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 phaser has an associated phase number. The phase + * number starts at zero, and advances when all parties arrive at the + * phaser, wrapping around to zero after reaching {@code + * Integer.MAX_VALUE}. The use of phase numbers enables independent + * control of actions upon arrival at a phaser and upon awaiting + * others, via two kinds of methods that may be invoked by any + * registered party: + * + *
Termination. A phaser may enter a termination + * state, that may be checked using method {@link #isTerminated}. Upon + * termination, all synchronization methods immediately return without + * waiting for advance, as indicated by a negative return value. + * Similarly, attempts to register upon termination have no effect. + * Termination is triggered when an invocation of {@code onAdvance} + * returns {@code true}. The default implementation returns {@code + * true} if a deregistration has caused the number of registered + * parties to become zero. 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., + * constructed in tree structures) to reduce contention. Phasers with + * large numbers of parties that would otherwise experience heavy + * synchronization contention costs may instead be set up so that + * groups of sub-phasers share a common parent. This may greatly + * increase throughput even though it incurs greater per-operation + * overhead. + * + *
In a tree of tiered phasers, registration and deregistration of + * child phasers with their parent are managed automatically. + * Whenever the number of registered parties of a child phaser becomes + * non-zero (as established in the {@link #Phaser(Phaser,int)} + * constructor, {@link #register}, or {@link #bulkRegister}), the + * child phaser is registered with its parent. Whenever the number of + * registered parties becomes zero as the result of an invocation of + * {@link #arriveAndDeregister}, the child phaser is deregistered + * from its parent. + * + *
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}) + * 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} + * to control a one-shot action serving a variable number of parties. + * The typical idiom is for the method setting this up to first + * register, then start the actions, then deregister, as in: + * + *
{@code
+ * void runTasks(List tasks) {
+ * final Phaser phaser = new Phaser(1); // "1" to register self
+ * // create and start threads
+ * for (final Runnable task : tasks) {
+ * phaser.register();
+ * new Thread() {
+ * public void run() {
+ * phaser.arriveAndAwaitAdvance(); // await all creation
+ * task.run();
+ * }
+ * }.start();
+ * }
+ *
+ * // allow threads to start and deregister self
+ * phaser.arriveAndDeregister();
+ * }}
+ *
+ * One way to cause a set of threads to repeatedly perform actions + * for a given number of iterations is to override {@code onAdvance}: + * + *
{@code
+ * void startTasks(List tasks, final int iterations) {
+ * final Phaser phaser = new Phaser() {
+ * protected boolean onAdvance(int phase, int registeredParties) {
+ * return phase >= iterations || registeredParties == 0;
+ * }
+ * };
+ * phaser.register();
+ * for (final Runnable task : tasks) {
+ * phaser.register();
+ * new Thread() {
+ * public void run() {
+ * do {
+ * task.run();
+ * phaser.arriveAndAwaitAdvance();
+ * } 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 {@code n} tasks using a tree of phasers, you + * could use code of the following form, assuming a Task class with a + * constructor accepting a {@code Phaser} that it registers with upon + * construction. After invocation of {@code build(new Task[n], 0, n, + * new Phaser())}, these tasks could then be started, for example by + * submitting to a pool: + * + *
{@code
+ * void build(Task[] tasks, 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(tasks, i, j, new Phaser(ph));
+ * }
+ * } else {
+ * for (int i = lo; i < hi; ++i)
+ * tasks[i] = new Task(ph);
+ * // assumes new Task(ph) performs ph.register()
+ * }
+ * }}
+ *
+ * The best value of {@code TASKS_PER_PHASER} depends mainly on
+ * expected synchronization rates. A value as low as four may
+ * be appropriate for extremely small per-phase task bodies (thus
+ * high rates), or up to hundreds for extremely large ones.
+ *
+ * Implementation notes: This implementation restricts the
+ * maximum number of parties to 65535. Attempts to register additional
+ * parties result in {@code IllegalStateException}. However, you can and
+ * should create tiered phasers to accommodate arbitrarily large sets
+ * of participants.
