1 /* 2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 3 * 4 * This code is free software; you can redistribute it and/or modify it 5 * under the terms of the GNU General Public License version 2 only, as 6 * published by the Free Software Foundation. Oracle designates this 7 * particular file as subject to the "Classpath" exception as provided 8 * by Oracle in the LICENSE file that accompanied this code. 9 * 10 * This code is distributed in the hope that it will be useful, but WITHOUT 11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 13 * version 2 for more details (a copy is included in the LICENSE file that 14 * accompanied this code). 15 * 16 * You should have received a copy of the GNU General Public License version 17 * 2 along with this work; if not, write to the Free Software Foundation, 18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 19 * 20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 21 * or visit www.oracle.com if you need additional information or have any 22 * questions. 23 */ 24 25 /* 26 * This file is available under and governed by the GNU General Public 27 * License version 2 only, as published by the Free Software Foundation. 28 * However, the following notice accompanied the original version of this 29 * file: 30 * 31 * Written by Doug Lea with assistance from members of JCP JSR-166 32 * Expert Group and released to the public domain, as explained at 33 * http://creativecommons.org/publicdomain/zero/1.0/ 34 */ 35 36 package java.util.concurrent; 37 38 import java.lang.invoke.MethodHandles; 39 import java.lang.invoke.VarHandle; 40 import java.util.concurrent.atomic.AtomicReference; 41 import java.util.concurrent.locks.LockSupport; 42 43 /** 44 * A reusable synchronization barrier, similar in functionality to 45 * {@link CyclicBarrier} and {@link CountDownLatch} but supporting 46 * more flexible usage. 47 * 48 * <p><b>Registration.</b> Unlike the case for other barriers, the 49 * number of parties <em>registered</em> to synchronize on a phaser 50 * may vary over time. Tasks may be registered at any time (using 51 * methods {@link #register}, {@link #bulkRegister}, or forms of 52 * constructors establishing initial numbers of parties), and 53 * optionally deregistered upon any arrival (using {@link 54 * #arriveAndDeregister}). As is the case with most basic 55 * synchronization constructs, registration and deregistration affect 56 * only internal counts; they do not establish any further internal 57 * bookkeeping, so tasks cannot query whether they are registered. 58 * (However, you can introduce such bookkeeping by subclassing this 59 * class.) 60 * 61 * <p><b>Synchronization.</b> Like a {@code CyclicBarrier}, a {@code 62 * Phaser} may be repeatedly awaited. Method {@link 63 * #arriveAndAwaitAdvance} has effect analogous to {@link 64 * java.util.concurrent.CyclicBarrier#await CyclicBarrier.await}. Each 65 * generation of a phaser has an associated phase number. The phase 66 * number starts at zero, and advances when all parties arrive at the 67 * phaser, wrapping around to zero after reaching {@code 68 * Integer.MAX_VALUE}. The use of phase numbers enables independent 69 * control of actions upon arrival at a phaser and upon awaiting 70 * others, via two kinds of methods that may be invoked by any 71 * registered party: 72 * 73 * <ul> 74 * 75 * <li><b>Arrival.</b> Methods {@link #arrive} and 76 * {@link #arriveAndDeregister} record arrival. These methods 77 * do not block, but return an associated <em>arrival phase 78 * number</em>; that is, the phase number of the phaser to which 79 * the arrival applied. When the final party for a given phase 80 * arrives, an optional action is performed and the phase 81 * advances. These actions are performed by the party 82 * triggering a phase advance, and are arranged by overriding 83 * method {@link #onAdvance(int, int)}, which also controls 84 * termination. Overriding this method is similar to, but more 85 * flexible than, providing a barrier action to a {@code 86 * CyclicBarrier}. 87 * 88 * <li><b>Waiting.</b> Method {@link #awaitAdvance} requires an 89 * argument indicating an arrival phase number, and returns when 90 * the phaser advances to (or is already at) a different phase. 91 * Unlike similar constructions using {@code CyclicBarrier}, 92 * method {@code awaitAdvance} continues to wait even if the 93 * waiting thread is interrupted. Interruptible and timeout 94 * versions are also available, but exceptions encountered while 95 * tasks wait interruptibly or with timeout do not change the 96 * state of the phaser. If necessary, you can perform any 97 * associated recovery within handlers of those exceptions, 98 * often after invoking {@code forceTermination}. Phasers may 99 * also be used by tasks executing in a {@link ForkJoinPool}. 100 * Progress is ensured if the pool's parallelism level can 101 * accommodate the maximum number of simultaneously blocked 102 * parties. 103 * 104 * </ul> 105 * 106 * <p><b>Termination.</b> A phaser may enter a <em>termination</em> 107 * state, that may be checked using method {@link #isTerminated}. Upon 108 * termination, all synchronization methods immediately return without 109 * waiting for advance, as indicated by a negative return value. 110 * Similarly, attempts to register upon termination have no effect. 