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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.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 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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* <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 parallelismLevel 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><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 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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* final Phaser phaser = new Phaser(1); // "1" to register self |
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* // create and start threads |
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* for (final Runnable task : tasks) { |
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* phaser.register(); |
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* new Thread() { |
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* public void run() { |
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* phaser.arriveAndAwaitAdvance(); // await all creation |
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* task.run(); |
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* } |
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* }.start(); |
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* } |
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* |
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* // allow threads to start and deregister self |
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* phaser.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, final int iterations) { |
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* final Phaser phaser = new Phaser() { |
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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.register(); |
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* for (final Runnable task : tasks) { |
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* phaser.register(); |
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* new Thread() { |
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* public void run() { |
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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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* } |
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* }.start(); |
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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();}</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 (U.compareAndSwapLong(this, STATE, 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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U.compareAndSwapLong(this, STATE, 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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U.compareAndSwapLong(this, STATE, 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; |
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} |
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} |
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} |
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|
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/** |
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* Implementation of register, bulkRegister. |
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* |
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* @param registrations number to add to both parties and |
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* unarrived fields. Must be greater than zero. |
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*/ |
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private int doRegister(int registrations) { |
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// adjustment to state |
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long adjust = ((long)registrations << PARTIES_SHIFT) | registrations; |
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final Phaser parent = this.parent; |
395 |
int phase; |
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for (;;) { |
