Java tutorial
/* * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ /* * This file is available under and governed by the GNU General Public * License version 2 only, as published by the Free Software Foundation. * However, the following notice accompanied the original version of this * file: * * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent; import java.lang.invoke.MethodHandles; import java.lang.invoke.VarHandle; import java.util.concurrent.atomic.AtomicReference; import java.util.concurrent.locks.LockSupport; /** * A reusable synchronization barrier, similar in functionality to * {@link CyclicBarrier} and {@link CountDownLatch} but supporting * more flexible usage. * * <p><b>Registration.</b> Unlike the case for other barriers, the * number of parties <em>registered</em> 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.) * * <p><b>Synchronization.</b> 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: * * <ul> * * <li><b>Arrival.</b> Methods {@link #arrive} and * {@link #arriveAndDeregister} record arrival. These methods * do not block, but return an associated <em>arrival phase * number</em>; that is, the phase number of the phaser to which * the arrival applied. When the final party for a given phase * arrives, an optional action is performed and the phase * advances. These actions are performed by the party * triggering a phase advance, and are arranged by overriding * method {@link #onAdvance(int, int)}, which also controls * termination. Overriding this method is similar to, but more * flexible than, providing a barrier action to a {@code * CyclicBarrier}. * * <li><b>Waiting.</b> Method {@link #awaitAdvance} requires an * argument indicating an arrival phase number, and returns when * the phaser advances to (or is already at) a different phase. * Unlike similar constructions using {@code CyclicBarrier}, * method {@code awaitAdvance} continues to wait even if the * waiting thread is interrupted. Interruptible and timeout * versions are also available, but exceptions encountered while * tasks wait interruptibly or with timeout do not change the * state of the phaser. If necessary, you can perform any * associated recovery within handlers of those exceptions, * often after invoking {@code forceTermination}. Phasers may * also be used by tasks executing in a {@link ForkJoinPool}. * Progress is ensured if the pool's parallelismLevel can * accommodate the maximum number of simultaneously blocked * parties. * * </ul> * * <p><b>Termination.</b> A phaser may enter a <em>termination</em> * 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. * * <p><b>Tiering.</b> Phasers may be <em>tiered</em> (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. * * <p>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. * * <p><b>Monitoring.</b> 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. * * <p><b>Sample usages:</b> * * <p>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 all the actions, then deregister, as in: * * <pre> {@code * void runTasks(List<Runnable> tasks) { * Phaser startingGate = new Phaser(1); // "1" to register self * // create and start threads * for (Runnable task : tasks) { * startingGate.register(); * new Thread(() -> { * startingGate.arriveAndAwaitAdvance(); * task.run(); * }).start(); * } * * // deregister self to allow threads to proceed * startingGate.arriveAndDeregister(); * }}</pre> * * <p>One way to cause a set of threads to repeatedly perform actions * for a given number of iterations is to override {@code onAdvance}: * * <pre> {@code * void startTasks(List<Runnable> tasks, int iterations) { * Phaser phaser = new Phaser() { * protected boolean onAdvance(int phase, int registeredParties) { * return phase >= iterations - 1 || registeredParties == 0; * } * }; * phaser.register(); * for (Runnable task : tasks) { * phaser.register(); * new Thread(() -> { * do { * task.run(); * phaser.arriveAndAwaitAdvance(); * } while (!phaser.isTerminated()); * }).start(); * } * // allow threads to proceed; don't wait for them * phaser.arriveAndDeregister(); * }}</pre> * * If the main task must later await termination, it * may re-register and then execute a similar loop: * <pre> {@code * // ... * phaser.register(); * while (!phaser.isTerminated()) * phaser.arriveAndAwaitAdvance();}</pre> * * <p>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: * * <pre> {@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(); * }}</pre> * * <p>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: * * <pre> {@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() * } * }}</pre> * * 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. * * <p><b>Implementation notes</b>: 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 