AbstractQueuedSynchronizer.java revision d7806194431c008c2ae2b546b988c0e819f7b47b
1/* 2 * Written by Doug Lea with assistance from members of JCP JSR-166 3 * Expert Group and released to the public domain, as explained at 4 * http://creativecommons.org/licenses/publicdomain 5 */ 6 7package java.util.concurrent.locks; 8import java.util.*; 9import java.util.concurrent.*; 10import java.util.concurrent.atomic.*; 11import sun.misc.Unsafe; 12 13// BEGIN android-note 14// Use older class level documentation to not @link to hasQueuedPredecessors 15// END android-changed 16 17/** 18 * Provides a framework for implementing blocking locks and related 19 * synchronizers (semaphores, events, etc) that rely on 20 * first-in-first-out (FIFO) wait queues. This class is designed to 21 * be a useful basis for most kinds of synchronizers that rely on a 22 * single atomic <tt>int</tt> value to represent state. Subclasses 23 * must define the protected methods that change this state, and which 24 * define what that state means in terms of this object being acquired 25 * or released. Given these, the other methods in this class carry 26 * out all queuing and blocking mechanics. Subclasses can maintain 27 * other state fields, but only the atomically updated <tt>int</tt> 28 * value manipulated using methods {@link #getState}, {@link 29 * #setState} and {@link #compareAndSetState} is tracked with respect 30 * to synchronization. 31 * 32 * <p>Subclasses should be defined as non-public internal helper 33 * classes that are used to implement the synchronization properties 34 * of their enclosing class. Class 35 * <tt>AbstractQueuedSynchronizer</tt> does not implement any 36 * synchronization interface. Instead it defines methods such as 37 * {@link #acquireInterruptibly} that can be invoked as 38 * appropriate by concrete locks and related synchronizers to 39 * implement their public methods. 40 * 41 * <p>This class supports either or both a default <em>exclusive</em> 42 * mode and a <em>shared</em> mode. When acquired in exclusive mode, 43 * attempted acquires by other threads cannot succeed. Shared mode 44 * acquires by multiple threads may (but need not) succeed. This class 45 * does not "understand" these differences except in the 46 * mechanical sense that when a shared mode acquire succeeds, the next 47 * waiting thread (if one exists) must also determine whether it can 48 * acquire as well. Threads waiting in the different modes share the 49 * same FIFO queue. Usually, implementation subclasses support only 50 * one of these modes, but both can come into play for example in a 51 * {@link ReadWriteLock}. Subclasses that support only exclusive or 52 * only shared modes need not define the methods supporting the unused mode. 53 * 54 * <p>This class defines a nested {@link ConditionObject} class that 55 * can be used as a {@link Condition} implementation by subclasses 56 * supporting exclusive mode for which method {@link 57 * #isHeldExclusively} reports whether synchronization is exclusively 58 * held with respect to the current thread, method {@link #release} 59 * invoked with the current {@link #getState} value fully releases 60 * this object, and {@link #acquire}, given this saved state value, 61 * eventually restores this object to its previous acquired state. No 62 * <tt>AbstractQueuedSynchronizer</tt> method otherwise creates such a 63 * condition, so if this constraint cannot be met, do not use it. The 64 * behavior of {@link ConditionObject} depends of course on the 65 * semantics of its synchronizer implementation. 66 * 67 * <p>This class provides inspection, instrumentation, and monitoring 68 * methods for the internal queue, as well as similar methods for 69 * condition objects. These can be exported as desired into classes 70 * using an <tt>AbstractQueuedSynchronizer</tt> for their 71 * synchronization mechanics. 72 * 73 * <p>Serialization of this class stores only the underlying atomic 74 * integer maintaining state, so deserialized objects have empty 75 * thread queues. Typical subclasses requiring serializability will 76 * define a <tt>readObject</tt> method that restores this to a known 77 * initial state upon deserialization. 78 * 79 * <h3>Usage</h3> 80 * 81 * <p>To use this class as the basis of a synchronizer, redefine the 82 * following methods, as applicable, by inspecting and/or modifying 83 * the synchronization state using {@link #getState}, {@link 84 * #setState} and/or {@link #compareAndSetState}: 85 * 86 * <ul> 87 * <li> {@link #tryAcquire} 88 * <li> {@link #tryRelease} 89 * <li> {@link #tryAcquireShared} 90 * <li> {@link #tryReleaseShared} 91 * <li> {@link #isHeldExclusively} 92 *</ul> 93 * 94 * Each of these methods by default throws {@link 95 * UnsupportedOperationException}. Implementations of these methods 96 * must be internally thread-safe, and should in general be short and 97 * not block. Defining these methods is the <em>only</em> supported 98 * means of using this class. All other methods are declared 99 * <tt>final</tt> because they cannot be independently varied. 100 * 101 * <p>You may also find the inherited methods from {@link 102 * AbstractOwnableSynchronizer} useful to keep track of the thread 103 * owning an exclusive synchronizer. You are encouraged to use them 104 * -- this enables monitoring and diagnostic tools to assist users in 105 * determining which threads hold locks. 106 * 107 * <p>Even though this class is based on an internal FIFO queue, it 108 * does not automatically enforce FIFO acquisition policies. The core 109 * of exclusive synchronization takes the form: 110 * 111 * <pre> 112 * Acquire: 113 * while (!tryAcquire(arg)) { 114 * <em>enqueue thread if it is not already queued</em>; 115 * <em>possibly block current thread</em>; 116 * } 117 * 118 * Release: 119 * if (tryRelease(arg)) 120 * <em>unblock the first queued thread</em>; 121 * </pre> 122 * 123 * (Shared mode is similar but may involve cascading signals.) 124 * 125 * <p><a name="barging">Because checks in acquire are invoked before 126 * enqueuing, a newly acquiring thread may <em>barge</em> ahead of 127 * others that are blocked and queued. However, you can, if desired, 128 * define <tt>tryAcquire</tt> and/or <tt>tryAcquireShared</tt> to 129 * disable barging by internally invoking one or more of the inspection 130 * methods. In particular, a strict FIFO lock can define 131 * <tt>tryAcquire</tt> to immediately return <tt>false</tt> if {@link 132 * #getFirstQueuedThread} does not return the current thread. A 133 * normally preferable non-strict fair version can immediately return 134 * <tt>false</tt> only if {@link #hasQueuedThreads} returns 135 * <tt>true</tt> and <tt>getFirstQueuedThread</tt> is not the current 136 * thread; or equivalently, that <tt>getFirstQueuedThread</tt> is both 137 * non-null and not the current thread. Further variations are 138 * possible. 139 * 140 * <p>Throughput and scalability are generally highest for the 141 * default barging (also known as <em>greedy</em>, 142 * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy. 143 * While this is not guaranteed to be fair or starvation-free, earlier 144 * queued threads are allowed to recontend before later queued 145 * threads, and each recontention has an unbiased chance to succeed 146 * against incoming threads. Also, while acquires do not 147 * "spin" in the usual sense, they may perform multiple 148 * invocations of <tt>tryAcquire</tt> interspersed with other 149 * computations before blocking. This gives most of the benefits of 150 * spins when exclusive synchronization is only briefly held, without 151 * most of the liabilities when it isn't. If so desired, you can 152 * augment this by preceding calls to acquire methods with 153 * "fast-path" checks, possibly prechecking {@link #hasContended} 154 * and/or {@link #hasQueuedThreads} to only do so if the synchronizer 155 * is likely not to be contended. 156 * 157 * <p>This class provides an efficient and scalable basis for 158 * synchronization in part by specializing its range of use to 159 * synchronizers that can rely on <tt>int</tt> state, acquire, and 160 * release parameters, and an internal FIFO wait queue. When this does 161 * not suffice, you can build synchronizers from a lower level using 162 * {@link java.util.concurrent.atomic atomic} classes, your own custom 163 * {@link java.util.Queue} classes, and {@link LockSupport} blocking 164 * support. 165 * 166 * <h3>Usage Examples</h3> 167 * 168 * <p>Here is a non-reentrant mutual exclusion lock class that uses 169 * the value zero to represent the unlocked state, and one to 170 * represent the locked state. While a non-reentrant lock 171 * does not strictly require recording of the current owner 172 * thread, this class does so anyway to make usage easier to monitor. 173 * It also supports conditions and exposes 174 * one of the instrumentation methods: 175 * 176 * <pre> 177 * class Mutex implements Lock, java.io.Serializable { 178 * 179 * // Our internal helper class 180 * private static class Sync extends AbstractQueuedSynchronizer { 181 * // Report whether in locked state 182 * protected boolean isHeldExclusively() { 183 * return getState() == 1; 184 * } 185 * 186 * // Acquire the lock if state is zero 187 * public boolean tryAcquire(int acquires) { 188 * assert acquires == 1; // Otherwise unused 189 * if (compareAndSetState(0, 1)) { 190 * setExclusiveOwnerThread(Thread.currentThread()); 191 * return true; 192 * } 193 * return false; 194 * } 195 * 196 * // Release the lock by setting state to zero 197 * protected boolean tryRelease(int releases) { 198 * assert releases == 1; // Otherwise unused 199 * if (getState() == 0) throw new IllegalMonitorStateException(); 200 * setExclusiveOwnerThread(null); 201 * setState(0); 202 * return true; 203 * } 204 * 205 * // Provide a Condition 206 * Condition newCondition() { return new ConditionObject(); } 207 * 208 * // Deserialize properly 209 * private void readObject(ObjectInputStream s) 210 * throws IOException, ClassNotFoundException { 211 * s.defaultReadObject(); 212 * setState(0); // reset to unlocked state 213 * } 214 * } 215 * 216 * // The sync object does all the hard work. We just forward to it. 217 * private final Sync sync = new Sync(); 218 * 219 * public void lock() { sync.acquire(1); } 220 * public boolean tryLock() { return sync.tryAcquire(1); } 221 * public void unlock() { sync.release(1); } 222 * public Condition newCondition() { return sync.newCondition(); } 223 * public boolean isLocked() { return sync.isHeldExclusively(); } 224 * public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); } 225 * public void lockInterruptibly() throws InterruptedException { 226 * sync.acquireInterruptibly(1); 227 * } 228 * public boolean tryLock(long timeout, TimeUnit unit) 229 * throws InterruptedException { 230 * return sync.tryAcquireNanos(1, unit.toNanos(timeout)); 231 * } 232 * } 233 * </pre> 234 * 235 * <p>Here is a latch class that is like a {@link CountDownLatch} 236 * except that it only requires a single <tt>signal</tt> to 237 * fire. Because a latch is non-exclusive, it uses the <tt>shared</tt> 238 * acquire and release methods. 