111 * Termination is triggered when an invocation of {@code onAdvance} 112 * returns {@code true}. The default implementation returns {@code 113 * true} if a deregistration has caused the number of registered 114 * parties to become zero. As illustrated below, when phasers control 115 * actions with a fixed number of iterations, it is often convenient 116 * to override this method to cause termination when the current phase 117 * number reaches a threshold. Method {@link #forceTermination} is 118 * also available to abruptly release waiting threads and allow them 119 * to terminate. 120 * 121 * <p><b>Tiering.</b> Phasers may be <em>tiered</em> (i.e., 122 * constructed in tree structures) to reduce contention. Phasers with 123 * large numbers of parties that would otherwise experience heavy 124 * synchronization contention costs may instead be set up so that 125 * groups of sub-phasers share a common parent. This may greatly 126 * increase throughput even though it incurs greater per-operation 127 * overhead. 128 * 129 * <p>In a tree of tiered phasers, registration and deregistration of 130 * child phasers with their parent are managed automatically. 131 * Whenever the number of registered parties of a child phaser becomes 132 * non-zero (as established in the {@link #Phaser(Phaser,int)} 133 * constructor, {@link #register}, or {@link #bulkRegister}), the 134 * child phaser is registered with its parent. Whenever the number of 135 * registered parties becomes zero as the result of an invocation of 136 * {@link #arriveAndDeregister}, the child phaser is deregistered 137 * from its parent. 138 * 139 * <p><b>Monitoring.</b> While synchronization methods may be invoked 140 * only by registered parties, the current state of a phaser may be 141 * monitored by any caller. At any given moment there are {@link 142 * #getRegisteredParties} parties in total, of which {@link 143 * #getArrivedParties} have arrived at the current phase ({@link 144 * #getPhase}). When the remaining ({@link #getUnarrivedParties}) 145 * parties arrive, the phase advances. The values returned by these 146 * methods may reflect transient states and so are not in general 147 * useful for synchronization control. Method {@link #toString} 148 * returns snapshots of these state queries in a form convenient for 149 * informal monitoring. 150 * 151 * <p><b>Sample usages:</b> 152 * 153 * <p>A {@code Phaser} may be used instead of a {@code CountDownLatch} 154 * to control a one-shot action serving a variable number of parties. 155 * The typical idiom is for the method setting this up to first 156 * register, then start all the actions, then deregister, as in: 157 * 158 * <pre> {@code 159 * void runTasks(List<Runnable> tasks) { 160 * Phaser startingGate = new Phaser(1); // "1" to register self 161 * // create and start threads 162 * for (Runnable task : tasks) { 163 * startingGate.register(); 164 * new Thread(() -> { 165 * startingGate.arriveAndAwaitAdvance(); 166 * task.run(); 167 * }).start(); 168 * } 169 * 170 * // deregister self to allow threads to proceed 171 * startingGate.arriveAndDeregister(); 172 * }}</pre> 173 * 174 * <p>One way to cause a set of threads to repeatedly perform actions 175 * for a given number of iterations is to override {@code onAdvance}: 176 * 177 * <pre> {@code 178 * void startTasks(List<Runnable> tasks, int iterations) { 179 * Phaser phaser = new Phaser() { 180 * protected boolean onAdvance(int phase, int registeredParties) { 181 * return phase >= iterations - 1 || registeredParties == 0; 182 * } 183 * }; 184 * phaser.register(); 185 * for (Runnable task : tasks) { 186 * phaser.register(); 187 * new Thread(() -> { 188 * do { 189 * task.run(); 190 * phaser.arriveAndAwaitAdvance(); 191 * } while (!phaser.isTerminated()); 192 * }).start(); 193 * } 194 * // allow threads to proceed; don't wait for them 195 * phaser.arriveAndDeregister(); 196 * }}</pre> 197 * 198 * If the main task must later await termination, it 199 * may re-register and then execute a similar loop: 200 * <pre> {@code 201 * // ... 202 * phaser.register(); 203 * while (!phaser.isTerminated()) 204 * phaser.arriveAndAwaitAdvance();}</pre> 205 * 206 * <p>Related constructions may be used to await particular phase numbers 207 * in contexts where you are sure that the phase will never wrap around 208 * {@code Integer.MAX_VALUE}. For example: 209 * 210 * <pre> {@code 211 * void awaitPhase(Phaser phaser, int phase) { 212 * int p = phaser.register(); // assumes caller not already registered 213 * while (p < phase) { 214 * if (phaser.isTerminated()) 215 * // ... deal with unexpected termination 216 * else 217 * p = phaser.arriveAndAwaitAdvance(); 218 * } 219 * phaser.arriveAndDeregister(); 220 * }}</pre> 221 * 222 * <p>To create a set of {@code n} tasks using a tree of phasers, you 223 * could use code of the following form, assuming a Task class with a 224 * constructor accepting a {@code Phaser} that it registers with upon 225 * construction. After invocation of {@code build(new Task[n], 0, n, 226 * new Phaser())}, these tasks could then be started, for example by 227 * submitting to a pool: 228 * 229 * <pre> {@code 230 * void build(Task[] tasks, int lo, int hi, Phaser ph) { 231 * if (hi - lo > TASKS_PER_PHASER) { 232 * for (int i = lo; i < hi; i += TASKS_PER_PHASER) { 233 * int j = Math.min(i + TASKS_PER_PHASER, hi); 234 * build(tasks, i, j, new Phaser(ph)); 235 * } 236 * } else { 237 * for (int i = lo; i < hi; ++i) 238 * tasks[i] = new Task(ph); 239 * // assumes new Task(ph) performs ph.register() 240 * } 241 * }}</pre> 242 * 243 * The best value of {@code TASKS_PER_PHASER} depends mainly on 244 * expected synchronization rates. A value as low as four may 245 * be appropriate for extremely small per-phase task bodies (thus 246 * high rates), or up to hundreds for extremely large ones. 247 * 248 * <p><b>Implementation notes</b>: This implementation restricts the 249 * maximum number of parties to 65535. Attempts to register additional 250 * parties result in {@code IllegalStateException}. However, you can and 251 * should create tiered phasers to accommodate arbitrarily large sets 252 * of participants. 