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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 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 <em>registered</em> to synchronize on a |
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* phaser may vary over time. Tasks may be registered at any time |
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* (using methods {@link #register}, {@link #bulkRegister}, or forms |
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* of constructors establishing initial numbers of parties), and may |
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* optionally be deregistered upon any arrival (using {@link |
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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 |
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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 number. The phase |
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* number starts at zero, amd advances when all parties arrive at the |
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* barrier, wrapping around to zero after reaching {@code |
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* 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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* 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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* an associated <em>arrival phase number</em>; |
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* that is, the phase number of the barrier to which the |
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* arrival applied. |
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* |
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* <li> Awaiting others. Method {@link #awaitAdvance} requires an |
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* argument indicating an arrival phase number, and returns |
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* when the barrier advances to a new phase. |
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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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< |
* |
| 93 |
< |
* <li>The current state of a phaser may be monitored. At any given |
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< |
* moment there are {@link #getRegisteredParties}, where {@link |
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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. |
| 96 |
> |
* |
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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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< |
* arrive, the phase advances. Method {@link #toString} returns |
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< |
* snapshots of these state queries in a form convenient for |
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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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– |
* </ul> |
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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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* <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: |
| 171 |
+ |
* |
| 172 |
+ |
* <pre> {@code |
| 173 |
+ |
* void awaitPhase(Phaser phaser, int phase) { |
| 174 |
+ |
* int p = phaser.register(); // assumes caller not already registered |
| 175 |
+ |
* while (p < phase) { |
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+ |
* if (phaser.isTerminated()) |
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+ |
* // ... deal with unexpected termination |
| 178 |
+ |
* else |
| 179 |
+ |
* p = phaser.arriveAndAwaitAdvance(); |
| 180 |
+ |
* } |
| 181 |
+ |
* phaser.arriveAndDeregister(); |
| 182 |
+ |
* } |
| 183 |
+ |
* }</pre> |
| 184 |
+ |
* |
| 185 |
+ |
* |
| 186 |
|
* <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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|
private volatile long state; |
| 251 |
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|
| 219 |
– |
private static final int ushortBits = 16; |
| 252 |
|
private static final int ushortMask = 0xffff; |
| 253 |
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private static final int phaseMask = 0x7fffffff; |
| 254 |
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|
| 458 |
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|
| 459 |
|
/** |
| 460 |
|
* Arrives at the barrier, but does not wait for others. (You can |
| 461 |
< |
* in turn wait for others via {@link #awaitAdvance}). |
| 461 |
> |
* in turn wait for others via {@link #awaitAdvance}). It is an |
| 462 |
> |
* unenforced usage error for an unregistered party to invoke this |
| 463 |
> |
* method. |
| 464 |
|
* |
| 465 |
|
* @return the arrival phase number, or a negative value if terminated |
| 466 |
|
* @throws IllegalStateException if not terminated and the number |
| 512 |
|
* required to trip the barrier in future phases. If this phaser |
| 513 |
|
* has a parent, and deregistration causes this phaser to have |
| 514 |
|
* zero parties, this phaser also arrives at and is deregistered |
| 515 |
< |
* from its parent. |
| 515 |
> |
* from its parent. It is an unenforced usage error for an |
| 516 |
> |
* unregistered party to invoke this method. |
| 517 |
|
* |
| 518 |
|
* @return the arrival phase number, or a negative value if terminated |
| 519 |
|
* @throws IllegalStateException if not terminated and the number |
| 569 |
|
* interruption or timeout, you can arrange this with an analogous |
| 570 |
|
* construction using one of the other forms of the awaitAdvance |
| 571 |
|
* method. If instead you need to deregister upon arrival use |
