()) |
1531 |
return true; |
1532 |
} |
1533 |
return false; |
1534 |
} |
1535 |
|
1536 |
// Instrumentation and monitoring methods |
1537 |
|
1538 |
/** |
1539 |
* Returns an estimate of the number of threads waiting to |
1540 |
* acquire. The value is only an estimate because the number of |
1541 |
* threads may change dynamically while this method traverses |
1542 |
* internal data structures. This method is designed for use in |
1543 |
* monitoring system state, not for synchronization control. |
1544 |
* |
1545 |
* @return the estimated number of threads waiting to acquire |
1546 |
*/ |
1547 |
public final int getQueueLength() { |
1548 |
int n = 0; |
1549 |
for (Node p = tail; p != null; p = p.prev) { |
1550 |
if (p.thread != null) |
1551 |
++n; |
1552 |
} |
1553 |
return n; |
1554 |
} |
1555 |
|
1556 |
/** |
1557 |
* Returns a collection containing threads that may be waiting to |
1558 |
* acquire. Because the actual set of threads may change |
1559 |
* dynamically while constructing this result, the returned |
1560 |
* collection is only a best-effort estimate. The elements of the |
1561 |
* returned collection are in no particular order. This method is |
1562 |
* designed to facilitate construction of subclasses that provide |
1563 |
* more extensive monitoring facilities. |
1564 |
* |
1565 |
* @return the collection of threads |
1566 |
*/ |
1567 |
public final Collection<Thread> getQueuedThreads() { |
1568 |
ArrayList<Thread> list = new ArrayList<>(); |
1569 |
for (Node p = tail; p != null; p = p.prev) { |
1570 |
Thread t = p.thread; |
1571 |
if (t != null) |
1572 |
list.add(t); |
1573 |
} |
1574 |
return list; |
1575 |
} |
1576 |
|
1577 |
/** |
1578 |
* Returns a collection containing threads that may be waiting to |
1579 |
* acquire in exclusive mode. This has the same properties |
1580 |
* as {@link #getQueuedThreads} except that it only returns |
1581 |
* those threads waiting due to an exclusive acquire. |
1582 |
* |
1583 |
* @return the collection of threads |
1584 |
*/ |
1585 |
public final Collection<Thread> getExclusiveQueuedThreads() { |
1586 |
ArrayList<Thread> list = new ArrayList<>(); |
1587 |
for (Node p = tail; p != null; p = p.prev) { |
1588 |
if (!p.isShared()) { |
1589 |
Thread t = p.thread; |
1590 |
if (t != null) |
1591 |
list.add(t); |
1592 |
} |
1593 |
} |
1594 |
return list; |
1595 |
} |
1596 |
|
1597 |
/** |
1598 |
* Returns a collection containing threads that may be waiting to |
1599 |
* acquire in shared mode. This has the same properties |
1600 |
* as {@link #getQueuedThreads} except that it only returns |
1601 |
* those threads waiting due to a shared acquire. |
1602 |
* |
1603 |
* @return the collection of threads |
1604 |
*/ |
1605 |
public final Collection<Thread> getSharedQueuedThreads() { |
1606 |
ArrayList<Thread> list = new ArrayList<>(); |
1607 |
for (Node p = tail; p != null; p = p.prev) { |
1608 |
if (p.isShared()) { |
1609 |
Thread t = p.thread; |
1610 |
if (t != null) |
1611 |
list.add(t); |
1612 |
} |
1613 |
} |
1614 |
return list; |
1615 |
} |
1616 |
|
1617 |
/** |
1618 |
* Returns a string identifying this synchronizer, as well as its state. |
1619 |
* The state, in brackets, includes the String {@code "State ="} |
1620 |
* followed by the current value of {@link #getState}, and either |
1621 |
* {@code "nonempty"} or {@code "empty"} depending on whether the |
1622 |
* queue is empty. |
1623 |
* |
1624 |
* @return a string identifying this synchronizer, as well as its state |
1625 |
*/ |
1626 |
public String toString() { |
1627 |
return super.toString() |
1628 |
+ "[State = " + getState() + ", " |
1629 |
+ (hasQueuedThreads() ? "non" : "") + "empty queue]"; |
1630 |
} |
1631 |
|
1632 |
|
1633 |
// Internal support methods for Conditions |
1634 |
|
1635 |
/** |
1636 |
