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import java.util.concurrent.locks.LockSupport; |
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|
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/** |
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* A reusable synchronization barrier, similar in functionality to a |
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* A reusable synchronization barrier, similar in functionality to |
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* {@link java.util.concurrent.CyclicBarrier CyclicBarrier} and |
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* {@link java.util.concurrent.CountDownLatch CountDownLatch} |
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* but supporting more flexible usage. |
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* |
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* <ul> |
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* |
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* <li> The number of parties synchronizing on a phaser may vary over |
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* time. A task may register to be a party at any time, and may |
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* deregister upon arriving at the barrier. As is the case with most |
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* basic synchronization constructs, registration and deregistration |
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* affect only internal counts; they do not establish any further |
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* internal bookkeeping, so tasks cannot query whether they are |
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* registered. (However, you can introduce such bookkeeping by |
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* subclassing this class.) |
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* |
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* <li> Each generation has an associated phase value, starting at |
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* zero, and advancing when all parties reach the barrier (wrapping |
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* around to zero after reaching {@code Integer.MAX_VALUE}). |
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* |
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* <li> Like a {@code CyclicBarrier}, a phaser may be repeatedly |
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* awaited. Method {@link #arriveAndAwaitAdvance} has effect |
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* analogous to {@link java.util.concurrent.CyclicBarrier#await |
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* CyclicBarrier.await}. However, phasers separate two aspects of |
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* coordination, which may also be invoked independently: |
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* <p> <b>Registration.</b> Unlike the case for other barriers, the |
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* number of parties <em>registered</em> to synchronize on a phaser |
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* may vary over time. Tasks may be registered at any time (using |
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* methods {@link #register}, {@link #bulkRegister}, or forms of |
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* constructors establishing initial numbers of parties), and |
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* optionally deregistered upon any arrival (using {@link |
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* #arriveAndDeregister}). As is the case with most basic |
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* synchronization constructs, registration and deregistration affect |
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* only internal counts; they do not establish any further internal |
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* bookkeeping, so tasks cannot query whether they are registered. |
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* (However, you can introduce such bookkeeping by subclassing this |
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* class.) |
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* |
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* <p> <b>Synchronization.</b> Like a {@code CyclicBarrier}, a {@code |
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* Phaser} may be repeatedly awaited. Method {@link |
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* #arriveAndAwaitAdvance} has effect analogous to {@link |
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* java.util.concurrent.CyclicBarrier#await CyclicBarrier.await}. Each |
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* generation of a {@code Phaser} has an associated phase number. The |
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> |
* phase number starts at zero, amd advances when all parties arrive |
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* at the barrier, wrapping around to zero after reaching {@code |
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> |
* Integer.MAX_VALUE}. The use of phase numbers enables independent |
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* control of actions upon arrival at a barrier and upon awaiting |
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* others, via two kinds of methods that may be invoked by any |