+ *
+ * @since 1.7
+ * @author Doug Lea
+ */
+public class Phaser {
+ /*
+ * This class implements an extension of X10 "clocks". Thanks to
+ * Vijay Saraswat for the idea, and to Vivek Sarkar for
+ * enhancements to extend functionality.
+ */
+
+ /**
+ * Primary state representation, holding four bit-fields:
+ *
+ * unarrived -- the number of parties yet to hit barrier (bits 0-15)
+ * parties -- the number of parties to wait (bits 16-31)
+ * phase -- the generation of the barrier (bits 32-62)
+ * terminated -- set if barrier is terminated (bit 63 / sign)
+ *
+ * Except that a phaser with no registered parties is
+ * distinguished by the otherwise illegal state of having zero
+ * parties and one unarrived parties (encoded as EMPTY below).
+ *
+ * To efficiently maintain atomicity, these values are packed into
+ * a single (atomic) long. Good performance relies on keeping
+ * state decoding and encoding simple, and keeping race windows
+ * short.
+ *
+ * All state updates are performed via CAS except initial
+ * registration of a sub-phaser (i.e., one with a non-null
+ * parent). In this (relatively rare) case, we use built-in
+ * synchronization to lock while first registering with its
+ * parent.
+ *
+ * The phase of a subphaser is allowed to lag that of its
+ * ancestors until it is actually accessed -- see method
+ * reconcileState.
+ */
+ private volatile long state;
+
+ private static final int MAX_PARTIES = 0xffff;
+ private static final int MAX_PHASE = Integer.MAX_VALUE;
+ private static final int PARTIES_SHIFT = 16;
+ private static final int PHASE_SHIFT = 32;
+ private static final int UNARRIVED_MASK = 0xffff; // to mask ints
+ private static final long PARTIES_MASK = 0xffff0000L; // to mask longs
+ private static final long TERMINATION_BIT = 1L << 63;
+
+ // some special values
+ private static final int ONE_ARRIVAL = 1;
+ private static final int ONE_PARTY = 1 << PARTIES_SHIFT;
+ private static final int EMPTY = 1;
+
+ // The following unpacking methods are usually manually inlined
+
+ private static int unarrivedOf(long s) {
+ int counts = (int)s;
+ return (counts == EMPTY) ? 0 : counts & UNARRIVED_MASK;
+ }
+
+ private static int partiesOf(long s) {
+ return (int)s >>> PARTIES_SHIFT;
+ }
+
+ private static int phaseOf(long s) {
+ return (int)(s >>> PHASE_SHIFT);
+ }
+
+ private static int arrivedOf(long s) {
+ int counts = (int)s;
+ return (counts == EMPTY) ? 0 :
+ (counts >>> PARTIES_SHIFT) - (counts & UNARRIVED_MASK);
+ }
+
+ /**
+ * The parent of this phaser, or null if none
+ */
+ private final Phaser parent;
+
+ /**
+ * The root of phaser tree. Equals this if not in a tree.
+ */
+ private final Phaser root;
+
+ /**
+ * Heads of Treiber stacks for waiting threads. To eliminate
+ * contention when releasing some threads while adding others, we
+ * use two of them, alternating across even and odd phases.
+ * Subphasers share queues with root to speed up releases.
+ */
+ private final AtomicReference It is a usage error for an unregistered party to invoke this
+ * method. However, this error may result in an {@code
+ * IllegalStateException} only upon some subsequent operation on
+ * this phaser, if ever.
+ *
+ * @return the arrival phase number, or a negative value if terminated
+ * @throws IllegalStateException if not terminated and the number
+ * of unarrived parties would become negative
+ */
+ public int arrive() {
+ return doArrive(false);
+ }
+
+ /**
+ * Arrives at this phaser and deregisters from it without waiting
+ * for others to arrive. Deregistration reduces the number of
+ * parties required to advance in future phases. If this phaser
+ * has a parent, and deregistration causes this phaser to have
+ * zero parties, this phaser is also deregistered from its parent.