* Returns true if a node, always one that was initially placed on |
1637 |
* a condition queue, is now waiting to reacquire on sync queue. |
1638 |
* @param node the node |
1639 |
* @return true if is reacquiring |
1640 |
*/ |
1641 |
final boolean isOnSyncQueue(Node node) { |
1642 |
if (node.waitStatus == Node.CONDITION || node.prev == null) |
1643 |
return false; |
1644 |
if (node.next != null) // If has successor, it must be on queue |
1645 |
return true; |
1646 |
/* |
1647 |
* node.prev can be non-null, but not yet on queue because |
1648 |
* the CAS to place it on queue can fail. So we have to |
1649 |
* traverse from tail to make sure it actually made it. It |
1650 |
* will always be near the tail in calls to this method, and |
1651 |
* unless the CAS failed (which is unlikely), it will be |
1652 |
* there, so we hardly ever traverse much. |
1653 |
*/ |
1654 |
return findNodeFromTail(node); |
1655 |
} |
1656 |
|
1657 |
/** |
1658 |
* Returns true if node is on sync queue by searching backwards from tail. |
1659 |
* Called only when needed by isOnSyncQueue. |
1660 |
* @return true if present |
1661 |
*/ |
1662 |
private boolean findNodeFromTail(Node node) { |
1663 |
// We check for node first, since it's likely to be at or near tail. |
1664 |
// tail is known to be non-null, so we could re-order to "save" |
1665 |
// one null check, but we leave it this way to help the VM. |
1666 |
for (Node p = tail;;) { |
1667 |
if (p == node) |
1668 |
return true; |
1669 |
if (p == null) |
1670 |
return false; |
1671 |
p = p.prev; |
1672 |
} |
1673 |
} |
1674 |
|
1675 |
/** |
1676 |
* Transfers a node from a condition queue onto sync queue. |
1677 |
* Returns true if successful. |
1678 |
* @param node the node |
1679 |
* @return true if successfully transferred (else the node was |
1680 |
* cancelled before signal) |
1681 |
*/ |
1682 |
final boolean transferForSignal(Node node) { |
1683 |
/* |
1684 |
* If cannot change waitStatus, the node has been cancelled. |
1685 |
*/ |
1686 |
if (!node.compareAndSetWaitStatus(Node.CONDITION, 0)) |
1687 |
return false; |
1688 |
|
1689 |
/* |
1690 |
* Splice onto queue and try to set waitStatus of predecessor to |
1691 |
* indicate that thread is (probably) waiting. If cancelled or |
1692 |
* attempt to set waitStatus fails, wake up to resync (in which |
1693 |
* case the waitStatus can be transiently and harmlessly wrong). |
1694 |
*/ |
1695 |
Node p = enq(node); |
1696 |
int ws = p.waitStatus; |
1697 |
if (ws > 0 || !p.compareAndSetWaitStatus(ws, Node.SIGNAL)) |
1698 |
LockSupport.unpark(node.thread); |
1699 |
return true; |
1700 |
} |
1701 |
|
1702 |
/** |
1703 |
* Transfers node, if necessary, to sync queue after a cancelled wait. |
1704 |
* Returns true if thread was cancelled before being signalled. |
1705 |
* |
1706 |
* @param node the node |
1707 |
* @return true if cancelled before the node was signalled |
1708 |
*/ |
1709 |
final boolean transferAfterCancelledWait(Node node) { |
1710 |
if (node.compareAndSetWaitStatus(Node.CONDITION, 0)) { |
1711 |
enq(node); |
1712 |
return true; |
1713 |
} |
1714 |
/* |
1715 |
* If we lost out to a signal(), then we can't proceed |
1716 |
* until it finishes its enq(). Cancelling during an |
1717 |
* incomplete transfer is both rare and transient, so just |
1718 |
* spin. |
1719 |
*/ |
1720 |
while (!isOnSyncQueue(node)) |
1721 |
Thread.yield(); |
1722 |
return false; |
1723 |
} |
1724 |
|
1725 |
/** |
1726 |
* Invokes release with current state value; returns saved state. |
1727 |
* Cancels node and throws exception on failure. |
1728 |
* @param node the condition node for this wait |
1729 |
* @return previous sync state |
1730 |
*/ |
1731 |
final int fullyRelease(Node node) { |
1732 |
try { |
1733 |
int savedState = getState(); |
1734 |
if (release(savedState)) |
1735 |