239 * 240 * <pre> 241 * class BooleanLatch { 242 * 243 * private static class Sync extends AbstractQueuedSynchronizer { 244 * boolean isSignalled() { return getState() != 0; } 245 * 246 * protected int tryAcquireShared(int ignore) { 247 * return isSignalled() ? 1 : -1; 248 * } 249 * 250 * protected boolean tryReleaseShared(int ignore) { 251 * setState(1); 252 * return true; 253 * } 254 * } 255 * 256 * private final Sync sync = new Sync(); 257 * public boolean isSignalled() { return sync.isSignalled(); } 258 * public void signal() { sync.releaseShared(1); } 259 * public void await() throws InterruptedException { 260 * sync.acquireSharedInterruptibly(1); 261 * } 262 * } 263 * </pre> 264 * 265 * @since 1.5 266 * @author Doug Lea 267 */ 268public abstract class AbstractQueuedSynchronizer 269 extends AbstractOwnableSynchronizer 270 implements java.io.Serializable { 271 272 private static final long serialVersionUID = 7373984972572414691L; 273 274 /** 275 * Creates a new <tt>AbstractQueuedSynchronizer</tt> instance 276 * with initial synchronization state of zero. 277 */ 278 protected AbstractQueuedSynchronizer() { } 279 280 /** 281 * Wait queue node class. 282 * 283 * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and 284 * Hagersten) lock queue. CLH locks are normally used for 285 * spinlocks. We instead use them for blocking synchronizers, but 286 * use the same basic tactic of holding some of the control 287 * information about a thread in the predecessor of its node. A 288 * "status" field in each node keeps track of whether a thread 289 * should block. A node is signalled when its predecessor 290 * releases. Each node of the queue otherwise serves as a 291 * specific-notification-style monitor holding a single waiting 292 * thread. The status field does NOT control whether threads are 293 * granted locks etc though. A thread may try to acquire if it is 294 * first in the queue. But being first does not guarantee success; 295 * it only gives the right to contend. So the currently released 296 * contender thread may need to rewait. 297 * 298 * <p>To enqueue into a CLH lock, you atomically splice it in as new 299 * tail. To dequeue, you just set the head field. 300 * <pre> 301 * +------+ prev +-----+ +-----+ 302 * head | | <---- | | <---- | | tail 303 * +------+ +-----+ +-----+ 304 * </pre> 305 * 306 * <p>Insertion into a CLH queue requires only a single atomic 307 * operation on "tail", so there is a simple atomic point of 308 * demarcation from unqueued to queued. Similarly, dequeing 309 * involves only updating the "head". However, it takes a bit 310 * more work for nodes to determine who their successors are, 311 * in part to deal with possible cancellation due to timeouts 312 * and interrupts. 313 * 314 * <p>The "prev" links (not used in original CLH locks), are mainly 315 * needed to handle cancellation. If a node is cancelled, its 316 * successor is (normally) relinked to a non-cancelled 317 * predecessor. For explanation of similar mechanics in the case 318 * of spin locks, see the papers by Scott and Scherer at 319 * http://www.cs.rochester.edu/u/scott/synchronization/ 320 * 321 * <p>We also use "next" links to implement blocking mechanics. 322 * The thread id for each node is kept in its own node, so a 323 * predecessor signals the next node to wake up by traversing 324 * next link to determine which thread it is. Determination of 325 * successor must avoid races with newly queued nodes to set 326 * the "next" fields of their predecessors. This is solved 327 * when necessary by checking backwards from the atomically 328 * updated "tail" when a node's successor appears to be null. 329 * (Or, said differently, the next-links are an optimization 330 * so that we don't usually need a backward scan.) 331 * 332 * <p>Cancellation introduces some conservatism to the basic 333 * algorithms. Since we must poll for cancellation of other 334 * nodes, we can miss noticing whether a cancelled node is 335 * ahead or behind us. This is dealt with by always unparking 336 * successors upon cancellation, allowing them to stabilize on 337 * a new predecessor, unless we can identify an uncancelled 338 * predecessor who will carry this responsibility. 339 * 340 * <p>CLH queues need a dummy header node to get started. But 341 * we don't create them on construction, because it would be wasted 342 * effort if there is never contention. Instead, the node 343 * is constructed and head and tail pointers are set upon first 344 * contention. 345 * 346 * <p>Threads waiting on Conditions use the same nodes, but 347 * use an additional link. Conditions only need to link nodes 348 * in simple (non-concurrent) linked queues because they are 349 * only accessed when exclusively held. Upon await, a node is 350 * inserted into a condition queue. Upon signal, the node is 351 * transferred to the main queue. A special value of status 352 * field is used to mark which queue a node is on. 353 * 354 * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill 355 * Scherer and Michael Scott, along with members of JSR-166 356 * expert group, for helpful ideas, discussions, and critiques 357 * on the design of this class. 358 */ 359 static final class Node { 360 /** Marker to indicate a node is waiting in shared mode */ 361 static final Node SHARED = new Node(); 362 /** Marker to indicate a node is waiting in exclusive mode */ 363 static final Node EXCLUSIVE = null; 364 365 /** waitStatus value to indicate thread has cancelled */ 366 static final int CANCELLED = 1; 367 /** waitStatus value to indicate successor's thread needs unparking */ 368 static final int SIGNAL = -1; 369 /** waitStatus value to indicate thread is waiting on condition */ 370 static final int CONDITION = -2; 371 /** 372 * waitStatus value to indicate the next acquireShared should 373 * unconditionally propagate 374 */ 375 static final int PROPAGATE = -3; 376 377 /** 378 * Status field, taking on only the values: 379 * SIGNAL: The successor of this node is (or will soon be) 380 * blocked (via park), so the current node must 381 * unpark its successor when it releases or 382 * cancels. To avoid races, acquire methods must 383 * first indicate they need a signal, 384 * then retry the atomic acquire, and then, 385 * on failure, block. 386 * CANCELLED: This node is cancelled due to timeout or interrupt. 387 * Nodes never leave this state. In particular, 388 * a thread with cancelled node never again blocks. 389 * CONDITION: This node is currently on a condition queue. 390 * It will not be used as a sync queue node 391 * until transferred, at which time the status 392 * will be set to 0. (Use of this value here has 393 * nothing to do with the other uses of the 394 * field, but simplifies mechanics.) 395 * PROPAGATE: A releaseShared should be propagated to other 396 * nodes. This is set (for head node only) in 397 * doReleaseShared to ensure propagation 398 * continues, even if other operations have 399 * since intervened. 400 * 0: None of the above 401 * 402 * The values are arranged numerically to simplify use. 403 * Non-negative values mean that a node doesn't need to 404 * signal. So, most code doesn't need to check for particular 405 * values, just for sign. 406 * 407 * The field is initialized to 0 for normal sync nodes, and 408 * CONDITION for condition nodes. It is modified using CAS 409 * (or when possible, unconditional volatile writes). 410 */ 411 volatile int waitStatus; 412 413 /** 414 * Link to predecessor node that current node/thread relies on 415 * for checking waitStatus. Assigned during enqueing, and nulled 416 * out (for sake of GC) only upon dequeuing. Also, upon 417 * cancellation of a predecessor, we short-circuit while 418 * finding a non-cancelled one, which will always exist 419 * because the head node is never cancelled: A node becomes 420 * head only as a result of successful acquire. A 421 * cancelled thread never succeeds in acquiring, and a thread only 422 * cancels itself, not any other node. 423 */ 424 volatile Node prev; 425 426 /** 427 * Link to the successor node that the current node/thread 428 * unparks upon release. Assigned during enqueuing, adjusted 429 * when bypassing cancelled predecessors, and nulled out (for 430 * sake of GC) when dequeued. The enq operation does not 431 * assign next field of a predecessor until after attachment, 432 * so seeing a null next field does not necessarily mean that 433 * node is at end of queue. However, if a next field appears 434 * to be null, we can scan prev's from the tail to 435 * double-check. The next field of cancelled nodes is set to 436 * point to the node itself instead of null, to make life 437 * easier for isOnSyncQueue. 438 */ 439 volatile Node next; 440 441 /** 442 * The thread that enqueued this node. Initialized on 443 * construction and nulled out after use. 444 */ 445 volatile Thread thread; 446 447 /** 448 * Link to next node waiting on condition, or the special 449 * value SHARED. Because condition queues are accessed only 450 * when holding in exclusive mode, we just need a simple 451 * linked queue to hold nodes while they are waiting on 452 * conditions. They are then transferred to the queue to 453 * re-acquire. And because conditions can only be exclusive, 454 * we save a field by using special value to indicate shared 455 * mode. 456 */ 457 Node nextWaiter; 458 459 /** 460 * Returns true if node is waiting in shared mode 461 */ 462 final boolean isShared() { 463 return nextWaiter == SHARED; 464 } 465 466 /** 467 * Returns previous node, or throws NullPointerException if null. 468 * Use when predecessor cannot be null. The null check could 469 * be elided, but is present to help the VM. 470 * 471 * @return the predecessor of this node 472 */ 473 final Node predecessor() throws NullPointerException { 474 Node p = prev; 475 if (p == null) 476 throw new NullPointerException(); 477 else 478 return p; 479 } 480 481 Node() { // Used to establish initial head or SHARED marker 482 } 483 484 Node(Thread thread, Node mode) { // Used by addWaiter 485 this.nextWaiter = mode; 486 this.thread = thread; 487 } 488 489 Node(Thread thread, int waitStatus) { // Used by Condition 490 this.waitStatus = waitStatus; 491 this.thread = thread; 492 } 493 } 494 495 /** 496 * Head of the wait queue, lazily initialized. Except for 497 * initialization, it is modified only via method setHead. Note: 498 * If head exists, its waitStatus is guaranteed not to be 499 * CANCELLED. 