397 |
long s = (parent == null) ? state : reconcileState(); |
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int counts = (int)s; |
399 |
int parties = counts >>> PARTIES_SHIFT; |
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int unarrived = counts & UNARRIVED_MASK; |
401 |
if (registrations > MAX_PARTIES - parties) |
402 |
throw new IllegalStateException(badRegister(s)); |
403 |
phase = (int)(s >>> PHASE_SHIFT); |
404 |
if (phase < 0) |
405 |
break; |
406 |
if (counts != EMPTY) { // not 1st registration |
407 |
if (parent == null || reconcileState() == s) { |
408 |
if (unarrived == 0) // wait out advance |
409 |
root.internalAwaitAdvance(phase, null); |
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else if (U.compareAndSwapLong(this, STATE, s, s + adjust)) |
411 |
break; |
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} |
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} |
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else if (parent == null) { // 1st root registration |
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long next = ((long)phase << PHASE_SHIFT) | adjust; |
416 |
if (U.compareAndSwapLong(this, STATE, s, next)) |
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break; |
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} |
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else { |
420 |
synchronized (this) { // 1st sub registration |
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if (state == s) { // recheck under lock |
422 |
phase = parent.doRegister(1); |
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if (phase < 0) |
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break; |
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// finish registration whenever parent registration |
426 |
// succeeded, even when racing with termination, |
427 |
// since these are part of the same "transaction". |
428 |
while (!U.compareAndSwapLong |
429 |
(this, STATE, s, |
430 |
((long)phase << PHASE_SHIFT) | adjust)) { |
431 |
s = state; |
432 |
phase = (int)(root.state >>> PHASE_SHIFT); |
433 |
// assert (int)s == EMPTY; |
434 |
} |
435 |
break; |
436 |
} |
437 |
} |
438 |
} |
439 |
} |
440 |
return phase; |
441 |
} |
442 |
|
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/** |
444 |
* Resolves lagged phase propagation from root if necessary. |
445 |
* Reconciliation normally occurs when root has advanced but |
446 |
* subphasers have not yet done so, in which case they must finish |
447 |
* their own advance by setting unarrived to parties (or if |
448 |
* parties is zero, resetting to unregistered EMPTY state). |
449 |
* |
450 |
* @return reconciled state |
451 |
*/ |
452 |
private long reconcileState() { |
453 |
final Phaser root = this.root; |
454 |
long s = state; |
455 |
if (root != this) { |
456 |
int phase, p; |
457 |
// CAS to root phase with current parties, tripping unarrived |
458 |
while ((phase = (int)(root.state >>> PHASE_SHIFT)) != |
459 |
(int)(s >>> PHASE_SHIFT) && |
460 |
!U.compareAndSwapLong |
461 |
(this, STATE, s, |
462 |
s = (((long)phase << PHASE_SHIFT) | |
463 |
((phase < 0) ? (s & COUNTS_MASK) : |
464 |
(((p = (int)s >>> PARTIES_SHIFT) == 0) ? EMPTY : |
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((s & PARTIES_MASK) | p)))))) |
466 |
s = state; |
467 |
} |
468 |
return s; |
469 |
} |
470 |
|
471 |
/** |
472 |
* Creates a new phaser with no initially registered parties, no |
473 |
* parent, and initial phase number 0. Any thread using this |
474 |
* phaser will need to first register for it. |
475 |
*/ |
476 |
public Phaser() { |
477 |
this(null, 0); |
478 |
} |
479 |
|
480 |
/** |
481 |
* Creates a new phaser with the given number of registered |
482 |
* unarrived parties, no parent, and initial phase number 0. |
483 |
* |
484 |
* @param parties the number of parties required to advance to the |
485 |
* next phase |
486 |
* @throws IllegalArgumentException if parties less than zero |
487 |
* or greater than the maximum number of parties supported |
488 |
*/ |
489 |
public Phaser(int parties) { |
490 |
this(null, parties); |
491 |
} |
492 |
|
