Registration. Unlike the case for other barriers, the + * number of parties registered to synchronize on a phaser + * may vary over time. Tasks may be registered at any time (using + * methods {@link #register}, {@link #bulkRegister}, or forms of + * constructors establishing initial numbers of parties), and + * optionally deregistered upon any arrival (using {@link * #arriveAndDeregister}). As is the case with most basic * synchronization constructs, registration and deregistration affect * only internal counts; they do not establish any further internal - * bookkeeping, so tasks cannot query whether they are - * registered. (However, you can introduce such bookkeeping by - * subclassing this class.) - * - *
Synchronization. Like a {@code CyclicBarrier}, a {@code + * Phaser} may be repeatedly awaited. Method {@link + * #arriveAndAwaitAdvance} has effect analogous to {@link + * java.util.concurrent.CyclicBarrier#await CyclicBarrier.await}. Each + * generation of a {@code Phaser} has an associated phase number. The + * phase number starts at zero, and advances when all parties arrive + * at the barrier, wrapping around to zero after reaching {@code + * Integer.MAX_VALUE}. The use of phase numbers enables independent + * control of actions upon arrival at a barrier and upon awaiting + * others, via two kinds of methods that may be invoked by any + * registered party: * *
Termination. A {@code Phaser} may enter a + * termination state in which all synchronization methods + * immediately return without updating phaser state or waiting for + * advance, and indicating (via a negative phase value) that execution + * is complete. Termination is triggered when an invocation of {@code + * onAdvance} returns {@code true}. As illustrated below, when + * phasers control actions with a fixed number of iterations, it is * often convenient to override this method to cause termination when * the current phase number reaches a threshold. Method {@link * #forceTermination} is also available to abruptly release waiting * threads and allow them to terminate. * - *
Tiering. Phasers may be tiered (i.e., arranged + * in tree structures) to reduce contention. Phasers with large * numbers of parties that would otherwise experience heavy - * synchronization contention costs may instead be arranged in trees. - * This will typically greatly increase throughput even though it - * incurs somewhat greater per-operation overhead. - * - *
Monitoring. While synchronization methods may be invoked + * only by registered parties, the current state of a phaser may be + * monitored by any caller. At any given moment there are {@link + * #getRegisteredParties} parties in total, of which {@link * #getArrivedParties} have arrived at the current phase ({@link - * #getPhase}). When the remaining {@link #getUnarrivedParties}) - * arrive, the phase advances. Method {@link #toString} returns - * snapshots of these state queries in a form convenient for + * #getPhase}). When the remaining ({@link #getUnarrivedParties}) + * parties arrive, the phase advances. The values returned by these + * methods may reflect transient states and so are not in general + * useful for synchronization control. Method {@link #toString} + * returns snapshots of these state queries in a form convenient for * informal monitoring. * - *
Sample usages: * *
A {@code Phaser} may be used instead of a {@code CountDownLatch} @@ -131,43 +137,66 @@ import java.util.concurrent.locks.LockSu *
{@code
* void startTasks(List tasks, final int iterations) {
* final Phaser phaser = new Phaser() {
- * public boolean onAdvance(int phase, int registeredParties) {
+ * protected boolean onAdvance(int phase, int registeredParties) {
* return phase >= iterations || registeredParties == 0;
* }
* };
* phaser.register();
- * for (Runnable task : tasks) {
+ * for (final Runnable task : tasks) {
* phaser.register();
* new Thread() {
* public void run() {
* do {
* task.run();
* phaser.arriveAndAwaitAdvance();
- * } while(!phaser.isTerminated();
+ * } while (!phaser.isTerminated());
* }
* }.start();
* }
* phaser.arriveAndDeregister(); // deregister self, don't wait
* }}
*
+ * If the main task must later await termination, it
+ * may re-register and then execute a similar loop:
+ * {@code
+ * // ...
+ * phaser.register();
+ * while (!phaser.isTerminated())
+ * phaser.arriveAndAwaitAdvance();}
+ *
+ * Related constructions may be used to await particular phase numbers + * in contexts where you are sure that the phase will never wrap around + * {@code Integer.MAX_VALUE}. For example: + * + *
{@code
+ * void awaitPhase(Phaser phaser, int phase) {
+ * int p = phaser.register(); // assumes caller not already registered
+ * while (p < phase) {
+ * if (phaser.isTerminated())
+ * // ... deal with unexpected termination
+ * else
+ * p = phaser.arriveAndAwaitAdvance();
+ * }
+ * phaser.arriveAndDeregister();
+ * }}
+ *
+ *
* To create a set of tasks using a tree of phasers, * you could use code of the following form, assuming a * Task class with a constructor accepting a phaser that * it registers for upon construction: + * *
{@code
- * void build(Task[] actions, int lo, int hi, Phaser b) {
- * int step = (hi - lo) / TASKS_PER_PHASER;
- * if (step > 1) {
- * int i = lo;
- * while (i < hi) {
- * int r = Math.min(i + step, hi);
- * build(actions, i, r, new Phaser(b));
- * i = r;
+ * void build(Task[] actions, int lo, int hi, Phaser ph) {
+ * if (hi - lo > TASKS_PER_PHASER) {
+ * for (int i = lo; i < hi; i += TASKS_PER_PHASER) {
+ * int j = Math.min(i + TASKS_PER_PHASER, hi);
+ * build(actions, i, j, new Phaser(ph));
* }
* } else {
* for (int i = lo; i < hi; ++i)
- * actions[i] = new Task(b);
- * // assumes new Task(b) performs b.register()
+ * actions[i] = new Task(ph);
+ * // assumes new Task(ph) performs ph.register()
* }
* }
* // .. initially called, for n tasks via
@@ -216,7 +245,6 @@ public class Phaser {
*/
private volatile long state;
- private static final int ushortBits = 16;
private static final int ushortMask = 0xffff;
private static final int phaseMask = 0x7fffffff;
@@ -426,7 +454,9 @@ public class Phaser {
/**
* Arrives at the barrier, but does not wait for others. (You can
- * in turn wait for others via {@link #awaitAdvance}).
