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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/licenses/publicdomain |
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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 a |
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* {@link java.util.concurrent.CyclicBarrier CyclicBarrier} and |
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* {@link java.util.concurrent.CountDownLatch CountDownLatch} |
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* but supporting more flexible usage. |
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* |
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* <ul> |
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* |
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* <li> The number of parties synchronizing on a phaser may vary over |
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* time. A task may register to be a party at any time, and may |
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* deregister upon arriving at the barrier. As is the case with most |
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* basic synchronization constructs, registration and deregistration |
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* affect only internal counts; they do not establish any further |
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* internal bookkeeping, so tasks cannot query whether they are |
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* registered. (However, you can introduce such bookkeeping by |
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* subclassing this class.) |
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* |
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* <li> Each generation has an associated phase value, starting at |
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* zero, and advancing when all parties reach the barrier (wrapping |
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* around to zero after reaching {@code Integer.MAX_VALUE}). |
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* |
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* <li> Like a {@code CyclicBarrier}, a phaser may be repeatedly |
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* awaited. Method {@link #arriveAndAwaitAdvance} has effect |
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* analogous to {@link java.util.concurrent.CyclicBarrier#await |
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* CyclicBarrier.await}. However, phasers separate two aspects of |
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* coordination, which may also be invoked independently: |
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* |
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* <ul> |
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* |
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* <li> Arriving at a barrier. Methods {@link #arrive} and |
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* {@link #arriveAndDeregister} do not block, but return |
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* the phase value current upon entry to the method. |
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* |
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* <li> Awaiting others. Method {@link #awaitAdvance} requires an |
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* argument indicating the entry phase, and returns when the |
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* barrier advances to a new phase. |
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* </ul> |
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* |
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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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* 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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* numbers of parties that would otherwise experience heavy |
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* synchronization contention costs may instead be arranged in trees. |
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* This will typically greatly increase throughput even though it |
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* incurs somewhat greater per-operation overhead. |
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* |
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* <li> By default, {@code awaitAdvance} continues to wait even if |
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* the waiting thread is interrupted. And unlike the case in |
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* {@code CyclicBarrier}, exceptions encountered while tasks wait |
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* interruptibly or with timeout do not change the state of the |
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* barrier. If necessary, you can perform any associated recovery |
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* within handlers of those exceptions, often after invoking |
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* {@code forceTermination}. |