return savedState; |
1736 |
throw new IllegalMonitorStateException(); |
1737 |
} catch (Throwable t) { |
1738 |
node.waitStatus = Node.CANCELLED; |
1739 |
throw t; |
1740 |
} |
1741 |
} |
1742 |
|
1743 |
// Instrumentation methods for conditions |
1744 |
|
1745 |
/** |
1746 |
* Queries whether the given ConditionObject |
1747 |
* uses this synchronizer as its lock. |
1748 |
* |
1749 |
* @param condition the condition |
1750 |
* @return {@code true} if owned |
1751 |
* @throws NullPointerException if the condition is null |
1752 |
*/ |
1753 |
public final boolean owns(ConditionObject condition) { |
1754 |
return condition.isOwnedBy(this); |
1755 |
} |
1756 |
|
1757 |
/** |
1758 |
* Queries whether any threads are waiting on the given condition |
1759 |
* associated with this synchronizer. Note that because timeouts |
1760 |
* and interrupts may occur at any time, a {@code true} return |
1761 |
* does not guarantee that a future {@code signal} will awaken |
1762 |
* any threads. This method is designed primarily for use in |
1763 |
* monitoring of the system state. |
1764 |
* |
1765 |
* @param condition the condition |
1766 |
* @return {@code true} if there are any waiting threads |
1767 |
* @throws IllegalMonitorStateException if exclusive synchronization |
1768 |
* is not held |
1769 |
* @throws IllegalArgumentException if the given condition is |
1770 |
* not associated with this synchronizer |
1771 |
* @throws NullPointerException if the condition is null |
1772 |
*/ |
1773 |
public final boolean hasWaiters(ConditionObject condition) { |
1774 |
if (!owns(condition)) |
1775 |
throw new IllegalArgumentException("Not owner"); |
1776 |
return condition.hasWaiters(); |
1777 |
} |
1778 |
|
1779 |
/** |
1780 |
* Returns an estimate of the number of threads waiting on the |
1781 |
* given condition associated with this synchronizer. Note that |
1782 |
* because timeouts and interrupts may occur at any time, the |
1783 |
* estimate serves only as an upper bound on the actual number of |
1784 |
* waiters. This method is designed for use in monitoring system |
1785 |
* state, not for synchronization control. |
1786 |
* |
1787 |
* @param condition the condition |
1788 |
* @return the estimated number of waiting threads |
1789 |
* @throws IllegalMonitorStateException if exclusive synchronization |
1790 |
* is not held |
1791 |
* @throws IllegalArgumentException if the given condition is |
1792 |
* not associated with this synchronizer |
1793 |
* @throws NullPointerException if the condition is null |
1794 |
*/ |
1795 |
public final int getWaitQueueLength(ConditionObject condition) { |
1796 |
if (!owns(condition)) |
1797 |
throw new IllegalArgumentException("Not owner"); |
1798 |
return condition.getWaitQueueLength(); |
1799 |
} |
1800 |
|
1801 |
/** |
1802 |
* Returns a collection containing those threads that may be |
1803 |
* waiting on the given condition associated with this |
1804 |
* synchronizer. Because the actual set of threads may change |
1805 |
* dynamically while constructing this result, the returned |
1806 |
* collection is only a best-effort estimate. The elements of the |
1807 |
* returned collection are in no particular order. |
1808 |
* |
1809 |
* @param condition the condition |
1810 |
* @return the collection of threads |
1811 |
* @throws IllegalMonitorStateException if exclusive synchronization |
1812 |
* is not held |
1813 |
* @throws IllegalArgumentException if the given condition is |
1814 |
* not associated with this synchronizer |
1815 |
* @throws NullPointerException if the condition is null |
1816 |
*/ |
1817 |
public final Collection<Thread> getWaitingThreads(ConditionObject condition) { |
1818 |
if (!owns(condition)) |
1819 |
throw new IllegalArgumentException("Not owner"); |
1820 |
return condition.getWaitingThreads(); |
