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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; |
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|
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import java.lang.invoke.MethodHandles; |
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import java.lang.invoke.VarHandle; |
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import java.util.concurrent.atomic.AtomicReference; |
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import java.util.concurrent.locks.LockSupport; |
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|
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/** |
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* A reusable synchronization barrier, similar in functionality to |
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* {@link CyclicBarrier} and {@link CountDownLatch} but supporting |
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* more flexible usage. |
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* |
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* <p><b>Registration.</b> Unlike the case for other barriers, the |
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* number of parties <em>registered</em> to synchronize on a phaser |
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* may vary over time. Tasks may be registered at any time (using |
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* methods {@link #register}, {@link #bulkRegister}, or forms of |
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* constructors establishing initial numbers of parties), and |
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* optionally deregistered upon any arrival (using {@link |
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* #arriveAndDeregister}). As is the case with most basic |
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* synchronization constructs, registration and deregistration affect |
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* only internal counts; they do not establish any further internal |
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* bookkeeping, so tasks cannot query whether they are registered. |
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* (However, you can introduce such bookkeeping by subclassing this |
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* class.) |
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* |
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* <p><b>Synchronization.</b> Like a {@code CyclicBarrier}, a {@code |
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* Phaser} may be repeatedly awaited. Method {@link |
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* #arriveAndAwaitAdvance} has effect analogous to {@link |
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* java.util.concurrent.CyclicBarrier#await CyclicBarrier.await}. Each |
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* generation of a phaser has an associated phase number. The phase |
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* number starts at zero, and advances when all parties arrive at the |
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* phaser, wrapping around to zero after reaching {@code |
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* Integer.MAX_VALUE}. The use of phase numbers enables independent |
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* control of actions upon arrival at a phaser and upon awaiting |
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* others, via two kinds of methods that may be invoked by any |
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* registered party: |
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* |
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* <ul> |
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* |
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* <li><b>Arrival.</b> Methods {@link #arrive} and |
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* {@link #arriveAndDeregister} record arrival. These methods |
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* do not block, but return an associated <em>arrival phase |
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* number</em>; that is, the phase number of the phaser to which |
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* the arrival applied. When the final party for a given phase |
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* arrives, an optional action is performed and the phase |
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* advances. These actions are performed by the party |
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* triggering a phase advance, and are arranged by overriding |
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* method {@link #onAdvance(int, int)}, which also controls |
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* termination. Overriding this method is similar to, but more |
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* flexible than, providing a barrier action to a {@code |
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* CyclicBarrier}. |
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* |
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* <li><b>Waiting.</b> Method {@link #awaitAdvance} requires an |
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* argument indicating an arrival phase number, and returns when |
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* the phaser advances to (or is already at) a different phase. |
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* Unlike similar constructions using {@code CyclicBarrier}, |
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* method {@code awaitAdvance} continues to wait even if the |
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* waiting thread is interrupted. Interruptible and timeout |
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* versions are also available, but exceptions encountered while |
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* tasks wait interruptibly or with timeout do not change the |
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* state of the phaser. If necessary, you can perform any |
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* associated recovery within handlers of those exceptions, |
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* often after invoking {@code forceTermination}. Phasers may |
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* also be used by tasks executing in a {@link ForkJoinPool}. |
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* Progress is ensured if the pool's parallelism level can |
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* accommodate the maximum number of simultaneously blocked |
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* parties. |
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* |
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* </ul> |
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* |
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* <p><b>Termination.</b> A phaser may enter a <em>termination</em> |
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* state, that may be checked using method {@link #isTerminated}. Upon |
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* termination, all synchronization methods immediately return without |
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* waiting for advance, as indicated by a negative return value. |
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* Similarly, attempts to register upon termination have no effect. |
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* Termination is triggered when an invocation of {@code onAdvance} |
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* returns {@code true}. The default implementation returns {@code |