| 572 |
< |
* {@code arriveAndDeregister}. |
| 572 |
> |
* {@code arriveAndDeregister}. It is an unenforced usage error |
| 573 |
> |
* for an unregistered party to invoke this method. |
| 574 |
|
* |
| 575 |
|
* @return the arrival phase number, or a negative number if terminated |
| 576 |
|
* @throws IllegalStateException if not terminated and the number |
| 584 |
|
* Awaits the phase of the barrier to advance from the given phase |
| 585 |
|
* value, returning immediately if the current phase of the |
| 586 |
|
* barrier is not equal to the given phase value or this barrier |
| 587 |
< |
* is terminated. |
| 587 |
> |
* is terminated. It is an unenforced usage error for an |
| 588 |
> |
* unregistered party to invoke this method. |
| 589 |
|
* |
| 590 |
|
* @param phase an arrival phase number, or negative value if |
| 591 |
|
* terminated; this argument is normally the value returned by a |
| 608 |
|
|
| 609 |
|
/** |
| 610 |
|
* Awaits the phase of the barrier to advance from the given phase |
| 611 |
< |
* value, throwing {@code InterruptedException} if interrupted while |
| 612 |
< |
* waiting, or returning immediately if the current phase of the |
| 613 |
< |
* barrier is not equal to the given phase value or this barrier |
| 614 |
< |
* is terminated. |
| 611 |
> |
* value, throwing {@code InterruptedException} if interrupted |
| 612 |
> |
* while waiting, or returning immediately if the current phase of |
| 613 |
> |
* the barrier is not equal to the given phase value or this |
| 614 |
> |
* barrier is terminated. It is an unenforced usage error for an |
| 615 |
> |
* unregistered party to invoke this method. |
| 616 |
|
* |
| 617 |
|
* @param phase an arrival phase number, or negative value if |
| 618 |
|
* terminated; this argument is normally the value returned by a |
| 636 |
|
|
| 637 |
|
/** |
| 638 |
|
* Awaits the phase of the barrier to advance from the given phase |
| 639 |
< |
* value or the given timeout to elapse, throwing |
| 640 |
< |
* {@code InterruptedException} if interrupted while waiting, or |
| 641 |
< |
* returning immediately if the current phase of the barrier is not |
| 642 |
< |
* equal to the given phase value or this barrier is terminated. |
| 639 |
> |
* value or the given timeout to elapse, throwing {@code |
| 640 |
> |
* InterruptedException} if interrupted while waiting, or |
| 641 |
> |
* returning immediately if the current phase of the barrier is |
| 642 |
> |
* not equal to the given phase value or this barrier is |
| 643 |
> |
* terminated. It is an unenforced usage error for an |
| 644 |
> |
* unregistered party to invoke this method. |
| 645 |
|
* |
| 646 |
|
* @param phase an arrival phase number, or negative value if |
| 647 |
|
* terminated; this argument is normally the value returned by a |
| 762 |
|
} |
| 763 |
|
|
| 764 |
|
/** |
| 765 |
< |
* Overridable method to perform an action upon phase advance, and |
| 766 |
< |
* to control termination. This method is invoked whenever the |
| 767 |
< |
* barrier is tripped (and thus all other waiting parties are |
| 768 |
< |
* dormant). If it returns {@code true}, then, rather than advance |
| 769 |
< |
* the phase number, this barrier will be set to a final |
| 770 |
< |
* termination state, and subsequent calls to {@link #isTerminated} |
| 771 |
< |
* will return true. |
| 765 |
> |
* Overridable method to perform an action upon impending phase |
| 766 |
> |
* advance, and to control termination. This method is invoked |
| 767 |
> |
* upon arrival of the party tripping the barrier (when all other |
| 768 |
> |
* waiting parties are dormant). If this method returns {@code |
| 769 |
> |
* true}, then, rather than advance the phase number, this barrier |
| 770 |
> |
* will be set to a final termination state, and subsequent calls |
| 771 |
> |
* to {@link #isTerminated} will return true. Any (unchecked) |
| 772 |
> |
* Exception or Error thrown by an invocation of this method is |
| 773 |
> |
* propagated to the party attempting to trip the barrier, in |
| 774 |
> |
* which case no advance occurs. |
| 775 |
> |
* |
| 776 |
> |
* <p>The arguments to this method provide the state of the phaser |
| 777 |
> |
* prevailing for the current transition. (When called from within |
| 778 |
> |
* an implementation of {@code onAdvance} the values returned by |
| 779 |
> |
* methods such as {@code getPhase} may or may not reliably |
| 780 |
> |
* indicate the state to which this transition applies.) |
| 781 |
|
* |
| 782 |
|
* <p>The default version returns {@code true} when the number of |
| 783 |
|
* registered parties is zero. Normally, overrides that arrange |
| 785 |
|
* property. |
| 786 |
|
* |
| 787 |
|
* <p>You may override this method to perform an action with side |
| 788 |
< |
* effects visible to participating tasks, but it is in general |
| 789 |
< |
* only sensible to do so in designs where all parties register |
| 790 |
< |
* before any arrive, and all {@link #awaitAdvance} at each phase. |
| 791 |
< |
* Otherwise, you cannot ensure lack of interference from other |
| 792 |
< |
* parties during the invocation of this method. |
| 788 |
> |
* effects visible to participating tasks, but doing so requires |
| 789 |
> |
* care: Method {@code onAdvance} may be invoked more than once |
| 790 |
> |
* per transition. Further, unless all parties register before |
| 791 |
> |
* any arrive, and all {@link #awaitAdvance} at each phase, then |
| 792 |
> |
* you cannot ensure lack of interference from other parties |
| 793 |
> |
* during the invocation of this method. |
| 794 |
|
* |
| 795 |
|
* @param phase the phase number on entering the barrier |
| 796 |
|
* @param registeredParties the current number of registered parties |