253 * 254 * @since 1.7 255 * @author Doug Lea 256 */ 257 public class Phaser { 258 /* 259 * This class implements an extension of X10 "clocks". Thanks to 260 * Vijay Saraswat for the idea, and to Vivek Sarkar for 261 * enhancements to extend functionality. 262 */ 263 264 /** 265 * Primary state representation, holding four bit-fields: 266 * 267 * unarrived -- the number of parties yet to hit barrier (bits 0-15) 268 * parties -- the number of parties to wait (bits 16-31) 269 * phase -- the generation of the barrier (bits 32-62) 270 * terminated -- set if barrier is terminated (bit 63 / sign) 271 * 272 * Except that a phaser with no registered parties is 273 * distinguished by the otherwise illegal state of having zero 274 * parties and one unarrived parties (encoded as EMPTY below). 275 * 276 * To efficiently maintain atomicity, these values are packed into 277 * a single (atomic) long. Good performance relies on keeping 278 * state decoding and encoding simple, and keeping race windows 279 * short. 280 * 281 * All state updates are performed via CAS except initial 282 * registration of a sub-phaser (i.e., one with a non-null 283 * parent). In this (relatively rare) case, we use built-in 284 * synchronization to lock while first registering with its 285 * parent. 286 * 287 * The phase of a subphaser is allowed to lag that of its 288 * ancestors until it is actually accessed -- see method 289 * reconcileState. 290 */ 291 private volatile long state; 292 293 private static final int MAX_PARTIES = 0xffff; 294 private static final int MAX_PHASE = Integer.MAX_VALUE; 295 private static final int PARTIES_SHIFT = 16; 296 private static final int PHASE_SHIFT = 32; 297 private static final int UNARRIVED_MASK = 0xffff; // to mask ints 298 private static final long PARTIES_MASK = 0xffff0000L; // to mask longs 299 private static final long COUNTS_MASK = 0xffffffffL; 300 private static final long TERMINATION_BIT = 1L << 63; 301 302 // some special values 303 private static final int ONE_ARRIVAL = 1; 304 private static final int ONE_PARTY = 1 << PARTIES_SHIFT; 305 private static final int ONE_DEREGISTER = ONE_ARRIVAL|ONE_PARTY; 306 private static final int EMPTY = 1; 307 308 // The following unpacking methods are usually manually inlined 309 unarrivedOf(long s)310 private static int unarrivedOf(long s) { 311 int counts = (int)s; 312 return (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK); 313 } 314 partiesOf(long s)315 private static int partiesOf(long s) { 316 return (int)s >>> PARTIES_SHIFT; 317 } 318 phaseOf(long s)319 private static int phaseOf(long s) { 320 return (int)(s >>> PHASE_SHIFT); 321 } 322 arrivedOf(long s)323 private static int arrivedOf(long s) { 324 int counts = (int)s; 325 return (counts == EMPTY) ? 0 : 326 (counts >>> PARTIES_SHIFT) - (counts & UNARRIVED_MASK); 327 } 328 329 /** 330 * The parent of this phaser, or null if none. 331 */ 332 private final Phaser parent; 333 334 /** 335 * The root of phaser tree. Equals this if not in a tree. 336 */ 337 private final Phaser root; 338 339 /** 340 * Heads of Treiber stacks for waiting threads. To eliminate 341 * contention when releasing some threads while adding others, we 342 * use two of them, alternating across even and odd phases. 343 * Subphasers share queues with root to speed up releases. 344 */ 345 private final AtomicReference<QNode> evenQ; 346 private final AtomicReference<QNode> oddQ; 347 348 /** 349 * Returns message string for bounds exceptions on arrival. 350 */ badArrive(long s)351 private String badArrive(long s) { 352 return "Attempted arrival of unregistered party for " + 353 stateToString(s); 354 } 355 356 /** 357 * Returns message string for bounds exceptions on registration. 358 */ badRegister(long s)359 private String badRegister(long s) { 360 return "Attempt to register more than " + 361 MAX_PARTIES + " parties for " + stateToString(s); 362 } 363 364 /** 365 * Main implementation for methods arrive and arriveAndDeregister. 366 * Manually tuned to speed up and minimize race windows for the 367 * common case of just decrementing unarrived field. 368 * 369 * @param adjust value to subtract from state; 370 * ONE_ARRIVAL for arrive, 371 * ONE_DEREGISTER for arriveAndDeregister 372 */ doArrive(int adjust)373 private int doArrive(int adjust) { 374 final Phaser root = this.root; 375 for (;;) { 376 long s = (root == this) ? state : reconcileState(); 377 int phase = (int)(s >>> PHASE_SHIFT); 378 if (phase < 0) 379 return phase; 380 int counts = (int)s; 381 int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK); 382 if (unarrived <= 0) 383 throw new IllegalStateException(badArrive(s)); 384 if (STATE.compareAndSet(this, s, s-=adjust)) { 385 if (unarrived == 1) { 386 long n = s & PARTIES_MASK; // base of next state 387 int nextUnarrived = (int)n >>> PARTIES_SHIFT; 388 if (root == this) { 389 if (onAdvance(phase, nextUnarrived)) 390 n |= TERMINATION_BIT; 391 else if (nextUnarrived == 0) 392 n |= EMPTY; 393 else 394 n |= nextUnarrived; 395 int nextPhase = (phase + 1) & MAX_PHASE; 396 n |= (long)nextPhase << PHASE_SHIFT; 397 STATE.compareAndSet(this, s, n); 398 releaseWaiters(phase); 399 } 400 else if (nextUnarrived == 0) { // propagate deregistration 401 phase = parent.doArrive(ONE_DEREGISTER); 402 STATE.compareAndSet(this, s, s | EMPTY); 403 } 404 else 405 phase = parent.doArrive(ONE_ARRIVAL); 406 } 407 return phase; 408 } 409 } 410 } 411 412 /** 413 * Implementation of register, bulkRegister. 