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* registered party: |
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* |
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* <ul> |
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* |
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* <li> Arriving at a barrier. Methods {@link #arrive} and |
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* {@link #arriveAndDeregister} do not block, but return |
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* the phase value current upon entry to the method. |
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* |
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* <li> Awaiting others. Method {@link #awaitAdvance} requires an |
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* argument indicating the entry phase, and returns when the |
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* barrier advances to a new phase. |
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* </ul> |
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> |
* <li> <b>Arrival.</b> Methods {@link #arrive} and |
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* {@link #arriveAndDeregister} record arrival at a |
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* barrier. These methods do not block, but return an associated |
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* <em>arrival phase number</em>; that is, the phase number of |
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* the barrier to which the arrival applied. When the final |
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> |
* party for a given phase arrives, an optional barrier action |
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* is performed and the phase advances. Barrier actions, |
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* performed by the party triggering a phase advance, are |
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* arranged by overriding method {@link #onAdvance(int, int)}, |
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* which also controls termination. Overriding this method is |
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> |
* similar to, but more flexible than, providing a barrier |
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* action to a {@code CyclicBarrier}. |
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> |
* |
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> |
* <li> <b>Waiting.</b> Method {@link #awaitAdvance} requires an |
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* argument indicating an arrival phase number, and returns when |
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* the barrier advances to (or is already at) a different phase. |
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* Unlike similar constructions using {@code CyclicBarrier}, |
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* method {@code awaitAdvance} continues to wait even if the |
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* waiting thread is interrupted. (Interruptible and timeout |
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* versions are also available.) Exceptions encountered while |
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* tasks wait interruptibly or with timeout do not change the |
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* state of the barrier. If necessary, you can perform any |
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* associated recovery within handlers of those exceptions, |
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* often after invoking {@code forceTermination}. Phasers may |
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* also be used by tasks executing in a {@link ForkJoinPool}, |
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* which will ensure sufficient parallelism to execute tasks |
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* when others are blocked waiting for a phase to advance. |
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* |
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* </ul> |
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* |
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< |
* <li> Barrier actions, performed by the task triggering a phase |
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* advance, are arranged by overriding method {@link #onAdvance(int, |
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< |
* int)}, which also controls termination. Overriding this method is |
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* similar to, but more flexible than, providing a barrier action to a |
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* {@code CyclicBarrier}. |
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* |
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* <li> Phasers may enter a <em>termination</em> state in which all |
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* actions immediately return without updating phaser state or waiting |
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< |
* for advance, and indicating (via a negative phase value) that |
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* execution is complete. Termination is triggered when an invocation |
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< |
* of {@code onAdvance} returns {@code true}. When a phaser is |
| 64 |
< |