500 */ 501 private transient volatile Node head; 502 503 /** 504 * Tail of the wait queue, lazily initialized. Modified only via 505 * method enq to add new wait node. 506 */ 507 private transient volatile Node tail; 508 509 /** 510 * The synchronization state. 511 */ 512 private volatile int state; 513 514 /** 515 * Returns the current value of synchronization state. 516 * This operation has memory semantics of a <tt>volatile</tt> read. 517 * @return current state value 518 */ 519 protected final int getState() { 520 return state; 521 } 522 523 /** 524 * Sets the value of synchronization state. 525 * This operation has memory semantics of a <tt>volatile</tt> write. 526 * @param newState the new state value 527 */ 528 protected final void setState(int newState) { 529 state = newState; 530 } 531 532 /** 533 * Atomically sets synchronization state to the given updated 534 * value if the current state value equals the expected value. 535 * This operation has memory semantics of a <tt>volatile</tt> read 536 * and write. 537 * 538 * @param expect the expected value 539 * @param update the new value 540 * @return true if successful. False return indicates that the actual 541 * value was not equal to the expected value. 542 */ 543 protected final boolean compareAndSetState(int expect, int update) { 544 // See below for intrinsics setup to support this 545 return unsafe.compareAndSwapInt(this, stateOffset, expect, update); 546 } 547 548 // Queuing utilities 549 550 /** 551 * The number of nanoseconds for which it is faster to spin 552 * rather than to use timed park. A rough estimate suffices 553 * to improve responsiveness with very short timeouts. 554 */ 555 static final long spinForTimeoutThreshold = 1000L; 556 557 /** 558 * Inserts node into queue, initializing if necessary. See picture above. 559 * @param node the node to insert 560 * @return node's predecessor 561 */ 562 private Node enq(final Node node) { 563 for (;;) { 564 Node t = tail; 565 if (t == null) { // Must initialize 566 if (compareAndSetHead(new Node())) 567 tail = head; 568 } else { 569 node.prev = t; 570 if (compareAndSetTail(t, node)) { 571 t.next = node; 572 return t; 573 } 574 } 575 } 576 } 577 578 /** 579 * Creates and enqueues node for current thread and given mode. 580 * 581 * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared 582 * @return the new node 583 */ 584 private Node addWaiter(Node mode) { 585 Node node = new Node(Thread.currentThread(), mode); 586 // Try the fast path of enq; backup to full enq on failure 587 Node pred = tail; 588 if (pred != null) { 589 node.prev = pred; 590 if (compareAndSetTail(pred, node)) { 591 pred.next = node; 592 return node; 593 } 594 } 595 enq(node); 596 return node; 597 } 598 599 /** 600 * Sets head of queue to be node, thus dequeuing. Called only by 601 * acquire methods. Also nulls out unused fields for sake of GC 602 * and to suppress unnecessary signals and traversals. 603 * 604 * @param node the node 605 */ 606 private void setHead(Node node) { 607 head = node; 608 node.thread = null; 609 node.prev = null; 610 } 611 612 /** 613 * Wakes up node's successor, if one exists. 614 * 615 * @param node the node 616 */ 617 private void unparkSuccessor(Node node) { 618 /* 619 * If status is negative (i.e., possibly needing signal) try 620 * to clear in anticipation of signalling. It is OK if this 621 * fails or if status is changed by waiting thread. 622 */ 623 int ws = node.waitStatus; 624 if (ws < 0) 625 compareAndSetWaitStatus(node, ws, 0); 626 627 /* 628 * Thread to unpark is held in successor, which is normally 629 * just the next node. But if cancelled or apparently null, 630 * traverse backwards from tail to find the actual 631 * non-cancelled successor. 632 */ 633 Node s = node.next; 634 if (s == null || s.waitStatus > 0) { 635 s = null; 636 for (Node t = tail; t != null && t != node; t = t.prev) 637 if (t.waitStatus <= 0) 638 s = t; 639 } 640 if (s != null) 641 LockSupport.unpark(s.thread); 642 } 643 644 /** 645 * Release action for shared mode -- signal successor and ensure 646 * propagation. (Note: For exclusive mode, release just amounts 647 * to calling unparkSuccessor of head if it needs signal.) 648 */ 649 private void doReleaseShared() { 650 /* 651 * Ensure that a release propagates, even if there are other 652 * in-progress acquires/releases. This proceeds in the usual 653 * way of trying to unparkSuccessor of head if it needs 654 * signal. But if it does not, status is set to PROPAGATE to 655 * ensure that upon release, propagation continues. 656 * Additionally, we must loop in case a new node is added 657 * while we are doing this. Also, unlike other uses of 658 * unparkSuccessor, we need to know if CAS to reset status 659 * fails, if so rechecking. 660 */ 661 for (;;) { 662 Node h = head; 663 if (h != null && h != tail) { 664 int ws = h.waitStatus; 665 if (ws == Node.SIGNAL) { 666 if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) 667 continue; // loop to recheck cases 668 unparkSuccessor(h); 669 } 670 else if (ws == 0 && 671 !compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) 672 continue; // loop on failed CAS 673 } 674 if (h == head) // loop if head changed 675 break; 676 } 677 } 678 679 /** 680 * Sets head of queue, and checks if successor may be waiting 681 * in shared mode, if so propagating if either propagate > 0 or 682 * PROPAGATE status was set. 683 * 684 * @param node the node 685 * @param propagate the return value from a tryAcquireShared 686 */ 687 private void setHeadAndPropagate(Node node, int propagate) { 688 Node h = head; // Record old head for check below 689 setHead(node); 690 /* 691 * Try to signal next queued node if: 692 * Propagation was indicated by caller, 693 * or was recorded (as h.waitStatus) by a previous operation 694 * (note: this uses sign-check of waitStatus because 695 * PROPAGATE status may transition to SIGNAL.) 696 * and 697 * The next node is waiting in shared mode, 698 * or we don't know, because it appears null 699 * 700 * The conservatism in both of these checks may cause 701 * unnecessary wake-ups, but only when there are multiple 702 * racing acquires/releases, so most need signals now or soon 703 * anyway. 704 */ 705 if (propagate > 0 || h == null || h.waitStatus < 0) { 706 Node s = node.next; 707 if (s == null || s.isShared()) 708 doReleaseShared(); 709 } 710 } 711 712 // Utilities for various versions of acquire 713 714 /** 715 * Cancels an ongoing attempt to acquire. 716 * 717 * @param node the node 718 */ 719 private void cancelAcquire(Node node) { 720 // Ignore if node doesn't exist 721 if (node == null) 722 return; 723 724 node.thread = null; 725 726 // Skip cancelled predecessors 727 Node pred = node.prev; 728 while (pred.waitStatus > 0) 729 node.prev = pred = pred.prev; 730 731 // predNext is the apparent node to unsplice. CASes below will 732 // fail if not, in which case, we lost race vs another cancel 733 // or signal, so no further action is necessary. 734 Node predNext = pred.next; 735 736 // Can use unconditional write instead of CAS here. 737 // After this atomic step, other Nodes can skip past us. 738 // Before, we are free of interference from other threads. 739 node.waitStatus = Node.CANCELLED; 740 741 // If we are the tail, remove ourselves. 742 if (node == tail && compareAndSetTail(node, pred)) { 743 compareAndSetNext(pred, predNext, null); 744 } else { 745 // If successor needs signal, try to set pred's next-link 746 // so it will get one. Otherwise wake it up to propagate. 747 int ws; 748 if (pred != head && 749 ((ws = pred.waitStatus) == Node.SIGNAL || 750 (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) && 751 pred.thread != null) { 752 Node next = node.next; 753 if (next != null && next.waitStatus <= 0) 754 compareAndSetNext(pred, predNext, next); 755 } else { 756 unparkSuccessor(node); 757 } 758 759 node.next = node; // help GC 760 } 761 } 762 763 /** 764 * Checks and updates status for a node that failed to acquire. 765 * Returns true if thread should block. This is the main signal 766 * control in all acquire loops. Requires that pred == node.prev 767 * 768 * @param pred node's predecessor holding status 769 * @param node the node 770 * @return {@code true} if thread should block 771 */ 772 private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) { 773 int ws = pred.waitStatus; 774 if (ws == Node.SIGNAL) 775 /* 776 * This node has already set status asking a release 777 * to signal it, so it can safely park. 778 */ 779 return true; 780 if (ws > 0) { 781 /* 782 * Predecessor was cancelled. Skip over predecessors and 783 * indicate retry. 784 */ 785 do { 786 node.prev = pred = pred.prev; 787 } while (pred.waitStatus > 0); 788 pred.next = node; 789 } else { 790 /* 791 * waitStatus must be 0 or PROPAGATE. Indicate that we 792 * need a signal, but don't park yet. Caller will need to 793 * retry to make sure it cannot acquire before parking. 794 */ 795 compareAndSetWaitStatus(pred, ws, Node.SIGNAL); 796 } 797 return false; 798 } 799 800 /** 801 * Convenience method to interrupt current thread. 802 */ 803 private static void selfInterrupt() { 804 Thread.currentThread().interrupt(); 805 } 806 807 /** 808 * Convenience method to park and then check if interrupted 809 * 810 * @return {@code true} if interrupted 811 */ 812 private final boolean parkAndCheckInterrupt() { 813 LockSupport.park(this); 814 return Thread.interrupted(); 815 } 816 817 /* 818 * Various flavors of acquire, varying in exclusive/shared and 819 * control modes. Each is mostly the same, but annoyingly 820 * different. Only a little bit of factoring is possible due to 821 * interactions of exception mechanics (including ensuring that we 822 * cancel if tryAcquire throws exception) and other control, at 823 * least not without hurting performance too much. 824 */ 825 826 /** 827 * Acquires in exclusive uninterruptible mode for thread already in 828 * queue. Used by condition wait methods as well as acquire. 