414 * 415 * @param registrations number to add to both parties and 416 * unarrived fields. Must be greater than zero. 417 */ doRegister(int registrations)418 private int doRegister(int registrations) { 419 // adjustment to state 420 long adjust = ((long)registrations << PARTIES_SHIFT) | registrations; 421 final Phaser parent = this.parent; 422 int phase; 423 for (;;) { 424 long s = (parent == null) ? state : reconcileState(); 425 int counts = (int)s; 426 int parties = counts >>> PARTIES_SHIFT; 427 int unarrived = counts & UNARRIVED_MASK; 428 if (registrations > MAX_PARTIES - parties) 429 throw new IllegalStateException(badRegister(s)); 430 phase = (int)(s >>> PHASE_SHIFT); 431 if (phase < 0) 432 break; 433 if (counts != EMPTY) { // not 1st registration 434 if (parent == null || reconcileState() == s) { 435 if (unarrived == 0) // wait out advance 436 root.internalAwaitAdvance(phase, null); 437 else if (STATE.compareAndSet(this, s, s + adjust)) 438 break; 439 } 440 } 441 else if (parent == null) { // 1st root registration 442 long next = ((long)phase << PHASE_SHIFT) | adjust; 443 if (STATE.compareAndSet(this, s, next)) 444 break; 445 } 446 else { 447 synchronized (this) { // 1st sub registration 448 if (state == s) { // recheck under lock 449 phase = parent.doRegister(1); 450 if (phase < 0) 451 break; 452 // finish registration whenever parent registration 453 // succeeded, even when racing with termination, 454 // since these are part of the same "transaction". 455 while (!STATE.weakCompareAndSet 456 (this, s, 457 ((long)phase << PHASE_SHIFT) | adjust)) { 458 s = state; 459 phase = (int)(root.state >>> PHASE_SHIFT); 460 // assert (int)s == EMPTY; 461 } 462 break; 463 } 464 } 465 } 466 } 467 return phase; 468 } 469 470 /** 471 * Resolves lagged phase propagation from root if necessary. 472 * Reconciliation normally occurs when root has advanced but 473 * subphasers have not yet done so, in which case they must finish 474 * their own advance by setting unarrived to parties (or if 475 * parties is zero, resetting to unregistered EMPTY state). 476 * 477 * @return reconciled state 478 */ reconcileState()479 private long reconcileState() { 480 final Phaser root = this.root; 481 long s = state; 482 if (root != this) { 483 int phase, p; 484 // CAS to root phase with current parties, tripping unarrived 485 while ((phase = (int)(root.state >>> PHASE_SHIFT)) != 486 (int)(s >>> PHASE_SHIFT) && 487 !STATE.weakCompareAndSet 488 (this, s, 489 s = (((long)phase << PHASE_SHIFT) | 490 ((phase < 0) ? (s & COUNTS_MASK) : 491 (((p = (int)s >>> PARTIES_SHIFT) == 0) ? EMPTY : 492 ((s & PARTIES_MASK) | p)))))) 493 s = state; 494 } 495 return s; 496 } 497 498 /** 499 * Creates a new phaser with no initially registered parties, no 500 * parent, and initial phase number 0. Any thread using this 501 * phaser will need to first register for it. 502 */ Phaser()503 public Phaser() { 504 this(null, 0); 505 } 506 507 /** 508 * Creates a new phaser with the given number of registered 509 * unarrived parties, no parent, and initial phase number 0. 510 * 511 * @param parties the number of parties required to advance to the 512 * next phase 513 * @throws IllegalArgumentException if parties less than zero 514 * or greater than the maximum number of parties supported 515 */ Phaser(int parties)516 public Phaser(int parties) { 517 this(null, parties); 518 } 519 520 /** 521 * Equivalent to {@link #Phaser(Phaser, int) Phaser(parent, 0)}. 522 * 523 * @param parent the parent phaser 524 */ Phaser(Phaser parent)525 public Phaser(Phaser parent) { 526 this(parent, 0); 527 } 528 529 /** 530 * Creates a new phaser with the given parent and number of 531 * registered unarrived parties. When the given parent is non-null 532 * and the given number of parties is greater than zero, this 533 * child phaser is registered with its parent. 534 * 535 * @param parent the parent phaser 536 * @param parties the number of parties required to advance to the 537 * next phase 538 * @throws IllegalArgumentException if parties less than zero 539 * or greater than the maximum number of parties supported 540 */ Phaser(Phaser parent, int parties)541 public Phaser(Phaser parent, int parties) { 542 if (parties >>> PARTIES_SHIFT != 0) 543 throw new IllegalArgumentException("Illegal number of parties"); 544 int phase = 0; 545 this.parent = parent; 546 if (parent != null) { 547 final Phaser root = parent.root; 548 this.root = root; 549 this.evenQ = root.evenQ; 550 this.oddQ = root.oddQ; 551 if (parties != 0) 552 phase = parent.doRegister(1); 553 } 554 else { 555 this.root = this; 556 this.evenQ = new AtomicReference<QNode>(); 557 this.oddQ = new AtomicReference<QNode>(); 558 } 559 this.state = (parties == 0) ? (long)EMPTY : 560 ((long)phase << PHASE_SHIFT) | 561 ((long)parties << PARTIES_SHIFT) | 562 ((long)parties); 563 } 564 565 /** 566 * Adds a new unarrived party to this phaser. If an ongoing 567 * invocation of {@link #onAdvance} is in progress, this method 568 * may await its completion before returning. If this phaser has 569 * a parent, and this phaser previously had no registered parties, 570 * this child phaser is also registered with its parent. If 571 * this phaser is terminated, the attempt to register has 572 * no effect, and a negative value is returned. 