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* true} if a deregistration has caused the number of registered |
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* parties to become zero. As illustrated below, when phasers control |
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* actions with a fixed number of iterations, it is often convenient |
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* to override this method to cause termination when the current phase |
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* number reaches a threshold. Method {@link #forceTermination} is |
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* also available to abruptly release waiting threads and allow them |
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* to terminate. |
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* |
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* <p><b>Tiering.</b> Phasers may be <em>tiered</em> (i.e., |
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* constructed in tree structures) to reduce contention. Phasers with |
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* large numbers of parties that would otherwise experience heavy |
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* synchronization contention costs may instead be set up so that |
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* groups of sub-phasers share a common parent. This may greatly |
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* increase throughput even though it incurs greater per-operation |
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* overhead. |
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* |
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* <p>In a tree of tiered phasers, registration and deregistration of |
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* child phasers with their parent are managed automatically. |
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* Whenever the number of registered parties of a child phaser becomes |
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* non-zero (as established in the {@link #Phaser(Phaser,int)} |
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* constructor, {@link #register}, or {@link #bulkRegister}), the |
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* child phaser is registered with its parent. Whenever the number of |
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* registered parties becomes zero as the result of an invocation of |
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* {@link #arriveAndDeregister}, the child phaser is deregistered |
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* from its parent. |
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* |
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* <p><b>Monitoring.</b> While synchronization methods may be invoked |
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* only by registered parties, the current state of a phaser may be |
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* monitored by any caller. At any given moment there are {@link |
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* #getRegisteredParties} parties in total, of which {@link |
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* #getArrivedParties} have arrived at the current phase ({@link |
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* #getPhase}). When the remaining ({@link #getUnarrivedParties}) |
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* parties arrive, the phase advances. The values returned by these |
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* methods may reflect transient states and so are not in general |
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* useful for synchronization control. Method {@link #toString} |
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* returns snapshots of these state queries in a form convenient for |
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* informal monitoring. |
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* |
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* <p>Memory consistency effects: Actions prior to any form of arrive |
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* method <a href="package-summary.html#MemoryVisibility"> |
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* <i>happen-before</i></a> a corresponding phase advance and |
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* onAdvance actions (if present), which in turn <i>happen-before</i> |
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* actions following the phase advance. |
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* |
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* <p><b>Sample usages:</b> |
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* |
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* <p>A {@code Phaser} may be used instead of a {@code CountDownLatch} |
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* to control a one-shot action serving a variable number of parties. |
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* The typical idiom is for the method setting this up to first |
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* register, then start all the actions, then deregister, as in: |
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* |
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* <pre> {@code |
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* void runTasks(List<Runnable> tasks) { |
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* Phaser startingGate = new Phaser(1); // "1" to register self |
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* // create and start threads |
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* for (Runnable task : tasks) { |
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* startingGate.register(); |
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* new Thread(() -> { |
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* startingGate.arriveAndAwaitAdvance(); |
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* task.run(); |
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* }).start(); |
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* } |
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* |
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* // deregister self to allow threads to proceed |
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* startingGate.arriveAndDeregister(); |
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* }}</pre> |
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* |
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* <p>One way to cause a set of threads to repeatedly perform actions |
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* for a given number of iterations is to override {@code onAdvance}: |
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* |
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* <pre> {@code |
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* void startTasks(List<Runnable> tasks, int iterations) { |
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* Phaser phaser = new Phaser() { |
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* protected boolean onAdvance(int phase, int registeredParties) { |
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* return phase >= iterations - 1 || registeredParties == 0; |
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* } |
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* }; |
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* phaser.register(); |
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* for (Runnable task : tasks) { |
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* phaser.register(); |
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* new Thread(() -> { |