829 * 830 * @param node the node 831 * @param arg the acquire argument 832 * @return {@code true} if interrupted while waiting 833 */ 834 final boolean acquireQueued(final Node node, int arg) { 835 boolean failed = true; 836 try { 837 boolean interrupted = false; 838 for (;;) { 839 final Node p = node.predecessor(); 840 if (p == head && tryAcquire(arg)) { 841 setHead(node); 842 p.next = null; // help GC 843 failed = false; 844 return interrupted; 845 } 846 if (shouldParkAfterFailedAcquire(p, node) && 847 parkAndCheckInterrupt()) 848 interrupted = true; 849 } 850 } finally { 851 if (failed) 852 cancelAcquire(node); 853 } 854 } 855 856 /** 857 * Acquires in exclusive interruptible mode. 858 * @param arg the acquire argument 859 */ 860 private void doAcquireInterruptibly(int arg) 861 throws InterruptedException { 862 final Node node = addWaiter(Node.EXCLUSIVE); 863 boolean failed = true; 864 try { 865 for (;;) { 866 final Node p = node.predecessor(); 867 if (p == head && tryAcquire(arg)) { 868 setHead(node); 869 p.next = null; // help GC 870 failed = false; 871 return; 872 } 873 if (shouldParkAfterFailedAcquire(p, node) && 874 parkAndCheckInterrupt()) 875 throw new InterruptedException(); 876 } 877 } finally { 878 if (failed) 879 cancelAcquire(node); 880 } 881 } 882 883 /** 884 * Acquires in exclusive timed mode. 885 * 886 * @param arg the acquire argument 887 * @param nanosTimeout max wait time 888 * @return {@code true} if acquired 889 */ 890 private boolean doAcquireNanos(int arg, long nanosTimeout) 891 throws InterruptedException { 892 long lastTime = System.nanoTime(); 893 final Node node = addWaiter(Node.EXCLUSIVE); 894 boolean failed = true; 895 try { 896 for (;;) { 897 final Node p = node.predecessor(); 898 if (p == head && tryAcquire(arg)) { 899 setHead(node); 900 p.next = null; // help GC 901 failed = false; 902 return true; 903 } 904 if (nanosTimeout <= 0) 905 return false; 906 if (shouldParkAfterFailedAcquire(p, node) && 907 nanosTimeout > spinForTimeoutThreshold) 908 LockSupport.parkNanos(this, nanosTimeout); 909 long now = System.nanoTime(); 910 nanosTimeout -= now - lastTime; 911 lastTime = now; 912 if (Thread.interrupted()) 913 throw new InterruptedException(); 914 } 915 } finally { 916 if (failed) 917 cancelAcquire(node); 918 } 919 } 920 921 /** 922 * Acquires in shared uninterruptible mode. 923 * @param arg the acquire argument 924 */ 925 private void doAcquireShared(int arg) { 926 final Node node = addWaiter(Node.SHARED); 927 boolean failed = true; 928 try { 929 boolean interrupted = false; 930 for (;;) { 931 final Node p = node.predecessor(); 932 if (p == head) { 933 int r = tryAcquireShared(arg); 934 if (r >= 0) { 935 setHeadAndPropagate(node, r); 936 p.next = null; // help GC 937 if (interrupted) 938 selfInterrupt(); 939 failed = false; 940 return; 941 } 942 } 943 if (shouldParkAfterFailedAcquire(p, node) && 944 parkAndCheckInterrupt()) 945 interrupted = true; 946 } 947 } finally { 948 if (failed) 949 cancelAcquire(node); 950 } 951 } 952 953 /** 954 * Acquires in shared interruptible mode. 955 * @param arg the acquire argument 956 */ 957 private void doAcquireSharedInterruptibly(int arg) 958 throws InterruptedException { 959 final Node node = addWaiter(Node.SHARED); 960 boolean failed = true; 961 try { 962 for (;;) { 963 final Node p = node.predecessor(); 964 if (p == head) { 965 int r = tryAcquireShared(arg); 966 if (r >= 0) { 967 setHeadAndPropagate(node, r); 968 p.next = null; // help GC 969 failed = false; 970 return; 971 } 972 } 973 if (shouldParkAfterFailedAcquire(p, node) && 974 parkAndCheckInterrupt()) 975 throw new InterruptedException(); 976 } 977 } finally { 978 if (failed) 979 cancelAcquire(node); 980 } 981 } 982 983 /** 984 * Acquires in shared timed mode. 985 * 986 * @param arg the acquire argument 987 * @param nanosTimeout max wait time 988 * @return {@code true} if acquired 989 */ 990 private boolean doAcquireSharedNanos(int arg, long nanosTimeout) 991 throws InterruptedException { 992 993 long lastTime = System.nanoTime(); 994 final Node node = addWaiter(Node.SHARED); 995 boolean failed = true; 996 try { 997 for (;;) { 998 final Node p = node.predecessor(); 999 if (p == head) { 1000 int r = tryAcquireShared(arg); 1001 if (r >= 0) { 1002 setHeadAndPropagate(node, r); 1003 p.next = null; // help GC 1004 failed = false; 1005 return true; 1006 } 1007 } 1008 if (nanosTimeout <= 0) 1009 return false; 1010 if (shouldParkAfterFailedAcquire(p, node) && 1011 nanosTimeout > spinForTimeoutThreshold) 1012 LockSupport.parkNanos(this, nanosTimeout); 1013 long now = System.nanoTime(); 1014 nanosTimeout -= now - lastTime; 1015 lastTime = now; 1016 if (Thread.interrupted()) 1017 throw new InterruptedException(); 1018 } 1019 } finally { 1020 if (failed) 1021 cancelAcquire(node); 1022 } 1023 } 1024 1025 // Main exported methods 1026 1027 /** 1028 * Attempts to acquire in exclusive mode. This method should query 1029 * if the state of the object permits it to be acquired in the 1030 * exclusive mode, and if so to acquire it. 1031 * 1032 * <p>This method is always invoked by the thread performing 1033 * acquire. If this method reports failure, the acquire method 1034 * may queue the thread, if it is not already queued, until it is 1035 * signalled by a release from some other thread. This can be used 1036 * to implement method {@link Lock#tryLock()}. 1037 * 1038 * <p>The default 1039 * implementation throws {@link UnsupportedOperationException}. 1040 * 1041 * @param arg the acquire argument. This value is always the one 1042 * passed to an acquire method, or is the value saved on entry 1043 * to a condition wait. The value is otherwise uninterpreted 1044 * and can represent anything you like. 1045 * @return {@code true} if successful. Upon success, this object has 1046 * been acquired. 1047 * @throws IllegalMonitorStateException if acquiring would place this 1048 * synchronizer in an illegal state. This exception must be 1049 * thrown in a consistent fashion for synchronization to work 1050 * correctly. 1051 * @throws UnsupportedOperationException if exclusive mode is not supported 1052 */ 1053 protected boolean tryAcquire(int arg) { 1054 throw new UnsupportedOperationException(); 1055 } 1056 1057 /** 1058 * Attempts to set the state to reflect a release in exclusive 1059 * mode. 1060 * 1061 * <p>This method is always invoked by the thread performing release. 1062 * 1063 * <p>The default implementation throws 1064 * {@link UnsupportedOperationException}. 1065 * 1066 * @param arg the release argument. This value is always the one 1067 * passed to a release method, or the current state value upon 1068 * entry to a condition wait. The value is otherwise 1069 * uninterpreted and can represent anything you like. 1070 * @return {@code true} if this object is now in a fully released 1071 * state, so that any waiting threads may attempt to acquire; 1072 * and {@code false} otherwise. 1073 * @throws IllegalMonitorStateException if releasing would place this 1074 * synchronizer in an illegal state. This exception must be 1075 * thrown in a consistent fashion for synchronization to work 1076 * correctly. 1077 * @throws UnsupportedOperationException if exclusive mode is not supported 1078 */ 1079 protected boolean tryRelease(int arg) { 1080 throw new UnsupportedOperationException(); 1081 } 1082 1083 /** 1084 * Attempts to acquire in shared mode. This method should query if 1085 * the state of the object permits it to be acquired in the shared 1086 * mode, and if so to acquire it. 1087 * 1088 * <p>This method is always invoked by the thread performing 1089 * acquire. If this method reports failure, the acquire method 1090 * may queue the thread, if it is not already queued, until it is 1091 * signalled by a release from some other thread. 1092 * 1093 * <p>The default implementation throws {@link 1094 * UnsupportedOperationException}. 1095 * 1096 * @param arg the acquire argument. This value is always the one 1097 * passed to an acquire method, or is the value saved on entry 1098 * to a condition wait. The value is otherwise uninterpreted 1099 * and can represent anything you like. 1100 * @return a negative value on failure; zero if acquisition in shared 1101 * mode succeeded but no subsequent shared-mode acquire can 1102 * succeed; and a positive value if acquisition in shared 1103 * mode succeeded and subsequent shared-mode acquires might 1104 * also succeed, in which case a subsequent waiting thread 1105 * must check availability. (Support for three different 1106 * return values enables this method to be used in contexts 1107 * where acquires only sometimes act exclusively.) Upon 1108 * success, this object has been acquired. 1109 * @throws IllegalMonitorStateException if acquiring would place this 1110 * synchronizer in an illegal state. This exception must be 1111 * thrown in a consistent fashion for synchronization to work 1112 * correctly. 1113 * @throws UnsupportedOperationException if shared mode is not supported 1114 */ 1115 protected int tryAcquireShared(int arg) { 1116 throw new UnsupportedOperationException(); 1117 } 1118 1119 /** 1120 * Attempts to set the state to reflect a release in shared mode. 1121 * 1122 * <p>This method is always invoked by the thread performing release. 1123 * 1124 * <p>The default implementation throws 1125 * {@link UnsupportedOperationException}. 1126 * 1127 * @param arg the release argument. This value is always the one 1128 * passed to a release method, or the current state value upon 1129 * entry to a condition wait. The value is otherwise 1130 * uninterpreted and can represent anything you like. 1131 * @return {@code true} if this release of shared mode may permit a 1132 * waiting acquire (shared or exclusive) to succeed; and 1133 * {@code false} otherwise 1134 * @throws IllegalMonitorStateException if releasing would place this 1135 * synchronizer in an illegal state. This exception must be 1136 * thrown in a consistent fashion for synchronization to work 1137 * correctly. 