573 * 574 * @return the arrival phase number to which this registration 575 * applied. If this value is negative, then this phaser has 576 * terminated, in which case registration has no effect. 577 * @throws IllegalStateException if attempting to register more 578 * than the maximum supported number of parties 579 */ register()580 public int register() { 581 return doRegister(1); 582 } 583 584 /** 585 * Adds the given number of new unarrived parties to this phaser. 586 * If an ongoing invocation of {@link #onAdvance} is in progress, 587 * this method may await its completion before returning. If this 588 * phaser has a parent, and the given number of parties is greater 589 * than zero, and this phaser previously had no registered 590 * parties, this child phaser is also registered with its parent. 591 * If this phaser is terminated, the attempt to register has no 592 * effect, and a negative value is returned. 593 * 594 * @param parties the number of additional parties required to 595 * advance to the next phase 596 * @return the arrival phase number to which this registration 597 * applied. If this value is negative, then this phaser has 598 * terminated, in which case registration has no effect. 599 * @throws IllegalStateException if attempting to register more 600 * than the maximum supported number of parties 601 * @throws IllegalArgumentException if {@code parties < 0} 602 */ bulkRegister(int parties)603 public int bulkRegister(int parties) { 604 if (parties < 0) 605 throw new IllegalArgumentException(); 606 if (parties == 0) 607 return getPhase(); 608 return doRegister(parties); 609 } 610 611 /** 612 * Arrives at this phaser, without waiting for others to arrive. 613 * 614 * <p>It is a usage error for an unregistered party to invoke this 615 * method. However, this error may result in an {@code 616 * IllegalStateException} only upon some subsequent operation on 617 * this phaser, if ever. 618 * 619 * @return the arrival phase number, or a negative value if terminated 620 * @throws IllegalStateException if not terminated and the number 621 * of unarrived parties would become negative 622 */ arrive()623 public int arrive() { 624 return doArrive(ONE_ARRIVAL); 625 } 626 627 /** 628 * Arrives at this phaser and deregisters from it without waiting 629 * for others to arrive. Deregistration reduces the number of 630 * parties required to advance in future phases. If this phaser 631 * has a parent, and deregistration causes this phaser to have 632 * zero parties, this phaser is also deregistered from its parent. 633 * 634 * <p>It is a usage error for an unregistered party to invoke this 635 * method. However, this error may result in an {@code 636 * IllegalStateException} only upon some subsequent operation on 637 * this phaser, if ever. 638 * 639 * @return the arrival phase number, or a negative value if terminated 640 * @throws IllegalStateException if not terminated and the number 641 * of registered or unarrived parties would become negative 642 */ arriveAndDeregister()643 public int arriveAndDeregister() { 644 return doArrive(ONE_DEREGISTER); 645 } 646 647 /** 648 * Arrives at this phaser and awaits others. Equivalent in effect 649 * to {@code awaitAdvance(arrive())}. If you need to await with 650 * interruption or timeout, you can arrange this with an analogous 651 * construction using one of the other forms of the {@code 652 * awaitAdvance} method. If instead you need to deregister upon 653 * arrival, use {@code awaitAdvance(arriveAndDeregister())}. 654 * 655 * <p>It is a usage error for an unregistered party to invoke this 656 * method. However, this error may result in an {@code 657 * IllegalStateException} only upon some subsequent operation on 658 * this phaser, if ever. 659 * 660 * @return the arrival phase number, or the (negative) 661 * {@linkplain #getPhase() current phase} if terminated 662 * @throws IllegalStateException if not terminated and the number 663 * of unarrived parties would become negative 664 */ arriveAndAwaitAdvance()665 public int arriveAndAwaitAdvance() { 666 // Specialization of doArrive+awaitAdvance eliminating some reads/paths 667 final Phaser root = this.root; 668 for (;;) { 669 long s = (root == this) ? state : reconcileState(); 670 int phase = (int)(s >>> PHASE_SHIFT); 671 if (phase < 0) 672 return phase; 673 int counts = (int)s; 674 int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK); 675 if (unarrived <= 0) 676 throw new IllegalStateException(badArrive(s)); 677 if (STATE.compareAndSet(this, s, s -= ONE_ARRIVAL)) { 678 if (unarrived > 1) 679 return root.internalAwaitAdvance(phase, null); 680 if (root != this) 681 return parent.arriveAndAwaitAdvance(); 682 long n = s & PARTIES_MASK; // base of next state 683 int nextUnarrived = (int)n >>> PARTIES_SHIFT; 684 if (onAdvance(phase, nextUnarrived)) 685 n |= TERMINATION_BIT; 686 else if (nextUnarrived == 0) 687 n |= EMPTY; 688 else 689 n |= nextUnarrived; 690 int nextPhase = (phase + 1) & MAX_PHASE; 691 n |= (long)nextPhase << PHASE_SHIFT; 692 if (!STATE.compareAndSet(this, s, n)) 693 return (int)(state >>> PHASE_SHIFT); // terminated 694 releaseWaiters(phase); 695 return nextPhase; 696 } 697 } 698 } 699 700 /** 701 * Awaits the phase of this phaser to advance from the given phase 702 * value, returning immediately if the current phase is not equal 703 * to the given phase value or this phaser is terminated. 