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* |
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* <li>Phasers may be used to coordinate tasks executing in a {@link |
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* ForkJoinPool}, which will ensure sufficient parallelism to execute |
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* tasks when others are blocked waiting for a phase to advance. |
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* |
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* </ul> |
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* |
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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 |
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* parties. The typical idiom is for the method setting this up to |
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* first 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 (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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* public 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 (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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* <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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* it registers for upon construction: |
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* <pre> {@code |
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* void build(Task[] actions, int lo, int hi, Phaser b) { |
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* int step = (hi - lo) / TASKS_PER_PHASER; |
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* if (step > 1) { |
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* int i = lo; |
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* while (i < hi) { |
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* int r = Math.min(i + step, hi); |
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* build(actions, i, r, new Phaser(b)); |
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* i = r; |
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* } |
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* } else { |
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* for (int i = lo; i < hi; ++i) |
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* actions[i] = new Task(b); |
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* // assumes new Task(b) performs b.register() |
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* } |
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* } |
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* // .. initially called, for n tasks via |
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* build(new Task[n], 0, n, new Phaser());}</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 barrier synchronization rates. A value as low as four may |
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* be appropriate for extremely small per-barrier 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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* </pre> |
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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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* Barrier state representation. Conceptually, a barrier contains |
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* four values: |
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* |
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* * parties -- the number of parties to wait (16 bits) |
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* * unarrived -- the number of parties yet to hit barrier (16 bits) |
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* * phase -- the generation of the barrier (31 bits) |
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* * terminated -- set if barrier is terminated (1 bit) |
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* |
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* However, to efficiently maintain atomicity, these values are |
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* packed into a single (atomic) long. Termination uses the sign |
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* bit of 32 bit representation of phase, so phase is set to -1 on |
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* termination. Good performance relies on keeping state decoding |
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* and encoding simple, and keeping race windows short. |
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* |
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* Note: there are some cheats in arrive() that rely on unarrived |
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* count being lowest 16 bits. |
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*/ |
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private volatile long state; |
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|
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private static final int ushortBits = 16; |
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private static final int ushortMask = 0xffff; |