1138 * @throws UnsupportedOperationException if shared mode is not supported 1139 */ 1140 protected boolean tryReleaseShared(int arg) { 1141 throw new UnsupportedOperationException(); 1142 } 1143 1144 /** 1145 * Returns {@code true} if synchronization is held exclusively with 1146 * respect to the current (calling) thread. This method is invoked 1147 * upon each call to a non-waiting {@link ConditionObject} method. 1148 * (Waiting methods instead invoke {@link #release}.) 1149 * 1150 * <p>The default implementation throws {@link 1151 * UnsupportedOperationException}. This method is invoked 1152 * internally only within {@link ConditionObject} methods, so need 1153 * not be defined if conditions are not used. 1154 * 1155 * @return {@code true} if synchronization is held exclusively; 1156 * {@code false} otherwise 1157 * @throws UnsupportedOperationException if conditions are not supported 1158 */ 1159 protected boolean isHeldExclusively() { 1160 throw new UnsupportedOperationException(); 1161 } 1162 1163 /** 1164 * Acquires in exclusive mode, ignoring interrupts. Implemented 1165 * by invoking at least once {@link #tryAcquire}, 1166 * returning on success. Otherwise the thread is queued, possibly 1167 * repeatedly blocking and unblocking, invoking {@link 1168 * #tryAcquire} until success. This method can be used 1169 * to implement method {@link Lock#lock}. 1170 * 1171 * @param arg the acquire argument. This value is conveyed to 1172 * {@link #tryAcquire} but is otherwise uninterpreted and 1173 * can represent anything you like. 1174 */ 1175 public final void acquire(int arg) { 1176 if (!tryAcquire(arg) && 1177 acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) 1178 selfInterrupt(); 1179 } 1180 1181 /** 1182 * Acquires in exclusive mode, aborting if interrupted. 1183 * Implemented by first checking interrupt status, then invoking 1184 * at least once {@link #tryAcquire}, returning on 1185 * success. Otherwise the thread is queued, possibly repeatedly 1186 * blocking and unblocking, invoking {@link #tryAcquire} 1187 * until success or the thread is interrupted. This method can be 1188 * used to implement method {@link Lock#lockInterruptibly}. 1189 * 1190 * @param arg the acquire argument. This value is conveyed to 1191 * {@link #tryAcquire} but is otherwise uninterpreted and 1192 * can represent anything you like. 1193 * @throws InterruptedException if the current thread is interrupted 1194 */ 1195 public final void acquireInterruptibly(int arg) 1196 throws InterruptedException { 1197 if (Thread.interrupted()) 1198 throw new InterruptedException(); 1199 if (!tryAcquire(arg)) 1200 doAcquireInterruptibly(arg); 1201 } 1202 1203 /** 1204 * Attempts to acquire in exclusive mode, aborting if interrupted, 1205 * and failing if the given timeout elapses. Implemented by first 1206 * checking interrupt status, then invoking at least once {@link 1207 * #tryAcquire}, returning on success. Otherwise, the thread is 1208 * queued, possibly repeatedly blocking and unblocking, invoking 1209 * {@link #tryAcquire} until success or the thread is interrupted 1210 * or the timeout elapses. This method can be used to implement 1211 * method {@link Lock#tryLock(long, TimeUnit)}. 1212 * 1213 * @param arg the acquire argument. This value is conveyed to 1214 * {@link #tryAcquire} but is otherwise uninterpreted and 1215 * can represent anything you like. 1216 * @param nanosTimeout the maximum number of nanoseconds to wait 1217 * @return {@code true} if acquired; {@code false} if timed out 1218 * @throws InterruptedException if the current thread is interrupted 1219 */ 1220 public final boolean tryAcquireNanos(int arg, long nanosTimeout) 1221 throws InterruptedException { 1222 if (Thread.interrupted()) 1223 throw new InterruptedException(); 1224 return tryAcquire(arg) || 1225 doAcquireNanos(arg, nanosTimeout); 1226 } 1227 1228 /** 1229 * Releases in exclusive mode. Implemented by unblocking one or 1230 * more threads if {@link #tryRelease} returns true. 1231 * This method can be used to implement method {@link Lock#unlock}. 1232 * 1233 * @param arg the release argument. This value is conveyed to 1234 * {@link #tryRelease} but is otherwise uninterpreted and 1235 * can represent anything you like. 1236 * @return the value returned from {@link #tryRelease} 1237 */ 1238 public final boolean release(int arg) { 1239 if (tryRelease(arg)) { 1240 Node h = head; 1241 if (h != null && h.waitStatus != 0) 1242 unparkSuccessor(h); 1243 return true; 1244 } 1245 return false; 1246 } 1247 1248 /** 1249 * Acquires in shared mode, ignoring interrupts. Implemented by 1250 * first invoking at least once {@link #tryAcquireShared}, 1251 * returning on success. Otherwise the thread is queued, possibly 1252 * repeatedly blocking and unblocking, invoking {@link 1253 * #tryAcquireShared} until success. 1254 * 1255 * @param arg the acquire argument. This value is conveyed to 1256 * {@link #tryAcquireShared} but is otherwise uninterpreted 1257 * and can represent anything you like. 1258 */ 1259 public final void acquireShared(int arg) { 1260 if (tryAcquireShared(arg) < 0) 1261 doAcquireShared(arg); 1262 } 1263 1264 /** 1265 * Acquires in shared mode, aborting if interrupted. Implemented 1266 * by first checking interrupt status, then invoking at least once 1267 * {@link #tryAcquireShared}, returning on success. Otherwise the 1268 * thread is queued, possibly repeatedly blocking and unblocking, 1269 * invoking {@link #tryAcquireShared} until success or the thread 1270 * is interrupted. 1271 * @param arg the acquire argument 1272 * This value is conveyed to {@link #tryAcquireShared} but is 1273 * otherwise uninterpreted and can represent anything 1274 * you like. 1275 * @throws InterruptedException if the current thread is interrupted 1276 */ 1277 public final void acquireSharedInterruptibly(int arg) 1278 throws InterruptedException { 1279 if (Thread.interrupted()) 1280 throw new InterruptedException(); 1281 if (tryAcquireShared(arg) < 0) 1282 doAcquireSharedInterruptibly(arg); 1283 } 1284 1285 /** 1286 * Attempts to acquire in shared mode, aborting if interrupted, and 1287 * failing if the given timeout elapses. Implemented by first 1288 * checking interrupt status, then invoking at least once {@link 1289 * #tryAcquireShared}, returning on success. Otherwise, the 1290 * thread is queued, possibly repeatedly blocking and unblocking, 1291 * invoking {@link #tryAcquireShared} until success or the thread 1292 * is interrupted or the timeout elapses. 1293 * 1294 * @param arg the acquire argument. This value is conveyed to 1295 * {@link #tryAcquireShared} but is otherwise uninterpreted 1296 * and can represent anything you like. 1297 * @param nanosTimeout the maximum number of nanoseconds to wait 1298 * @return {@code true} if acquired; {@code false} if timed out 1299 * @throws InterruptedException if the current thread is interrupted 1300 */ 1301 public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) 1302 throws InterruptedException { 1303 if (Thread.interrupted()) 1304 throw new InterruptedException(); 1305 return tryAcquireShared(arg) >= 0 || 1306 doAcquireSharedNanos(arg, nanosTimeout); 1307 } 1308 1309 /** 1310 * Releases in shared mode. Implemented by unblocking one or more 1311 * threads if {@link #tryReleaseShared} returns true. 1312 * 1313 * @param arg the release argument. This value is conveyed to 1314 * {@link #tryReleaseShared} but is otherwise uninterpreted 1315 * and can represent anything you like. 1316 * @return the value returned from {@link #tryReleaseShared} 1317 */ 1318 public final boolean releaseShared(int arg) { 1319 if (tryReleaseShared(arg)) { 1320 doReleaseShared(); 1321 return true; 1322 } 1323 return false; 1324 } 1325 1326 // Queue inspection methods 1327 1328 /** 1329 * Queries whether any threads are waiting to acquire. Note that 1330 * because cancellations due to interrupts and timeouts may occur 1331 * at any time, a {@code true} return does not guarantee that any 1332 * other thread will ever acquire. 1333 * 1334 * <p>In this implementation, this operation returns in 1335 * constant time. 1336 * 1337 * @return {@code true} if there may be other threads waiting to acquire 1338 */ 1339 public final boolean hasQueuedThreads() { 1340 return head != tail; 1341 } 1342 1343 /** 1344 * Queries whether any threads have ever contended to acquire this 1345 * synchronizer; that is if an acquire method has ever blocked. 1346 * 1347 * <p>In this implementation, this operation returns in 1348 * constant time. 1349 * 1350 * @return {@code true} if there has ever been contention 1351 */ 1352 public final boolean hasContended() { 1353 return head != null; 1354 } 1355 1356 /** 1357 * Returns the first (longest-waiting) thread in the queue, or 1358 * {@code null} if no threads are currently queued. 1359 * 1360 * <p>In this implementation, this operation normally returns in 1361 * constant time, but may iterate upon contention if other threads are 1362 * concurrently modifying the queue. 1363 * 1364 * @return the first (longest-waiting) thread in the queue, or 1365 * {@code null} if no threads are currently queued 1366 */ 1367 public final Thread getFirstQueuedThread() { 1368 // handle only fast path, else relay 1369 return (head == tail) ? null : fullGetFirstQueuedThread(); 1370 } 1371 1372 /** 1373 * Version of getFirstQueuedThread called when fastpath fails 1374 */ 1375 private Thread fullGetFirstQueuedThread() { 1376 /* 1377 * The first node is normally head.next. Try to get its 1378 * thread field, ensuring consistent reads: If thread 1379 * field is nulled out or s.prev is no longer head, then 1380 * some other thread(s) concurrently performed setHead in 1381 * between some of our reads. We try this twice before 1382 * resorting to traversal. 1383 */ 1384 Node h, s; 1385 Thread st; 1386 if (((h = head) != null && (s = h.next) != null && 1387 s.prev == head && (st = s.thread) != null) || 1388 ((h = head) != null && (s = h.next) != null && 1389 s.prev == head && (st = s.thread) != null)) 1390 return st; 1391 1392 /* 1393 * Head's next field might not have been set yet, or may have 1394 * been unset after setHead. So we must check to see if tail 1395 * is actually first node. If not, we continue on, safely 1396 * traversing from tail back to head to find first, 1397 * guaranteeing termination. 1398 */ 1399 1400 Node t = tail; 1401 Thread firstThread = null; 1402 while (t != null && t != head) { 1403 Thread tt = t.thread; 1404 if (tt != null) 1405 firstThread = tt; 1406 t = t.prev; 1407 } 1408 return firstThread; 1409 } 1410 1411 /** 1412 * Returns true if the given thread is currently queued. 