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* do { |
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* task.run(); |
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* phaser.arriveAndAwaitAdvance(); |
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* } while (!phaser.isTerminated()); |
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* }).start(); |
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* } |
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* // allow threads to proceed; don't wait for them |
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* phaser.arriveAndDeregister(); |
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* }}</pre> |
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* |
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* If the main task must later await termination, it |
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* may re-register and then execute a similar loop: |
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* <pre> {@code |
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* // ... |
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* phaser.register(); |
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* while (!phaser.isTerminated()) |
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* phaser.arriveAndAwaitAdvance();}</pre> |
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* |
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* <p>Related constructions may be used to await particular phase numbers |
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* in contexts where you are sure that the phase will never wrap around |
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* {@code Integer.MAX_VALUE}. For example: |
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* |
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* <pre> {@code |
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* void awaitPhase(Phaser phaser, int phase) { |
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* int p = phaser.register(); // assumes caller not already registered |
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* while (p < phase) { |
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* if (phaser.isTerminated()) |
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* // ... deal with unexpected termination |
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* else |
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* p = phaser.arriveAndAwaitAdvance(); |
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* } |
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* phaser.arriveAndDeregister(); |
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* }}</pre> |
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* |
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* <p>To create a set of {@code n} tasks using a tree of phasers, you |
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* could use code of the following form, assuming a Task class with a |
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* constructor accepting a {@code Phaser} that it registers with upon |
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* construction. After invocation of {@code build(new Task[n], 0, n, |
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* new Phaser())}, these tasks could then be started, for example by |
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* submitting to a pool: |
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* |
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* <pre> {@code |
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* void build(Task[] tasks, int lo, int hi, Phaser ph) { |
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* if (hi - lo > TASKS_PER_PHASER) { |
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* for (int i = lo; i < hi; i += TASKS_PER_PHASER) { |
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* int j = Math.min(i + TASKS_PER_PHASER, hi); |
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* build(tasks, i, j, new Phaser(ph)); |
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* } |
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* } else { |
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* for (int i = lo; i < hi; ++i) |
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* tasks[i] = new Task(ph); |
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* // assumes new Task(ph) performs ph.register() |
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* } |
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* }}</pre> |
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* |
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* The best value of {@code TASKS_PER_PHASER} depends mainly on |
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* expected synchronization rates. A value as low as four may |
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* be appropriate for extremely small per-phase task bodies (thus |
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* high rates), or up to hundreds for extremely large ones. |
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* |
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* <p><b>Implementation notes:</b> This implementation restricts the |
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* maximum number of parties to 65535. Attempts to register additional |
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* parties result in {@code IllegalStateException}. However, you can and |
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* should create tiered phasers to accommodate arbitrarily large sets |
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* of participants. |
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* |
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* @since 1.7 |
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* @author Doug Lea |
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*/ |
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public class Phaser { |
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/* |
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* This class implements an extension of X10 "clocks". Thanks to |
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* Vijay Saraswat for the idea, and to Vivek Sarkar for |
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* enhancements to extend functionality. |
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*/ |
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|
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/** |
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* Primary state representation, holding four bit-fields: |
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* |
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* unarrived -- the number of parties yet to hit barrier (bits 0-15) |
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* parties -- the number of parties to wait (bits 16-31) |
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* phase -- the generation of the barrier (bits 32-62) |
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* terminated -- set if barrier is terminated (bit 63 / sign) |
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* |
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* Except that a phaser with no registered parties is |
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* distinguished by the otherwise illegal state of having zero |
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* parties and one unarrived parties (encoded as EMPTY below). |
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* |
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* To efficiently maintain atomicity, these values are packed into |
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* a single (atomic) long. Good performance relies on keeping |