1821 |
} |
1822 |
|
1823 |
/** |
1824 |
* Condition implementation for a {@link AbstractQueuedSynchronizer} |
1825 |
* serving as the basis of a {@link Lock} implementation. |
1826 |
* |
1827 |
* <p>Method documentation for this class describes mechanics, |
1828 |
* not behavioral specifications from the point of view of Lock |
1829 |
* and Condition users. Exported versions of this class will in |
1830 |
* general need to be accompanied by documentation describing |
1831 |
* condition semantics that rely on those of the associated |
1832 |
* {@code AbstractQueuedSynchronizer}. |
1833 |
* |
1834 |
* <p>This class is Serializable, but all fields are transient, |
1835 |
* so deserialized conditions have no waiters. |
1836 |
*/ |
1837 |
public class ConditionObject implements Condition, java.io.Serializable { |
1838 |
private static final long serialVersionUID = 1173984872572414699L; |
1839 |
/** First node of condition queue. */ |
1840 |
private transient Node firstWaiter; |
1841 |
/** Last node of condition queue. */ |
1842 |
private transient Node lastWaiter; |
1843 |
|
1844 |
/** |
1845 |
* Creates a new {@code ConditionObject} instance. |
1846 |
*/ |
1847 |
public ConditionObject() { } |
1848 |
|
1849 |
// Internal methods |
1850 |
|
1851 |
/** |
1852 |
* Adds a new waiter to wait queue. |
1853 |
* @return its new wait node |
1854 |
*/ |
1855 |
private Node addConditionWaiter() { |
1856 |
if (!isHeldExclusively()) |
1857 |
throw new IllegalMonitorStateException(); |
1858 |
Node t = lastWaiter; |
1859 |
// If lastWaiter is cancelled, clean out. |
1860 |
if (t != null && t.waitStatus != Node.CONDITION) { |
1861 |
unlinkCancelledWaiters(); |
1862 |
t = lastWaiter; |
1863 |
} |
1864 |
|
1865 |
Node node = new Node(Node.CONDITION); |
1866 |
|
1867 |
if (t == null) |
1868 |
firstWaiter = node; |
1869 |
else |
1870 |
t.nextWaiter = node; |
1871 |
lastWaiter = node; |
1872 |
return node; |
1873 |
} |
1874 |
|
1875 |
/** |
1876 |
* Removes and transfers nodes until hit non-cancelled one or |
1877 |
* null. Split out from signal in part to encourage compilers |
1878 |
* to inline the case of no waiters. |
1879 |
* @param first (non-null) the first node on condition queue |
1880 |
*/ |
1881 |
private void doSignal(Node first) { |
1882 |
do { |
1883 |
if ( (firstWaiter = first.nextWaiter) == null) |
1884 |
lastWaiter = null; |
1885 |
first.nextWaiter = null; |
1886 |
} while (!transferForSignal(first) && |
1887 |
(first = firstWaiter) != null); |
1888 |
} |
1889 |
|
1890 |
/** |
1891 |
* Removes and transfers all nodes. |
1892 |
* @param first (non-null) the first node on condition queue |
1893 |
*/ |
1894 |
private void doSignalAll(Node first) { |
1895 |
lastWaiter = firstWaiter = null; |
1896 |
do { |
1897 |
Node next = first.nextWaiter; |
1898 |
first.nextWaiter = null; |
1899 |
transferForSignal(first); |
1900 |
first = next; |
1901 |
} while (first != null); |
1902 |
} |
1903 |
|
1904 |
/** |
1905 |
* Unlinks cancelled waiter nodes from condition queue. |
1906 |
* Called only while holding lock. This is called when |
1907 |
* cancellation occurred during condition wait, and upon |
1908 |
* insertion of a new waiter when lastWaiter is seen to have |
1909 |
* been cancelled. This method is needed to avoid garbage |
1910 |
* retention in the absence of signals. So even though it may |
1911 |
* require a full traversal, it comes into play only when |
1912 |
* timeouts or cancellations occur in the absence of |
1913 |
* signals. It traverses all nodes rather than stopping at a |
1914 |
* particular target to unlink all pointers to garbage nodes |
1915 |
* without requiring many re-traversals during cancellation |
1916 |
* storms. |
1917 |
*/ |
1918 |
private void unlinkCancelledWaiters() { |
1919 |
Node t = firstWaiter; |
1920 |