+ *
+ * It is a usage error for an unregistered party to invoke this
+ * method. However, this error may result in an {@code
+ * IllegalStateException} only upon some subsequent operation on
+ * this phaser, if ever.
+ *
+ * @return the arrival phase number, or a negative value if terminated
+ * @throws IllegalStateException if not terminated and the number
+ * of registered or unarrived parties would become negative
+ */
+ public int arriveAndDeregister() {
+ return doArrive(true);
+ }
+
+ /**
+ * Arrives at this phaser and awaits others. Equivalent in effect
+ * to {@code awaitAdvance(arrive())}. If you need to await with
+ * interruption or timeout, you can arrange this with an analogous
+ * construction using one of the other forms of the {@code
+ * awaitAdvance} method. If instead you need to deregister upon
+ * arrival, use {@code awaitAdvance(arriveAndDeregister())}.
+ *
+ * It is a usage error for an unregistered party to invoke this
+ * method. However, this error may result in an {@code
+ * IllegalStateException} only upon some subsequent operation on
+ * this phaser, if ever.
+ *
+ * @return the arrival phase number, or the (negative)
+ * {@linkplain #getPhase() current phase} if terminated
+ * @throws IllegalStateException if not terminated and the number
+ * of unarrived parties would become negative
+ */
+ public int arriveAndAwaitAdvance() {
+ // Specialization of doArrive+awaitAdvance eliminating some reads/paths
+ final Phaser root = this.root;
+ for (;;) {
+ long s = (root == this) ? state : reconcileState();
+ int phase = (int)(s >>> PHASE_SHIFT);
+ int counts = (int)s;
+ int unarrived = (counts & UNARRIVED_MASK) - 1;
+ if (phase < 0)
+ return phase;
+ else if (counts == EMPTY || unarrived < 0) {
+ if (reconcileState() == s)
+ throw new IllegalStateException(badArrive(s));
+ }
+ else if (UNSAFE.compareAndSwapLong(this, stateOffset, s,
+ s -= ONE_ARRIVAL)) {
+ if (unarrived != 0)
+ return root.internalAwaitAdvance(phase, null);
+ if (root != this)
+ return parent.arriveAndAwaitAdvance();
+ long n = s & PARTIES_MASK; // base of next state
+ int nextUnarrived = (int)n >>> PARTIES_SHIFT;
+ if (onAdvance(phase, nextUnarrived))
+ n |= TERMINATION_BIT;
+ else if (nextUnarrived == 0)
+ n |= EMPTY;
+ else
+ n |= nextUnarrived;
+ int nextPhase = (phase + 1) & MAX_PHASE;
+ n |= (long)nextPhase << PHASE_SHIFT;
+ if (!UNSAFE.compareAndSwapLong(this, stateOffset, s, n))
+ return (int)(state >>> PHASE_SHIFT); // terminated
+ releaseWaiters(phase);
+ return nextPhase;
+ }
+ }
+ }
+
+ /**
+ * Awaits the phase of this phaser to advance from the given phase
+ * value, returning immediately if the current phase is not equal
+ * to the given phase value or this phaser is terminated.
+ *
+ * @param phase an arrival phase number, or negative value if
+ * terminated; this argument is normally the value returned by a
+ * previous call to {@code arrive} or {@code arriveAndDeregister}.
+ * @return the next arrival phase number, or the argument if it is
+ * negative, or the (negative) {@linkplain #getPhase() current phase}
+ * if terminated
+ */
+ public int awaitAdvance(int phase) {
+ final Phaser root = this.root;
+ long s = (root == this) ? state : reconcileState();
+ int p = (int)(s >>> PHASE_SHIFT);
+ if (phase < 0)
+ return phase;
+ if (p == phase)
+ return root.internalAwaitAdvance(phase, null);
+ return p;
+ }
+
+ /**
+ * Awaits the phase of this phaser to advance from the given phase
+ * value, throwing {@code InterruptedException} if interrupted
+ * while waiting, or returning immediately if the current phase is
+ * not equal to the given phase value or this phaser is
+ * terminated.