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* state decoding and encoding simple, and keeping race windows |
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* short. |
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* |
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* All state updates are performed via CAS except initial |
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* registration of a sub-phaser (i.e., one with a non-null |
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* parent). In this (relatively rare) case, we use built-in |
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* synchronization to lock while first registering with its |
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* parent. |
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* |
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* The phase of a subphaser is allowed to lag that of its |
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* ancestors until it is actually accessed -- see method |
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* reconcileState. |
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*/ |
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private volatile long state; |
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|
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private static final int MAX_PARTIES = 0xffff; |
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private static final int MAX_PHASE = Integer.MAX_VALUE; |
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private static final int PARTIES_SHIFT = 16; |
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private static final int PHASE_SHIFT = 32; |
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private static final int UNARRIVED_MASK = 0xffff; // to mask ints |
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private static final long PARTIES_MASK = 0xffff0000L; // to mask longs |
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private static final long COUNTS_MASK = 0xffffffffL; |
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private static final long TERMINATION_BIT = 1L << 63; |
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|
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// some special values |
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private static final int ONE_ARRIVAL = 1; |
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private static final int ONE_PARTY = 1 << PARTIES_SHIFT; |
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private static final int ONE_DEREGISTER = ONE_ARRIVAL|ONE_PARTY; |
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private static final int EMPTY = 1; |
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|
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// The following unpacking methods are usually manually inlined |
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|
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private static int unarrivedOf(long s) { |
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int counts = (int)s; |
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return (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK); |
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} |
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|
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private static int partiesOf(long s) { |
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return (int)s >>> PARTIES_SHIFT; |
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} |
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|
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private static int phaseOf(long s) { |
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return (int)(s >>> PHASE_SHIFT); |
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} |
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|
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private static int arrivedOf(long s) { |
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int counts = (int)s; |
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return (counts == EMPTY) ? 0 : |
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(counts >>> PARTIES_SHIFT) - (counts & UNARRIVED_MASK); |
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} |
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|
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/** |
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* The parent of this phaser, or null if none. |
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*/ |
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private final Phaser parent; |
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|
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/** |
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* The root of phaser tree. Equals this if not in a tree. |
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*/ |
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private final Phaser root; |
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|
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/** |
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* Heads of Treiber stacks for waiting threads. To eliminate |
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* contention when releasing some threads while adding others, we |
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* use two of them, alternating across even and odd phases. |
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* Subphasers share queues with root to speed up releases. |
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*/ |
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private final AtomicReference<QNode> evenQ; |
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private final AtomicReference<QNode> oddQ; |
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|
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/** |
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* Returns message string for bounds exceptions on arrival. |
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*/ |
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private String badArrive(long s) { |
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return "Attempted arrival of unregistered party for " + |
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stateToString(s); |
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} |
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|
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/** |
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* Returns message string for bounds exceptions on registration. |
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*/ |
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private String badRegister(long s) { |
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return "Attempt to register more than " + |
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MAX_PARTIES + " parties for " + stateToString(s); |
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} |
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|
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/** |
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* Main implementation for methods arrive and arriveAndDeregister. |
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* Manually tuned to speed up and minimize race windows for the |
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* common case of just decrementing unarrived field. |
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* |
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* @param adjust value to subtract from state; |
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* ONE_ARRIVAL for arrive, |
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* ONE_DEREGISTER for arriveAndDeregister |