+ * in turn wait for others via {@link #awaitAdvance}). It is an
+ * unenforced usage error for an unregistered party to invoke this
+ * method.
*
* @return the arrival phase number, or a negative value if terminated
* @throws IllegalStateException if not terminated and the number
@@ -478,7 +508,8 @@ public class Phaser {
* required to trip the barrier in future phases. If this phaser
* has a parent, and deregistration causes this phaser to have
* zero parties, this phaser also arrives at and is deregistered
- * from its parent.
+ * from its parent. It is an unenforced usage error for an
+ * unregistered party to invoke this method.
*
* @return the arrival phase number, or a negative value if terminated
* @throws IllegalStateException if not terminated and the number
@@ -534,7 +565,8 @@ public class Phaser {
* interruption or timeout, you can arrange this with an analogous
* construction using one of the other forms of the awaitAdvance
* method. If instead you need to deregister upon arrival use
- * {@code arriveAndDeregister}.
+ * {@code arriveAndDeregister}. It is an unenforced usage error
+ * for an unregistered party to invoke this method.
*
* @return the arrival phase number, or a negative number if terminated
* @throws IllegalStateException if not terminated and the number
@@ -548,7 +580,8 @@ public class Phaser {
* Awaits the phase of the barrier to advance from the given phase
* value, returning immediately if the current phase of the
* barrier is not equal to the given phase value or this barrier
- * is terminated.
+ * is terminated. It is an unenforced usage error for an
+ * unregistered party to invoke this method.
*
* @param phase an arrival phase number, or negative value if
* terminated; this argument is normally the value returned by a
@@ -571,10 +604,11 @@ public class Phaser {
/**
* Awaits the phase of the barrier to advance from the given phase
- * value, throwing {@code InterruptedException} if interrupted while
- * waiting, or returning immediately if the current phase of the
- * barrier is not equal to the given phase value or this barrier
- * is terminated.
+ * value, throwing {@code InterruptedException} if interrupted
+ * while waiting, or returning immediately if the current phase of
+ * the barrier is not equal to the given phase value or this
+ * barrier is terminated. It is an unenforced usage error for an
+ * unregistered party to invoke this method.
*
* @param phase an arrival phase number, or negative value if
* terminated; this argument is normally the value returned by a
@@ -598,10 +632,12 @@ public class Phaser {
/**
* Awaits the phase of the barrier to advance from the given phase
- * value or the given timeout to elapse, throwing
- * {@code InterruptedException} if interrupted while waiting, or
- * returning immediately if the current phase of the barrier is not
- * equal to the given phase value or this barrier is terminated.
+ * value or the given timeout to elapse, throwing {@code
+ * InterruptedException} if interrupted while waiting, or
+ * returning immediately if the current phase of the barrier is
+ * not equal to the given phase value or this barrier is
+ * terminated. It is an unenforced usage error for an
+ * unregistered party to invoke this method.
*
* @param phase an arrival phase number, or negative value if
* terminated; this argument is normally the value returned by a
@@ -722,13 +758,22 @@ public class Phaser {
}
/**
- * Overridable method to perform an action upon phase advance, and
- * to control termination. This method is invoked whenever the
- * barrier is tripped (and thus all other waiting parties are
- * dormant). If it returns {@code true}, then, rather than advance
- * the phase number, this barrier will be set to a final
- * termination state, and subsequent calls to {@link #isTerminated}
- * will return true.
+ * Overridable method to perform an action upon impending phase
+ * advance, and to control termination. This method is invoked
+ * upon arrival of the party tripping the barrier (when all other
+ * waiting parties are dormant). If this method returns {@code
+ * true}, then, rather than advance the phase number, this barrier
+ * will be set to a final termination state, and subsequent calls
+ * to {@link #isTerminated} will return true. Any (unchecked)
+ * Exception or Error thrown by an invocation of this method is
+ * propagated to the party attempting to trip the barrier, in
+ * which case no advance occurs.
+ *
+ * The arguments to this method provide the state of the phaser
+ * prevailing for the current transition. (When called from within
+ * an implementation of {@code onAdvance} the values returned by
+ * methods such as {@code getPhase} may or may not reliably
+ * indicate the state to which this transition applies.)
*
*
The default version returns {@code true} when the number of
* registered parties is zero. Normally, overrides that arrange
@@ -736,11 +781,12 @@ public class Phaser {
* property.
*
*
You may override this method to perform an action with side
- * effects visible to participating tasks, but it is in general
- * only sensible to do so in designs where all parties register
- * before any arrive, and all {@link #awaitAdvance} at each phase.
- * Otherwise, you cannot ensure lack of interference from other
- * parties during the invocation of this method.
+ * effects visible to participating tasks, but doing so requires
+ * care: Method {@code onAdvance} may be invoked more than once
+ * per transition. Further, unless all parties register before
+ * any arrive, and all {@link #awaitAdvance} at each phase, then
+ * you cannot ensure lack of interference from other parties
+ * during the invocation of this method.
*
* @param phase the phase number on entering the barrier
* @param registeredParties the current number of registered parties