Node trail = null; |
1921 |
while (t != null) { |
1922 |
Node next = t.nextWaiter; |
1923 |
if (t.waitStatus != Node.CONDITION) { |
1924 |
t.nextWaiter = null; |
1925 |
if (trail == null) |
1926 |
firstWaiter = next; |
1927 |
else |
1928 |
trail.nextWaiter = next; |
1929 |
if (next == null) |
1930 |
lastWaiter = trail; |
1931 |
} |
1932 |
else |
1933 |
trail = t; |
1934 |
t = next; |
1935 |
} |
1936 |
} |
1937 |
|
1938 |
// public methods |
1939 |
|
1940 |
/** |
1941 |
* Moves the longest-waiting thread, if one exists, from the |
1942 |
* wait queue for this condition to the wait queue for the |
1943 |
* owning lock. |
1944 |
* |
1945 |
* @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
1946 |
* returns {@code false} |
1947 |
*/ |
1948 |
public final void signal() { |
1949 |
if (!isHeldExclusively()) |
1950 |
throw new IllegalMonitorStateException(); |
1951 |
Node first = firstWaiter; |
1952 |
if (first != null) |
1953 |
doSignal(first); |
1954 |
} |
1955 |
|
1956 |
/** |
1957 |
* Moves all threads from the wait queue for this condition to |
1958 |
* the wait queue for the owning lock. |
1959 |
* |
1960 |
* @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
1961 |
* returns {@code false} |
1962 |
*/ |
1963 |
public final void signalAll() { |
1964 |
if (!isHeldExclusively()) |
1965 |
throw new IllegalMonitorStateException(); |
1966 |
Node first = firstWaiter; |
1967 |
if (first != null) |
1968 |
doSignalAll(first); |
1969 |
} |
1970 |
|
1971 |
/** |
1972 |
* Implements uninterruptible condition wait. |
1973 |
* <ol> |
1974 |
* <li>Save lock state returned by {@link #getState}. |
1975 |
* <li>Invoke {@link #release} with saved state as argument, |
1976 |
* throwing IllegalMonitorStateException if it fails. |
1977 |
* <li>Block until signalled. |
1978 |
* <li>Reacquire by invoking specialized version of |
1979 |
* {@link #acquire} with saved state as argument. |
1980 |
* </ol> |
1981 |
*/ |
1982 |
public final void awaitUninterruptibly() { |
1983 |
Node node = addConditionWaiter(); |
1984 |
int savedState = fullyRelease(node); |
1985 |
boolean interrupted = false; |
1986 |
while (!isOnSyncQueue(node)) { |
1987 |
LockSupport.park(this); |
1988 |
if (Thread.interrupted()) |
1989 |
interrupted = true; |
1990 |
} |
1991 |
if (acquireQueued(node, savedState) || interrupted) |
1992 |
selfInterrupt(); |
1993 |
} |
1994 |
|
1995 |
/* |
1996 |
* For interruptible waits, we need to track whether to throw |
1997 |
* InterruptedException, if interrupted while blocked on |
1998 |
* condition, versus reinterrupt current thread, if |
1999 |
* interrupted while blocked waiting to re-acquire. |
2000 |
*/ |
2001 |
|
2002 |
/** Mode meaning to reinterrupt on exit from wait */ |
2003 |
private static final int REINTERRUPT = 1; |
2004 |
/** Mode meaning to throw InterruptedException on exit from wait */ |
2005 |
private static final int THROW_IE = -1; |
2006 |
|
2007 |
/** |
2008 |
* Checks for interrupt, returning THROW_IE if interrupted |
2009 |
* before signalled, REINTERRUPT if after signalled, or |
2010 |
* 0 if not interrupted. |
2011 |
*/ |
2012 |
private int checkInterruptWhileWaiting(Node node) { |
2013 |
return Thread.interrupted() ? |
2014 |
(transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) : |
2015 |
0; |
2016 |
} |
2017 |
|
2018 |
/** |
2019 |
* Throws InterruptedException, reinterrupts current thread, or |
2020 |
* does nothing, depending on mode. |
2021 |
*/ |
2022 |
private void reportInterruptAfterWait(int interruptMode) |
2023 |
throws InterruptedException { |
2024 |
if (interruptMode == THROW_IE) |
2025 |
throw new InterruptedException(); |
2026 |
else if (interruptMode == REINTERRUPT) |
2027 |
selfInterrupt(); |
2028 |
} |
2029 |
|
2030 |
/** |
2031 |
* Implements interruptible condition wait. |
2032 |
* <ol> |
2033 |