493 |
/** |
494 |
* Equivalent to {@link #Phaser(Phaser, int) Phaser(parent, 0)}. |
495 |
* |
496 |
* @param parent the parent phaser |
497 |
*/ |
498 |
public Phaser(Phaser parent) { |
499 |
this(parent, 0); |
500 |
} |
501 |
|
502 |
/** |
503 |
* Creates a new phaser with the given parent and number of |
504 |
* registered unarrived parties. When the given parent is non-null |
505 |
* and the given number of parties is greater than zero, this |
506 |
* child phaser is registered with its parent. |
507 |
* |
508 |
* @param parent the parent phaser |
509 |
* @param parties the number of parties required to advance to the |
510 |
* next phase |
511 |
* @throws IllegalArgumentException if parties less than zero |
512 |
* or greater than the maximum number of parties supported |
513 |
*/ |
514 |
public Phaser(Phaser parent, int parties) { |
515 |
if (parties >>> PARTIES_SHIFT != 0) |
516 |
throw new IllegalArgumentException("Illegal number of parties"); |
517 |
int phase = 0; |
518 |
this.parent = parent; |
519 |
if (parent != null) { |
520 |
final Phaser root = parent.root; |
521 |
this.root = root; |
522 |
this.evenQ = root.evenQ; |
523 |
this.oddQ = root.oddQ; |
524 |
if (parties != 0) |
525 |
phase = parent.doRegister(1); |
526 |
} |
527 |
else { |
528 |
this.root = this; |
529 |
this.evenQ = new AtomicReference<QNode>(); |
530 |
this.oddQ = new AtomicReference<QNode>(); |
531 |
} |
532 |
this.state = (parties == 0) ? (long)EMPTY : |
533 |
((long)phase << PHASE_SHIFT) | |
534 |
((long)parties << PARTIES_SHIFT) | |
535 |
((long)parties); |
536 |
} |
537 |
|
538 |
/** |
539 |
* Adds a new unarrived party to this phaser. If an ongoing |
540 |
* invocation of {@link #onAdvance} is in progress, this method |
541 |
* may await its completion before returning. If this phaser has |
542 |
* a parent, and this phaser previously had no registered parties, |
543 |
* this child phaser is also registered with its parent. If |
544 |
* this phaser is terminated, the attempt to register has |
545 |
* no effect, and a negative value is returned. |
546 |
* |
547 |
* @return the arrival phase number to which this registration |
548 |
* applied. If this value is negative, then this phaser has |
549 |
* terminated, in which case registration has no effect. |
550 |
* @throws IllegalStateException if attempting to register more |
551 |
* than the maximum supported number of parties |
552 |
*/ |
553 |
public int register() { |
554 |
return doRegister(1); |
555 |
} |
556 |
|
557 |
/** |
558 |
* Adds the given number of new unarrived parties to this phaser. |
559 |
* If an ongoing invocation of {@link #onAdvance} is in progress, |
560 |
* this method may await its completion before returning. If this |
561 |
* phaser has a parent, and the given number of parties is greater |
562 |
* than zero, and this phaser previously had no registered |
563 |
* parties, this child phaser is also registered with its parent. |
564 |
* If this phaser is terminated, the attempt to register has no |
565 |
* effect, and a negative value is returned. |
566 |
* |
567 |
* @param parties the number of additional parties required to |
568 |
* advance to the next phase |
569 |
* @return the arrival phase number to which this registration |
570 |
* applied. If this value is negative, then this phaser has |
571 |
* terminated, in which case registration has no effect. |
572 |
* @throws IllegalStateException if attempting to register more |
573 |
* than the maximum supported number of parties |
574 |
* @throws IllegalArgumentException if {@code parties < 0} |
575 |
*/ |
576 |
public int bulkRegister(int parties) { |
577 |
if (parties < 0) |
578 |
throw new IllegalArgumentException(); |
579 |
if (parties == 0) |
580 |
return getPhase(); |
581 |
return doRegister(parties); |
582 |
} |
583 |
|
584 |
/** |
585 |
* Arrives at this phaser, without waiting for others to arrive. |
586 |
* |
587 |
* <p>It is a usage error for an unregistered party to invoke this |
588 |
* method. However, this error may result in an {@code |
589 |
* IllegalStateException} only upon some subsequent operation on |
590 |
* this phaser, if ever. |
591 |
* |
592 |