+ *
+ * @param phase an arrival phase number, or negative value if
+ * terminated; this argument is normally the value returned by a
+ * previous call to {@code arrive} or {@code arriveAndDeregister}.
+ * @return the next arrival phase number, or the argument if it is
+ * negative, or the (negative) {@linkplain #getPhase() current phase}
+ * if terminated
+ * @throws InterruptedException if thread interrupted while waiting
+ */
+ public int awaitAdvanceInterruptibly(int phase)
+ throws InterruptedException {
+ final Phaser root = this.root;
+ long s = (root == this) ? state : reconcileState();
+ int p = (int)(s >>> PHASE_SHIFT);
+ if (phase < 0)
+ return phase;
+ if (p == phase) {
+ QNode node = new QNode(this, phase, true, false, 0L);
+ p = root.internalAwaitAdvance(phase, node);
+ if (node.wasInterrupted)
+ throw new InterruptedException();
+ }
+ return p;
+ }
+
+ /**
+ * Awaits the phase of this phaser 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 is not equal to the
+ * given phase value or this phaser is terminated.
+ *
+ * @param phase an arrival phase number, or negative value if
+ * terminated; this argument is normally the value returned by a
+ * previous call to {@code arrive} or {@code arriveAndDeregister}.
+ * @param timeout how long to wait before giving up, in units of
+ * {@code unit}
+ * @param unit a {@code TimeUnit} determining how to interpret the
+ * {@code timeout} parameter
+ * @return the next arrival phase number, or the argument if it is
+ * negative, or the (negative) {@linkplain #getPhase() current phase}
+ * if terminated
+ * @throws InterruptedException if thread interrupted while waiting
+ * @throws TimeoutException if timed out while waiting
+ */
+ public int awaitAdvanceInterruptibly(int phase,
+ long timeout, TimeUnit unit)
+ throws InterruptedException, TimeoutException {
+ long nanos = unit.toNanos(timeout);
+ final Phaser root = this.root;
+ long s = (root == this) ? state : reconcileState();
+ int p = (int)(s >>> PHASE_SHIFT);
+ if (phase < 0)
+ return phase;
+ if (p == phase) {
+ QNode node = new QNode(this, phase, true, true, nanos);
+ p = root.internalAwaitAdvance(phase, node);
+ if (node.wasInterrupted)
+ throw new InterruptedException();
+ else if (p == phase)
+ throw new TimeoutException();
+ }
+ return p;
+ }
+
+ /**
+ * Forces this phaser to enter termination state. Counts of
+ * registered parties are unaffected. If this phaser is a member
+ * of a tiered set of phasers, then all of the phasers in the set
+ * are terminated. If this phaser is already terminated, this
+ * method has no effect. This method may be useful for
+ * coordinating recovery after one or more tasks encounter
+ * unexpected exceptions.
+ */
+ public void forceTermination() {
+ // Only need to change root state
+ final Phaser root = this.root;
+ long s;
+ while ((s = root.state) >= 0) {
+ if (UNSAFE.compareAndSwapLong(root, stateOffset,
+ s, s | TERMINATION_BIT)) {
+ // signal all threads
+ releaseWaiters(0);
+ releaseWaiters(1);
+ return;
+ }
+ }
+ }
+
+ /**
+ * Returns the current phase number. The maximum phase number is
+ * {@code Integer.MAX_VALUE}, after which it restarts at
+ * zero. Upon termination, the phase number is negative,
+ * in which case the prevailing phase prior to termination
+ * may be obtained via {@code getPhase() + Integer.MIN_VALUE}.
+ *
+ * @return the phase number, or a negative value if terminated
+ */
+ public final int getPhase() {
+ return (int)(root.state >>> PHASE_SHIFT);
+ }
+
+ /**
+ * Returns the number of parties registered at this phaser.