* <li>If current thread is interrupted, throw InterruptedException. |
2034 |
* <li>Save lock state returned by {@link #getState}. |
2035 |
* <li>Invoke {@link #release} with saved state as argument, |
2036 |
* throwing IllegalMonitorStateException if it fails. |
2037 |
* <li>Block until signalled or interrupted. |
2038 |
* <li>Reacquire by invoking specialized version of |
2039 |
* {@link #acquire} with saved state as argument. |
2040 |
* <li>If interrupted while blocked in step 4, throw InterruptedException. |
2041 |
* </ol> |
2042 |
*/ |
2043 |
public final void await() throws InterruptedException { |
2044 |
if (Thread.interrupted()) |
2045 |
throw new InterruptedException(); |
2046 |
Node node = addConditionWaiter(); |
2047 |
int savedState = fullyRelease(node); |
2048 |
int interruptMode = 0; |
2049 |
while (!isOnSyncQueue(node)) { |
2050 |
LockSupport.park(this); |
2051 |
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
2052 |
break; |
2053 |
} |
2054 |
if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
2055 |
interruptMode = REINTERRUPT; |
2056 |
if (node.nextWaiter != null) // clean up if cancelled |
2057 |
unlinkCancelledWaiters(); |
2058 |
if (interruptMode != 0) |
2059 |
reportInterruptAfterWait(interruptMode); |
2060 |
} |
2061 |
|
2062 |
/** |
2063 |
* Implements timed condition wait. |
2064 |
* <ol> |
2065 |
* <li>If current thread is interrupted, throw InterruptedException. |
2066 |
* <li>Save lock state returned by {@link #getState}. |
2067 |
* <li>Invoke {@link #release} with saved state as argument, |
2068 |
* throwing IllegalMonitorStateException if it fails. |
2069 |
* <li>Block until signalled, interrupted, or timed out. |
2070 |
* <li>Reacquire by invoking specialized version of |
2071 |
* {@link #acquire} with saved state as argument. |
2072 |
* <li>If interrupted while blocked in step 4, throw InterruptedException. |
2073 |
* </ol> |
2074 |
*/ |
2075 |
public final long awaitNanos(long nanosTimeout) |
2076 |
throws InterruptedException { |
2077 |
if (Thread.interrupted()) |
2078 |
throw new InterruptedException(); |
2079 |
// We don't check for nanosTimeout <= 0L here, to allow |
2080 |
// awaitNanos(0) as a way to "yield the lock". |
2081 |
final long deadline = System.nanoTime() + nanosTimeout; |
2082 |
long initialNanos = nanosTimeout; |
2083 |
Node node = addConditionWaiter(); |
2084 |
int savedState = fullyRelease(node); |
2085 |
int interruptMode = 0; |
2086 |
while (!isOnSyncQueue(node)) { |
2087 |
if (nanosTimeout <= 0L) { |
2088 |
transferAfterCancelledWait(node); |
2089 |
break; |
2090 |
} |
2091 |
if (nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD) |
2092 |
LockSupport.parkNanos(this, nanosTimeout); |
2093 |
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
2094 |
break; |
2095 |
nanosTimeout = deadline - System.nanoTime(); |
2096 |
} |
2097 |
if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
2098 |
interruptMode = REINTERRUPT; |
2099 |
if (node.nextWaiter != null) |
2100 |
unlinkCancelledWaiters(); |
2101 |
if (interruptMode != 0) |
2102 |
reportInterruptAfterWait(interruptMode); |
2103 |
long remaining = deadline - System.nanoTime(); // avoid overflow |
2104 |
return (remaining <= initialNanos) ? remaining : Long.MIN_VALUE; |
2105 |
} |
2106 |
|
2107 |
/** |
2108 |
* Implements absolute timed condition wait. |
2109 |
* <ol> |
2110 |
* <li>If current thread is interrupted, throw InterruptedException. |
2111 |
* <li>Save lock state returned by {@link #getState}. |
2112 |
* <li>Invoke {@link #release} with saved state as argument, |
2113 |
* throwing IllegalMonitorStateException if it fails. |
2114 |
* <li>Block until signalled, interrupted, or timed out. |
2115 |
* <li>Reacquire by invoking specialized version of |
2116 |
* {@link #acquire} with saved state as argument. |
2117 |