COUNTS_MASK = 0xffffffffL; 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 ONE_DEREGISTER = ONE_ARRIVAL | ONE_PARTY; 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<QNode> evenQ; private final AtomicReference<QNode> oddQ; /** * Returns message string for bounds exceptions on arrival. */ private String badArrive(long s) { return "Attempted arrival of unregistered party for " + stateToString(s); } /** * Returns message string for bounds exceptions on registration. */ private String badRegister(long s) { return "Attempt to register more than " + MAX_PARTIES + " parties for " + stateToString(s); } /** * Main implementation for methods arrive and arriveAndDeregister. * Manually tuned to speed up and minimize race windows for the * common case of just decrementing unarrived field. * * @param adjust value to subtract from state; * ONE_ARRIVAL for arrive, * ONE_DEREGISTER for arriveAndDeregister */ private int doArrive(int adjust) { final Phaser root = this.root; for (;;) { long s = (root == this) ? state : reconcileState(); int phase = (int) (s >>> PHASE_SHIFT); if (phase < 0) return phase; int counts = (int) s; int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK); if (unarrived <= 0) throw new IllegalStateException(badArrive(s)); if (STATE.compareAndSet(this, s, s -= adjust)) { if (unarrived == 1) { long n = s & PARTIES_MASK; // base of next state int nextUnarrived = (int) n >>> PARTIES_SHIFT; if (root == this) { 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; STATE.compareAndSet(this, s, n); releaseWaiters(phase); } else if (nextUnarrived == 0) { // propagate deregistration phase = parent.doArrive(ONE_DEREGISTER); STATE.compareAndSet(this, s, s | EMPTY); } else phase = parent.doArrive(ONE_ARRIVAL); } return phase; } } } /** * Implementation of register, bulkRegister. * * @param registrations number to add to both parties and * unarrived fields. Must be greater than zero. */ private int doRegister(int registrations) { // adjustment to state long adjust = ((long) registrations << PARTIES_SHIFT) | registrations; final Phaser parent = this.parent; int phase; for (;;) { long s = (parent == null) ? state : reconcileState(); int counts = (int) s; int parties = counts >>> PARTIES_SHIFT; int unarrived = counts & UNARRIVED_MASK; if (registrations > MAX_PARTIES - parties) throw new IllegalStateException(badRegister(s)); phase = (int) (s >>> PHASE_SHIFT); if (phase < 0) break; if (counts != EMPTY) { // not 1st registration if (parent == null || reconcileState() == s) { if (unarrived == 0) // wait out advance root.internalAwaitAdvance(phase, null); else if (STATE.compareAndSet(this, s, s + adjust)) break; } } else if (parent == null) { // 1st root registration long next = ((long) phase << PHASE_SHIFT) | adjust; if (STATE.compareAndSet(this, s, next)) break; } else { synchronized (this) { // 1st sub registration if (state == s) { // recheck under lock phase = parent.doRegister(1); if (phase < 0) break; // finish registration whenever parent registration // succeeded, even when racing with termination, // since these are part of the same "transaction". while (!STATE.weakCompareAndSet(this, s, ((long) phase << PHASE_SHIFT) | adjust)) { s = state; phase = (int) (root.state >>> PHASE_SHIFT); // assert (int)s == EMPTY; } break; } } } } return phase; } /** * Resolves lagged phase propagation from root if necessary. * Reconciliation normally occurs when root has advanced but * subphasers have not yet done so, in which case they must finish * their own advance by setting unarrived to parties (or if * parties is zero, resetting to unregistered EMPTY state). * * @return reconciled state */ private long reconcileState() { final Phaser root = this.root; long s = state; if (root != this) { int phase, p; // CAS to root phase with current parties, tripping unarrived while ((phase = (int) (root.state >>> PHASE_SHIFT)) != (int) (s >>> PHASE_SHIFT) && !STATE.weakCompareAndSet(this, s, s = (((long) phase << PHASE_SHIFT) | ((phase < 0) ? (s & COUNTS_MASK) : (((p = (int) s >>> PARTIES_SHIFT) == 0) ? EMPTY : ((s & PARTIES_MASK) | p)))))) s = state; } return s; } /** * Creates a new phaser with no initially registered parties, no * parent, and initial phase number 0. Any thread using this * phaser will need to first register for it. */ public Phaser() { this(null, 0); } /** * Creates a new phaser with the given number of registered * unarrived parties, no parent, and initial phase number 0. * * @param parties the number of parties required to advance to the * next phase * @throws IllegalArgumentException if parties less than zero * or greater than the maximum number of parties supported */ public Phaser(int parties) { this(null, parties); } /** * Equivalent to {@link #Phaser(Phaser, int) Phaser(parent, 0)}. * * @param parent the parent phaser */ public Phaser(Phaser parent) { this(parent, 0); } /** * Creates a new phaser with the given parent and number of * registered unarrived parties. When the given parent is non-null * and the given number of parties is greater than zero, this * child phaser is registered with its parent. * * @param parent the parent phaser * @param parties the number of parties required to advance to the * next phase * @throws IllegalArgumentException if parties less than zero * or greater than the maximum number of parties supported */ public Phaser(Phaser parent, int parties) { if (parties >>> PARTIES_SHIFT != 0) throw new IllegalArgumentException("Illegal number of parties"); int phase = 0; this.parent = parent; if (parent != null) { final Phaser root = parent.root; this.root = root; this.evenQ = root.evenQ; this.oddQ = root.oddQ; if (parties != 0) phase = parent.doRegister(1); } else { this.root = this; this.evenQ = new AtomicReference<QNode>(); this.oddQ = new AtomicReference<QNode>(); } this.state = (parties == 0) ? (long) EMPTY : ((long) phase << PHASE_SHIFT) | ((long) parties << PARTIES_SHIFT) | ((long) parties); } /** * Adds a new unarrived party to this phaser. If an ongoing * invocation of {@link #onAdvance} is in progress, this method * may await its completion before returning. If this phaser has * a parent, and this phaser previously had no registered parties, * this child phaser is also registered with its parent. If * this phaser is terminated, the attempt to register has * no effect, and a negative value is returned. * * @return the arrival phase number to which this registration * applied. If this value is negative, then this phaser has * terminated, in which case registration has no effect. * @throws IllegalStateException if attempting to register more * than the maximum supported number of parties */ public int register() { return doRegister(1); } /** * Adds the given number of new unarrived parties to this phaser. * If an ongoing invocation of {@link #onAdvance} is in progress, * this method may await its completion before returning. If this * phaser has a parent, and the given number of parties is greater * than zero, and this phaser previously had no registered * parties, this child phaser is also registered with its parent. * If this phaser is terminated, the attempt to register has no * effect, and a negative value is returned. * * @param parties the number of additional parties required to * advance to the next phase * @return the arrival phase number to which this registration * applied. If this value is negative, then this phaser has * terminated, in which case registration has no effect. * @throws IllegalStateException if attempting to register more * than the maximum supported number of parties * @throws IllegalArgumentException if {@code parties < 0} */ public int bulkRegister(int parties) { if (parties < 0) throw new IllegalArgumentException(); if (parties == 0) return getPhase(); return doRegister(parties); } /** * Arrives at this phaser, without waiting for others to arrive. * * <p>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(ONE_ARRIVAL); } /** * 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. * * <p>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(ONE_DEREGISTER); } /** * 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())}. * * <p>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); if (phase < 0) return phase; int counts = (int) s; int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK); if (unarrived <= 0) throw new IllegalStateException(badArrive(s)); if (STATE.compareAndSet(this, s, s -= ONE_ARRIVAL)) { if (unarrived > 1) 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 (!STATE.compareAndSet(this, 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 (STATE.compareAndSet(root, s, s | TERMINATION_BIT)) { // signal all threads releaseWaiters(0); // Waiters on evenQ releaseWaiters(1); // Waiters on oddQ 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. * * <p>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. * * <p>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. * * <p>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}: * * <pre> {@code * Phaser phaser = new Phaser() { * protected boolean onAdvance(int phase, int parties) { return false; } * }}</pre> * * @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<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; while ((q = head.get()) != null && q.phase != (int) (root.state >>> PHASE_SHIFT)) { if (head.compareAndSet(q, q.next) && (t = q.thread) != null) { q.thread = null; LockSupport.unpark(t); } } } /** * Variant of releaseWaiters that additionally tries