* 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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> |
* <p> <b>Termination.</b> A {@code Phaser} may enter a |
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> |
* <em>termination</em> state in which all actions immediately return |
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> |
* without updating phaser state or waiting for advance, and |
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> |
* indicating (via a negative phase value) that execution is complete. |
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> |
* Termination is triggered when an invocation of {@code onAdvance} |
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> |
* returns {@code true}. 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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< |
* <li> Phasers may be tiered to reduce contention. Phasers with large |
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> |
* <p> <b>Tiering.</b> Phasers may be <em>tiered</em> (i.e., arranged |
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> |
* in tree structures) to reduce contention. Phasers with large |
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* numbers of parties that would otherwise experience heavy |
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< |
* synchronization contention costs may instead be arranged in trees. |
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< |
* This will typically greatly increase throughput even though it |
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< |
* incurs somewhat greater per-operation overhead. |
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< |
* |
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< |
* <li> By default, {@code awaitAdvance} continues to wait even if |
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< |
* the waiting thread is interrupted. And unlike the case in |
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< |
* {@code CyclicBarrier}, exceptions encountered while tasks wait |
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< |
* interruptibly or with timeout do not change the state of the |
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< |
* barrier. If necessary, you can perform any associated recovery |
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< |
* within handlers of those exceptions, often after invoking |
| 102 |
< |
* {@code forceTermination}. |
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< |
* |
| 104 |
< |
* <li>Phasers may be used to coordinate tasks executing in a {@link |
| 85 |
< |
* ForkJoinPool}, which will ensure sufficient parallelism to execute |
| 86 |
< |
* tasks when others are blocked waiting for a phase to advance. |
| 87 |
< |
* |
| 88 |
< |
* </ul> |
| 92 |
> |
* synchronization contention costs may instead be set up so that |
| 93 |
> |
* groups of sub-phasers share a common parent. This may greatly |
| 94 |
> |
* increase throughput even though it incurs greater per-operation |
| 95 |
> |
* overhead. |
| 96 |
> |
* |
| 97 |
> |
* <p><b>Monitoring.</b> While synchronization methods may be invoked |
| 98 |
> |
* only by registered parties, the current state of a phaser may be |
| 99 |
> |
* monitored by any caller. At any given moment there are {@link |
| 100 |
> |
* #getRegisteredParties}, where {@link #getArrivedParties} have |
| 101 |
> |
* arrived at the current phase ({@link #getPhase}). When the |
| 102 |
> |
* remaining {@link #getUnarrivedParties}) arrive, the phase |
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> |
* advances. Method {@link #toString} returns snapshots of these state |
| 104 |
> |
* queries in a form convenient for informal monitoring. |
| 105 |
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* |
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* <p><b>Sample usages:</b> |
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* |
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* first register, then start the actions, then deregister, as in: |
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* |
| 113 |
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* <pre> {@code |
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< |
* void runTasks(List<Runnable> list) { |
| 114 |
> |
* void runTasks(List<Runnable> tasks) { |
| 115 |
|
* final Phaser phaser = new Phaser(1); // "1" to register self |
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* // create and start threads |
| 117 |
< |
* for (Runnable r : list) { |
| 117 |
> |
* for (Runnable task : tasks) { |
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* phaser.register(); |
| 119 |
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* new Thread() { |
| 120 |
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* public void run() { |
| 121 |
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* phaser.arriveAndAwaitAdvance(); // await all creation |
| 122 |
< |
* r.run(); |
| 122 |
> |
* task.run(); |
| 123 |
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* } |
| 124 |
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* }.start(); |
| 125 |
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* } |
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* for a given number of iterations is to override {@code onAdvance}: |
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* |
| 134 |
|
* <pre> {@code |
| 135 |
< |
* void startTasks(List<Runnable> list, final int iterations) { |
