J. Duke | 319a3b9 | 2007-12-01 00:00:00 +0000 | [diff] [blame^] | 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. Sun designates this |
| 7 | * particular file as subject to the "Classpath" exception as provided |
| 8 | * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, |
| 21 | * CA 95054 USA or visit www.sun.com if you need additional information or |
| 22 | * have any 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/licenses/publicdomain |
| 34 | */ |
| 35 | |
| 36 | package java.util.concurrent.locks; |
| 37 | import java.util.*; |
| 38 | import java.util.concurrent.*; |
| 39 | import java.util.concurrent.atomic.*; |
| 40 | import sun.misc.Unsafe; |
| 41 | |
| 42 | /** |
| 43 | * Provides a framework for implementing blocking locks and related |
| 44 | * synchronizers (semaphores, events, etc) that rely on |
| 45 | * first-in-first-out (FIFO) wait queues. This class is designed to |
| 46 | * be a useful basis for most kinds of synchronizers that rely on a |
| 47 | * single atomic <tt>int</tt> value to represent state. Subclasses |
| 48 | * must define the protected methods that change this state, and which |
| 49 | * define what that state means in terms of this object being acquired |
| 50 | * or released. Given these, the other methods in this class carry |
| 51 | * out all queuing and blocking mechanics. Subclasses can maintain |
| 52 | * other state fields, but only the atomically updated <tt>int</tt> |
| 53 | * value manipulated using methods {@link #getState}, {@link |
| 54 | * #setState} and {@link #compareAndSetState} is tracked with respect |
| 55 | * to synchronization. |
| 56 | * |
| 57 | * <p>Subclasses should be defined as non-public internal helper |
| 58 | * classes that are used to implement the synchronization properties |
| 59 | * of their enclosing class. Class |
| 60 | * <tt>AbstractQueuedSynchronizer</tt> does not implement any |
| 61 | * synchronization interface. Instead it defines methods such as |
| 62 | * {@link #acquireInterruptibly} that can be invoked as |
| 63 | * appropriate by concrete locks and related synchronizers to |
| 64 | * implement their public methods. |
| 65 | * |
| 66 | * <p>This class supports either or both a default <em>exclusive</em> |
| 67 | * mode and a <em>shared</em> mode. When acquired in exclusive mode, |
| 68 | * attempted acquires by other threads cannot succeed. Shared mode |
| 69 | * acquires by multiple threads may (but need not) succeed. This class |
| 70 | * does not "understand" these differences except in the |
| 71 | * mechanical sense that when a shared mode acquire succeeds, the next |
| 72 | * waiting thread (if one exists) must also determine whether it can |
| 73 | * acquire as well. Threads waiting in the different modes share the |
| 74 | * same FIFO queue. Usually, implementation subclasses support only |
| 75 | * one of these modes, but both can come into play for example in a |
| 76 | * {@link ReadWriteLock}. Subclasses that support only exclusive or |
| 77 | * only shared modes need not define the methods supporting the unused mode. |
| 78 | * |
| 79 | * <p>This class defines a nested {@link ConditionObject} class that |
| 80 | * can be used as a {@link Condition} implementation by subclasses |
| 81 | * supporting exclusive mode for which method {@link |
| 82 | * #isHeldExclusively} reports whether synchronization is exclusively |
| 83 | * held with respect to the current thread, method {@link #release} |
| 84 | * invoked with the current {@link #getState} value fully releases |
| 85 | * this object, and {@link #acquire}, given this saved state value, |
| 86 | * eventually restores this object to its previous acquired state. No |
| 87 | * <tt>AbstractQueuedSynchronizer</tt> method otherwise creates such a |
| 88 | * condition, so if this constraint cannot be met, do not use it. The |
| 89 | * behavior of {@link ConditionObject} depends of course on the |
| 90 | * semantics of its synchronizer implementation. |
| 91 | * |
| 92 | * <p>This class provides inspection, instrumentation, and monitoring |
| 93 | * methods for the internal queue, as well as similar methods for |