* <li>If interrupted while blocked in step 4, throw InterruptedException. |
2118 |
* <li>If timed out while blocked in step 4, return false, else true. |
2119 |
* </ol> |
2120 |
*/ |
2121 |
public final boolean awaitUntil(Date deadline) |
2122 |
throws InterruptedException { |
2123 |
long abstime = deadline.getTime(); |
2124 |
if (Thread.interrupted()) |
2125 |
throw new InterruptedException(); |
2126 |
Node node = addConditionWaiter(); |
2127 |
int savedState = fullyRelease(node); |
2128 |
boolean timedout = false; |
2129 |
int interruptMode = 0; |
2130 |
while (!isOnSyncQueue(node)) { |
2131 |
if (System.currentTimeMillis() >= abstime) { |
2132 |
timedout = transferAfterCancelledWait(node); |
2133 |
break; |
2134 |
} |
2135 |
LockSupport.parkUntil(this, abstime); |
2136 |
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
2137 |
break; |
2138 |
} |
2139 |
if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
2140 |
interruptMode = REINTERRUPT; |
2141 |
if (node.nextWaiter != null) |
2142 |
unlinkCancelledWaiters(); |
2143 |
if (interruptMode != 0) |
2144 |
reportInterruptAfterWait(interruptMode); |
2145 |
return !timedout; |
2146 |
} |
2147 |
|
2148 |
/** |
2149 |
* Implements timed condition wait. |
2150 |
* <ol> |
2151 |
* <li>If current thread is interrupted, throw InterruptedException. |
2152 |
* <li>Save lock state returned by {@link #getState}. |
2153 |
* <li>Invoke {@link #release} with saved state as argument, |
2154 |
* throwing IllegalMonitorStateException if it fails. |
2155 |
* <li>Block until signalled, interrupted, or timed out. |
2156 |
* <li>Reacquire by invoking specialized version of |
2157 |
* {@link #acquire} with saved state as argument. |
2158 |
* <li>If interrupted while blocked in step 4, throw InterruptedException. |
2159 |
* <li>If timed out while blocked in step 4, return false, else true. |
2160 |
* </ol> |
2161 |
*/ |
2162 |
public final boolean await(long time, TimeUnit unit) |
2163 |
throws InterruptedException { |
2164 |
long nanosTimeout = unit.toNanos(time); |
2165 |
if (Thread.interrupted()) |
2166 |
throw new InterruptedException(); |
2167 |
// We don't check for nanosTimeout <= 0L here, to allow |
2168 |
// await(0, unit) as a way to "yield the lock". |
2169 |
final long deadline = System.nanoTime() + nanosTimeout; |
2170 |
Node node = addConditionWaiter(); |
2171 |
int savedState = fullyRelease(node); |
2172 |
boolean timedout = false; |
2173 |
int interruptMode = 0; |
2174 |
while (!isOnSyncQueue(node)) { |
2175 |
if (nanosTimeout <= 0L) { |
2176 |
timedout = transferAfterCancelledWait(node); |
2177 |
break; |
2178 |
} |
2179 |
if (nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD) |
2180 |
LockSupport.parkNanos(this, nanosTimeout); |
2181 |
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
2182 |
break; |
2183 |
nanosTimeout = deadline - System.nanoTime(); |
2184 |
} |
2185 |
if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
2186 |
interruptMode = REINTERRUPT; |
2187 |
if (node.nextWaiter != null) |
2188 |
unlinkCancelledWaiters(); |
2189 |
if (interruptMode != 0) |
2190 |
reportInterruptAfterWait(interruptMode); |
2191 |
return !timedout; |
2192 |
} |
2193 |
|
2194 |
// support for instrumentation |
2195 |
|
2196 |
/** |
2197 |
* Returns true if this condition was created by the given |
2198 |
* synchronization object. |
2199 |
* |
2200 |
* @return {@code true} if owned |
2201 |
*/ |
2202 |
final boolean isOwnedBy(AbstractQueuedSynchronizer sync) { |
2203 |
return sync == AbstractQueuedSynchronizer.this; |
2204 |
} |
2205 |
|
2206 |
/** |
2207 |
* Queries whether any threads are waiting on this condition. |
2208 |
* Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}. |
2209 |
* |
2210 |
* @return {@code true} if there are any waiting threads |
2211 |