to remove any * nodes no longer waiting for advance due to timeout or * interrupt. Currently, nodes are removed only if they are at * head of queue, which suffices to reduce memory footprint in * most usages. * * @return current phase on exit */ private int abortWait(int phase) { AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; for (;;) { Thread t; QNode q = head.get(); int p = (int) (root.state >>> PHASE_SHIFT); if (q == null || ((t = q.thread) != null && q.phase == p)) return p; if (head.compareAndSet(q, q.next) && t != null) { q.thread = null; LockSupport.unpark(t); } } } /** The number of CPUs, for spin control */ private static final int NCPU = Runtime.getRuntime().availableProcessors(); /** * The number of times to spin before blocking while waiting for * advance, per arrival while waiting. On multiprocessors, fully * blocking and waking up a large number of threads all at once is * usually a very slow process, so we use rechargeable spins to * avoid it when threads regularly arrive: When a thread in * internalAwaitAdvance notices another arrival before blocking, * and there appear to be enough CPUs available, it spins * SPINS_PER_ARRIVAL more times before blocking. The value trades * off good-citizenship vs big unnecessary slowdowns. */ static final int SPINS_PER_ARRIVAL = (NCPU < 2) ? 1 : 1 << 8; /** * Possibly blocks and waits for phase to advance unless aborted. * Call only on root phaser. * * @param phase current phase * @param node if non-null, the wait node to track interrupt and timeout; * if null, denotes noninterruptible wait * @return current phase */ private int internalAwaitAdvance(int phase, QNode node) { // assert root == this; releaseWaiters(phase - 1); // ensure old queue clean boolean queued = false; // true when node is enqueued int lastUnarrived = 0; // to increase spins upon change int spins = SPINS_PER_ARRIVAL; long s; int p; while ((p = (int) ((s = state) >>> PHASE_SHIFT)) == phase) { if (node == null) { // spinning in noninterruptible mode int unarrived = (int) s & UNARRIVED_MASK; if (unarrived != lastUnarrived && (lastUnarrived = unarrived) < NCPU) spins += SPINS_PER_ARRIVAL; boolean interrupted = Thread.interrupted(); if (interrupted || --spins < 0) { // need node to record intr node = new QNode(this, phase, false, false, 0L); node.wasInterrupted = interrupted; } else Thread.onSpinWait(); } else if (node.isReleasable()) // done or aborted break; else if (!queued) { // push onto queue AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; QNode q = node.next = head.get(); if ((q == null || q.phase == phase) && (int) (state >>> PHASE_SHIFT) == phase) // avoid stale enq queued = head.compareAndSet(q, node); } else { try { ForkJoinPool.managedBlock(node); } catch (InterruptedException cantHappen) { node.wasInterrupted = true; } } } if (node != null) { if (node.thread != null) node.thread = null; // avoid need for unpark() if (node.wasInterrupted && !node.interruptible) Thread.currentThread().interrupt(); if (p == phase && (p = (int) (state >>> PHASE_SHIFT)) == phase) return abortWait(phase); // possibly clean up on abort } releaseWaiters(phase); return p; } /** * Wait nodes for Treiber stack representing wait queue. */ static final class QNode implements ForkJoinPool.ManagedBlocker { final Phaser phaser; final int phase; final boolean interruptible; final boolean timed; boolean wasInterrupted; long nanos; final long deadline; volatile Thread thread; // nulled to cancel wait QNode next; QNode(Phaser phaser, int phase, boolean interruptible, boolean timed, long nanos) { this.phaser = phaser; this.phase = phase; this.interruptible = interruptible; this.nanos = nanos; this.timed = timed; this.deadline = timed ? System.nanoTime() + nanos : 0L; thread = Thread.currentThread(); } public boolean isReleasable() { if (thread == null) return true; if (phaser.getPhase() != phase) { thread = null; return true; } if (Thread.interrupted()) wasInterrupted = true; if (wasInterrupted && interruptible) { thread = null; return true; } if (timed && (nanos <= 0L || (nanos = deadline - System.nanoTime()) <= 0L)) { thread = null; return true; } return false; } public boolean block() { while (!isReleasable()) { if (timed) LockSupport.parkNanos(this, nanos); else LockSupport.park(this); } return true; } } // VarHandle mechanics private static final VarHandle STATE; static { try { MethodHandles.Lookup l = MethodHandles.lookup(); STATE = l.findVarHandle(Phaser.class, "state", long.class); } catch (ReflectiveOperationException e) { throw new ExceptionInInitializerError(e); } // Reduce the risk of rare disastrous classloading in first call to // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773 Class<?> ensureLoaded = LockSupport.class; } }