| 94 | * condition objects. These can be exported as desired into classes |
| 95 | * using an <tt>AbstractQueuedSynchronizer</tt> for their |
| 96 | * synchronization mechanics. |
| 97 | * |
| 98 | * <p>Serialization of this class stores only the underlying atomic |
| 99 | * integer maintaining state, so deserialized objects have empty |
| 100 | * thread queues. Typical subclasses requiring serializability will |
| 101 | * define a <tt>readObject</tt> method that restores this to a known |
| 102 | * initial state upon deserialization. |
| 103 | * |
| 104 | * <h3>Usage</h3> |
| 105 | * |
| 106 | * <p>To use this class as the basis of a synchronizer, redefine the |
| 107 | * following methods, as applicable, by inspecting and/or modifying |
| 108 | * the synchronization state using {@link #getState}, {@link |
| 109 | * #setState} and/or {@link #compareAndSetState}: |
| 110 | * |
| 111 | * <ul> |
| 112 | * <li> {@link #tryAcquire} |
| 113 | * <li> {@link #tryRelease} |
| 114 | * <li> {@link #tryAcquireShared} |
| 115 | * <li> {@link #tryReleaseShared} |
| 116 | * <li> {@link #isHeldExclusively} |
| 117 | *</ul> |
| 118 | * |
| 119 | * Each of these methods by default throws {@link |
| 120 | * UnsupportedOperationException}. Implementations of these methods |
| 121 | * must be internally thread-safe, and should in general be short and |
| 122 | * not block. Defining these methods is the <em>only</em> supported |
| 123 | * means of using this class. All other methods are declared |
| 124 | * <tt>final</tt> because they cannot be independently varied. |
| 125 | * |
| 126 | * <p>You may also find the inherited methods from {@link |
| 127 | * AbstractOwnableSynchronizer} useful to keep track of the thread |
| 128 | * owning an exclusive synchronizer. You are encouraged to use them |
| 129 | * -- this enables monitoring and diagnostic tools to assist users in |
| 130 | * determining which threads hold locks. |
| 131 | * |
| 132 | * <p>Even though this class is based on an internal FIFO queue, it |
| 133 | * does not automatically enforce FIFO acquisition policies. The core |
| 134 | * of exclusive synchronization takes the form: |
| 135 | * |
| 136 | * <pre> |
| 137 | * Acquire: |
| 138 | * while (!tryAcquire(arg)) { |
| 139 | * <em>enqueue thread if it is not already queued</em>; |
| 140 | * <em>possibly block current thread</em>; |
| 141 | * } |
| 142 | * |
| 143 | * Release: |
| 144 | * if (tryRelease(arg)) |
| 145 | * <em>unblock the first queued thread</em>; |
| 146 | * </pre> |
| 147 | * |
| 148 | * (Shared mode is similar but may involve cascading signals.) |
| 149 | * |
| 150 | * <p><a name="barging">Because checks in acquire are invoked before |
| 151 | * enqueuing, a newly acquiring thread may <em>barge</em> ahead of |
| 152 | * others that are blocked and queued. However, you can, if desired, |
| 153 | * define <tt>tryAcquire</tt> and/or <tt>tryAcquireShared</tt> to |
| 154 | * disable barging by internally invoking one or more of the inspection |
| 155 | * methods, thereby providing a <em>fair</em> FIFO acquisition order. |
| 156 | * In particular, most fair synchronizers can define <tt>tryAcquire</tt> |
| 157 | * to return <tt>false</tt> if {@link #hasQueuedPredecessors} (a method |
| 158 | * specifically designed to be used by fair synchronizers) returns |
| 159 | * <tt>true</tt>. Other variations are possible. |
| 160 | * |
| 161 | * <p>Throughput and scalability are generally highest for the |
| 162 | * default barging (also known as <em>greedy</em>, |
| 163 | * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy. |
| 164 | * While this is not guaranteed to be fair or starvation-free, earlier |
| 165 | * queued threads are allowed to recontend before later queued |
| 166 | * threads, and each recontention has an unbiased chance to succeed |
| 167 | * against incoming threads. Also, while acquires do not |
| 168 | * "spin" in the usual sense, they may perform multiple |
| 169 | * invocations of <tt>tryAcquire</tt> interspersed with other |
| 170 | * computations before blocking. This gives most of the benefits of |
| 171 | * spins when exclusive synchronization is only briefly held, without |