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*/ |
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private int doArrive(int adjust) { |
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final Phaser root = this.root; |
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for (;;) { |
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long s = (root == this) ? state : reconcileState(); |
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int phase = (int)(s >>> PHASE_SHIFT); |
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if (phase < 0) |
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return phase; |
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int counts = (int)s; |
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int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK); |
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if (unarrived <= 0) |
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throw new IllegalStateException(badArrive(s)); |
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if (STATE.compareAndSet(this, s, s-=adjust)) { |
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if (unarrived == 1) { |
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long n = s & PARTIES_MASK; // base of next state |
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int nextUnarrived = (int)n >>> PARTIES_SHIFT; |
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if (root == this) { |
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if (onAdvance(phase, nextUnarrived)) |
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n |= TERMINATION_BIT; |
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else if (nextUnarrived == 0) |
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n |= EMPTY; |
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else |
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n |= nextUnarrived; |
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int nextPhase = (phase + 1) & MAX_PHASE; |
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n |= (long)nextPhase << PHASE_SHIFT; |
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STATE.compareAndSet(this, s, n); |
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releaseWaiters(phase); |
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} |
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else if (nextUnarrived == 0) { // propagate deregistration |
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phase = parent.doArrive(ONE_DEREGISTER); |
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STATE.compareAndSet(this, s, s | EMPTY); |
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} |
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else |
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phase = parent.doArrive(ONE_ARRIVAL); |
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} |
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return phase; |
385 |
} |
386 |
} |
387 |
} |
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|
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/** |
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* Implementation of register, bulkRegister. |
391 |
* |
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* @param registrations number to add to both parties and |
393 |
* unarrived fields. Must be greater than zero. |
394 |
*/ |
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private int doRegister(int registrations) { |
396 |
// adjustment to state |
397 |
long adjust = ((long)registrations << PARTIES_SHIFT) | registrations; |
398 |
final Phaser parent = this.parent; |
399 |
int phase; |
400 |
for (;;) { |
401 |
long s = (parent == null) ? state : reconcileState(); |
402 |
int counts = (int)s; |
403 |
int parties = counts >>> PARTIES_SHIFT; |
404 |
int unarrived = counts & UNARRIVED_MASK; |
405 |
if (registrations > MAX_PARTIES - parties) |
406 |
throw new IllegalStateException(badRegister(s)); |
407 |
phase = (int)(s >>> PHASE_SHIFT); |
408 |
if (phase < 0) |
409 |
break; |
410 |
if (counts != EMPTY) { // not 1st registration |
411 |
if (parent == null || reconcileState() == s) { |
412 |
if (unarrived == 0) // wait out advance |
413 |
root.internalAwaitAdvance(phase, null); |
414 |
else if (STATE.compareAndSet(this, s, s + adjust)) |
415 |
break; |
416 |
} |
417 |
} |
418 |
else if (parent == null) { // 1st root registration |
419 |
long next = ((long)phase << PHASE_SHIFT) | adjust; |
420 |
if (STATE.compareAndSet(this, s, next)) |
421 |
break; |
422 |
} |
423 |
else { |
424 |
synchronized (this) { // 1st sub registration |
425 |
if (state == s) { // recheck under lock |
426 |
phase = parent.doRegister(1); |
427 |
if (phase < 0) |
428 |
break; |
429 |
// finish registration whenever parent registration |
430 |
// succeeded, even when racing with termination, |
431 |
// since these are part of the same "transaction". |
432 |
while (!STATE.weakCompareAndSet |
433 |
(this, s, |
434 |
((long)phase << PHASE_SHIFT) | adjust)) { |
435 |
s = state; |
436 |
phase = (int)(root.state >>> PHASE_SHIFT); |
437 |
// assert (int)s == EMPTY; |
438 |
} |
439 |
break; |
440 |
} |
441 |
} |
442 |
} |
443 |
} |
444 |
return phase; |
445 |
} |
446 |
|
447 |
/** |
448 |
* Resolves lagged phase propagation from root if necessary. |
449 |
* Reconciliation normally occurs when root has advanced but |
450 |
* subphasers have not yet done so, in which case they must finish |
451 |
* their own advance by setting unarrived to parties (or if |
452 |
* parties is zero, resetting to unregistered EMPTY state). |
453 |
* |
454 |
* @return reconciled state |
455 |
*/ |
456 |
private long reconcileState() { |
457 |
final Phaser root = this.root; |
458 |
long s = state; |
459 |
if (root != this) { |
460 |
int phase, p; |
461 |
// CAS to root phase with current parties, tripping unarrived |
462 |
while ((phase = (int)(root.state >>> PHASE_SHIFT)) != |
463 |
(int)(s >>> PHASE_SHIFT) && |
464 |
!STATE.weakCompareAndSet |
465 |
(this, s, |
466 |
s = (((long)phase << PHASE_SHIFT) | |
467 |
((phase < 0) ? (s & COUNTS_MASK) : |
468 |
(((p = (int)s >>> PARTIES_SHIFT) == 0) ? EMPTY : |
469 |
((s & PARTIES_MASK) | p)))))) |
470 |
s = state; |
471 |
} |
472 |
return s; |
473 |
} |
474 |
|
475 |
/** |
476 |
* Creates a new phaser with no initially registered parties, no |
477 |
* parent, and initial phase number 0. Any thread using this |
478 |
* phaser will need to first register for it. |
479 |
*/ |
480 |
public Phaser() { |
481 |
this(null, 0); |
482 |
} |
483 |
|
484 |
/** |
485 |
* Creates a new phaser with the given number of registered |
486 |
* unarrived parties, no parent, and initial phase number 0. |
487 |
* |
488 |
* @param parties the number of parties required to advance to the |
489 |
* next phase |
490 |
* @throws IllegalArgumentException if parties less than zero |
491 |
* or greater than the maximum number of parties supported |
492 |
*/ |
493 |
public Phaser(int parties) { |
494 |
this(null, parties); |
495 |
} |
496 |
|
497 |
/** |
498 |
* Equivalent to {@link #Phaser(Phaser, int) Phaser(parent, 0)}. |
499 |
* |
500 |
* @param parent the parent phaser |
501 |
*/ |
502 |
public Phaser(Phaser parent) { |
503 |
this(parent, 0); |
504 |
} |
505 |
|
506 |
/** |
507 |
* Creates a new phaser with the given parent and number of |
508 |
* registered unarrived parties. When the given parent is non-null |
509 |
* and the given number of parties is greater than zero, this |
510 |
* child phaser is registered with its parent. |
511 |
* |
512 |
* @param parent the parent phaser |
513 |
* @param parties the number of parties required to advance to the |
514 |
* next phase |
515 |
* @throws IllegalArgumentException if parties less than zero |
516 |
* or greater than the maximum number of parties supported |
517 |
*/ |
518 |
public Phaser(Phaser parent, int parties) { |
519 |
if (parties >>> PARTIES_SHIFT != 0) |
520 |
throw new IllegalArgumentException("Illegal number of parties"); |
