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import java.util.concurrent.locks.LockSupport; |
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/** |
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* A reusable synchronization barrier, similar in functionality to a |
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* A reusable synchronization barrier, similar in functionality to |
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* {@link java.util.concurrent.CyclicBarrier CyclicBarrier} and |
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* {@link java.util.concurrent.CountDownLatch CountDownLatch} |
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* but supporting more flexible usage. |
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* |
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* <ul> |
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* |
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* <li> The number of parties synchronizing on a phaser may vary over |
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* time. A task may register to be a party at any time, and may |
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* deregister upon arriving at the barrier. As is the case with most |
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* basic synchronization constructs, registration and deregistration |
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* affect only internal counts; they do not establish any further |
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* internal bookkeeping, so tasks cannot query whether they are |
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* registered. (However, you can introduce such bookkeeping by |
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* subclassing this class.) |
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* |
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* <li> Each generation has an associated phase value, starting at |
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* zero, and advancing when all parties reach the barrier (wrapping |
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* around to zero after reaching {@code Integer.MAX_VALUE}). |
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* |
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* <li> Like a {@code CyclicBarrier}, a phaser may be repeatedly |
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* awaited. Method {@link #arriveAndAwaitAdvance} has effect |
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* analogous to {@link java.util.concurrent.CyclicBarrier#await |
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* CyclicBarrier.await}. However, phasers separate two aspects of |
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* coordination, which may also be invoked independently: |
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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 {@code Phaser} has an associated phase number. The |
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* phase number starts at zero, and advances when all parties arrive |
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* at the barrier, 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 barrier 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> Arriving at a barrier. Methods {@link #arrive} and |
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* {@link #arriveAndDeregister} do not block, but return |
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* the phase value current upon entry to the method. |
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* |
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* <li> Awaiting others. Method {@link #awaitAdvance} requires an |
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* argument indicating the entry phase, and returns when the |
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* barrier advances to a new phase. |
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* </ul> |
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* <li> <b>Arrival.</b> Methods {@link #arrive} and |
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* {@link #arriveAndDeregister} record arrival at a |
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* barrier. These methods do not block, but return an associated |
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* <em>arrival phase number</em>; that is, the phase number of |
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* the barrier to which the arrival applied. When the final |
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* party for a given phase arrives, an optional barrier action |
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* is performed and the phase advances. Barrier actions, |
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* performed by the party triggering a phase advance, are |
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* arranged by overriding method {@link #onAdvance(int, int)}, |
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* which also controls termination. Overriding this method is |
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* similar to, but more flexible than, providing a barrier |
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* action to a {@code 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 barrier 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 barrier. 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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* which will ensure sufficient parallelism to execute tasks |
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* when others are blocked waiting for a phase to advance. |
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* |
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* </ul> |
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* |
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* <li> Barrier actions, performed by the task triggering a phase |
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* advance, are arranged by overriding method {@link #onAdvance(int, |
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* int)}, which also controls termination. Overriding this method is |
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* similar to, but more flexible than, providing a barrier action to a |
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* {@code CyclicBarrier}. |
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* |
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* <li> Phasers may enter a <em>termination</em> state in which all |
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* actions immediately return without updating phaser state or waiting |
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* for advance, and indicating (via a negative phase value) that |
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* execution is complete. Termination is triggered when an invocation |
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* of {@code onAdvance} returns {@code true}. When a phaser is |
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* controlling an action with a fixed number of iterations, it is |
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* <p> <b>Termination.</b> A {@code Phaser} may enter a |
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* <em>termination</em> state in which all synchronization methods |
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* immediately return without updating phaser state or waiting for |
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* advance, and indicating (via a negative phase value) that execution |
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* is complete. Termination is triggered when an invocation of {@code |
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* onAdvance} returns {@code true}. As illustrated below, when |
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* phasers control actions with a fixed number of iterations, it is |
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* often convenient to override this method to cause termination when |
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* the current phase number reaches a threshold. Method {@link |
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* #forceTermination} is also available to abruptly release waiting |
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* threads and allow them to terminate. |
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* |
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* <li> Phasers may be tiered to reduce contention. Phasers with large |
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* <p> <b>Tiering.</b> Phasers may be <em>tiered</em> (i.e., arranged |
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* in tree structures) to reduce contention. Phasers with large |