| 172 | * most of the liabilities when it isn't. If so desired, you can |
| 173 | * augment this by preceding calls to acquire methods with |
| 174 | * "fast-path" checks, possibly prechecking {@link #hasContended} |
| 175 | * and/or {@link #hasQueuedThreads} to only do so if the synchronizer |
| 176 | * is likely not to be contended. |
| 177 | * |
| 178 | * <p>This class provides an efficient and scalable basis for |
| 179 | * synchronization in part by specializing its range of use to |
| 180 | * synchronizers that can rely on <tt>int</tt> state, acquire, and |
| 181 | * release parameters, and an internal FIFO wait queue. When this does |
| 182 | * not suffice, you can build synchronizers from a lower level using |
| 183 | * {@link java.util.concurrent.atomic atomic} classes, your own custom |
| 184 | * {@link java.util.Queue} classes, and {@link LockSupport} blocking |
| 185 | * support. |
| 186 | * |
| 187 | * <h3>Usage Examples</h3> |
| 188 | * |
| 189 | * <p>Here is a non-reentrant mutual exclusion lock class that uses |
| 190 | * the value zero to represent the unlocked state, and one to |
| 191 | * represent the locked state. While a non-reentrant lock |
| 192 | * does not strictly require recording of the current owner |
| 193 | * thread, this class does so anyway to make usage easier to monitor. |
| 194 | * It also supports conditions and exposes |
| 195 | * one of the instrumentation methods: |
| 196 | * |
| 197 | * <pre> |
| 198 | * class Mutex implements Lock, java.io.Serializable { |
| 199 | * |
| 200 | * // Our internal helper class |
| 201 | * private static class Sync extends AbstractQueuedSynchronizer { |
| 202 | * // Report whether in locked state |
| 203 | * protected boolean isHeldExclusively() { |
| 204 | * return getState() == 1; |
| 205 | * } |
| 206 | * |
| 207 | * // Acquire the lock if state is zero |
| 208 | * public boolean tryAcquire(int acquires) { |
| 209 | * assert acquires == 1; // Otherwise unused |
| 210 | * if (compareAndSetState(0, 1)) { |
| 211 | * setExclusiveOwnerThread(Thread.currentThread()); |
| 212 | * return true; |
| 213 | * } |
| 214 | * return false; |
| 215 | * } |
| 216 | * |
| 217 | * // Release the lock by setting state to zero |
| 218 | * protected boolean tryRelease(int releases) { |
| 219 | * assert releases == 1; // Otherwise unused |
| 220 | * if (getState() == 0) throw new IllegalMonitorStateException(); |
| 221 | * setExclusiveOwnerThread(null); |
| 222 | * setState(0); |
| 223 | * return true; |
| 224 | * } |
| 225 | * |
| 226 | * // Provide a Condition |
| 227 | * Condition newCondition() { return new ConditionObject(); } |
| 228 | * |
| 229 | * // Deserialize properly |
| 230 | * private void readObject(ObjectInputStream s) |
| 231 | * throws IOException, ClassNotFoundException { |
| 232 | * s.defaultReadObject(); |
| 233 | * setState(0); // reset to unlocked state |
| 234 | * } |
| 235 | * } |
| 236 | * |
| 237 | * // The sync object does all the hard work. We just forward to it. |
| 238 | * private final Sync sync = new Sync(); |
| 239 | * |
| 240 | * public void lock() { sync.acquire(1); } |
| 241 | * public boolean tryLock() { return sync.tryAcquire(1); } |
| 242 | * public void unlock() { sync.release(1); } |
| 243 | * public Condition newCondition() { return sync.newCondition(); } |
| 244 | * public boolean isLocked() { return sync.isHeldExclusively(); } |
| 245 | * public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); } |
| 246 | * public void lockInterruptibly() throws InterruptedException { |
| 247 | * sync.acquireInterruptibly(1); |
| 248 | * } |
| 249 | * public boolean tryLock(long timeout, TimeUnit unit) |
| 250 | * throws InterruptedException { |
| 251 | * return sync.tryAcquireNanos(1, unit.toNanos(timeout)); |
| 252 | * } |
| 253 | * } |
| 254 | * </pre> |
| 255 | * |
| 256 | * <p>Here is a latch class that is like a {@link CountDownLatch} |
| 257 | * except that it only requires a single <tt>signal</tt> to |
| 258 | * fire. Because a latch is non-exclusive, it uses the <tt>shared</tt> |
| 259 | * acquire and release methods. |
| 260 | * |
| 261 | * <pre> |
| 262 | * class BooleanLatch { |
| 263 | * |