521 |
int phase = 0; |
522 |
this.parent = parent; |
523 |
if (parent != null) { |
524 |
final Phaser root = parent.root; |
525 |
this.root = root; |
526 |
this.evenQ = root.evenQ; |
527 |
this.oddQ = root.oddQ; |
528 |
if (parties != 0) |
529 |
phase = parent.doRegister(1); |
530 |
} |
531 |
else { |
532 |
this.root = this; |
533 |
this.evenQ = new AtomicReference<QNode>(); |
534 |
this.oddQ = new AtomicReference<QNode>(); |
535 |
} |
536 |
this.state = (parties == 0) ? (long)EMPTY : |
537 |
((long)phase << PHASE_SHIFT) | |
538 |
((long)parties << PARTIES_SHIFT) | |
539 |
((long)parties); |
540 |
} |
541 |
|
542 |
/** |
543 |
* Adds a new unarrived party to this phaser. If an ongoing |
544 |
* invocation of {@link #onAdvance} is in progress, this method |
545 |
* may await its completion before returning. If this phaser has |
546 |
* a parent, and this phaser previously had no registered parties, |
547 |
* this child phaser is also registered with its parent. If |
548 |
* this phaser is terminated, the attempt to register has |
549 |
* no effect, and a negative value is returned. |
550 |
* |
551 |
* @return the arrival phase number to which this registration |
552 |
* applied. If this value is negative, then this phaser has |
553 |
* terminated, in which case registration has no effect. |
554 |
* @throws IllegalStateException if attempting to register more |
555 |
* than the maximum supported number of parties |
556 |
*/ |
557 |
public int register() { |
558 |
return doRegister(1); |
559 |
} |
560 |
|
561 |
/** |
562 |
* Adds the given number of new unarrived parties to this phaser. |
563 |
* If an ongoing invocation of {@link #onAdvance} is in progress, |
564 |
* this method may await its completion before returning. If this |
565 |
* phaser has a parent, and the given number of parties is greater |
566 |
* than zero, and this phaser previously had no registered |
567 |
* parties, this child phaser is also registered with its parent. |
568 |
* If this phaser is terminated, the attempt to register has no |
569 |
* effect, and a negative value is returned. |
570 |
* |
571 |
* @param parties the number of additional parties required to |
572 |
* advance to the next phase |
573 |
* @return the arrival phase number to which this registration |
574 |
* applied. If this value is negative, then this phaser has |
575 |
* terminated, in which case registration has no effect. |
576 |
* @throws IllegalStateException if attempting to register more |
577 |
* than the maximum supported number of parties |
578 |
* @throws IllegalArgumentException if {@code parties < 0} |
579 |
*/ |
580 |
public int bulkRegister(int parties) { |
581 |
if (parties < 0) |
582 |
throw new IllegalArgumentException(); |
583 |
if (parties == 0) |
584 |
return getPhase(); |
585 |
return doRegister(parties); |
586 |
} |
587 |
|
588 |
/** |
589 |
* Arrives at this phaser, without waiting for others to arrive. |
590 |
* |
591 |
* <p>It is a usage error for an unregistered party to invoke this |
592 |
* method. However, this error may result in an {@code |
593 |
* IllegalStateException} only upon some subsequent operation on |
594 |
* this phaser, if ever. |
595 |
* |
596 |
* @return the arrival phase number, or a negative value if terminated |
597 |
* @throws IllegalStateException if not terminated and the number |
598 |
* of unarrived parties would become negative |
599 |
*/ |
600 |
public int arrive() { |
601 |
return doArrive(ONE_ARRIVAL); |
602 |
} |
603 |
|
604 |
/** |
605 |
* Arrives at this phaser and deregisters from it without waiting |
606 |
* for others to arrive. Deregistration reduces the number of |
607 |
* parties required to advance in future phases. If this phaser |
608 |
* has a parent, and deregistration causes this phaser to have |
609 |
* zero parties, this phaser is also deregistered from its parent. |
610 |
* |
611 |
* <p>It is a usage error for an unregistered party to invoke this |
612 |
* method. However, this error may result in an {@code |
613 |
* IllegalStateException} only upon some subsequent operation on |
614 |
* this phaser, if ever. |
615 |
* |
616 |
* @return the arrival phase number, or a negative value if terminated |
617 |
* @throws IllegalStateException if not terminated and the number |
618 |
* of registered or unarrived parties would become negative |
619 |
*/ |
620 |
public int arriveAndDeregister() { |
621 |
return doArrive(ONE_DEREGISTER); |
622 |
} |
623 |
|
624 |
/** |
625 |
* Arrives at this phaser and awaits others. Equivalent in effect |
626 |
* to {@code awaitAdvance(arrive())}. If you need to await with |
627 |
* interruption or timeout, you can arrange this with an analogous |
628 |
* construction using one of the other forms of the {@code |
629 |
* awaitAdvance} method. If instead you need to deregister upon |
630 |
* arrival, use {@code awaitAdvance(arriveAndDeregister())}. |
631 |
* |
632 |
* <p>It is a usage error for an unregistered party to invoke this |
633 |
* method. However, this error may result in an {@code |
634 |
* IllegalStateException} only upon some subsequent operation on |
635 |
* this phaser, if ever. |
636 |
* |
637 |
* @return the arrival phase number, or the (negative) |
638 |
* {@linkplain #getPhase() current phase} if terminated |
639 |
* @throws IllegalStateException if not terminated and the number |
640 |
* of unarrived parties would become negative |
641 |
*/ |
642 |
public int arriveAndAwaitAdvance() { |
643 |
// Specialization of doArrive+awaitAdvance eliminating some reads/paths |
644 |
final Phaser root = this.root; |
645 |
for (;;) { |
646 |
long s = (root == this) ? state : reconcileState(); |
647 |
int phase = (int)(s >>> PHASE_SHIFT); |
648 |
if (phase < 0) |
649 |
return phase; |
650 |
int counts = (int)s; |
651 |
int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK); |
652 |
if (unarrived <= 0) |
653 |
throw new IllegalStateException(badArrive(s)); |
654 |
if (STATE.compareAndSet(this, s, s -= ONE_ARRIVAL)) { |
655 |
if (unarrived > 1) |
656 |
return root.internalAwaitAdvance(phase, null); |
657 |
if (root != this) |
658 |
return parent.arriveAndAwaitAdvance(); |
659 |
long n = s & PARTIES_MASK; // base of next state |
660 |
int nextUnarrived = (int)n >>> PARTIES_SHIFT; |
661 |
if (onAdvance(phase, nextUnarrived)) |
662 |
n |= TERMINATION_BIT; |
663 |
else if (nextUnarrived == 0) |
664 |
n |= EMPTY; |
665 |
else |
666 |
n |= nextUnarrived; |
667 |
int nextPhase = (phase + 1) & MAX_PHASE; |
668 |
n |= (long)nextPhase << PHASE_SHIFT; |
669 |
if (!STATE.compareAndSet(this, s, n)) |
670 |
return (int)(state >>> PHASE_SHIFT); // terminated |
671 |
releaseWaiters(phase); |
672 |
return nextPhase; |
673 |
} |
674 |
} |
675 |
} |
676 |
|
677 |
/** |
678 |
* Awaits the phase of this phaser to advance from the given phase |
679 |
* value, returning immediately if the current phase is not equal |
680 |
* to the given phase value or this phaser is terminated. |
681 |
* |
682 |
* @param phase an arrival phase number, or negative value if |
683 |
* terminated; this argument is normally the value returned by a |
684 |
* previous call to {@code arrive} or {@code arriveAndDeregister}. |
685 |
* @return the next arrival phase number, or the argument if it is |
686 |
* negative, or the (negative) {@linkplain #getPhase() current phase} |
687 |
* if terminated |
688 |
*/ |
689 |
public int awaitAdvance(int phase) { |
690 |
final Phaser root = this.root; |
691 |
long s = (root == this) ? state : reconcileState(); |
692 |
int p = (int)(s >>> PHASE_SHIFT); |
693 |
if (phase < 0) |
694 |
return phase; |
695 |
if (p == phase) |
696 |
return root.internalAwaitAdvance(phase, null); |
697 |
return p; |
698 |
} |
699 |
|
700 |
/** |
701 |
* Awaits the phase of this phaser to advance from the given phase |
702 |
* value, throwing {@code InterruptedException} if interrupted |
703 |
* while waiting, or returning immediately if the current phase is |
704 |
* not equal to the given phase value or this phaser is |
705 |