* @return the arrival phase number, or a negative value if terminated |
593 |
* @throws IllegalStateException if not terminated and the number |
594 |
* of unarrived parties would become negative |
595 |
*/ |
596 |
public int arrive() { |
597 |
return doArrive(ONE_ARRIVAL); |
598 |
} |
599 |
|
600 |
/** |
601 |
* Arrives at this phaser and deregisters from it without waiting |
602 |
* for others to arrive. Deregistration reduces the number of |
603 |
* parties required to advance in future phases. If this phaser |
604 |
* has a parent, and deregistration causes this phaser to have |
605 |
* zero parties, this phaser is also deregistered from its parent. |
606 |
* |
607 |
* <p>It is a usage error for an unregistered party to invoke this |
608 |
* method. However, this error may result in an {@code |
609 |
* IllegalStateException} only upon some subsequent operation on |
610 |
* this phaser, if ever. |
611 |
* |
612 |
* @return the arrival phase number, or a negative value if terminated |
613 |
* @throws IllegalStateException if not terminated and the number |
614 |
* of registered or unarrived parties would become negative |
615 |
*/ |
616 |
public int arriveAndDeregister() { |
617 |
return doArrive(ONE_DEREGISTER); |
618 |
} |
619 |
|
620 |
/** |
621 |
* Arrives at this phaser and awaits others. Equivalent in effect |
622 |
* to {@code awaitAdvance(arrive())}. If you need to await with |
623 |
* interruption or timeout, you can arrange this with an analogous |
624 |
* construction using one of the other forms of the {@code |
625 |
* awaitAdvance} method. If instead you need to deregister upon |
626 |
* arrival, use {@code awaitAdvance(arriveAndDeregister())}. |
627 |
* |
628 |
* <p>It is a usage error for an unregistered party to invoke this |
629 |
* method. However, this error may result in an {@code |
630 |
* IllegalStateException} only upon some subsequent operation on |
631 |
* this phaser, if ever. |
632 |
* |
633 |
* @return the arrival phase number, or the (negative) |
634 |
* {@linkplain #getPhase() current phase} if terminated |
635 |
* @throws IllegalStateException if not terminated and the number |
636 |
* of unarrived parties would become negative |
637 |
*/ |
638 |
public int arriveAndAwaitAdvance() { |
639 |
// Specialization of doArrive+awaitAdvance eliminating some reads/paths |
640 |
final Phaser root = this.root; |
641 |
for (;;) { |
642 |
long s = (root == this) ? state : reconcileState(); |
643 |
int phase = (int)(s >>> PHASE_SHIFT); |
644 |
if (phase < 0) |
645 |
return phase; |
646 |
int counts = (int)s; |
647 |
int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK); |
648 |
if (unarrived <= 0) |
649 |
throw new IllegalStateException(badArrive(s)); |
650 |
if (U.compareAndSwapLong(this, STATE, s, s -= ONE_ARRIVAL)) { |
651 |
if (unarrived > 1) |
652 |
return root.internalAwaitAdvance(phase, null); |
653 |
if (root != this) |
654 |
return parent.arriveAndAwaitAdvance(); |
655 |
long n = s & PARTIES_MASK; // base of next state |
656 |
int nextUnarrived = (int)n >>> PARTIES_SHIFT; |
657 |
if (onAdvance(phase, nextUnarrived)) |
658 |
n |= TERMINATION_BIT; |
659 |
else if (nextUnarrived == 0) |
660 |
n |= EMPTY; |
661 |
else |
662 |
n |= nextUnarrived; |
663 |
int nextPhase = (phase + 1) & MAX_PHASE; |
664 |
n |= (long)nextPhase << PHASE_SHIFT; |
665 |
if (!U.compareAndSwapLong(this, STATE, s, n)) |
666 |
return (int)(state >>> PHASE_SHIFT); // terminated |
667 |
releaseWaiters(phase); |
668 |
return nextPhase; |
669 |
} |
670 |
} |
671 |
} |
672 |
|
673 |
/** |
674 |
* Awaits the phase of this phaser to advance from the given phase |
675 |
* value, returning immediately if the current phase is not equal |
676 |
* to the given phase value or this phaser is terminated. |
677 |
* |
678 |
* @param phase an arrival phase number, or negative value if |
679 |
* terminated; this argument is normally the value returned by a |
680 |
* previous call to {@code arrive} or {@code arriveAndDeregister}. |
681 |
* @return the next arrival phase number, or the argument if it is |
682 |
* negative, or the (negative) {@linkplain #getPhase() current phase} |
683 |
* if terminated |