704 * 705 * @param phase an arrival phase number, or negative value if 706 * terminated; this argument is normally the value returned by a 707 * previous call to {@code arrive} or {@code arriveAndDeregister}. 708 * @return the next arrival phase number, or the argument if it is 709 * negative, or the (negative) {@linkplain #getPhase() current phase} 710 * if terminated 711 */ awaitAdvance(int phase)712 public int awaitAdvance(int phase) { 713 final Phaser root = this.root; 714 long s = (root == this) ? state : reconcileState(); 715 int p = (int)(s >>> PHASE_SHIFT); 716 if (phase < 0) 717 return phase; 718 if (p == phase) 719 return root.internalAwaitAdvance(phase, null); 720 return p; 721 } 722 723 /** 724 * Awaits the phase of this phaser to advance from the given phase 725 * value, throwing {@code InterruptedException} if interrupted 726 * while waiting, or returning immediately if the current phase is 727 * not equal to the given phase value or this phaser is 728 * terminated. 729 * 730 * @param phase an arrival phase number, or negative value if 731 * terminated; this argument is normally the value returned by a 732 * previous call to {@code arrive} or {@code arriveAndDeregister}. 733 * @return the next arrival phase number, or the argument if it is 734 * negative, or the (negative) {@linkplain #getPhase() current phase} 735 * if terminated 736 * @throws InterruptedException if thread interrupted while waiting 737 */ awaitAdvanceInterruptibly(int phase)738 public int awaitAdvanceInterruptibly(int phase) 739 throws InterruptedException { 740 final Phaser root = this.root; 741 long s = (root == this) ? state : reconcileState(); 742 int p = (int)(s >>> PHASE_SHIFT); 743 if (phase < 0) 744 return phase; 745 if (p == phase) { 746 QNode node = new QNode(this, phase, true, false, 0L); 747 p = root.internalAwaitAdvance(phase, node); 748 if (node.wasInterrupted) 749 throw new InterruptedException(); 750 } 751 return p; 752 } 753 754 /** 755 * Awaits the phase of this phaser to advance from the given phase 756 * value or the given timeout to elapse, throwing {@code 757 * InterruptedException} if interrupted while waiting, or 758 * returning immediately if the current phase is not equal to the 759 * given phase value or this phaser is terminated. 760 * 761 * @param phase an arrival phase number, or negative value if 762 * terminated; this argument is normally the value returned by a 763 * previous call to {@code arrive} or {@code arriveAndDeregister}. 764 * @param timeout how long to wait before giving up, in units of 765 * {@code unit} 766 * @param unit a {@code TimeUnit} determining how to interpret the 767 * {@code timeout} parameter 768 * @return the next arrival phase number, or the argument if it is 769 * negative, or the (negative) {@linkplain #getPhase() current phase} 770 * if terminated 771 * @throws InterruptedException if thread interrupted while waiting 772 * @throws TimeoutException if timed out while waiting 773 */ awaitAdvanceInterruptibly(int phase, long timeout, TimeUnit unit)774 public int awaitAdvanceInterruptibly(int phase, 775 long timeout, TimeUnit unit) 776 throws InterruptedException, TimeoutException { 777 long nanos = unit.toNanos(timeout); 778 final Phaser root = this.root; 779 long s = (root == this) ? state : reconcileState(); 780 int p = (int)(s >>> PHASE_SHIFT); 781 if (phase < 0) 782 return phase; 783 if (p == phase) { 784 QNode node = new QNode(this, phase, true, true, nanos); 785 p = root.internalAwaitAdvance(phase, node); 786 if (node.wasInterrupted) 787 throw new InterruptedException(); 788 else if (p == phase) 789 throw new TimeoutException(); 790 } 791 return p; 792 } 793 794 /** 795 * Forces this phaser to enter termination state. Counts of 796 * registered parties are unaffected. If this phaser is a member 797 * of a tiered set of phasers, then all of the phasers in the set 798 * are terminated. If this phaser is already terminated, this 799 * method has no effect. This method may be useful for 800 * coordinating recovery after one or more tasks encounter 801 * unexpected exceptions. 802 */ forceTermination()803 public void forceTermination() { 804 // Only need to change root state 805 final Phaser root = this.root; 806 long s; 807 while ((s = root.state) >= 0) { 808 if (STATE.compareAndSet(root, s, s | TERMINATION_BIT)) { 809 // signal all threads 810 releaseWaiters(0); // Waiters on evenQ 811 releaseWaiters(1); // Waiters on oddQ 812 return; 813 } 814 } 815 } 816 817 /** 818 * Returns the current phase number. The maximum phase number is 819 * {@code Integer.MAX_VALUE}, after which it restarts at 820 * zero. Upon termination, the phase number is negative, 821 * in which case the prevailing phase prior to termination 822 * may be obtained via {@code getPhase() + Integer.MIN_VALUE}. 823 * 824 * @return the phase number, or a negative value if terminated 825 */ getPhase()826 public final int getPhase() { 827 return (int)(root.state >>> PHASE_SHIFT); 828 } 829 830 /** 831 * Returns the number of parties registered at this phaser. 