* terminated. |
706 |
* |
707 |
* @param phase an arrival phase number, or negative value if |
708 |
* terminated; this argument is normally the value returned by a |
709 |
* previous call to {@code arrive} or {@code arriveAndDeregister}. |
710 |
* @return the next arrival phase number, or the argument if it is |
711 |
* negative, or the (negative) {@linkplain #getPhase() current phase} |
712 |
* if terminated |
713 |
* @throws InterruptedException if thread interrupted while waiting |
714 |
*/ |
715 |
public int awaitAdvanceInterruptibly(int phase) |
716 |
throws InterruptedException { |
717 |
final Phaser root = this.root; |
718 |
long s = (root == this) ? state : reconcileState(); |
719 |
int p = (int)(s >>> PHASE_SHIFT); |
720 |
if (phase < 0) |
721 |
return phase; |
722 |
if (p == phase) { |
723 |
QNode node = new QNode(this, phase, true, false, 0L); |
724 |
p = root.internalAwaitAdvance(phase, node); |
725 |
if (node.wasInterrupted) |
726 |
throw new InterruptedException(); |
727 |
} |
728 |
return p; |
729 |
} |
730 |
|
731 |
/** |
732 |
* Awaits the phase of this phaser to advance from the given phase |
733 |
* value or the given timeout to elapse, throwing {@code |
734 |
* InterruptedException} if interrupted while waiting, or |
735 |
* returning immediately if the current phase is not equal to the |
736 |
* given phase value or this phaser is terminated. |
737 |
* |
738 |
* @param phase an arrival phase number, or negative value if |
739 |
* terminated; this argument is normally the value returned by a |
740 |
* previous call to {@code arrive} or {@code arriveAndDeregister}. |
741 |
* @param timeout how long to wait before giving up, in units of |
742 |
* {@code unit} |
743 |
* @param unit a {@code TimeUnit} determining how to interpret the |
744 |
* {@code timeout} parameter |
745 |
* @return the next arrival phase number, or the argument if it is |
746 |
* negative, or the (negative) {@linkplain #getPhase() current phase} |
747 |
* if terminated |
748 |
* @throws InterruptedException if thread interrupted while waiting |
749 |
* @throws TimeoutException if timed out while waiting |
750 |
*/ |
751 |
public int awaitAdvanceInterruptibly(int phase, |
752 |
long timeout, TimeUnit unit) |
753 |
throws InterruptedException, TimeoutException { |
754 |
long nanos = unit.toNanos(timeout); |
755 |
final Phaser root = this.root; |
756 |
long s = (root == this) ? state : reconcileState(); |
757 |
int p = (int)(s >>> PHASE_SHIFT); |
758 |
if (phase < 0) |
759 |
return phase; |
760 |
if (p == phase) { |
761 |
QNode node = new QNode(this, phase, true, true, nanos); |
762 |
p = root.internalAwaitAdvance(phase, node); |
763 |
if (node.wasInterrupted) |
764 |
throw new InterruptedException(); |
765 |
else if (p == phase) |
766 |
throw new TimeoutException(); |
767 |
} |
768 |
return p; |
769 |
} |
770 |
|
771 |
/** |
772 |
* Forces this phaser to enter termination state. Counts of |
773 |
* registered parties are unaffected. If this phaser is a member |
774 |
* of a tiered set of phasers, then all of the phasers in the set |
775 |
* are terminated. If this phaser is already terminated, this |
776 |
* method has no effect. This method may be useful for |
777 |
* coordinating recovery after one or more tasks encounter |
778 |
* unexpected exceptions. |
779 |
*/ |
780 |
public void forceTermination() { |
781 |
// Only need to change root state |
782 |
final Phaser root = this.root; |
783 |
long s; |
784 |
while ((s = root.state) >= 0) { |
785 |
if (STATE.compareAndSet(root, s, s | TERMINATION_BIT)) { |
786 |
// signal all threads |
787 |
releaseWaiters(0); // Waiters on evenQ |
788 |
releaseWaiters(1); // Waiters on oddQ |
789 |
return; |
790 |
} |
791 |
} |
792 |
} |
793 |
|
794 |
/** |
795 |
* Returns the current phase number. The maximum phase number is |
796 |
* {@code Integer.MAX_VALUE}, after which it restarts at |
797 |
* zero. Upon termination, the phase number is negative, |
798 |
* in which case the prevailing phase prior to termination |
799 |
* may be obtained via {@code getPhase() + Integer.MIN_VALUE}. |
800 |
* |
801 |
* @return the phase number, or a negative value if terminated |
802 |
*/ |
803 |
public final int getPhase() { |
804 |
return (int)(root.state >>> PHASE_SHIFT); |
805 |
} |
806 |
|
807 |
/** |
808 |
* Returns the number of parties registered at this phaser. |
809 |
* |
810 |
* @return the number of parties |
811 |
*/ |
812 |
public int getRegisteredParties() { |
813 |
return partiesOf(state); |
814 |
} |
815 |
|
816 |
/** |
817 |
* Returns the number of registered parties that have arrived at |
818 |
* the current phase of this phaser. If this phaser has terminated, |
819 |
* the returned value is meaningless and arbitrary. |
820 |
* |
821 |
* @return the number of arrived parties |
822 |
*/ |
823 |
public int getArrivedParties() { |
824 |
return arrivedOf(reconcileState()); |
825 |
} |
826 |
|
827 |
/** |
828 |
* Returns the number of registered parties that have not yet |
829 |
* arrived at the current phase of this phaser. If this phaser has |
830 |
* terminated, the returned value is meaningless and arbitrary. |
831 |
* |
832 |
* @return the number of unarrived parties |
833 |
*/ |
834 |
public int getUnarrivedParties() { |
835 |
return unarrivedOf(reconcileState()); |
836 |
} |
837 |
|
838 |
/** |
839 |
* Returns the parent of this phaser, or {@code null} if none. |
840 |
* |
841 |
* @return the parent of this phaser, or {@code null} if none |
842 |
*/ |
843 |
public Phaser getParent() { |
844 |
return parent; |
845 |
} |
846 |
|
847 |
/** |
848 |
* Returns the root ancestor of this phaser, which is the same as |
849 |
* this phaser if it has no parent. |
850 |
* |
851 |
* @return the root ancestor of this phaser |
852 |
*/ |
853 |
public Phaser getRoot() { |
854 |
return root; |
855 |
} |
856 |
|
857 |
/** |
858 |
* Returns {@code true} if this phaser has been terminated. |
859 |
* |
860 |
* @return {@code true} if this phaser has been terminated |
861 |
*/ |
862 |
public boolean isTerminated() { |
863 |
return root.state < 0L; |
864 |
} |
865 |
|
866 |
/** |
867 |
* Overridable method to perform an action upon impending phase |
868 |
* advance, and to control termination. This method is invoked |
869 |
* upon arrival of the party advancing this phaser (when all other |
870 |
* waiting parties are dormant). If this method returns {@code |
871 |
* true}, this phaser will be set to a final termination state |
872 |
* upon advance, and subsequent calls to {@link #isTerminated} |
873 |
* will return true. Any (unchecked) Exception or Error thrown by |
874 |
* an invocation of this method is propagated to the party |
875 |
* attempting to advance this phaser, in which case no advance |
876 |
* occurs. |
877 |
* |
878 |
* <p>The arguments to this method provide the state of the phaser |
879 |
* prevailing for the current transition. The effects of invoking |
880 |
* arrival, registration, and waiting methods on this phaser from |
881 |
* within {@code onAdvance} are unspecified and should not be |
882 |
* relied on. |
883 |
* |
884 |
* <p>If this phaser is a member of a tiered set of phasers, then |
885 |
* {@code onAdvance} is invoked only for its root phaser on each |
886 |
* advance. |
887 |
* |
888 |
* <p>To support the most common use cases, the default |
889 |
* implementation of this method returns {@code true} when the |
890 |
* number of registered parties has become zero as the result of a |
891 |
* party invoking {@code arriveAndDeregister}. You can disable |
892 |
* this behavior, thus enabling continuation upon future |
893 |
* registrations, by overriding this method to always return |
894 |
* {@code false}: |
895 |
* |
896 |
* <pre> {@code |
897 |
* Phaser phaser = new Phaser() { |
898 |
* protected boolean onAdvance(int phase, int parties) { return false; } |
899 |
* };}</pre> |
900 |
* |
901 |
* @param phase the current phase number on entry to this method, |
902 |
* before this phaser is advanced |
903 |