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private static final int phaseMask = 0x7fffffff; |
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|
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private static int unarrivedOf(long s) { |
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return (int) (s & ushortMask); |
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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) >>> 16; |
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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 >>> 32); |
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} |
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|
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private static int arrivedOf(long s) { |
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return partiesOf(s) - unarrivedOf(s); |
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} |
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|
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private static long stateFor(int phase, int parties, int unarrived) { |
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return ((((long) phase) << 32) | (((long) parties) << 16) | |
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(long) unarrived); |
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} |
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|
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private static long trippedStateFor(int phase, int parties) { |
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long lp = (long) parties; |
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return (((long) phase) << 32) | (lp << 16) | lp; |
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} |
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|
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/** |
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* Returns message string for bad bounds exceptions. |
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*/ |
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private static String badBounds(int parties, int unarrived) { |
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return ("Attempt to set " + unarrived + |
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" unarrived of " + parties + " parties"); |
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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. Used to |
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* support faster state push-down. |
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*/ |
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private final Phaser root; |
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|
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// Wait queues |
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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 while 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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*/ |
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private final AtomicReference<QNode> evenQ = new AtomicReference<QNode>(); |
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private final AtomicReference<QNode> oddQ = new AtomicReference<QNode>(); |
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|
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private AtomicReference<QNode> queueFor(int phase) { |
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return ((phase & 1) == 0) ? evenQ : oddQ; |
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} |
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|
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/** |
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* Returns current state, first resolving lagged propagation from |
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* root if necessary. |
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*/ |
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private long getReconciledState() { |
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return (parent == null) ? state : reconcileState(); |
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} |
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|
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/** |
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* Recursively resolves state. |
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*/ |
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private long reconcileState() { |
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Phaser p = parent; |
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long s = state; |
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if (p != null) { |
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while (unarrivedOf(s) == 0 && phaseOf(s) != phaseOf(root.state)) { |
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long parentState = p.getReconciledState(); |
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int parentPhase = phaseOf(parentState); |
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int phase = phaseOf(s = state); |
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if (phase != parentPhase) { |