1413 * 1414 * <p>This implementation traverses the queue to determine 1415 * presence of the given thread. 1416 * 1417 * @param thread the thread 1418 * @return {@code true} if the given thread is on the queue 1419 * @throws NullPointerException if the thread is null 1420 */ 1421 public final boolean isQueued(Thread thread) { 1422 if (thread == null) 1423 throw new NullPointerException(); 1424 for (Node p = tail; p != null; p = p.prev) 1425 if (p.thread == thread) 1426 return true; 1427 return false; 1428 } 1429 1430 /** 1431 * Returns {@code true} if the apparent first queued thread, if one 1432 * exists, is waiting in exclusive mode. If this method returns 1433 * {@code true}, and the current thread is attempting to acquire in 1434 * shared mode (that is, this method is invoked from {@link 1435 * #tryAcquireShared}) then it is guaranteed that the current thread 1436 * is not the first queued thread. Used only as a heuristic in 1437 * ReentrantReadWriteLock. 1438 */ 1439 final boolean apparentlyFirstQueuedIsExclusive() { 1440 Node h, s; 1441 return (h = head) != null && 1442 (s = h.next) != null && 1443 !s.isShared() && 1444 s.thread != null; 1445 } 1446 1447 /** 1448 * Queries whether any threads have been waiting to acquire longer 1449 * than the current thread. 1450 * 1451 * <p>An invocation of this method is equivalent to (but may be 1452 * more efficient than): 1453 * <pre> {@code 1454 * getFirstQueuedThread() != Thread.currentThread() && 1455 * hasQueuedThreads()}</pre> 1456 * 1457 * <p>Note that because cancellations due to interrupts and 1458 * timeouts may occur at any time, a {@code true} return does not 1459 * guarantee that some other thread will acquire before the current 1460 * thread. Likewise, it is possible for another thread to win a 1461 * race to enqueue after this method has returned {@code false}, 1462 * due to the queue being empty. 1463 * 1464 * <p>This method is designed to be used by a fair synchronizer to 1465 * avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>. 1466 * Such a synchronizer's {@link #tryAcquire} method should return 1467 * {@code false}, and its {@link #tryAcquireShared} method should 1468 * return a negative value, if this method returns {@code true} 1469 * (unless this is a reentrant acquire). For example, the {@code 1470 * tryAcquire} method for a fair, reentrant, exclusive mode 1471 * synchronizer might look like this: 1472 * 1473 * <pre> {@code 1474 * protected boolean tryAcquire(int arg) { 1475 * if (isHeldExclusively()) { 1476 * // A reentrant acquire; increment hold count 1477 * return true; 1478 * } else if (hasQueuedPredecessors()) { 1479 * return false; 1480 * } else { 1481 * // try to acquire normally 1482 * } 1483 * }}</pre> 1484 * 1485 * @return {@code true} if there is a queued thread preceding the 1486 * current thread, and {@code false} if the current thread 1487 * is at the head of the queue or the queue is empty 1488 * @since 1.7 1489 * @hide 1490 */ 1491 public final boolean hasQueuedPredecessors() { 1492 // The correctness of this depends on head being initialized 1493 // before tail and on head.next being accurate if the current 1494 // thread is first in queue. 1495 Node t = tail; // Read fields in reverse initialization order 1496 Node h = head; 1497 Node s; 1498 return h != t && 1499 ((s = h.next) == null || s.thread != Thread.currentThread()); 1500 } 1501 1502 1503 // Instrumentation and monitoring methods 1504 1505 /** 1506 * Returns an estimate of the number of threads waiting to 1507 * acquire. The value is only an estimate because the number of 1508 * threads may change dynamically while this method traverses 1509 * internal data structures. This method is designed for use in 1510 * monitoring system state, not for synchronization 1511 * control. 1512 * 1513 * @return the estimated number of threads waiting to acquire 1514 */ 1515 public final int getQueueLength() { 1516 int n = 0; 1517 for (Node p = tail; p != null; p = p.prev) { 1518 if (p.thread != null) 1519 ++n; 1520 } 1521 return n; 1522 } 1523 1524 /** 1525 * Returns a collection containing threads that may be waiting to 1526 * acquire. Because the actual set of threads may change 1527 * dynamically while constructing this result, the returned 1528 * collection is only a best-effort estimate. The elements of the 1529 * returned collection are in no particular order. This method is 1530 * designed to facilitate construction of subclasses that provide 1531 * more extensive monitoring facilities. 1532 * 1533 * @return the collection of threads 1534 */ 1535 public final Collection<Thread> getQueuedThreads() { 1536 ArrayList<Thread> list = new ArrayList<Thread>(); 1537 for (Node p = tail; p != null; p = p.prev) { 1538 Thread t = p.thread; 1539 if (t != null) 1540 list.add(t); 1541 } 1542 return list; 1543 } 1544 1545 /** 1546 * Returns a collection containing threads that may be waiting to 1547 * acquire in exclusive mode. This has the same properties 1548 * as {@link #getQueuedThreads} except that it only returns 1549 * those threads waiting due to an exclusive acquire. 1550 * 1551 * @return the collection of threads 1552 */ 1553 public final Collection<Thread> getExclusiveQueuedThreads() { 1554 ArrayList<Thread> list = new ArrayList<Thread>(); 1555 for (Node p = tail; p != null; p = p.prev) { 1556 if (!p.isShared()) { 1557 Thread t = p.thread; 1558 if (t != null) 1559 list.add(t); 1560 } 1561 } 1562 return list; 1563 } 1564 1565 /** 1566 * Returns a collection containing threads that may be waiting to 1567 * acquire in shared mode. This has the same properties 1568 * as {@link #getQueuedThreads} except that it only returns 1569 * those threads waiting due to a shared acquire. 1570 * 1571 * @return the collection of threads 1572 */ 1573 public final Collection<Thread> getSharedQueuedThreads() { 1574 ArrayList<Thread> list = new ArrayList<Thread>(); 1575 for (Node p = tail; p != null; p = p.prev) { 1576 if (p.isShared()) { 1577 Thread t = p.thread; 1578 if (t != null) 1579 list.add(t); 1580 } 1581 } 1582 return list; 1583 } 1584 1585 /** 1586 * Returns a string identifying this synchronizer, as well as its state. 1587 * The state, in brackets, includes the String {@code "State ="} 1588 * followed by the current value of {@link #getState}, and either 1589 * {@code "nonempty"} or {@code "empty"} depending on whether the 1590 * queue is empty. 1591 * 1592 * @return a string identifying this synchronizer, as well as its state 1593 */ 1594 public String toString() { 1595 int s = getState(); 1596 String q = hasQueuedThreads() ? "non" : ""; 1597 return super.toString() + 1598 "[State = " + s + ", " + q + "empty queue]"; 1599 } 1600 1601 1602 // Internal support methods for Conditions 1603 1604 /** 1605 * Returns true if a node, always one that was initially placed on 1606 * a condition queue, is now waiting to reacquire on sync queue. 1607 * @param node the node 1608 * @return true if is reacquiring 1609 */ 1610 final boolean isOnSyncQueue(Node node) { 1611 if (node.waitStatus == Node.CONDITION || node.prev == null) 1612 return false; 1613 if (node.next != null) // If has successor, it must be on queue 1614 return true; 1615 /* 1616 * node.prev can be non-null, but not yet on queue because 1617 * the CAS to place it on queue can fail. So we have to 1618 * traverse from tail to make sure it actually made it. It 1619 * will always be near the tail in calls to this method, and 1620 * unless the CAS failed (which is unlikely), it will be 1621 * there, so we hardly ever traverse much. 1622 */ 1623 return findNodeFromTail(node); 1624 } 1625 1626 /** 1627 * Returns true if node is on sync queue by searching backwards from tail. 1628 * Called only when needed by isOnSyncQueue. 1629 * @return true if present 1630 */ 1631 private boolean findNodeFromTail(Node node) { 1632 Node t = tail; 1633 for (;;) { 1634 if (t == node) 1635 return true; 1636 if (t == null) 1637 return false; 1638 t = t.prev; 1639 } 1640 } 1641 1642 /** 1643 * Transfers a node from a condition queue onto sync queue. 1644 * Returns true if successful. 1645 * @param node the node 1646 * @return true if successfully transferred (else the node was 1647 * cancelled before signal). 1648 */ 1649 final boolean transferForSignal(Node node) { 1650 /* 1651 * If cannot change waitStatus, the node has been cancelled. 1652 */ 1653 if (!compareAndSetWaitStatus(node, Node.CONDITION, 0)) 1654 return false; 1655 1656 /* 1657 * Splice onto queue and try to set waitStatus of predecessor to 1658 * indicate that thread is (probably) waiting. If cancelled or 1659 * attempt to set waitStatus fails, wake up to resync (in which 1660 * case the waitStatus can be transiently and harmlessly wrong). 1661 */ 1662 Node p = enq(node); 1663 int ws = p.waitStatus; 1664 if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL)) 1665 LockSupport.unpark(node.thread); 1666 return true; 1667 } 1668 1669 /** 1670 * Transfers node, if necessary, to sync queue after a cancelled 1671 * wait. Returns true if thread was cancelled before being 1672 * signalled. 1673 * @param node its node 1674 * @return true if cancelled before the node was signalled 1675 */ 1676 final boolean transferAfterCancelledWait(Node node) { 1677 if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) { 1678 enq(node); 1679 return true; 1680 } 1681 /* 1682 * If we lost out to a signal(), then we can't proceed 1683 * until it finishes its enq(). Cancelling during an 1684 * incomplete transfer is both rare and transient, so just 1685 * spin. 1686 */ 1687 while (!isOnSyncQueue(node)) 1688 Thread.yield(); 1689 return false; 1690 } 1691 1692 /** 1693 * Invokes release with current state value; returns saved state. 1694 * Cancels node and throws exception on failure. 