832 * 833 * @return the number of parties 834 */ getRegisteredParties()835 public int getRegisteredParties() { 836 return partiesOf(state); 837 } 838 839 /** 840 * Returns the number of registered parties that have arrived at 841 * the current phase of this phaser. If this phaser has terminated, 842 * the returned value is meaningless and arbitrary. 843 * 844 * @return the number of arrived parties 845 */ getArrivedParties()846 public int getArrivedParties() { 847 return arrivedOf(reconcileState()); 848 } 849 850 /** 851 * Returns the number of registered parties that have not yet 852 * arrived at the current phase of this phaser. If this phaser has 853 * terminated, the returned value is meaningless and arbitrary. 854 * 855 * @return the number of unarrived parties 856 */ getUnarrivedParties()857 public int getUnarrivedParties() { 858 return unarrivedOf(reconcileState()); 859 } 860 861 /** 862 * Returns the parent of this phaser, or {@code null} if none. 863 * 864 * @return the parent of this phaser, or {@code null} if none 865 */ getParent()866 public Phaser getParent() { 867 return parent; 868 } 869 870 /** 871 * Returns the root ancestor of this phaser, which is the same as 872 * this phaser if it has no parent. 873 * 874 * @return the root ancestor of this phaser 875 */ getRoot()876 public Phaser getRoot() { 877 return root; 878 } 879 880 /** 881 * Returns {@code true} if this phaser has been terminated. 882 * 883 * @return {@code true} if this phaser has been terminated 884 */ isTerminated()885 public boolean isTerminated() { 886 return root.state < 0L; 887 } 888 889 /** 890 * Overridable method to perform an action upon impending phase 891 * advance, and to control termination. This method is invoked 892 * upon arrival of the party advancing this phaser (when all other 893 * waiting parties are dormant). If this method returns {@code 894 * true}, this phaser will be set to a final termination state 895 * upon advance, and subsequent calls to {@link #isTerminated} 896 * will return true. Any (unchecked) Exception or Error thrown by 897 * an invocation of this method is propagated to the party 898 * attempting to advance this phaser, in which case no advance 899 * occurs. 900 * 901 * <p>The arguments to this method provide the state of the phaser 902 * prevailing for the current transition. The effects of invoking 903 * arrival, registration, and waiting methods on this phaser from 904 * within {@code onAdvance} are unspecified and should not be 905 * relied on. 906 * 907 * <p>If this phaser is a member of a tiered set of phasers, then 908 * {@code onAdvance} is invoked only for its root phaser on each 909 * advance. 910 * 911 * <p>To support the most common use cases, the default 912 * implementation of this method returns {@code true} when the 913 * number of registered parties has become zero as the result of a 914 * party invoking {@code arriveAndDeregister}. You can disable 915 * this behavior, thus enabling continuation upon future 916 * registrations, by overriding this method to always return 917 * {@code false}: 918 * 919 * <pre> {@code 920 * Phaser phaser = new Phaser() { 921 * protected boolean onAdvance(int phase, int parties) { return false; } 922 * }}</pre> 923 * 924 * @param phase the current phase number on entry to this method, 925 * before this phaser is advanced 926 * @param registeredParties the current number of registered parties 927 * @return {@code true} if this phaser should terminate 928 */ onAdvance(int phase, int registeredParties)929 protected boolean onAdvance(int phase, int registeredParties) { 930 return registeredParties == 0; 931 } 932 933 /** 934 * Returns a string identifying this phaser, as well as its 935 * state. The state, in brackets, includes the String {@code 936 * "phase = "} followed by the phase number, {@code "parties = "} 937 * followed by the number of registered parties, and {@code 938 * "arrived = "} followed by the number of arrived parties. 939 * 940 * @return a string identifying this phaser, as well as its state 941 */ toString()942 public String toString() { 943 return stateToString(reconcileState()); 944 } 945 946 /** 947 * Implementation of toString and string-based error messages. 948 */ stateToString(long s)949 private String stateToString(long s) { 950 return super.toString() + 951 "[phase = " + phaseOf(s) + 952 " parties = " + partiesOf(s) + 953 " arrived = " + arrivedOf(s) + "]"; 954 } 955 956 // Waiting mechanics 957 958 /** 959 * Removes and signals threads from queue for phase. 960 */ releaseWaiters(int phase)961 private void releaseWaiters(int phase) { 962 QNode q; // first element of queue 963 Thread t; // its thread 964 AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; 965 while ((q = head.get()) != null && 966 q.phase != (int)(root.state >>> PHASE_SHIFT)) { 967 if (head.compareAndSet(q, q.next) && 968 (t = q.thread) != null) { 969 q.thread = null; 970 LockSupport.unpark(t); 971 } 972 } 973 } 974 975 /** 976 * Variant of releaseWaiters that additionally tries to remove any 977 * nodes no longer waiting for advance due to timeout or 978 * interrupt. Currently, nodes are removed only if they are at 979 * head of queue, which suffices to reduce memory footprint in 980 * most usages. 