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long next = trippedStateFor(parentPhase, partiesOf(s)); |
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if (casState(s, next)) { |
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releaseWaiters(phase); |
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s = next; |
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} |
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} |
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} |
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} |
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return s; |
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} |
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|
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/** |
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* Creates a new phaser without any initially registered parties, |
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* initial phase number 0, and no parent. Any thread using this |
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* phaser will need to first register for it. |
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*/ |
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public Phaser() { |
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this(null); |
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} |
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|
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/** |
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* Creates a new phaser with the given numbers of registered |
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* unarrived parties, initial phase number 0, and no parent. |
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* |
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* @param parties the number of parties required to trip barrier |
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* @throws IllegalArgumentException if parties less than zero |
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* or greater than the maximum number of parties supported |
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*/ |
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public Phaser(int parties) { |
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this(null, parties); |
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} |
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|
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/** |
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* Creates a new phaser with the given parent, without any |
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* initially registered parties. If parent is non-null this phaser |
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* is registered with the parent and its initial phase number is |
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* the same as that of parent phaser. |
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* |
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* @param parent the parent phaser |
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*/ |
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public Phaser(Phaser parent) { |
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int phase = 0; |
329 |
this.parent = parent; |
330 |
if (parent != null) { |
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this.root = parent.root; |
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phase = parent.register(); |
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} |
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else |
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this.root = this; |
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this.state = trippedStateFor(phase, 0); |
337 |
} |
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|
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/** |
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* Creates a new phaser with the given parent and numbers of |
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* registered unarrived parties. If parent is non-null, this phaser |
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* is registered with the parent and its initial phase number is |
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* the same as that of parent phaser. |
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* |
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* @param parent the parent phaser |
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* @param parties the number of parties required to trip barrier |
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* @throws IllegalArgumentException if parties less than zero |
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* or greater than the maximum number of parties supported |
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*/ |
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public Phaser(Phaser parent, int parties) { |
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if (parties < 0 || parties > ushortMask) |
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throw new IllegalArgumentException("Illegal number of parties"); |