684 |
*/ |
685 |
public int awaitAdvance(int phase) { |
686 |
final Phaser root = this.root; |
687 |
long s = (root == this) ? state : reconcileState(); |
688 |
int p = (int)(s >>> PHASE_SHIFT); |
689 |
if (phase < 0) |
690 |
return phase; |
691 |
if (p == phase) |
692 |
return root.internalAwaitAdvance(phase, null); |
693 |
return p; |
694 |
} |
695 |
|
696 |
/** |
697 |
* Awaits the phase of this phaser to advance from the given phase |
698 |
* value, throwing {@code InterruptedException} if interrupted |
699 |
* while waiting, or returning immediately if the current phase is |
700 |
* not equal to the given phase value or this phaser is |
701 |
* terminated. |
702 |
* |
703 |
* @param phase an arrival phase number, or negative value if |
704 |
* terminated; this argument is normally the value returned by a |
705 |
* previous call to {@code arrive} or {@code arriveAndDeregister}. |
706 |
* @return the next arrival phase number, or the argument if it is |
707 |
* negative, or the (negative) {@linkplain #getPhase() current phase} |
708 |
* if terminated |
709 |
* @throws InterruptedException if thread interrupted while waiting |
710 |
*/ |
711 |
public int awaitAdvanceInterruptibly(int phase) |
712 |
throws InterruptedException { |
713 |
final Phaser root = this.root; |
714 |
long s = (root == this) ? state : reconcileState(); |
715 |
int p = (int)(s >>> PHASE_SHIFT); |
716 |
if (phase < 0) |
717 |
return phase; |
718 |
if (p == phase) { |
719 |
QNode node = new QNode(this, phase, true, false, 0L); |
720 |
p = root.internalAwaitAdvance(phase, node); |
721 |
if (node.wasInterrupted) |
722 |
throw new InterruptedException(); |
723 |
} |
724 |
return p; |
725 |
} |
726 |
|
727 |
/** |
728 |
* Awaits the phase of this phaser to advance from the given phase |
729 |
* value or the given timeout to elapse, throwing {@code |
730 |
* InterruptedException} if interrupted while waiting, or |
731 |
* returning immediately if the current phase is not equal to the |
732 |
* given phase value or this phaser is terminated. |
733 |
* |
734 |
* @param phase an arrival phase number, or negative value if |
735 |
* terminated; this argument is normally the value returned by a |
736 |
* previous call to {@code arrive} or {@code arriveAndDeregister}. |
737 |
* @param timeout how long to wait before giving up, in units of |
738 |
* {@code unit} |
739 |
* @param unit a {@code TimeUnit} determining how to interpret the |
740 |
* {@code timeout} parameter |
741 |
* @return the next arrival phase number, or the argument if it is |
742 |
* negative, or the (negative) {@linkplain #getPhase() current phase} |
743 |
* if terminated |
744 |
* @throws InterruptedException if thread interrupted while waiting |
745 |
* @throws TimeoutException if timed out while waiting |
746 |
*/ |
747 |
public int awaitAdvanceInterruptibly(int phase, |
748 |
long timeout, TimeUnit unit) |
749 |
throws InterruptedException, TimeoutException { |
750 |
long nanos = unit.toNanos(timeout); |
751 |
final Phaser root = this.root; |
752 |
long s = (root == this) ? state : reconcileState(); |
753 |
int p = (int)(s >>> PHASE_SHIFT); |
754 |
if (phase < 0) |
755 |
return phase; |
756 |
if (p == phase) { |
757 |
QNode node = new QNode(this, phase, true, true, nanos); |
758 |
p = root.internalAwaitAdvance(phase, node); |
759 |
if (node.wasInterrupted) |
760 |
throw new InterruptedException(); |
761 |
else if (p == phase) |
762 |
throw new TimeoutException(); |
763 |
} |
764 |
return p; |
765 |
} |
766 |
|
767 |
/** |
768 |
* Forces this phaser to enter termination state. Counts of |
769 |
* registered parties are unaffected. If this phaser is a member |
770 |
* of a tiered set of phasers, then all of the phasers in the set |
771 |
* are terminated. If this phaser is already terminated, this |
772 |
* method has no effect. This method may be useful for |
773 |
* coordinating recovery after one or more tasks encounter |
774 |
* unexpected exceptions. |
775 |
*/ |
776 |
public void forceTermination() { |
777 |
// Only need to change root state |
778 |
final Phaser root = this.root; |
779 |
long s; |
780 |
while ((s = root.state) >= 0) { |
781 |