981 * 982 * @return current phase on exit 983 */ abortWait(int phase)984 private int abortWait(int phase) { 985 AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; 986 for (;;) { 987 Thread t; 988 QNode q = head.get(); 989 int p = (int)(root.state >>> PHASE_SHIFT); 990 if (q == null || ((t = q.thread) != null && q.phase == p)) 991 return p; 992 if (head.compareAndSet(q, q.next) && t != null) { 993 q.thread = null; 994 LockSupport.unpark(t); 995 } 996 } 997 } 998 999 /** The number of CPUs, for spin control */ 1000 private static final int NCPU = Runtime.getRuntime().availableProcessors(); 1001 1002 /** 1003 * The number of times to spin before blocking while waiting for 1004 * advance, per arrival while waiting. On multiprocessors, fully 1005 * blocking and waking up a large number of threads all at once is 1006 * usually a very slow process, so we use rechargeable spins to 1007 * avoid it when threads regularly arrive: When a thread in 1008 * internalAwaitAdvance notices another arrival before blocking, 1009 * and there appear to be enough CPUs available, it spins 1010 * SPINS_PER_ARRIVAL more times before blocking. The value trades 1011 * off good-citizenship vs big unnecessary slowdowns. 1012 */ 1013 static final int SPINS_PER_ARRIVAL = (NCPU < 2) ? 1 : 1 << 8; 1014 1015 /** 1016 * Possibly blocks and waits for phase to advance unless aborted. 1017 * Call only on root phaser. 1018 * 1019 * @param phase current phase 1020 * @param node if non-null, the wait node to track interrupt and timeout; 1021 * if null, denotes noninterruptible wait 1022 * @return current phase 1023 */ internalAwaitAdvance(int phase, QNode node)1024 private int internalAwaitAdvance(int phase, QNode node) { 1025 // assert root == this; 1026 releaseWaiters(phase-1); // ensure old queue clean 1027 boolean queued = false; // true when node is enqueued 1028 int lastUnarrived = 0; // to increase spins upon change 1029 int spins = SPINS_PER_ARRIVAL; 1030 long s; 1031 int p; 1032 while ((p = (int)((s = state) >>> PHASE_SHIFT)) == phase) { 1033 if (node == null) { // spinning in noninterruptible mode 1034 int unarrived = (int)s & UNARRIVED_MASK; 1035 if (unarrived != lastUnarrived && 1036 (lastUnarrived = unarrived) < NCPU) 1037 spins += SPINS_PER_ARRIVAL; 1038 boolean interrupted = Thread.interrupted(); 1039 if (interrupted || --spins < 0) { // need node to record intr 1040 node = new QNode(this, phase, false, false, 0L); 1041 node.wasInterrupted = interrupted; 1042 } 1043 else 1044 Thread.onSpinWait(); 1045 } 1046 else if (node.isReleasable()) // done or aborted 1047 break; 1048 else if (!queued) { // push onto queue 1049 AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; 1050 QNode q = node.next = head.get(); 1051 if ((q == null || q.phase == phase) && 1052 (int)(state >>> PHASE_SHIFT) == phase) // avoid stale enq 1053 queued = head.compareAndSet(q, node); 1054 } 1055 else { 1056 try { 1057 ForkJoinPool.managedBlock(node); 1058 } catch (InterruptedException cantHappen) { 1059 node.wasInterrupted = true; 1060 } 1061 } 1062 } 1063 1064 if (node != null) { 1065 if (node.thread != null) 1066 node.thread = null; // avoid need for unpark() 1067 if (node.wasInterrupted && !node.interruptible) 1068 Thread.currentThread().interrupt(); 1069 if (p == phase && (p = (int)(state >>> PHASE_SHIFT)) == phase) 1070 return abortWait(phase); // possibly clean up on abort 1071 } 1072 releaseWaiters(phase); 1073 return p; 1074 } 1075 1076 /** 1077 * Wait nodes for Treiber stack representing wait queue. 1078 */ 1079 static final class QNode implements ForkJoinPool.ManagedBlocker { 1080 final Phaser phaser; 1081 final int phase; 1082 final boolean interruptible; 1083 final boolean timed; 1084 boolean wasInterrupted; 1085 long nanos; 1086 final long deadline; 1087 volatile Thread thread; // nulled to cancel wait 1088 QNode next; 1089 QNode(Phaser phaser, int phase, boolean interruptible, boolean timed, long nanos)1090 QNode(Phaser phaser, int phase, boolean interruptible, 1091 boolean timed, long nanos) { 1092 this.phaser = phaser; 1093 this.phase = phase; 1094 this.interruptible = interruptible; 1095 this.nanos = nanos; 1096 this.timed = timed; 1097 this.deadline = timed ? System.nanoTime() + nanos : 0L; 1098 thread = Thread.currentThread(); 1099 } 1100 isReleasable()1101 public boolean isReleasable() { 1102 if (thread == null) 1103 return true; 1104 if (phaser.getPhase() != phase) { 1105 thread = null; 1106 return true; 1107 } 1108 if (Thread.interrupted()) 1109 wasInterrupted = true; 1110 if (wasInterrupted && interruptible) { 1111 thread = null; 1112 return true; 1113 } 1114 if (timed && 1115 (nanos <= 0L || (nanos = deadline - System.nanoTime()) <= 0L)) { 1116 thread = null; 1117 return true; 1118 } 1119 return false; 1120 } 1121 block()1122 public boolean block() { 1123 while (!isReleasable()) { 1124 if (timed) 1125 LockSupport.parkNanos(this, nanos); 1126 else 1127 LockSupport.park(this); 1128 } 1129 return true; 1130 } 1131 } 1132 1133 // VarHandle mechanics 1134 private static final VarHandle STATE; 1135 static { 1136 try { 1137 MethodHandles.Lookup l = MethodHandles.lookup(); 1138 STATE = l.findVarHandle(Phaser.class, "state", long.class); 1139 } catch (ReflectiveOperationException e) { 1140 throw new ExceptionInInitializerError(e); 1141 } 1142 1143 // Reduce the risk of rare disastrous classloading in first call to 1144 // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773 1145 Class<?> ensureLoaded = LockSupport.class; 1146 } 1147 } 1148