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int phase = 0; |
354 |
this.parent = parent; |
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if (parent != null) { |
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this.root = parent.root; |
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phase = parent.register(); |
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} |
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else |
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this.root = this; |
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this.state = trippedStateFor(phase, parties); |
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} |
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|
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/** |
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* Adds a new unarrived party to this phaser. |
366 |
* |
367 |
* @return the current barrier phase number upon registration |
368 |
* @throws IllegalStateException if attempting to register more |
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* than the maximum supported number of parties |
370 |
*/ |
371 |
public int register() { |
372 |
return doRegister(1); |
373 |
} |
374 |
|
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/** |
376 |
* Adds the given number of new unarrived parties to this phaser. |
377 |
* |
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* @param parties the number of parties required to trip barrier |
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* @return the current barrier phase number upon registration |
380 |
* @throws IllegalStateException if attempting to register more |
381 |
* than the maximum supported number of parties |
382 |
*/ |
383 |
public int bulkRegister(int parties) { |
384 |
if (parties < 0) |
385 |
throw new IllegalArgumentException(); |
386 |
if (parties == 0) |
387 |
return getPhase(); |
388 |
return doRegister(parties); |
389 |
} |
390 |
|
391 |
/** |
392 |
* Shared code for register, bulkRegister |
393 |
*/ |
394 |
private int doRegister(int registrations) { |
395 |
int phase; |
396 |
for (;;) { |
397 |
long s = getReconciledState(); |
398 |
phase = phaseOf(s); |
399 |
int unarrived = unarrivedOf(s) + registrations; |
400 |
int parties = partiesOf(s) + registrations; |
401 |
if (phase < 0) |
402 |
break; |
403 |
if (parties > ushortMask || unarrived > ushortMask) |
404 |
throw new IllegalStateException(badBounds(parties, unarrived)); |
405 |
if (phase == phaseOf(root.state) && |
406 |
casState(s, stateFor(phase, parties, unarrived))) |
407 |
break; |
408 |
} |
409 |
return phase; |
410 |
} |
411 |
|
412 |
/** |
413 |
* Arrives at the barrier, but does not wait for others. (You can |
414 |
* in turn wait for others via {@link #awaitAdvance}). |
415 |
* |
416 |
* @return the barrier phase number upon entry to this method, or a |
417 |
* negative value if terminated |
418 |
* @throws IllegalStateException if not terminated and the number |
419 |
* of unarrived parties would become negative |
420 |
*/ |
421 |
public int arrive() { |
422 |
int phase; |
423 |
for (;;) { |
424 |
long s = state; |
425 |
phase = phaseOf(s); |
426 |
if (phase < 0) |
427 |
break; |
428 |
int parties = partiesOf(s); |
429 |
int unarrived = unarrivedOf(s) - 1; |
430 |
if (unarrived > 0) { // Not the last arrival |
431 |
if (casState(s, s - 1)) // s-1 adds one arrival |
432 |
break; |
433 |
} |
434 |
else if (unarrived == 0) { // the last arrival |
435 |
Phaser par = parent; |
436 |
if (par == null) { // directly trip |
437 |
if (casState |
438 |
(s, |
439 |
trippedStateFor(onAdvance(phase, parties) ? -1 : |
440 |
((phase + 1) & phaseMask), parties))) { |
441 |
releaseWaiters(phase); |
442 |
break; |
443 |
} |
444 |
} |
445 |
else { // cascade to parent |
446 |
if (casState(s, s - 1)) { // zeroes unarrived |
447 |
par.arrive(); |
448 |
reconcileState(); |
449 |
break; |
450 |
} |
451 |
} |
452 |
} |
453 |
else if (phase != phaseOf(root.state)) // or if unreconciled |
454 |
reconcileState(); |
455 |
else |
456 |
throw new IllegalStateException(badBounds(parties, unarrived)); |
457 |
} |
458 |
return phase; |
459 |
} |
460 |
|
461 |
/** |
462 |
* Arrives at the barrier and deregisters from it without waiting |
463 |
* for others. Deregistration reduces the number of parties |
464 |
* required to trip the barrier in future phases. If this phaser |
465 |
* has a parent, and deregistration causes this phaser to have |
466 |
* zero parties, this phaser also arrives at and is deregistered |
467 |
* from its parent. |
468 |
* |
469 |
* @return the current barrier phase number upon entry to |
470 |
* this method, or a negative value if terminated |
471 |
* @throws IllegalStateException if not terminated and the number |
472 |
* of registered or unarrived parties would become negative |
473 |
*/ |
474 |
public int arriveAndDeregister() { |
475 |
// similar code to arrive, but too different to merge |
476 |
Phaser par = parent; |
477 |
int phase; |
478 |
for (;;) { |
479 |
long s = state; |
480 |
phase = phaseOf(s); |
481 |
if (phase < 0) |
482 |
break; |
483 |
int parties = partiesOf(s) - 1; |
484 |
int unarrived = unarrivedOf(s) - 1; |
485 |
if (parties >= 0) { |
486 |
if (unarrived > 0 || (unarrived == 0 && par != null)) { |
487 |