1695 * @param node the condition node for this wait 1696 * @return previous sync state 1697 */ 1698 final int fullyRelease(Node node) { 1699 boolean failed = true; 1700 try { 1701 int savedState = getState(); 1702 if (release(savedState)) { 1703 failed = false; 1704 return savedState; 1705 } else { 1706 throw new IllegalMonitorStateException(); 1707 } 1708 } finally { 1709 if (failed) 1710 node.waitStatus = Node.CANCELLED; 1711 } 1712 } 1713 1714 // Instrumentation methods for conditions 1715 1716 /** 1717 * Queries whether the given ConditionObject 1718 * uses this synchronizer as its lock. 1719 * 1720 * @param condition the condition 1721 * @return <tt>true</tt> if owned 1722 * @throws NullPointerException if the condition is null 1723 */ 1724 public final boolean owns(ConditionObject condition) { 1725 if (condition == null) 1726 throw new NullPointerException(); 1727 return condition.isOwnedBy(this); 1728 } 1729 1730 /** 1731 * Queries whether any threads are waiting on the given condition 1732 * associated with this synchronizer. Note that because timeouts 1733 * and interrupts may occur at any time, a <tt>true</tt> return 1734 * does not guarantee that a future <tt>signal</tt> will awaken 1735 * any threads. This method is designed primarily for use in 1736 * monitoring of the system state. 1737 * 1738 * @param condition the condition 1739 * @return <tt>true</tt> if there are any waiting threads 1740 * @throws IllegalMonitorStateException if exclusive synchronization 1741 * is not held 1742 * @throws IllegalArgumentException if the given condition is 1743 * not associated with this synchronizer 1744 * @throws NullPointerException if the condition is null 1745 */ 1746 public final boolean hasWaiters(ConditionObject condition) { 1747 if (!owns(condition)) 1748 throw new IllegalArgumentException("Not owner"); 1749 return condition.hasWaiters(); 1750 } 1751 1752 /** 1753 * Returns an estimate of the number of threads waiting on the 1754 * given condition associated with this synchronizer. Note that 1755 * because timeouts and interrupts may occur at any time, the 1756 * estimate serves only as an upper bound on the actual number of 1757 * waiters. This method is designed for use in monitoring of the 1758 * system state, not for synchronization control. 1759 * 1760 * @param condition the condition 1761 * @return the estimated number of waiting threads 1762 * @throws IllegalMonitorStateException if exclusive synchronization 1763 * is not held 1764 * @throws IllegalArgumentException if the given condition is 1765 * not associated with this synchronizer 1766 * @throws NullPointerException if the condition is null 1767 */ 1768 public final int getWaitQueueLength(ConditionObject condition) { 1769 if (!owns(condition)) 1770 throw new IllegalArgumentException("Not owner"); 1771 return condition.getWaitQueueLength(); 1772 } 1773 1774 /** 1775 * Returns a collection containing those threads that may be 1776 * waiting on the given condition associated with this 1777 * synchronizer. Because the actual set of threads may change 1778 * dynamically while constructing this result, the returned 1779 * collection is only a best-effort estimate. The elements of the 1780 * returned collection are in no particular order. 1781 * 1782 * @param condition the condition 1783 * @return the collection of threads 1784 * @throws IllegalMonitorStateException if exclusive synchronization 1785 * is not held 1786 * @throws IllegalArgumentException if the given condition is 1787 * not associated with this synchronizer 1788 * @throws NullPointerException if the condition is null 1789 */ 1790 public final Collection<Thread> getWaitingThreads(ConditionObject condition) { 1791 if (!owns(condition)) 1792 throw new IllegalArgumentException("Not owner"); 1793 return condition.getWaitingThreads(); 1794 } 1795 1796 /** 1797 * Condition implementation for a {@link 1798 * AbstractQueuedSynchronizer} serving as the basis of a {@link 1799 * Lock} implementation. 1800 * 1801 * <p>Method documentation for this class describes mechanics, 1802 * not behavioral specifications from the point of view of Lock 1803 * and Condition users. Exported versions of this class will in 1804 * general need to be accompanied by documentation describing 1805 * condition semantics that rely on those of the associated 1806 * <tt>AbstractQueuedSynchronizer</tt>. 1807 * 1808 * <p>This class is Serializable, but all fields are transient, 1809 * so deserialized conditions have no waiters. 1810 */ 1811 public class ConditionObject implements Condition, java.io.Serializable { 1812 private static final long serialVersionUID = 1173984872572414699L; 1813 /** First node of condition queue. */ 1814 private transient Node firstWaiter; 1815 /** Last node of condition queue. */ 1816 private transient Node lastWaiter; 1817 1818 /** 1819 * Creates a new <tt>ConditionObject</tt> instance. 1820 */ 1821 public ConditionObject() { } 1822 1823 // Internal methods 1824 1825 /** 1826 * Adds a new waiter to wait queue. 1827 * @return its new wait node 1828 */ 1829 private Node addConditionWaiter() { 1830 Node t = lastWaiter; 1831 // If lastWaiter is cancelled, clean out. 1832 if (t != null && t.waitStatus != Node.CONDITION) { 1833 unlinkCancelledWaiters(); 1834 t = lastWaiter; 1835 } 1836 Node node = new Node(Thread.currentThread(), Node.CONDITION); 1837 if (t == null) 1838 firstWaiter = node; 1839 else 1840 t.nextWaiter = node; 1841 lastWaiter = node; 1842 return node; 1843 } 1844 1845 /** 1846 * Removes and transfers nodes until hit non-cancelled one or 1847 * null. Split out from signal in part to encourage compilers 1848 * to inline the case of no waiters. 1849 * @param first (non-null) the first node on condition queue 1850 */ 1851 private void doSignal(Node first) { 1852 do { 1853 if ( (firstWaiter = first.nextWaiter) == null) 1854 lastWaiter = null; 1855 first.nextWaiter = null; 1856 } while (!transferForSignal(first) && 1857 (first = firstWaiter) != null); 1858 } 1859 1860 /** 1861 * Removes and transfers all nodes. 1862 * @param first (non-null) the first node on condition queue 1863 */ 1864 private void doSignalAll(Node first) { 1865 lastWaiter = firstWaiter = null; 1866 do { 1867 Node next = first.nextWaiter; 1868 first.nextWaiter = null; 1869 transferForSignal(first); 1870 first = next; 1871 } while (first != null); 1872 } 1873 1874 /** 1875 * Unlinks cancelled waiter nodes from condition queue. 1876 * Called only while holding lock. This is called when 1877 * cancellation occurred during condition wait, and upon 1878 * insertion of a new waiter when lastWaiter is seen to have 1879 * been cancelled. This method is needed to avoid garbage 1880 * retention in the absence of signals. So even though it may 1881 * require a full traversal, it comes into play only when 1882 * timeouts or cancellations occur in the absence of 1883 * signals. It traverses all nodes rather than stopping at a 1884 * particular target to unlink all pointers to garbage nodes 1885 * without requiring many re-traversals during cancellation 1886 * storms. 1887 */ 1888 private void unlinkCancelledWaiters() { 1889 Node t = firstWaiter; 1890 Node trail = null; 1891 while (t != null) { 1892 Node next = t.nextWaiter; 1893 if (t.waitStatus != Node.CONDITION) { 1894 t.nextWaiter = null; 1895 if (trail == null) 1896 firstWaiter = next; 1897 else 1898 trail.nextWaiter = next; 1899 if (next == null) 1900 lastWaiter = trail; 1901 } 1902 else 1903 trail = t; 1904 t = next; 1905 } 1906 } 1907 1908 // public methods 1909 1910 /** 1911 * Moves the longest-waiting thread, if one exists, from the 1912 * wait queue for this condition to the wait queue for the 1913 * owning lock. 1914 * 1915 * @throws IllegalMonitorStateException if {@link #isHeldExclusively} 1916 * returns {@code false} 1917 */ 1918 public final void signal() { 1919 if (!isHeldExclusively()) 1920 throw new IllegalMonitorStateException(); 1921 Node first = firstWaiter; 1922 if (first != null) 1923 doSignal(first); 1924 } 1925 1926 /** 1927 * Moves all threads from the wait queue for this condition to 1928 * the wait queue for the owning lock. 1929 * 1930 * @throws IllegalMonitorStateException if {@link #isHeldExclusively} 1931 * returns {@code false} 1932 */ 1933 public final void signalAll() { 1934 if (!isHeldExclusively()) 1935 throw new IllegalMonitorStateException(); 1936 Node first = firstWaiter; 1937 if (first != null) 1938 doSignalAll(first); 1939 } 1940 1941 /** 1942 * Implements uninterruptible condition wait. 1943 * <ol> 1944 * <li> Save lock state returned by {@link #getState}. 1945 * <li> Invoke {@link #release} with 1946 * saved state as argument, throwing 1947 * IllegalMonitorStateException if it fails. 1948 * <li> Block until signalled. 1949 * <li> Reacquire by invoking specialized version of 1950 * {@link #acquire} with saved state as argument. 1951 * </ol> 1952 */ 1953 public final void awaitUninterruptibly() { 1954 Node node = addConditionWaiter(); 1955 int savedState = fullyRelease(node); 1956 boolean interrupted = false; 1957 while (!isOnSyncQueue(node)) { 1958 LockSupport.park(this); 1959 if (Thread.interrupted()) 1960 interrupted = true; 1961 } 1962 if (acquireQueued(node, savedState) || interrupted) 1963 selfInterrupt(); 1964 } 1965 1966 /* 1967 * For interruptible waits, we need to track whether to throw 1968 * InterruptedException, if interrupted while blocked on 1969 * condition, versus reinterrupt current thread, if 1970 * interrupted while blocked waiting to re-acquire. 1971 */ 1972 1973 /** Mode meaning to reinterrupt on exit from wait */ 1974 private static final int REINTERRUPT = 1; 1975 /** Mode meaning to throw InterruptedException on exit from wait */ 1976 private static final int THROW_IE = -1; 1977 1978 /** 1979 * Checks for interrupt, returning THROW_IE if interrupted 1980 * before signalled, REINTERRUPT if after signalled, or 1981 * 0 if not interrupted. 1982 */ 1983 private int checkInterruptWhileWaiting(Node node) { 1984 return Thread.interrupted() ? 1985 (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) : 1986 0; 1987 } 1988 1989 /** 1990 * Throws InterruptedException, reinterrupts current thread, or 1991 * does nothing, depending on mode. 1992 */ 1993 private void reportInterruptAfterWait(int interruptMode) 1994 throws InterruptedException { 1995 if (interruptMode == THROW_IE) 1996 throw new InterruptedException(); 1997 else if (interruptMode == REINTERRUPT) 1998 selfInterrupt(); 1999 } 2000 2001 /** 2002 * Implements interruptible condition wait. 2003 * <ol> 2004 * <li> If current thread is interrupted, throw InterruptedException. 2005 * <li> Save lock state returned by {@link #getState}. 2006 * <li> Invoke {@link #release} with 2007 * saved state as argument, throwing 2008 * IllegalMonitorStateException if it fails. 2009 * <li> Block until signalled or interrupted. 2010 * <li> Reacquire by invoking specialized version of 2011 * {@link #acquire} with saved state as argument. 