| 264 | * private static class Sync extends AbstractQueuedSynchronizer { |
| 265 | * boolean isSignalled() { return getState() != 0; } |
| 266 | * |
| 267 | * protected int tryAcquireShared(int ignore) { |
| 268 | * return isSignalled()? 1 : -1; |
| 269 | * } |
| 270 | * |
| 271 | * protected boolean tryReleaseShared(int ignore) { |
| 272 | * setState(1); |
| 273 | * return true; |
| 274 | * } |
| 275 | * } |
| 276 | * |
| 277 | * private final Sync sync = new Sync(); |
| 278 | * public boolean isSignalled() { return sync.isSignalled(); } |
| 279 | * public void signal() { sync.releaseShared(1); } |
| 280 | * public void await() throws InterruptedException { |
| 281 | * sync.acquireSharedInterruptibly(1); |
| 282 | * } |
| 283 | * } |
| 284 | * </pre> |
| 285 | * |
| 286 | * @since 1.5 |
| 287 | * @author Doug Lea |
| 288 | */ |
| 289 | public abstract class AbstractQueuedSynchronizer |
| 290 | extends AbstractOwnableSynchronizer |
| 291 | implements java.io.Serializable { |
| 292 | |
| 293 | private static final long serialVersionUID = 7373984972572414691L; |
| 294 | |
| 295 | /** |
| 296 | * Creates a new <tt>AbstractQueuedSynchronizer</tt> instance |
| 297 | * with initial synchronization state of zero. |
| 298 | */ |
| 299 | protected AbstractQueuedSynchronizer() { } |
| 300 | |
| 301 | /** |
| 302 | * Wait queue node class. |
| 303 | * |
| 304 | * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and |
| 305 | * Hagersten) lock queue. CLH locks are normally used for |
| 306 | * spinlocks. We instead use them for blocking synchronizers, but |
| 307 | * use the same basic tactic of holding some of the control |
| 308 | * information about a thread in the predecessor of its node. A |
| 309 | * "status" field in each node keeps track of whether a thread |
| 310 | * should block. A node is signalled when its predecessor |
| 311 | * releases. Each node of the queue otherwise serves as a |
| 312 | * specific-notification-style monitor holding a single waiting |
| 313 | * thread. The status field does NOT control whether threads are |
| 314 | * granted locks etc though. A thread may try to acquire if it is |
| 315 | * first in the queue. But being first does not guarantee success; |
| 316 | * it only gives the right to contend. So the currently released |
| 317 | * contender thread may need to rewait. |
| 318 | * |
| 319 | * <p>To enqueue into a CLH lock, you atomically splice it in as new |
| 320 | * tail. To dequeue, you just set the head field. |
| 321 | * <pre> |
| 322 | * +------+ prev +-----+ +-----+ |
| 323 | * head | | <---- | | <---- | | tail |
| 324 | * +------+ +-----+ +-----+ |
| 325 | * </pre> |
| 326 | * |
| 327 | * <p>Insertion into a CLH queue requires only a single atomic |
| 328 | * operation on "tail", so there is a simple atomic point of |
| 329 | * demarcation from unqueued to queued. Similarly, dequeing |
| 330 | * involves only updating the "head". However, it takes a bit |
| 331 | * more work for nodes to determine who their successors are, |
| 332 | * in part to deal with possible cancellation due to timeouts |
| 333 | * and interrupts. |
| 334 | * |
| 335 | * <p>The "prev" links (not used in original CLH locks), are mainly |
| 336 | * needed to handle cancellation. If a node is cancelled, its |
| 337 | * successor is (normally) relinked to a non-cancelled |
| 338 | * predecessor. For explanation of similar mechanics in the case |
| 339 | * of spin locks, see the papers by Scott and Scherer at |
| 340 | * http://www.cs.rochester.edu/u/scott/synchronization/ |
| 341 | * |
| 342 | * <p>We also use "next" links to implement blocking mechanics. |
| 343 | * The thread id for each node is kept in its own node, so a |
| 344 | * predecessor signals the next node to wake up by traversing |
| 345 | * next link to determine which thread it is. Determination of |
| 346 | * successor must avoid races with newly queued nodes to set |
| 347 | * the "next" fields of their predecessors. This is solved |
| 348 | * when necessary by checking backwards from the atomically |
| 349 | * updated "tail" when a node's successor appears to be null. |