if (casState |
488 |
(s, |
489 |
stateFor(phase, parties, unarrived))) { |
490 |
if (unarrived == 0) { |
491 |
par.arriveAndDeregister(); |
492 |
reconcileState(); |
493 |
} |
494 |
break; |
495 |
} |
496 |
continue; |
497 |
} |
498 |
if (unarrived == 0) { |
499 |
if (casState |
500 |
(s, |
501 |
trippedStateFor(onAdvance(phase, parties) ? -1 : |
502 |
((phase + 1) & phaseMask), parties))) { |
503 |
releaseWaiters(phase); |
504 |
break; |
505 |
} |
506 |
continue; |
507 |
} |
508 |
if (par != null && phase != phaseOf(root.state)) { |
509 |
reconcileState(); |
510 |
continue; |
511 |
} |
512 |
} |
513 |
throw new IllegalStateException(badBounds(parties, unarrived)); |
514 |
} |
515 |
return phase; |
516 |
} |
517 |
|
518 |
/** |
519 |
* Arrives at the barrier and awaits others. Equivalent in effect |
520 |
* to {@code awaitAdvance(arrive())}. If you need to await with |
521 |
* interruption or timeout, you can arrange this with an analogous |
522 |
* construction using one of the other forms of the awaitAdvance |
523 |
* method. If instead you need to deregister upon arrival use |
524 |
* {@code arriveAndDeregister}. |
525 |
* |
526 |
* @return the phase on entry to this method |
527 |
* @throws IllegalStateException if not terminated and the number |
528 |
* of unarrived parties would become negative |
529 |
*/ |
530 |
public int arriveAndAwaitAdvance() { |
531 |
return awaitAdvance(arrive()); |
532 |
} |
533 |
|
534 |
/** |
535 |
* Awaits the phase of the barrier to advance from the given phase |
536 |
* value, returning immediately if the current phase of the |
537 |
* barrier is not equal to the given phase value or this barrier |
538 |
* is terminated. |
539 |
* |
540 |
* @param phase the phase on entry to this method |
541 |
* @return the current barrier phase number upon exit of |
542 |
* this method, or a negative value if terminated or |
543 |
* argument is negative |
544 |
*/ |
545 |
public int awaitAdvance(int phase) { |
546 |
if (phase < 0) |
547 |
return phase; |
548 |
long s = getReconciledState(); |
549 |
int p = phaseOf(s); |
550 |
if (p != phase) |
551 |
return p; |
552 |
if (unarrivedOf(s) == 0 && parent != null) |
553 |
parent.awaitAdvance(phase); |
554 |
// Fall here even if parent waited, to reconcile and help release |
555 |
return untimedWait(phase); |
556 |
} |
557 |
|
558 |
/** |
559 |
* Awaits the phase of the barrier to advance from the given phase |
560 |
* value, throwing {@code InterruptedException} if interrupted while |
561 |
* waiting, or returning immediately if the current phase of the |
562 |
* barrier is not equal to the given phase value or this barrier |
563 |
* is terminated. |
564 |
* |
565 |
* @param phase the phase on entry to this method |
566 |
* @return the current barrier phase number upon exit of |
567 |
* this method, or a negative value if terminated or |
568 |
* argument is negative |
569 |
* @throws InterruptedException if thread interrupted while waiting |
570 |
*/ |
571 |
public int awaitAdvanceInterruptibly(int phase) |
572 |
throws InterruptedException { |
573 |
if (phase < 0) |
574 |
return phase; |
575 |
long s = getReconciledState(); |
576 |
int p = phaseOf(s); |
577 |
if (p != phase) |
578 |
return p; |
579 |
if (unarrivedOf(s) == 0 && parent != null) |
580 |
parent.awaitAdvanceInterruptibly(phase); |
581 |
return interruptibleWait(phase); |
582 |
} |
583 |
|
584 |
/** |
585 |
* Awaits the phase of the barrier to advance from the given phase |
586 |
* value or the given timeout to elapse, throwing |
587 |
* {@code InterruptedException} if interrupted while waiting, or |
588 |
* returning immediately if the current phase of the barrier is not |
589 |
* equal to the given phase value or this barrier is terminated. |
590 |
* |
591 |
* @param phase the phase on entry to this method |
592 |
* @param timeout how long to wait before giving up, in units of |
593 |
* {@code unit} |
594 |
* @param unit a {@code TimeUnit} determining how to interpret the |
595 |
* {@code timeout} parameter |
596 |
* @return the current barrier phase number upon exit of |
597 |
* this method, or a negative value if terminated or |
598 |
* argument is negative |
599 |
* @throws InterruptedException if thread interrupted while waiting |
600 |
* @throws TimeoutException if timed out while waiting |
601 |
*/ |
602 |
public int awaitAdvanceInterruptibly(int phase, |
603 |
long timeout, TimeUnit unit) |
604 |
throws InterruptedException, TimeoutException { |
605 |
if (phase < 0) |
606 |
return phase; |
607 |
long s = getReconciledState(); |
608 |
int p = phaseOf(s); |
609 |
if (p != phase) |
610 |
return p; |
611 |
if (unarrivedOf(s) == 0 && parent != null) |
612 |
parent.awaitAdvanceInterruptibly(phase, timeout, unit); |
613 |
return timedWait(phase, unit.toNanos(timeout)); |
614 |
} |
615 |
|
616 |
/** |
617 |
* Forces this barrier to enter termination state. Counts of |
618 |
* arrived and registered parties are unaffected. If this phaser |
619 |
* has a parent, it too is terminated. This method may be useful |
620 |
* for coordinating recovery after one or more tasks encounter |
621 |
* unexpected exceptions. |
622 |
*/ |
623 |
public void forceTermination() { |
624 |
for (;;) { |
625 |
long s = getReconciledState(); |
626 |
int phase = phaseOf(s); |
627 |