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* numbers of parties that would otherwise experience heavy |
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* synchronization contention costs may instead be arranged in trees. |
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* This will typically greatly increase throughput even though it |
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* incurs somewhat greater per-operation overhead. |
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* |
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* <li> By default, {@code awaitAdvance} continues to wait even if |
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* the waiting thread is interrupted. And unlike the case in |
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* {@code CyclicBarrier}, exceptions encountered while tasks wait |
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* interruptibly or with timeout do not change the state of the |
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* barrier. If necessary, you can perform any associated recovery |
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* within handlers of those exceptions, often after invoking |
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* {@code forceTermination}. |
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* |
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* <li>Phasers may be used to coordinate tasks executing in a {@link |
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* ForkJoinPool}, which will ensure sufficient parallelism to execute |
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* tasks when others are blocked waiting for a phase to advance. |
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* |
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* </ul> |
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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><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}, where {@link #getArrivedParties} have |
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* arrived at the current phase ({@link #getPhase}). When the |
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* remaining {@link #getUnarrivedParties}) arrive, the phase |
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* advances. Method {@link #toString} returns snapshots of these state |
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* queries in a form convenient for informal monitoring. |
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* |
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* <p><b>Sample usages:</b> |
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* |
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* <pre> {@code |
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* void startTasks(List<Runnable> tasks, final int iterations) { |
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* final Phaser phaser = new Phaser() { |
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* public boolean onAdvance(int phase, int registeredParties) { |
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* protected boolean onAdvance(int phase, int registeredParties) { |
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* return phase >= iterations || registeredParties == 0; |
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* } |
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* }; |
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* phaser.arriveAndDeregister(); // deregister self, don't wait |
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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(); |
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* }</pre> |
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* |
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* 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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* } |
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* }</pre> |
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* |
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* |
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* <p>To create a set of tasks using a tree of phasers, |
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* you could use code of the following form, assuming a |
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* Task class with a constructor accepting a phaser that |
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/** |
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* Adds a new unarrived party to this phaser. |
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* |
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* @return the current barrier phase number upon registration |
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* @return the arrival phase number to which this registration applied |
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* @throws IllegalStateException if attempting to register more |
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* than the maximum supported number of parties |
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*/ |
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* Adds the given number of new unarrived parties to this phaser. |
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* |
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* @param parties the number of parties required to trip barrier |
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* @return the current barrier phase number upon registration |
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* @return the arrival phase number to which this registration applied |
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* @throws IllegalStateException if attempting to register more |
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* than the maximum supported number of parties |
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*/ |
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|
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/** |
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* Arrives at the barrier, but does not wait for others. (You can |
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* in turn wait for others via {@link #awaitAdvance}). |
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* in turn wait for others via {@link #awaitAdvance}). It is an |
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* unenforced usage error for an unregistered party to invoke this |
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* method. |
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* |
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* @return the barrier phase number upon entry to this method, or a |
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* negative value if terminated |
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* @return the arrival phase number, or a negative value if terminated |
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* @throws IllegalStateException if not terminated and the number |
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* of unarrived parties would become negative |
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*/ |
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* required to trip the barrier in future phases. If this phaser |
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* has a parent, and deregistration causes this phaser to have |
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* zero parties, this phaser also arrives at and is deregistered |
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* from its parent. |
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* from its parent. It is an unenforced usage error for an |
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* unregistered party to invoke this method. |
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* |
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* @return the current barrier phase number upon entry to |
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* this method, or a negative value if terminated |
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* @return the arrival phase number, or a negative value if terminated |
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* @throws IllegalStateException if not terminated and the number |
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* of registered or unarrived parties would become negative |
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*/ |
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* interruption or timeout, you can arrange this with an analogous |
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* construction using one of the other forms of the awaitAdvance |
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* method. If instead you need to deregister upon arrival use |
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* {@code arriveAndDeregister}. |