if (U.compareAndSwapLong(root, STATE, s, s | TERMINATION_BIT)) { |
782 |
// signal all threads |
783 |
releaseWaiters(0); // Waiters on evenQ |
784 |
releaseWaiters(1); // Waiters on oddQ |
785 |
return; |
786 |
} |
787 |
} |
788 |
} |
789 |
|
790 |
/** |
791 |
* Returns the current phase number. The maximum phase number is |
792 |
* {@code Integer.MAX_VALUE}, after which it restarts at |
793 |
* zero. Upon termination, the phase number is negative, |
794 |
* in which case the prevailing phase prior to termination |
795 |
* may be obtained via {@code getPhase() + Integer.MIN_VALUE}. |
796 |
* |
797 |
* @return the phase number, or a negative value if terminated |
798 |
*/ |
799 |
public final int getPhase() { |
800 |
return (int)(root.state >>> PHASE_SHIFT); |
801 |
} |
802 |
|
803 |
/** |
804 |
* Returns the number of parties registered at this phaser. |
805 |
* |
806 |
* @return the number of parties |
807 |
*/ |
808 |
public int getRegisteredParties() { |
809 |
return partiesOf(state); |
810 |
} |
811 |
|
812 |
/** |
813 |
* Returns the number of registered parties that have arrived at |
814 |
* the current phase of this phaser. If this phaser has terminated, |
815 |
* the returned value is meaningless and arbitrary. |
816 |
* |
817 |
* @return the number of arrived parties |
818 |
*/ |
819 |
public int getArrivedParties() { |
820 |
return arrivedOf(reconcileState()); |
821 |
} |
822 |
|
823 |
/** |
824 |
* Returns the number of registered parties that have not yet |
825 |
* arrived at the current phase of this phaser. If this phaser has |
826 |
* terminated, the returned value is meaningless and arbitrary. |
827 |
* |
828 |
* @return the number of unarrived parties |
829 |
*/ |
830 |
public int getUnarrivedParties() { |
831 |
return unarrivedOf(reconcileState()); |
832 |
} |
833 |
|
834 |
/** |
835 |
* Returns the parent of this phaser, or {@code null} if none. |
836 |
* |
837 |
* @return the parent of this phaser, or {@code null} if none |
838 |
*/ |
839 |
public Phaser getParent() { |
840 |
return parent; |
841 |
} |
842 |
|
843 |
/** |
844 |
* Returns the root ancestor of this phaser, which is the same as |
845 |
* this phaser if it has no parent. |
846 |
* |
847 |
* @return the root ancestor of this phaser |
848 |
*/ |
849 |
public Phaser getRoot() { |
850 |
return root; |
851 |
} |
852 |
|
853 |
/** |
854 |
* Returns {@code true} if this phaser has been terminated. |
855 |
* |
856 |
* @return {@code true} if this phaser has been terminated |
857 |
*/ |
858 |
public boolean isTerminated() { |
859 |
return root.state < 0L; |
860 |
} |
861 |
|
862 |
/** |
863 |
* Overridable method to perform an action upon impending phase |
864 |
* advance, and to control termination. This method is invoked |
865 |
* upon arrival of the party advancing this phaser (when all other |
866 |
* waiting parties are dormant). If this method returns {@code |
867 |
* true}, this phaser will be set to a final termination state |
868 |
* upon advance, and subsequent calls to {@link #isTerminated} |
869 |
* will return true. Any (unchecked) Exception or Error thrown by |
870 |
* an invocation of this method is propagated to the party |
871 |
* attempting to advance this phaser, in which case no advance |
872 |
* occurs. |
873 |
* |
874 |
* <p>The arguments to this method provide the state of the phaser |
875 |
* prevailing for the current transition. The effects of invoking |
876 |
* arrival, registration, and waiting methods on this phaser from |
877 |
* within {@code onAdvance} are unspecified and should not be |
878 |
* relied on. |
879 |
* |
880 |
* <p>If this phaser is a member of a tiered set of phasers, then |
881 |
* {@code onAdvance} is invoked only for its root phaser on each |
882 |
* advance. |
883 |
* |
884 |
* <p>To support the most common use cases, the default |
885 |
* implementation of this method returns {@code true} when the |
886 |
* number of registered parties has become zero as the result of a |
887 |
* party invoking {@code arriveAndDeregister}. You can disable |
888 |
* this behavior, thus enabling continuation upon future |
889 |
* registrations, by overriding this method to always return |