| 135 |
> |
* void startTasks(List<Runnable> tasks, final int iterations) { |
| 136 |
|
* final Phaser phaser = new Phaser() { |
| 137 |
< |
* public boolean onAdvance(int phase, int registeredParties) { |
| 137 |
> |
* protected boolean onAdvance(int phase, int registeredParties) { |
| 138 |
|
* return phase >= iterations || registeredParties == 0; |
| 139 |
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* } |
| 140 |
|
* }; |
| 141 |
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* phaser.register(); |
| 142 |
< |
* for (Runnable r : list) { |
| 142 |
> |
* for (Runnable task : tasks) { |
| 143 |
|
* phaser.register(); |
| 144 |
|
* new Thread() { |
| 145 |
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* public void run() { |
| 146 |
|
* do { |
| 147 |
< |
* r.run(); |
| 147 |
> |
* task.run(); |
| 148 |
|
* phaser.arriveAndAwaitAdvance(); |
| 149 |
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* } while(!phaser.isTerminated(); |
| 150 |
|
* } |
| 153 |
|
* phaser.arriveAndDeregister(); // deregister self, don't wait |
| 154 |
|
* }}</pre> |
| 155 |
|
* |
| 156 |
+ |
* If the main task must later await termination, it |
| 157 |
+ |
* may re-register and then execute a similar loop: |
| 158 |
+ |
* <pre> {@code |
| 159 |
+ |
* // ... |
| 160 |
+ |
* phaser.register(); |
| 161 |
+ |
* while (!phaser.isTerminated()) |
| 162 |
+ |
* phaser.arriveAndAwaitAdvance(); |
| 163 |
+ |
* }</pre> |
| 164 |
+ |
* |
| 165 |
+ |
* Related constructions may be used to await particular phase numbers |
| 166 |
+ |
* in contexts where you are sure that the phase will never wrap around |
| 167 |
+ |
* {@code Integer.MAX_VALUE}. For example: |
| 168 |
+ |
* |
| 169 |
+ |
* <pre> {@code |
| 170 |
+ |
* void awaitPhase(Phaser phaser, int phase) { |
| 171 |
+ |
* int p = phaser.register(); // assumes caller not already registered |
| 172 |
+ |
* while (p < phase) { |
| 173 |
+ |
* if (phaser.isTerminated()) |
| 174 |
+ |
* // ... deal with unexpected termination |
| 175 |
+ |
* else |
| 176 |
+ |
* p = phaser.arriveAndAwaitAdvance(); |
| 177 |
+ |
* } |
| 178 |
+ |
* phaser.arriveAndDeregister(); |
| 179 |
+ |
* } |
| 180 |
+ |
* }</pre> |
| 181 |
+ |
* |
| 182 |
+ |
* |
| 183 |
|
* <p>To create a set of tasks using a tree of phasers, |
| 184 |
|
* you could use code of the following form, assuming a |
| 185 |
|
* Task class with a constructor accepting a phaser that |
| 212 |
|
* |
| 213 |
|
* <p><b>Implementation notes</b>: This implementation restricts the |
| 214 |
|
* maximum number of parties to 65535. Attempts to register additional |
| 215 |
< |
* parties result in IllegalStateExceptions. However, you can and |
| 215 |
> |
* parties result in {@code IllegalStateException}. However, you can and |
| 216 |
|
* should create tiered phasers to accommodate arbitrarily large sets |
| 217 |
|
* of participants. |
| 218 |
|
* |
| 409 |
|
/** |
| 410 |
|
* Adds a new unarrived party to this phaser. |
| 411 |
|
* |
| 412 |
< |
* @return the current barrier phase number upon registration |
| 412 |
> |
* @return the arrival phase number to which this registration applied |
| 413 |
|
* @throws IllegalStateException if attempting to register more |
| 414 |
|
* than the maximum supported number of parties |
| 415 |
|
*/ |
| 421 |
|
* Adds the given number of new unarrived parties to this phaser. |
| 422 |
|
* |
| 423 |
|
* @param parties the number of parties required to trip barrier |
| 424 |
< |
* @return the current barrier phase number upon registration |
| 424 |
> |
* @return the arrival phase number to which this registration applied |
| 425 |
|
* @throws IllegalStateException if attempting to register more |
| 426 |
|
* than the maximum supported number of parties |
| 427 |
|
*/ |
| 456 |
|
|
| 457 |
|
/** |
| 458 |
|
* Arrives at the barrier, but does not wait for others. (You can |
| 459 |
< |
* in turn wait for others via {@link #awaitAdvance}). |
| 459 |
> |
* in turn wait for others via {@link #awaitAdvance}). It is an |
| 460 |
> |
* unenforced usage error for an unregistered party to invoke this |
| 461 |
> |
* method. |
| 462 |
|
* |
| 463 |
< |
* @return the barrier phase number upon entry to this method, or a |
| 419 |
< |
* negative value if terminated |
| 463 |
> |
* @return the arrival phase number, or a negative value if terminated |
| 464 |
|
* @throws IllegalStateException if not terminated and the number |
| 465 |
|
* of unarrived parties would become negative |
| 466 |
|
*/ |
| 510 |
|
* required to trip the barrier in future phases. If this phaser |
| 511 |
|
* has a parent, and deregistration causes this phaser to have |
| 512 |
|
* zero parties, this phaser also arrives at and is deregistered |
| 513 |
< |
* from its parent. |
| 513 |
> |
* from its parent. It is an unenforced usage error for an |
| 514 |
> |
* unregistered party to invoke this method. |
| 515 |
|
* |
| 516 |
< |
* @return the current barrier phase number upon entry to |
| 472 |
< |
* this method, or a negative value if terminated |
| 516 |
> |
* @return the arrival phase number, or a negative value if terminated |
| 517 |
|
* @throws IllegalStateException if not terminated and the number |
| 518 |
|
* of registered or unarrived parties would become negative |
| 519 |
|
*/ |
| 567 |
|
* interruption or timeout, you can arrange this with an analogous |
| 568 |
|
* construction using one of the other forms of the awaitAdvance |
| 569 |
|
* method. If instead you need to deregister upon arrival use |
| 570 |
< |
* {@code arriveAndDeregister}. |
| 570 |