2012 * <li> If interrupted while blocked in step 4, throw InterruptedException. 2013 * </ol> 2014 */ 2015 public final void await() throws InterruptedException { 2016 if (Thread.interrupted()) 2017 throw new InterruptedException(); 2018 Node node = addConditionWaiter(); 2019 int savedState = fullyRelease(node); 2020 int interruptMode = 0; 2021 while (!isOnSyncQueue(node)) { 2022 LockSupport.park(this); 2023 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) 2024 break; 2025 } 2026 if (acquireQueued(node, savedState) && interruptMode != THROW_IE) 2027 interruptMode = REINTERRUPT; 2028 if (node.nextWaiter != null) // clean up if cancelled 2029 unlinkCancelledWaiters(); 2030 if (interruptMode != 0) 2031 reportInterruptAfterWait(interruptMode); 2032 } 2033 2034 /** 2035 * Implements timed condition wait. 2036 * <ol> 2037 * <li> If current thread is interrupted, throw InterruptedException. 2038 * <li> Save lock state returned by {@link #getState}. 2039 * <li> Invoke {@link #release} with 2040 * saved state as argument, throwing 2041 * IllegalMonitorStateException if it fails. 2042 * <li> Block until signalled, interrupted, or timed out. 2043 * <li> Reacquire by invoking specialized version of 2044 * {@link #acquire} with saved state as argument. 2045 * <li> If interrupted while blocked in step 4, throw InterruptedException. 2046 * </ol> 2047 */ 2048 public final long awaitNanos(long nanosTimeout) 2049 throws InterruptedException { 2050 if (Thread.interrupted()) 2051 throw new InterruptedException(); 2052 Node node = addConditionWaiter(); 2053 int savedState = fullyRelease(node); 2054 long lastTime = System.nanoTime(); 2055 int interruptMode = 0; 2056 while (!isOnSyncQueue(node)) { 2057 if (nanosTimeout <= 0L) { 2058 transferAfterCancelledWait(node); 2059 break; 2060 } 2061 LockSupport.parkNanos(this, nanosTimeout); 2062 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) 2063 break; 2064 2065 long now = System.nanoTime(); 2066 nanosTimeout -= now - lastTime; 2067 lastTime = now; 2068 } 2069 if (acquireQueued(node, savedState) && interruptMode != THROW_IE) 2070 interruptMode = REINTERRUPT; 2071 if (node.nextWaiter != null) 2072 unlinkCancelledWaiters(); 2073 if (interruptMode != 0) 2074 reportInterruptAfterWait(interruptMode); 2075 return nanosTimeout - (System.nanoTime() - lastTime); 2076 } 2077 2078 /** 2079 * Implements absolute timed condition wait. 2080 * <ol> 2081 * <li> If current thread is interrupted, throw InterruptedException. 2082 * <li> Save lock state returned by {@link #getState}. 2083 * <li> Invoke {@link #release} with 2084 * saved state as argument, throwing 2085 * IllegalMonitorStateException if it fails. 2086 * <li> Block until signalled, interrupted, or timed out. 2087 * <li> Reacquire by invoking specialized version of 2088 * {@link #acquire} with saved state as argument. 2089 * <li> If interrupted while blocked in step 4, throw InterruptedException. 2090 * <li> If timed out while blocked in step 4, return false, else true. 2091 * </ol> 2092 */ 2093 public final boolean awaitUntil(Date deadline) 2094 throws InterruptedException { 2095 if (deadline == null) 2096 throw new NullPointerException(); 2097 long abstime = deadline.getTime(); 2098 if (Thread.interrupted()) 2099 throw new InterruptedException(); 2100 Node node = addConditionWaiter(); 2101 int savedState = fullyRelease(node); 2102 boolean timedout = false; 2103 int interruptMode = 0; 2104 while (!isOnSyncQueue(node)) { 2105 if (System.currentTimeMillis() > abstime) { 2106 timedout = transferAfterCancelledWait(node); 2107 break; 2108 } 2109 LockSupport.parkUntil(this, abstime); 2110 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) 2111 break; 2112 } 2113 if (acquireQueued(node, savedState) && interruptMode != THROW_IE) 2114 interruptMode = REINTERRUPT; 2115 if (node.nextWaiter != null) 2116 unlinkCancelledWaiters(); 2117 if (interruptMode != 0) 2118 reportInterruptAfterWait(interruptMode); 2119 return !timedout; 2120 } 2121 2122 /** 2123 * Implements timed condition wait. 2124 * <ol> 2125 * <li> If current thread is interrupted, throw InterruptedException. 2126 * <li> Save lock state returned by {@link #getState}. 2127 * <li> Invoke {@link #release} with 2128 * saved state as argument, throwing 2129 * IllegalMonitorStateException if it fails. 2130 * <li> Block until signalled, interrupted, or timed out. 2131 * <li> Reacquire by invoking specialized version of 2132 * {@link #acquire} with saved state as argument. 2133 * <li> If interrupted while blocked in step 4, throw InterruptedException. 2134 * <li> If timed out while blocked in step 4, return false, else true. 2135 * </ol> 2136 */ 2137 public final boolean await(long time, TimeUnit unit) 2138 throws InterruptedException { 2139 if (unit == null) 2140 throw new NullPointerException(); 2141 long nanosTimeout = unit.toNanos(time); 2142 if (Thread.interrupted()) 2143 throw new InterruptedException(); 2144 Node node = addConditionWaiter(); 2145 int savedState = fullyRelease(node); 2146 long lastTime = System.nanoTime(); 2147 boolean timedout = false; 2148 int interruptMode = 0; 2149 while (!isOnSyncQueue(node)) { 2150 if (nanosTimeout <= 0L) { 2151 timedout = transferAfterCancelledWait(node); 2152 break; 2153 } 2154 if (nanosTimeout >= spinForTimeoutThreshold) 2155 LockSupport.parkNanos(this, nanosTimeout); 2156 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) 2157 break; 2158 long now = System.nanoTime(); 2159 nanosTimeout -= now - lastTime; 2160 lastTime = now; 2161 } 2162 if (acquireQueued(node, savedState) && interruptMode != THROW_IE) 2163 interruptMode = REINTERRUPT; 2164 if (node.nextWaiter != null) 2165 unlinkCancelledWaiters(); 2166 if (interruptMode != 0) 2167 reportInterruptAfterWait(interruptMode); 2168 return !timedout; 2169 } 2170 2171 // support for instrumentation 2172 2173 /** 2174 * Returns true if this condition was created by the given 2175 * synchronization object. 2176 * 2177 * @return {@code true} if owned 2178 */ 2179 final boolean isOwnedBy(AbstractQueuedSynchronizer sync) { 2180 return sync == AbstractQueuedSynchronizer.this; 2181 } 2182 2183 /** 2184 * Queries whether any threads are waiting on this condition. 2185 * Implements {@link AbstractQueuedSynchronizer#hasWaiters}. 2186 * 2187 * @return {@code true} if there are any waiting threads 2188 * @throws IllegalMonitorStateException if {@link #isHeldExclusively} 2189 * returns {@code false} 2190 */ 2191 protected final boolean hasWaiters() { 2192 if (!isHeldExclusively()) 2193 throw new IllegalMonitorStateException(); 2194 for (Node w = firstWaiter; w != null; w = w.nextWaiter) { 2195 if (w.waitStatus == Node.CONDITION) 2196 return true; 2197 } 2198 return false; 2199 } 2200 2201 /** 2202 * Returns an estimate of the number of threads waiting on 2203 * this condition. 2204 * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength}. 2205 * 2206 * @return the estimated number of waiting threads 2207 * @throws IllegalMonitorStateException if {@link #isHeldExclusively} 2208 * returns {@code false} 2209 */ 2210 protected final int getWaitQueueLength() { 2211 if (!isHeldExclusively()) 2212 throw new IllegalMonitorStateException(); 2213 int n = 0; 2214 for (Node w = firstWaiter; w != null; w = w.nextWaiter) { 2215 if (w.waitStatus == Node.CONDITION) 2216 ++n; 2217 } 2218 return n; 2219 } 2220 2221 /** 2222 * Returns a collection containing those threads that may be 2223 * waiting on this Condition. 2224 * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads}. 2225 * 2226 * @return the collection of threads 2227 * @throws IllegalMonitorStateException if {@link #isHeldExclusively} 2228 * returns {@code false} 2229 */ 2230 protected final Collection<Thread> getWaitingThreads() { 2231 if (!isHeldExclusively()) 2232 throw new IllegalMonitorStateException(); 2233 ArrayList<Thread> list = new ArrayList<Thread>(); 2234 for (Node w = firstWaiter; w != null; w = w.nextWaiter) { 2235 if (w.waitStatus == Node.CONDITION) { 2236 Thread t = w.thread; 2237 if (t != null) 2238 list.add(t); 2239 } 2240 } 2241 return list; 2242 } 2243 } 2244 2245 /** 2246 * Setup to support compareAndSet. We need to natively implement 2247 * this here: For the sake of permitting future enhancements, we 2248 * cannot explicitly subclass AtomicInteger, which would be 2249 * efficient and useful otherwise. So, as the lesser of evils, we 2250 * natively implement using hotspot intrinsics API. And while we 2251 * are at it, we do the same for other CASable fields (which could 2252 * otherwise be done with atomic field updaters). 2253 */ 2254 // BEGIN android-changed 2255 private static final Unsafe unsafe = UnsafeAccess.THE_ONE; 2256 // END android-changed 2257 private static final long stateOffset; 2258 private static final long headOffset; 2259 private static final long tailOffset; 2260 private static final long waitStatusOffset; 2261 private static final long nextOffset; 2262 2263 static { 2264 try { 2265 stateOffset = unsafe.objectFieldOffset 2266 (AbstractQueuedSynchronizer.class.getDeclaredField("state")); 2267 headOffset = unsafe.objectFieldOffset 2268 (AbstractQueuedSynchronizer.class.getDeclaredField("head")); 2269 tailOffset = unsafe.objectFieldOffset 2270 (AbstractQueuedSynchronizer.class.getDeclaredField("tail")); 2271 waitStatusOffset = unsafe.objectFieldOffset 2272 (Node.class.getDeclaredField("waitStatus")); 2273 nextOffset = unsafe.objectFieldOffset 2274 (Node.class.getDeclaredField("next")); 2275 2276 } catch (Exception ex) { throw new Error(ex); } 2277 } 2278 2279 /** 2280 * CAS head field. Used only by enq. 2281 */ 2282 private final boolean compareAndSetHead(Node update) { 2283 return unsafe.compareAndSwapObject(this, headOffset, null, update); 2284 } 2285 2286 /** 2287 * CAS tail field. Used only by enq. 2288 */ 2289 private final boolean compareAndSetTail(Node expect, Node update) { 2290 return unsafe.compareAndSwapObject(this, tailOffset, expect, update); 2291 } 2292 2293 /** 2294 * CAS waitStatus field of a node. 2295 */ 2296 private static final boolean compareAndSetWaitStatus(Node node, 2297 int expect, 2298 int update) { 2299 return unsafe.compareAndSwapInt(node, waitStatusOffset, 2300 expect, update); 2301 } 2302 2303 /** 2304 * CAS next field of a node. 2305 */ 2306 private static final boolean compareAndSetNext(Node node, 2307 Node expect, 2308 Node update) { 2309 return unsafe.compareAndSwapObject(node, nextOffset, expect, update); 2310 } 2311} 2312