890 |
* {@code false}: |
891 |
* |
892 |
* <pre> {@code |
893 |
* Phaser phaser = new Phaser() { |
894 |
* protected boolean onAdvance(int phase, int parties) { return false; } |
895 |
* }}</pre> |
896 |
* |
897 |
* @param phase the current phase number on entry to this method, |
898 |
* before this phaser is advanced |
899 |
* @param registeredParties the current number of registered parties |
900 |
* @return {@code true} if this phaser should terminate |
901 |
*/ |
902 |
protected boolean onAdvance(int phase, int registeredParties) { |
903 |
return registeredParties == 0; |
904 |
} |
905 |
|
906 |
/** |
907 |
* Returns a string identifying this phaser, as well as its |
908 |
* state. The state, in brackets, includes the String {@code |
909 |
* "phase = "} followed by the phase number, {@code "parties = "} |
910 |
* followed by the number of registered parties, and {@code |
911 |
* "arrived = "} followed by the number of arrived parties. |
912 |
* |
913 |
* @return a string identifying this phaser, as well as its state |
914 |
*/ |
915 |
public String toString() { |
916 |
return stateToString(reconcileState()); |
917 |
} |
918 |
|
919 |
/** |
920 |
* Implementation of toString and string-based error messages. |
921 |
*/ |
922 |
private String stateToString(long s) { |
923 |
return super.toString() + |
924 |
"[phase = " + phaseOf(s) + |
925 |
" parties = " + partiesOf(s) + |
926 |
" arrived = " + arrivedOf(s) + "]"; |
927 |
} |
928 |
|
929 |
// Waiting mechanics |
930 |
|
931 |
/** |
932 |
* Removes and signals threads from queue for phase. |
933 |
*/ |
934 |
private void releaseWaiters(int phase) { |
935 |
QNode q; // first element of queue |
936 |
Thread t; // its thread |
937 |
AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; |
938 |
while ((q = head.get()) != null && |
939 |
q.phase != (int)(root.state >>> PHASE_SHIFT)) { |
940 |
if (head.compareAndSet(q, q.next) && |
941 |
(t = q.thread) != null) { |
942 |
q.thread = null; |
943 |
LockSupport.unpark(t); |
944 |
} |
945 |
} |
946 |
} |
947 |
|
948 |
/** |
949 |
* Variant of releaseWaiters that additionally tries to remove any |
950 |
* nodes no longer waiting for advance due to timeout or |
951 |
* interrupt. Currently, nodes are removed only if they are at |
952 |
* head of queue, which suffices to reduce memory footprint in |
953 |
* most usages. |
954 |
* |
955 |
* @return current phase on exit |
956 |
*/ |
957 |
private int abortWait(int phase) { |
958 |
AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; |
959 |
for (;;) { |
960 |
Thread t; |
961 |
QNode q = head.get(); |
962 |
int p = (int)(root.state >>> PHASE_SHIFT); |
963 |
if (q == null || ((t = q.thread) != null && q.phase == p)) |
964 |
return p; |
965 |
if (head.compareAndSet(q, q.next) && t != null) { |
966 |
q.thread = null; |
967 |
LockSupport.unpark(t); |
968 |
} |
969 |
} |
970 |
} |
971 |
|
972 |
/** The number of CPUs, for spin control */ |
973 |
private static final int NCPU = Runtime.getRuntime().availableProcessors(); |
974 |
|
975 |
/** |
976 |
* The number of times to spin before blocking while waiting for |
977 |
* advance, per arrival while waiting. On multiprocessors, fully |
978 |
* blocking and waking up a large number of threads all at once is |
979 |
* usually a very slow process, so we use rechargeable spins to |
980 |
* avoid it when threads regularly arrive: When a thread in |
981 |
* internalAwaitAdvance notices another arrival before blocking, |
982 |
* and there appear to be enough CPUs available, it spins |
983 |
* SPINS_PER_ARRIVAL more times before blocking. The value trades |
984 |
* off good-citizenship vs big unnecessary slowdowns. |
985 |
*/ |
986 |
static final int SPINS_PER_ARRIVAL = (NCPU < 2) ? 1 : 1 << 8; |
987 |
|
988 |
/** |
989 |
* Possibly blocks and waits for phase to advance unless aborted. |
990 |
* Call only on root phaser. |
991 |
* |
992 |
* @param phase current phase |
993 |
* @param node if non-null, the wait node to track interrupt and timeout; |
994 |
* if null, denotes noninterruptible wait |
995 |
* @return current phase |
996 |
*/ |
997 |
private int internalAwaitAdvance(int phase, QNode node) { |