| 350 | * (Or, said differently, the next-links are an optimization |
| 351 | * so that we don't usually need a backward scan.) |
| 352 | * |
| 353 | * <p>Cancellation introduces some conservatism to the basic |
| 354 | * algorithms. Since we must poll for cancellation of other |
| 355 | * nodes, we can miss noticing whether a cancelled node is |
| 356 | * ahead or behind us. This is dealt with by always unparking |
| 357 | * successors upon cancellation, allowing them to stabilize on |
| 358 | * a new predecessor, unless we can identify an uncancelled |
| 359 | * predecessor who will carry this responsibility. |
| 360 | * |
| 361 | * <p>CLH queues need a dummy header node to get started. But |
| 362 | * we don't create them on construction, because it would be wasted |
| 363 | * effort if there is never contention. Instead, the node |
| 364 | * is constructed and head and tail pointers are set upon first |
| 365 | * contention. |
| 366 | * |
| 367 | * <p>Threads waiting on Conditions use the same nodes, but |
| 368 | * use an additional link. Conditions only need to link nodes |
| 369 | * in simple (non-concurrent) linked queues because they are |
| 370 | * only accessed when exclusively held. Upon await, a node is |
| 371 | * inserted into a condition queue. Upon signal, the node is |
| 372 | * transferred to the main queue. A special value of status |
| 373 | * field is used to mark which queue a node is on. |
| 374 | * |
| 375 | * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill |
| 376 | * Scherer and Michael Scott, along with members of JSR-166 |
| 377 | * expert group, for helpful ideas, discussions, and critiques |
| 378 | * on the design of this class. |
| 379 | */ |
| 380 | static final class Node { |
| 381 | /** Marker to indicate a node is waiting in shared mode */ |
| 382 | static final Node SHARED = new Node(); |
| 383 | /** Marker to indicate a node is waiting in exclusive mode */ |
| 384 | static final Node EXCLUSIVE = null; |
| 385 | |
| 386 | /** waitStatus value to indicate thread has cancelled */ |
| 387 | static final int CANCELLED = 1; |
| 388 | /** waitStatus value to indicate successor's thread needs unparking */ |
| 389 | static final int SIGNAL = -1; |
| 390 | /** waitStatus value to indicate thread is waiting on condition */ |
| 391 | static final int CONDITION = -2; |
| 392 | |
| 393 | /** |
| 394 | * Status field, taking on only the values: |
| 395 | * SIGNAL: The successor of this node is (or will soon be) |
| 396 | * blocked (via park), so the current node must |
| 397 | * unpark its successor when it releases or |
| 398 | * cancels. To avoid races, acquire methods must |
| 399 | * first indicate they need a signal, |
| 400 | * then retry the atomic acquire, and then, |
| 401 | * on failure, block. |
| 402 | * CANCELLED: This node is cancelled due to timeout or interrupt. |
| 403 | * Nodes never leave this state. In particular, |
| 404 | * a thread with cancelled node never again blocks. |
| 405 | * CONDITION: This node is currently on a condition queue. |
| 406 | * It will not be used as a sync queue node until |
| 407 | * transferred. (Use of this value here |
| 408 | * has nothing to do with the other uses |
| 409 | * of the field, but simplifies mechanics.) |
| 410 | * 0: None of the above |
| 411 | * |
| 412 | * The values are arranged numerically to simplify use. |
| 413 | * Non-negative values mean that a node doesn't need to |
| 414 | * signal. So, most code doesn't need to check for particular |
| 415 | * values, just for sign. |
| 416 | * |
| 417 | * The field is initialized to 0 for normal sync nodes, and |
| 418 | * CONDITION for condition nodes. It is modified using CAS |
| 419 | * (or when possible, unconditional volatile writes). |
| 420 | */ |
| 421 | volatile int waitStatus; |
| 422 | |
| 423 | /** |
| 424 | * Link to predecessor node that current node/thread relies on |
| 425 | * for checking waitStatus. Assigned during enqueing, and nulled |
| 426 | * out (for sake of GC) only upon dequeuing. Also, upon |
| 427 | * cancellation of a predecessor, we short-circuit while |
| 428 | * finding a non-cancelled one, which will always exist |