+ *
+ * @return the number of parties
+ */
+ public int getRegisteredParties() {
+ return partiesOf(state);
+ }
+
+ /**
+ * Returns the number of registered parties that have arrived at
+ * the current phase of this phaser. If this phaser has terminated,
+ * the returned value is meaningless and arbitrary.
+ *
+ * @return the number of arrived parties
+ */
+ public int getArrivedParties() {
+ return arrivedOf(reconcileState());
+ }
+
+ /**
+ * Returns the number of registered parties that have not yet
+ * arrived at the current phase of this phaser. If this phaser has
+ * terminated, the returned value is meaningless and arbitrary.
+ *
+ * @return the number of unarrived parties
+ */
+ public int getUnarrivedParties() {
+ return unarrivedOf(reconcileState());
+ }
+
+ /**
+ * Returns the parent of this phaser, or {@code null} if none.
+ *
+ * @return the parent of this phaser, or {@code null} if none
+ */
+ public Phaser getParent() {
+ return parent;
+ }
+
+ /**
+ * Returns the root ancestor of this phaser, which is the same as
+ * this phaser if it has no parent.
+ *
+ * @return the root ancestor of this phaser
+ */
+ public Phaser getRoot() {
+ return root;
+ }
+
+ /**
+ * Returns {@code true} if this phaser has been terminated.
+ *
+ * @return {@code true} if this phaser has been terminated
+ */
+ public boolean isTerminated() {
+ return root.state < 0L;
+ }
+
+ /**
+ * Overridable method to perform an action upon impending phase
+ * advance, and to control termination. This method is invoked
+ * upon arrival of the party advancing this phaser (when all other
+ * waiting parties are dormant). If this method returns {@code
+ * true}, this phaser will be set to a final termination state
+ * upon advance, 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 advance this phaser, in which case no advance
+ * occurs.
+ *
+ * The arguments to this method provide the state of the phaser
+ * prevailing for the current transition. The effects of invoking
+ * arrival, registration, and waiting methods on this phaser from
+ * within {@code onAdvance} are unspecified and should not be
+ * relied on.
+ *
+ * If this phaser is a member of a tiered set of phasers, then
+ * {@code onAdvance} is invoked only for its root phaser on each
+ * advance.
+ *
+ * To support the most common use cases, the default
+ * implementation of this method returns {@code true} when the
+ * number of registered parties has become zero as the result of a
+ * party invoking {@code arriveAndDeregister}. You can disable
+ * this behavior, thus enabling continuation upon future
+ * registrations, by overriding this method to always return
+ * {@code false}:
+ *
+ * {@code
+ * Phaser phaser = new Phaser() {
+ * protected boolean onAdvance(int phase, int parties) { return false; }
+ * }}
+ *
+ * @param phase the current phase number on entry to this method,
+ * before this phaser is advanced
+ * @param registeredParties the current number of registered parties
+ * @return {@code true} if this phaser should terminate
+ */
+ protected boolean onAdvance(int phase, int registeredParties) {
+ return registeredParties == 0;
+ }
+
+ /**
+ * Returns a string identifying this phaser, as well as its
+ * state. The state, in brackets, includes the String {@code
+ * "phase = "} followed by the phase number, {@code "parties = "}
+ * followed by the number of registered parties, and {@code
+ * "arrived = "} followed by the number of arrived parties.
+ *
+ * @return a string identifying this phaser, as well as its state
+ */
+ public String toString() {
+ return stateToString(reconcileState());
+ }
+
+ /**
+ * Implementation of toString and string-based error messages
+ */
+ private String stateToString(long s) {
+ return super.toString() +
+ "[phase = " + phaseOf(s) +
+ " parties = " + partiesOf(s) +
+ " arrived = " + arrivedOf(s) + "]";
+ }
+
+ // Waiting mechanics
+
+ /**
+ * Removes and signals threads from queue for phase.
+ */
+ private void releaseWaiters(int phase) {
+ QNode q; // first element of queue
+ Thread t; // its thread
+ AtomicReference