* @param registeredParties the current number of registered parties |
904 |
* @return {@code true} if this phaser should terminate |
905 |
*/ |
906 |
protected boolean onAdvance(int phase, int registeredParties) { |
907 |
return registeredParties == 0; |
908 |
} |
909 |
|
910 |
/** |
911 |
* Returns a string identifying this phaser, as well as its |
912 |
* state. The state, in brackets, includes the String {@code |
913 |
* "phase = "} followed by the phase number, {@code "parties = "} |
914 |
* followed by the number of registered parties, and {@code |
915 |
* "arrived = "} followed by the number of arrived parties. |
916 |
* |
917 |
* @return a string identifying this phaser, as well as its state |
918 |
*/ |
919 |
public String toString() { |
920 |
return stateToString(reconcileState()); |
921 |
} |
922 |
|
923 |
/** |
924 |
* Implementation of toString and string-based error messages. |
925 |
*/ |
926 |
private String stateToString(long s) { |
927 |
return super.toString() + |
928 |
"[phase = " + phaseOf(s) + |
929 |
" parties = " + partiesOf(s) + |
930 |
" arrived = " + arrivedOf(s) + "]"; |
931 |
} |
932 |
|
933 |
// Waiting mechanics |
934 |
|
935 |
/** |
936 |
* Removes and signals threads from queue for phase. |
937 |
*/ |
938 |
private void releaseWaiters(int phase) { |
939 |
QNode q; // first element of queue |
940 |
Thread t; // its thread |
941 |
AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; |
942 |
while ((q = head.get()) != null && |
943 |
q.phase != (int)(root.state >>> PHASE_SHIFT)) { |
944 |
if (head.compareAndSet(q, q.next) && |
945 |
(t = q.thread) != null) { |
946 |
q.thread = null; |
947 |
LockSupport.unpark(t); |
948 |
} |
949 |
} |
950 |
} |
951 |
|
952 |
/** |
953 |
* Variant of releaseWaiters that additionally tries to remove any |
954 |
* nodes no longer waiting for advance due to timeout or |
955 |
* interrupt. Currently, nodes are removed only if they are at |
956 |
* head of queue, which suffices to reduce memory footprint in |
957 |
* most usages. |
958 |
* |
959 |
* @return current phase on exit |
960 |
*/ |
961 |
private int abortWait(int phase) { |
962 |
AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; |
963 |
for (;;) { |
964 |
Thread t; |
965 |
QNode q = head.get(); |
966 |
int p = (int)(root.state >>> PHASE_SHIFT); |
967 |
if (q == null || ((t = q.thread) != null && q.phase == p)) |
968 |
return p; |
969 |
if (head.compareAndSet(q, q.next) && t != null) { |
970 |
q.thread = null; |
971 |
LockSupport.unpark(t); |
972 |
} |
973 |
} |
974 |
} |
975 |
|
976 |
/** The number of CPUs, for spin control */ |
977 |
private static final int NCPU = Runtime.getRuntime().availableProcessors(); |
978 |
|
979 |
/** |
980 |
* The number of times to spin before blocking while waiting for |
981 |
* advance, per arrival while waiting. On multiprocessors, fully |
982 |
* blocking and waking up a large number of threads all at once is |
983 |
* usually a very slow process, so we use rechargeable spins to |
984 |
* avoid it when threads regularly arrive: When a thread in |
985 |
* internalAwaitAdvance notices another arrival before blocking, |
986 |
* and there appear to be enough CPUs available, it spins |
987 |
* SPINS_PER_ARRIVAL more times before blocking. The value trades |
988 |
* off good-citizenship vs big unnecessary slowdowns. |
989 |
*/ |
990 |
static final int SPINS_PER_ARRIVAL = (NCPU < 2) ? 1 : 1 << 8; |
991 |
|
992 |
/** |
993 |
* Possibly blocks and waits for phase to advance unless aborted. |
994 |
* Call only on root phaser. |
995 |
* |
996 |
* @param phase current phase |
997 |
* @param node if non-null, the wait node to track interrupt and timeout; |
998 |
* if null, denotes noninterruptible wait |
999 |
* @return current phase |
1000 |
*/ |
1001 |
private int internalAwaitAdvance(int phase, QNode node) { |
1002 |
// assert root == this; |
1003 |
releaseWaiters(phase-1); // ensure old queue clean |
1004 |
boolean queued = false; // true when node is enqueued |
1005 |
int lastUnarrived = 0; // to increase spins upon change |
1006 |
int spins = SPINS_PER_ARRIVAL; |
1007 |
long s; |
1008 |
int p; |
1009 |
while ((p = (int)((s = state) >>> PHASE_SHIFT)) == phase) { |
1010 |
if (node == null) { // spinning in noninterruptible mode |
1011 |
int unarrived = (int)s & UNARRIVED_MASK; |
1012 |
if (unarrived != lastUnarrived && |
1013 |
(lastUnarrived = unarrived) < NCPU) |
1014 |
spins += SPINS_PER_ARRIVAL; |
1015 |
boolean interrupted = Thread.interrupted(); |
1016 |
if (interrupted || --spins < 0) { // need node to record intr |
1017 |
node = new QNode(this, phase, false, false, 0L); |
1018 |
node.wasInterrupted = interrupted; |
1019 |
} |
1020 |
else |
1021 |
Thread.onSpinWait(); |
1022 |
} |
1023 |
else if (node.isReleasable()) // done or aborted |
1024 |
break; |
1025 |
else if (!queued) { // push onto queue |
1026 |
AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; |
1027 |
QNode q = node.next = head.get(); |
1028 |
if ((q == null || q.phase == phase) && |
1029 |
(int)(state >>> PHASE_SHIFT) == phase) // avoid stale enq |
1030 |
queued = head.compareAndSet(q, node); |
1031 |
} |
1032 |
else { |
1033 |
try { |
1034 |
ForkJoinPool.managedBlock(node); |
1035 |
} catch (InterruptedException cantHappen) { |
1036 |
node.wasInterrupted = true; |
1037 |
} |
1038 |
} |
1039 |
} |
1040 |
|
1041 |
if (node != null) { |
1042 |
if (node.thread != null) |
1043 |
node.thread = null; // avoid need for unpark() |
1044 |
if (node.wasInterrupted && !node.interruptible) |
1045 |
Thread.currentThread().interrupt(); |
1046 |
if (p == phase && (p = (int)(state >>> PHASE_SHIFT)) == phase) |
1047 |
return abortWait(phase); // possibly clean up on abort |
1048 |
} |
1049 |
releaseWaiters(phase); |
1050 |
return p; |
1051 |
} |
1052 |
|
1053 |
/** |
1054 |
* Wait nodes for Treiber stack representing wait queue. |
1055 |
*/ |
1056 |
static final class QNode implements ForkJoinPool.ManagedBlocker { |
1057 |
final Phaser phaser; |
1058 |
final int phase; |
1059 |
final boolean interruptible; |
1060 |
final boolean timed; |
1061 |
boolean wasInterrupted; |
1062 |
long nanos; |
1063 |
final long deadline; |
1064 |
volatile Thread thread; // nulled to cancel wait |
1065 |
QNode next; |
1066 |
|
1067 |
QNode(Phaser phaser, int phase, boolean interruptible, |
1068 |
boolean timed, long nanos) { |
1069 |
this.phaser = phaser; |
1070 |
this.phase = phase; |
1071 |
this.interruptible = interruptible; |
1072 |
this.nanos = nanos; |
1073 |
this.timed = timed; |
1074 |
this.deadline = timed ? System.nanoTime() + nanos : 0L; |
1075 |
thread = Thread.currentThread(); |
1076 |
} |
1077 |
|
1078 |
public boolean isReleasable() { |
1079 |
if (thread == null) |
1080 |
return true; |
1081 |
if (phaser.getPhase() != phase) { |
1082 |
thread = null; |
1083 |
return true; |
1084 |
} |
1085 |
if (Thread.interrupted()) |
1086 |
wasInterrupted = true; |
1087 |
if (wasInterrupted && interruptible) { |
1088 |
thread = null; |
1089 |
return true; |
1090 |
} |
1091 |
if (timed && |
1092 |
(nanos <= 0L || (nanos = deadline - System.nanoTime()) <= 0L)) { |
1093 |
thread = null; |
1094 |
return true; |
1095 |
} |
1096 |
return false; |
1097 |
} |
1098 |
|
1099 |
public boolean block() { |
1100 |
while (!isReleasable()) { |
1101 |
if (timed) |
1102 |
LockSupport.parkNanos(this, nanos); |
1103 |
else |
1104 |
LockSupport.park(this); |
1105 |
} |
1106 |
return true; |
1107 |
} |
1108 |
} |
1109 |
|
1110 |
// VarHandle mechanics |
1111 |
private static final VarHandle STATE; |
1112 |
static { |
1113 |
try { |
1114 |
MethodHandles.Lookup l = MethodHandles.lookup(); |
1115 |
STATE = l.findVarHandle(Phaser.class, "state", long.class); |
1116 |
} catch (ReflectiveOperationException e) { |
1117 |
throw new ExceptionInInitializerError(e); |
1118 |
} |
1119 |
|
1120 |
// Reduce the risk of rare disastrous classloading in first call to |
1121 |
// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773 |
1122 |
Class<?> ensureLoaded = LockSupport.class; |
1123 |
} |
1124 |
} |