* @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
2212 |
* returns {@code false} |
2213 |
*/ |
2214 |
protected final boolean hasWaiters() { |
2215 |
if (!isHeldExclusively()) |
2216 |
throw new IllegalMonitorStateException(); |
2217 |
for (Node w = firstWaiter; w != null; w = w.nextWaiter) { |
2218 |
if (w.waitStatus == Node.CONDITION) |
2219 |
return true; |
2220 |
} |
2221 |
return false; |
2222 |
} |
2223 |
|
2224 |
/** |
2225 |
* Returns an estimate of the number of threads waiting on |
2226 |
* this condition. |
2227 |
* Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}. |
2228 |
* |
2229 |
* @return the estimated number of waiting threads |
2230 |
* @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
2231 |
* returns {@code false} |
2232 |
*/ |
2233 |
protected final int getWaitQueueLength() { |
2234 |
if (!isHeldExclusively()) |
2235 |
throw new IllegalMonitorStateException(); |
2236 |
int n = 0; |
2237 |
for (Node w = firstWaiter; w != null; w = w.nextWaiter) { |
2238 |
if (w.waitStatus == Node.CONDITION) |
2239 |
++n; |
2240 |
} |
2241 |
return n; |
2242 |
} |
2243 |
|
2244 |
/** |
2245 |
* Returns a collection containing those threads that may be |
2246 |
* waiting on this Condition. |
2247 |
* Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}. |
2248 |
* |
2249 |
* @return the collection of threads |
2250 |
* @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
2251 |
* returns {@code false} |
2252 |
*/ |
2253 |
protected final Collection<Thread> getWaitingThreads() { |
2254 |
if (!isHeldExclusively()) |
2255 |
throw new IllegalMonitorStateException(); |
2256 |
ArrayList<Thread> list = new ArrayList<>(); |
2257 |
for (Node w = firstWaiter; w != null; w = w.nextWaiter) { |
2258 |
if (w.waitStatus == Node.CONDITION) { |
2259 |
Thread t = w.thread; |
2260 |
if (t != null) |
2261 |
list.add(t); |
2262 |
} |
2263 |
} |
2264 |
return list; |
2265 |
} |
2266 |
} |
2267 |
|
2268 |
/** |
2269 |
* Setup to support compareAndSet. We need to natively implement |
2270 |
* this here: For the sake of permitting future enhancements, we |
2271 |
* cannot explicitly subclass AtomicInteger, which would be |
2272 |
* efficient and useful otherwise. So, as the lesser of evils, we |
2273 |
* natively implement using hotspot intrinsics API. And while we |
2274 |
* are at it, we do the same for other CASable fields (which could |
2275 |
* otherwise be done with atomic field updaters). |
2276 |
*/ |
2277 |
private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); |
2278 |
private static final long STATE; |
2279 |
private static final long HEAD; |
2280 |
private static final long TAIL; |
2281 |
|
2282 |
static { |
2283 |
try { |
2284 |
STATE = U.objectFieldOffset |
2285 |
(AbstractQueuedSynchronizer.class.getDeclaredField("state")); |
2286 |
HEAD = U.objectFieldOffset |
2287 |
(AbstractQueuedSynchronizer.class.getDeclaredField("head")); |
2288 |
TAIL = U.objectFieldOffset |
2289 |
(AbstractQueuedSynchronizer.class.getDeclaredField("tail")); |
2290 |
} catch (ReflectiveOperationException e) { |
2291 |
throw new Error(e); |
2292 |
} |
2293 |
|
2294 |
// Reduce the risk of rare disastrous classloading in first call to |
2295 |
// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773 |
2296 |
Class<?> ensureLoaded = LockSupport.class; |
2297 |
} |
2298 |
|
2299 |
/** |
2300 |
* Initializes head and tail fields on first contention. |
2301 |
*/ |
2302 |
private final void initializeSyncQueue() { |
2303 |
Node h; |
2304 |
if (U.compareAndSwapObject(this, HEAD, null, (h = new Node()))) |
2305 |
tail = h; |
2306 |
} |
2307 |
|
2308 |
/** |
2309 |
* CASes tail field. |
2310 |
*/ |
2311 |
private final boolean compareAndSetTail(Node expect, Node update) { |
2312 |
return U.compareAndSwapObject(this, TAIL, expect, update); |
2313 |
} |
2314 |
} |