| 429 | * because the head node is never cancelled: A node becomes |
| 430 | * head only as a result of successful acquire. A |
| 431 | * cancelled thread never succeeds in acquiring, and a thread only |
| 432 | * cancels itself, not any other node. |
| 433 | */ |
| 434 | volatile Node prev; |
| 435 | |
| 436 | /** |
| 437 | * Link to the successor node that the current node/thread |
| 438 | * unparks upon release. Assigned during enqueuing, adjusted |
| 439 | * when bypassing cancelled predecessors, and nulled out (for |
| 440 | * sake of GC) when dequeued. The enq operation does not |
| 441 | * assign next field of a predecessor until after attachment, |
| 442 | * so seeing a null next field does not necessarily mean that |
| 443 | * node is at end of queue. However, if a next field appears |
| 444 | * to be null, we can scan prev's from the tail to |
| 445 | * double-check. The next field of cancelled nodes is set to |
| 446 | * point to the node itself instead of null, to make life |
| 447 | * easier for isOnSyncQueue. |
| 448 | */ |
| 449 | volatile Node next; |
| 450 | |
| 451 | /** |
| 452 | * The thread that enqueued this node. Initialized on |
| 453 | * construction and nulled out after use. |
| 454 | */ |
| 455 | volatile Thread thread; |
| 456 | |
| 457 | /** |
| 458 | * Link to next node waiting on condition, or the special |
| 459 | * value SHARED. Because condition queues are accessed only |
| 460 | * when holding in exclusive mode, we just need a simple |
| 461 | * linked queue to hold nodes while they are waiting on |
| 462 | * conditions. They are then transferred to the queue to |
| 463 | * re-acquire. And because conditions can only be exclusive, |
| 464 | * we save a field by using special value to indicate shared |
| 465 | * mode. |
| 466 | */ |
| 467 | Node nextWaiter; |
| 468 | |
| 469 | /** |
| 470 | * Returns true if node is waiting in shared mode |
| 471 | */ |
| 472 | final boolean isShared() { |
| 473 | return nextWaiter == SHARED; |
| 474 | } |
| 475 | |
| 476 | /** |
| 477 | * Returns previous node, or throws NullPointerException if null. |
| 478 | * Use when predecessor cannot be null. The null check could |
| 479 | * be elided, but is present to help the VM. |
| 480 | * |
| 481 | * @return the predecessor of this node |
| 482 | */ |
| 483 | final Node predecessor() throws NullPointerException { |
| 484 | Node p = prev; |
| 485 | if (p == null) |
| 486 | throw new NullPointerException(); |
| 487 | else |
| 488 | return p; |
| 489 | } |
| 490 | |
| 491 | Node() { // Used to establish initial head or SHARED marker |
| 492 | } |
| 493 | |
| 494 | Node(Thread thread, Node mode) { // Used by addWaiter |
| 495 | this.nextWaiter = mode; |
| 496 | this.thread = thread; |
| 497 | } |
| 498 | |
| 499 | Node(Thread thread, int waitStatus) { // Used by Condition |
| 500 | this.waitStatus = waitStatus; |
| 501 | this.thread = thread; |
| 502 | } |
| 503 | } |
| 504 | |
| 505 | /** |
| 506 | * Head of the wait queue, lazily initialized. Except for |
| 507 | * initialization, it is modified only via method setHead. Note: |
| 508 | * If head exists, its waitStatus is guaranteed not to be |
| 509 | * CANCELLED. |
| 510 | */ |
| 511 | private transient volatile Node head; |
| 512 | |
| 513 | /** |
| 514 | * Tail of the wait queue, lazily initialized. Modified only via |
| 515 | * method enq to add new wait node. |
| 516 | */ |
| 517 | private transient volatile Node tail; |
| 518 | |
| 519 | /** |
| 520 | * The synchronization state. |
| 521 | */ |
| 522 | private volatile int state; |
| 523 | |
| 524 | /** |
| 525 | * Returns the current value of synchronization state. |
| 526 | * This operation has memory semantics of a <tt>volatile</tt> read. |
| 527 | * @return current state value |
| 528 | */ |
| 529 | protected final int getState() { |
| 530 | return state; |
| 531 | } |
| 532 | |
| 533 | /** |
| 534 | * Sets the value of synchronization state. |
| 535 | * This operation has memory semantics of a <tt>volatile</tt> write. |
| 536 | * @param newState the new state value |
| 537 | */ |
| 538 | protected final void setState(int newState) { |
| 539 | state = newState; |
| 540 | } |
| 541 | |