> |
* {@code arriveAndDeregister}. It is an unenforced usage error |
| 571 |
> |
* for an unregistered party to invoke this method. |
| 572 |
|
* |
| 573 |
< |
* @return the phase on entry to this method |
| 573 |
> |
* @return the arrival phase number, or a negative number if terminated |
| 574 |
|
* @throws IllegalStateException if not terminated and the number |
| 575 |
|
* of unarrived parties would become negative |
| 576 |
|
*/ |
| 580 |
|
|
| 581 |
|
/** |
| 582 |
|
* Awaits the phase of the barrier to advance from the given phase |
| 583 |
< |
* value, or returns immediately if the current phase of the barrier |
| 584 |
< |
* is not equal to the given phase value or this barrier is |
| 585 |
< |
* terminated. |
| 586 |
< |
* |
| 587 |
< |
* @param phase the phase on entry to this method |
| 588 |
< |
* @return the phase on exit from this method |
| 583 |
> |
* value, returning immediately if the current phase of the |
| 584 |
> |
* barrier is not equal to the given phase value or this barrier |
| 585 |
> |
* is terminated. It is an unenforced usage error for an |
| 586 |
> |
* unregistered party to invoke this method. |
| 587 |
> |
* |
| 588 |
> |
* @param phase an arrival phase number, or negative value if |
| 589 |
> |
* terminated; this argument is normally the value returned by a |
| 590 |
> |
* previous call to {@code arrive} or its variants |
| 591 |
> |
* @return the next arrival phase number, or a negative value |
| 592 |
> |
* if terminated or argument is negative |
| 593 |
|
*/ |
| 594 |
|
public int awaitAdvance(int phase) { |
| 595 |
|
if (phase < 0) |
| 605 |
|
} |
| 606 |
|
|
| 607 |
|
/** |
| 608 |
< |
* Awaits the phase of the barrier to advance from the given |
| 609 |
< |
* value, or returns immediately if argument is negative or this |
| 610 |
< |
* barrier is terminated, or throws InterruptedException if |
| 611 |
< |
* interrupted while waiting. |
| 612 |
< |
* |
| 613 |
< |
* @param phase the phase on entry to this method |
| 614 |
< |
* @return the phase on exit from this method |
| 608 |
> |
* Awaits the phase of the barrier to advance from the given phase |
| 609 |
> |
* value, throwing {@code InterruptedException} if interrupted |
| 610 |
> |
* while waiting, or returning immediately if the current phase of |
| 611 |
> |
* the barrier is not equal to the given phase value or this |
| 612 |
> |
* barrier is terminated. It is an unenforced usage error for an |
| 613 |
> |
* unregistered party to invoke this method. |
| 614 |
> |
* |
| 615 |
> |
* @param phase an arrival phase number, or negative value if |
| 616 |
> |
* terminated; this argument is normally the value returned by a |
| 617 |
> |
* previous call to {@code arrive} or its variants |
| 618 |
> |
* @return the next arrival phase number, or a negative value |
| 619 |
> |
* if terminated or argument is negative |
| 620 |
|
* @throws InterruptedException if thread interrupted while waiting |
| 621 |
|
*/ |
| 622 |
|
public int awaitAdvanceInterruptibly(int phase) |
| 633 |
|
} |
| 634 |
|
|
| 635 |
|
/** |
| 636 |
< |
* Awaits the phase of the barrier to advance from the given value |
| 637 |
< |
* or the given timeout elapses, or returns immediately if |
| 638 |
< |
* argument is negative or this barrier is terminated. |
| 639 |
< |
* |
| 640 |
< |
* @param phase the phase on entry to this method |
| 641 |
< |
* @return the phase on exit from this method |
| 636 |
> |
* Awaits the phase of the barrier to advance from the given phase |
| 637 |
> |
* value or the given timeout to elapse, throwing {@code |
| 638 |
> |
* InterruptedException} if interrupted while waiting, or |
| 639 |
> |
* returning immediately if the current phase of the barrier is |
| 640 |
> |
* not equal to the given phase value or this barrier is |
| 641 |
> |
* terminated. It is an unenforced usage error for an |
| 642 |
> |
* unregistered party to invoke this method. |
| 643 |
> |
* |
| 644 |
> |
* @param phase an arrival phase number, or negative value if |
| 645 |
> |
* terminated; this argument is normally the value returned by a |
| 646 |
> |
* previous call to {@code arrive} or its variants |
| 647 |
> |
* @param timeout how long to wait before giving up, in units of |
| 648 |
> |
* {@code unit} |
| 649 |
> |
* @param unit a {@code TimeUnit} determining how to interpret the |
| 650 |
> |
* {@code timeout} parameter |
| 651 |
> |
* @return the next arrival phase number, or a negative value |
| 652 |
> |
* if terminated or argument is negative |
| 653 |
|
* @throws InterruptedException if thread interrupted while waiting |
| 654 |
|
* @throws TimeoutException if timed out while waiting |
| 655 |
|
*/ |
| 712 |
|
} |
| 713 |
|
|
| 714 |
|
/** |
| 715 |
< |
* Returns the number of parties that have arrived at the current |
| 716 |
< |
* phase of this barrier. |
| 715 |
> |
* Returns the number of registered parties that have arrived at |
| 716 |
> |
* the current phase of this barrier. |
| 717 |
|
* |
| 718 |
|
* @return the number of arrived parties |
| 719 |
|
*/ |
| 778 |
|
* only sensible to do so in designs where all parties register |
| 779 |
|
* before any arrive, and all {@link #awaitAdvance} at each phase. |
| 780 |
|
* Otherwise, you cannot ensure lack of interference from other |
| 781 |
< |
* parties during the the invocation of this method. |
| 781 |
> |
* parties during the invocation of this method. |
| 782 |
|
* |
| 783 |
|
* @param phase the phase number on entering the barrier |
| 784 |
|
* @param registeredParties the current number of registered parties |