998 |
// assert root == this; |
999 |
releaseWaiters(phase-1); // ensure old queue clean |
1000 |
boolean queued = false; // true when node is enqueued |
1001 |
int lastUnarrived = 0; // to increase spins upon change |
1002 |
int spins = SPINS_PER_ARRIVAL; |
1003 |
long s; |
1004 |
int p; |
1005 |
while ((p = (int)((s = state) >>> PHASE_SHIFT)) == phase) { |
1006 |
if (node == null) { // spinning in noninterruptible mode |
1007 |
int unarrived = (int)s & UNARRIVED_MASK; |
1008 |
if (unarrived != lastUnarrived && |
1009 |
(lastUnarrived = unarrived) < NCPU) |
1010 |
spins += SPINS_PER_ARRIVAL; |
1011 |
boolean interrupted = Thread.interrupted(); |
1012 |
if (interrupted || --spins < 0) { // need node to record intr |
1013 |
node = new QNode(this, phase, false, false, 0L); |
1014 |
node.wasInterrupted = interrupted; |
1015 |
} |
1016 |
} |
1017 |
else if (node.isReleasable()) // done or aborted |
1018 |
break; |
1019 |
else if (!queued) { // push onto queue |
1020 |
AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; |
1021 |
QNode q = node.next = head.get(); |
1022 |
if ((q == null || q.phase == phase) && |
1023 |
(int)(state >>> PHASE_SHIFT) == phase) // avoid stale enq |
1024 |
queued = head.compareAndSet(q, node); |
1025 |
} |
1026 |
else { |
1027 |
try { |
1028 |
ForkJoinPool.managedBlock(node); |
1029 |
} catch (InterruptedException cantHappen) { |
1030 |
node.wasInterrupted = true; |
1031 |
} |
1032 |
} |
1033 |
} |
1034 |
|
1035 |
if (node != null) { |
1036 |
if (node.thread != null) |
1037 |
node.thread = null; // avoid need for unpark() |
1038 |
if (node.wasInterrupted && !node.interruptible) |
1039 |
Thread.currentThread().interrupt(); |
1040 |
if (p == phase && (p = (int)(state >>> PHASE_SHIFT)) == phase) |
1041 |
return abortWait(phase); // possibly clean up on abort |
1042 |
} |
1043 |
releaseWaiters(phase); |
1044 |
return p; |
1045 |
} |
1046 |
|
1047 |
/** |
1048 |
* Wait nodes for Treiber stack representing wait queue. |
1049 |
*/ |
1050 |
static final class QNode implements ForkJoinPool.ManagedBlocker { |
1051 |
final Phaser phaser; |
1052 |
final int phase; |
1053 |
final boolean interruptible; |
1054 |
final boolean timed; |
1055 |
boolean wasInterrupted; |
1056 |
long nanos; |
1057 |
final long deadline; |
1058 |
volatile Thread thread; // nulled to cancel wait |
1059 |
QNode next; |
1060 |
|
1061 |
QNode(Phaser phaser, int phase, boolean interruptible, |
1062 |
boolean timed, long nanos) { |
1063 |
this.phaser = phaser; |
1064 |
this.phase = phase; |
1065 |
this.interruptible = interruptible; |
1066 |
this.nanos = nanos; |
1067 |
this.timed = timed; |
1068 |
this.deadline = timed ? System.nanoTime() + nanos : 0L; |
1069 |
thread = Thread.currentThread(); |
1070 |
} |
1071 |
|
1072 |
public boolean isReleasable() { |
1073 |
if (thread == null) |
1074 |
return true; |
1075 |
if (phaser.getPhase() != phase) { |
1076 |
thread = null; |
1077 |
return true; |
1078 |
} |
1079 |
if (Thread.interrupted()) |
1080 |
wasInterrupted = true; |
1081 |
if (wasInterrupted && interruptible) { |
1082 |
thread = null; |
1083 |
return true; |
1084 |
} |
1085 |
if (timed && |
1086 |
(nanos <= 0L || (nanos = deadline - System.nanoTime()) <= 0L)) { |
1087 |
thread = null; |
1088 |
return true; |
1089 |
} |
1090 |
return false; |
1091 |
} |
1092 |
|
1093 |
public boolean block() { |
1094 |
while (!isReleasable()) { |
1095 |
if (timed) |
1096 |
LockSupport.parkNanos(this, nanos); |
1097 |
else |
1098 |
LockSupport.park(this); |
1099 |
} |
1100 |
return true; |
1101 |
} |
1102 |
} |
1103 |
|
1104 |
// Unsafe mechanics |
1105 |
|
1106 |
private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); |
1107 |
private static final long STATE; |
1108 |
static { |
1109 |
try { |
1110 |
STATE = U.objectFieldOffset |
1111 |
(Phaser.class.getDeclaredField("state")); |
1112 |
} catch (ReflectiveOperationException e) { |
1113 |
throw new Error(e); |
1114 |
} |
1115 |
|
1116 |
// Reduce the risk of rare disastrous classloading in first call to |
1117 |
// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773 |
1118 |
Class<?> ensureLoaded = LockSupport.class; |
1119 |
} |
1120 |
} |