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* {@code arriveAndDeregister}. It is an unenforced usage error |
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* for an unregistered party to invoke this method. |
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* |
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* @return the phase on entry to this method |
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* @return the arrival phase number, or a negative number if terminated |
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* @throws IllegalStateException if not terminated and the number |
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* of unarrived parties would become negative |
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*/ |
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* Awaits the phase of the barrier to advance from the given phase |
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* value, returning immediately if the current phase of the |
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* barrier is not equal to the given phase value or this barrier |
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* is terminated. |
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* is terminated. It is an unenforced usage error for an |
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* unregistered party to invoke this method. |
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* |
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* @param phase the phase on entry to this method |
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* @return the current barrier phase number upon exit of |
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* this method, or a negative value if terminated or |
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* argument is negative |
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* @param phase an arrival phase number, or negative value if |
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* terminated; this argument is normally the value returned by a |
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* previous call to {@code arrive} or its variants |
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* @return the next arrival phase number, or a negative value |
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* if terminated or argument is negative |
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*/ |
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public int awaitAdvance(int phase) { |
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if (phase < 0) |
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|
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/** |
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* Awaits the phase of the barrier to advance from the given phase |
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* value, throwing {@code InterruptedException} if interrupted while |
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* waiting, or returning immediately if the current phase of the |
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* barrier is not equal to the given phase value or this barrier |
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* is terminated. |
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* |
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* @param phase the phase on entry to this method |
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* @return the current barrier phase number upon exit of |
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* this method, or a negative value if terminated or |
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* argument is negative |
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* value, throwing {@code InterruptedException} if interrupted |
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* while waiting, or returning immediately if the current phase of |
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* the barrier is not equal to the given phase value or this |
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* barrier is terminated. It is an unenforced usage error for an |
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* unregistered party to invoke this method. |
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* |
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* @param phase an arrival phase number, or negative value if |
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* terminated; this argument is normally the value returned by a |
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* previous call to {@code arrive} or its variants |
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* @return the next arrival phase number, or a negative value |
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* if terminated or argument is negative |
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* @throws InterruptedException if thread interrupted while waiting |
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*/ |
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public int awaitAdvanceInterruptibly(int phase) |
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|
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/** |
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* Awaits the phase of the barrier to advance from the given phase |
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* value or the given timeout to elapse, throwing |
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* {@code InterruptedException} if interrupted while waiting, or |
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* returning immediately if the current phase of the barrier is not |
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* equal to the given phase value or this barrier is terminated. |
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* |
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* @param phase the phase on entry to this method |
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* value or the given timeout to elapse, throwing {@code |
638 |
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* InterruptedException} if interrupted while waiting, or |
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* returning immediately if the current phase of the barrier is |
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* not equal to the given phase value or this barrier is |
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* terminated. It is an unenforced usage error for an |
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* unregistered party to invoke this method. |
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* |
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* @param phase an arrival phase number, or negative value if |
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* terminated; this argument is normally the value returned by a |
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* previous call to {@code arrive} or its variants |
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* @param timeout how long to wait before giving up, in units of |
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* {@code unit} |
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* @param unit a {@code TimeUnit} determining how to interpret the |
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* {@code timeout} parameter |
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* @return the current barrier phase number upon exit of |
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* this method, or a negative value if terminated or |
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* argument is negative |
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* @return the next arrival phase number, or a negative value |
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* if terminated or argument is negative |
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* @throws InterruptedException if thread interrupted while waiting |
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* @throws TimeoutException if timed out while waiting |
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*/ |
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} |
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|
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/** |
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* Returns the number of parties that have arrived at the current |
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* phase of this barrier. |
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* Returns the number of registered parties that have arrived at |
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* the current phase of this barrier. |
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* |
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* @return the number of arrived parties |
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*/ |