int parties = partiesOf(s); |
628 |
int unarrived = unarrivedOf(s); |
629 |
if (phase < 0 || |
630 |
casState(s, stateFor(-1, parties, unarrived))) { |
631 |
releaseWaiters(0); |
632 |
releaseWaiters(1); |
633 |
if (parent != null) |
634 |
parent.forceTermination(); |
635 |
return; |
636 |
} |
637 |
} |
638 |
} |
639 |
|
640 |
/** |
641 |
* Returns the current phase number. The maximum phase number is |
642 |
* {@code Integer.MAX_VALUE}, after which it restarts at |
643 |
* zero. Upon termination, the phase number is negative. |
644 |
* |
645 |
* @return the phase number, or a negative value if terminated |
646 |
*/ |
647 |
public final int getPhase() { |
648 |
return phaseOf(getReconciledState()); |
649 |
} |
650 |
|
651 |
/** |
652 |
* Returns the number of parties registered at this barrier. |
653 |
* |
654 |
* @return the number of parties |
655 |
*/ |
656 |
public int getRegisteredParties() { |
657 |
return partiesOf(state); |
658 |
} |
659 |
|
660 |
/** |
661 |
* Returns the number of parties that have arrived at the current |
662 |
* phase of this barrier. |
663 |
* |
664 |
* @return the number of arrived parties |
665 |
*/ |
666 |
public int getArrivedParties() { |
667 |
return arrivedOf(state); |
668 |
} |
669 |
|
670 |
/** |
671 |
* Returns the number of registered parties that have not yet |
672 |
* arrived at the current phase of this barrier. |
673 |
* |
674 |
* @return the number of unarrived parties |
675 |
*/ |
676 |
public int getUnarrivedParties() { |
677 |
return unarrivedOf(state); |
678 |
} |
679 |
|
680 |
/** |
681 |
* Returns the parent of this phaser, or {@code null} if none. |
682 |
* |
683 |
* @return the parent of this phaser, or {@code null} if none |
684 |
*/ |
685 |
public Phaser getParent() { |
686 |
return parent; |
687 |
} |
688 |
|
689 |
/** |
690 |
* Returns the root ancestor of this phaser, which is the same as |
691 |
* this phaser if it has no parent. |
692 |
* |
693 |
* @return the root ancestor of this phaser |
694 |
*/ |
695 |
public Phaser getRoot() { |
696 |
return root; |
697 |
} |
698 |
|
699 |
/** |
700 |
* Returns {@code true} if this barrier has been terminated. |
701 |
* |
702 |
* @return {@code true} if this barrier has been terminated |
703 |
*/ |
704 |
public boolean isTerminated() { |
705 |
return getPhase() < 0; |
706 |
} |
707 |
|
708 |
/** |
709 |
* Overridable method to perform an action upon phase advance, and |
710 |
* to control termination. This method is invoked whenever the |
711 |
* barrier is tripped (and thus all other waiting parties are |
712 |
* dormant). If it returns {@code true}, then, rather than advance |
713 |
* the phase number, this barrier will be set to a final |
714 |
* termination state, and subsequent calls to {@link #isTerminated} |
715 |
* will return true. |
716 |
* |
717 |
* <p>The default version returns {@code true} when the number of |
718 |
* registered parties is zero. Normally, overrides that arrange |
719 |
* termination for other reasons should also preserve this |
720 |
* property. |
721 |
* |
722 |
* <p>You may override this method to perform an action with side |
723 |
* effects visible to participating tasks, but it is in general |
724 |
* only sensible to do so in designs where all parties register |
725 |
* before any arrive, and all {@link #awaitAdvance} at each phase. |
726 |
* Otherwise, you cannot ensure lack of interference from other |
727 |
* parties during the invocation of this method. |
728 |
* |
729 |
* @param phase the phase number on entering the barrier |
730 |
* @param registeredParties the current number of registered parties |
731 |
* @return {@code true} if this barrier should terminate |
732 |
*/ |
733 |
protected boolean onAdvance(int phase, int registeredParties) { |
734 |
return registeredParties <= 0; |
735 |
} |
736 |
|
737 |
/** |
738 |
* Returns a string identifying this phaser, as well as its |
739 |
* state. The state, in brackets, includes the String {@code |
740 |
* "phase = "} followed by the phase number, {@code "parties = "} |
741 |
* followed by the number of registered parties, and {@code |
742 |
* "arrived = "} followed by the number of arrived parties. |
743 |
* |
744 |
* @return a string identifying this barrier, as well as its state |
745 |
*/ |
746 |
public String toString() { |
747 |
long s = getReconciledState(); |
748 |
return super.toString() + |
749 |
"[phase = " + phaseOf(s) + |
750 |
" parties = " + partiesOf(s) + |
751 |
" arrived = " + arrivedOf(s) + "]"; |
752 |
} |
753 |
|
754 |
// methods for waiting |
755 |
|
756 |
/** |
757 |
* Wait nodes for Treiber stack representing wait queue |
758 |
*/ |
759 |
static final class QNode implements ForkJoinPool.ManagedBlocker { |
760 |
final Phaser phaser; |
761 |
final int phase; |
762 |
final long startTime; |
763 |
final long nanos; |
764 |
final boolean timed; |
765 |
final boolean interruptible; |
766 |
volatile boolean wasInterrupted = false; |
767 |
volatile Thread thread; // nulled to cancel wait |
768 |
QNode next; |
769 |
QNode(Phaser phaser, int phase, boolean interruptible, |
770 |
boolean timed, long startTime, long nanos) { |
771 |
this.phaser = phaser; |
772 |
this.phase = phase; |
773 |
this.timed = timed; |
774 |
this.interruptible = interruptible; |
775 |
this.startTime = startTime; |
776 |
this.nanos = nanos; |
777 |
thread = Thread.currentThread(); |
778 |
} |
779 |
public boolean isReleasable() { |
780 |
return (thread == null || |
781 |
phaser.getPhase() != phase || |
782 |
(interruptible && wasInterrupted) || |
783 |
(timed && (nanos - (System.nanoTime() - startTime)) <= 0)); |
784 |
} |
785 |
public boolean block() { |
786 |
if (Thread.interrupted()) { |
787 |
wasInterrupted = true; |
788 |
if (interruptible) |
789 |
return true; |
790 |
} |
791 |
if (!timed) |
792 |
LockSupport.park(this); |
793 |
else { |
794 |
long waitTime = nanos - (System.nanoTime() - startTime); |
795 |
if (waitTime <= 0) |
796 |
return true; |
797 |
LockSupport.parkNanos(this, waitTime); |
798 |
} |
799 |
return isReleasable(); |
800 |
} |
801 |
void signal() { |
802 |
Thread t = thread; |
803 |
if (t != null) { |
804 |
thread = null; |
805 |
LockSupport.unpark(t); |
806 |
} |
807 |
} |
808 |
boolean doWait() { |
809 |
if (thread != null) { |
810 |
try { |
811 |
ForkJoinPool.managedBlock(this, false); |
812 |
} catch (InterruptedException ie) { |
813 |
} |
814 |
} |
815 |
return wasInterrupted; |
816 |
} |
817 |
|
818 |
} |
819 |
|
820 |
/** |
821 |
* Removes and signals waiting threads from wait queue. |
822 |
*/ |
823 |
private void releaseWaiters(int phase) { |
824 |
AtomicReference<QNode> head = queueFor(phase); |
825 |
QNode q; |
826 |
while ((q = head.get()) != null) { |
827 |
if (head.compareAndSet(q, q.next)) |
828 |
q.signal(); |
829 |
} |
830 |
} |
831 |
|
832 |
/** |
833 |
* Tries to enqueue given node in the appropriate wait queue. |
834 |
* |
835 |
* @return true if successful |
836 |
*/ |
837 |
private boolean tryEnqueue(QNode node) { |
838 |
AtomicReference<QNode> head = queueFor(node.phase); |
839 |
return head.compareAndSet(node.next = head.get(), node); |
840 |
} |
841 |
|
842 |
/** |
843 |
* Enqueues node and waits unless aborted or signalled. |
844 |
* |
845 |
* @return current phase |
846 |
*/ |
847 |
private int untimedWait(int phase) { |
848 |
QNode node = null; |
849 |
boolean queued = false; |
850 |
boolean interrupted = false; |
851 |
int p; |
852 |
while ((p = getPhase()) == phase) { |
853 |
if (Thread.interrupted()) |
854 |
interrupted = true; |
855 |
else if (node == null) |
856 |
node = new QNode(this, phase, false, false, 0, 0); |
857 |
else if (!queued) |
858 |
queued = tryEnqueue(node); |
859 |
else |
860 |
interrupted = node.doWait(); |
861 |
} |
862 |
if (node != null) |
863 |
node.thread = null; |
864 |
releaseWaiters(phase); |
865 |
if (interrupted) |
866 |
Thread.currentThread().interrupt(); |
867 |
return p; |
868 |
} |
869 |
|
870 |
/** |
871 |
* Interruptible version |
872 |
* @return current phase |
873 |
*/ |
874 |
private int interruptibleWait(int phase) throws InterruptedException { |
875 |
QNode node = null; |
876 |
boolean queued = false; |
877 |
boolean interrupted = false; |
878 |
int p; |
879 |
while ((p = getPhase()) == phase && !interrupted) { |
880 |
if (Thread.interrupted()) |
881 |
interrupted = true; |
882 |
else if (node == null) |
883 |
node = new QNode(this, phase, true, false, 0, 0); |
884 |
else if (!queued) |
885 |
queued = tryEnqueue(node); |
886 |
else |
887 |
interrupted = node.doWait(); |
888 |
} |
889 |
if (node != null) |
890 |
node.thread = null; |
891 |
if (p != phase || (p = getPhase()) != phase) |
892 |
releaseWaiters(phase); |
893 |
if (interrupted) |
894 |
throw new InterruptedException(); |
895 |
return p; |
896 |
} |
897 |
|
898 |
/** |
899 |
* Timeout version. |
900 |
* @return current phase |
901 |
*/ |
902 |
private int timedWait(int phase, long nanos) |
903 |
throws InterruptedException, TimeoutException { |
904 |
long startTime = System.nanoTime(); |
905 |
QNode node = null; |
906 |
boolean queued = false; |
907 |
boolean interrupted = false; |
908 |
int p; |
909 |
while ((p = getPhase()) == phase && !interrupted) { |
910 |
if (Thread.interrupted()) |
911 |
interrupted = true; |
912 |
else if (nanos - (System.nanoTime() - startTime) <= 0) |
913 |
break; |
914 |
else if (node == null) |
915 |
node = new QNode(this, phase, true, true, startTime, nanos); |
916 |
else if (!queued) |
917 |
queued = tryEnqueue(node); |
918 |
else |
919 |
interrupted = node.doWait(); |
920 |
} |
921 |
if (node != null) |
922 |
node.thread = null; |
923 |
if (p != phase || (p = getPhase()) != phase) |
924 |
releaseWaiters(phase); |
925 |
if (interrupted) |
926 |
throw new InterruptedException(); |
927 |
if (p == phase) |
928 |
throw new TimeoutException(); |
929 |
return p; |
930 |
} |
931 |
|
932 |
// Unsafe mechanics |
933 |
|
934 |
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe(); |
935 |
private static final long stateOffset = |
936 |
objectFieldOffset("state", Phaser.class); |
937 |
|
938 |
private final boolean casState(long cmp, long val) { |
939 |
return UNSAFE.compareAndSwapLong(this, stateOffset, cmp, val); |
940 |
} |
941 |
|
942 |
private static long objectFieldOffset(String field, Class<?> klazz) { |
943 |
try { |
944 |
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
945 |
} catch (NoSuchFieldException e) { |
946 |
// Convert Exception to corresponding Error |
947 |
NoSuchFieldError error = new NoSuchFieldError(field); |
948 |
error.initCause(e); |
949 |
throw error; |
950 |
} |
951 |
} |
952 |
} |