| 542 | /** |
| 543 | * Atomically sets synchronization state to the given updated |
| 544 | * value if the current state value equals the expected value. |
| 545 | * This operation has memory semantics of a <tt>volatile</tt> read |
| 546 | * and write. |
| 547 | * |
| 548 | * @param expect the expected value |
| 549 | * @param update the new value |
| 550 | * @return true if successful. False return indicates that the actual |
| 551 | * value was not equal to the expected value. |
| 552 | */ |
| 553 | protected final boolean compareAndSetState(int expect, int update) { |
| 554 | // See below for intrinsics setup to support this |
| 555 | return unsafe.compareAndSwapInt(this, stateOffset, expect, update); |
| 556 | } |
| 557 | |
| 558 | // Queuing utilities |
| 559 | |
| 560 | /** |
| 561 | * The number of nanoseconds for which it is faster to spin |
| 562 | * rather than to use timed park. A rough estimate suffices |
| 563 | * to improve responsiveness with very short timeouts. |
| 564 | */ |
| 565 | static final long spinForTimeoutThreshold = 1000L; |
| 566 | |
| 567 | /** |
| 568 | * Inserts node into queue, initializing if necessary. See picture above. |
| 569 | * @param node the node to insert |
| 570 | * @return node's predecessor |
| 571 | */ |
| 572 | private Node enq(final Node node) { |
| 573 | for (;;) { |
| 574 | Node t = tail; |
| 575 | if (t == null) { // Must initialize |
| 576 | if (compareAndSetHead(new Node())) |
| 577 | tail = head; |
| 578 | } else { |
| 579 | node.prev = t; |
| 580 | if (compareAndSetTail(t, node)) { |
| 581 | t.next = node; |
| 582 | return t; |
| 583 | } |
| 584 | } |
| 585 | } |
| 586 | } |
| 587 | |
| 588 | /** |
| 589 | * Creates and enqueues node for current thread and given mode. |
| 590 | * |
| 591 | * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared |
| 592 | * @return the new node |
| 593 | */ |
| 594 | private Node addWaiter(Node mode) { |
| 595 | Node node = new Node(Thread.currentThread(), mode); |
| 596 | // Try the fast path of enq; backup to full enq on failure |
| 597 | Node pred = tail; |
| 598 | if (pred != null) { |
| 599 | node.prev = pred; |
| 600 | if (compareAndSetTail(pred, node)) { |
| 601 | pred.next = node; |
| 602 | return node; |
| 603 | } |
| 604 | } |
| 605 | enq(node); |
| 606 | return node; |
| 607 | } |
| 608 | |
| 609 | /** |
| 610 | * Sets head of queue to be node, thus dequeuing. Called only by |
| 611 | * acquire methods. Also nulls out unused fields for sake of GC |
| 612 | * and to suppress unnecessary signals and traversals. |
| 613 | * |
| 614 | * @param node the node |
| 615 | */ |
| 616 | private void setHead(Node node) { |
| 617 | head = node; |
| 618 | node.thread = null; |
| 619 | node.prev = null; |
| 620 | } |
| 621 | |
| 622 | /** |
| 623 | * Wakes up node's successor, if one exists. |
| 624 | * |
| 625 | * @param node the node |
| 626 | */ |
| 627 | private void unparkSuccessor(Node node) { |
| 628 | /* |
| 629 | * Try to clear status in anticipation of signalling. It is |
| 630 | * OK if this fails or if status is changed by waiting thread. |
| 631 | */ |
| 632 | compareAndSetWaitStatus(node, Node.SIGNAL, 0); |
| 633 | |
| 634 | /* |
| 635 | * Thread to unpark is held in successor, which is normally |
| 636 | * just the next node. But if cancelled or apparently null, |
| 637 | * traverse backwards from tail to find the actual |
| 638 | * non-cancelled successor. |
| 639 | */ |
| 640 | Node s = node.next; |
| 641 | if (s == null || s.waitStatus > 0) { |
| 642 | s = null; |
| 643 | for (Node t = tail; t != null && t != node; t = t.prev) |
| 644 | if (t.waitStatus <= 0) |
| 645 | s = t; |
| 646 | } |
| 647 | if (s != null) |
| 648 | LockSupport.unpark(s.thread); |
| 649 | } |
| 650 | |
| 651 | /** |
| 652 | * Sets head of queue, and checks if successor may be waiting |
| 653 | * in shared mode, if so propagating if propagate > 0. |
| 654 | * |
| 655 | * @param pred the node holding waitStatus for node |
| 656 | * @param node the node |
| 657 | * @param propagate the return value from a tryAcquireShared |
| 658 | */ |
| 659 | private void setHeadAndPropagate(Node node, int propagate) { |
| 660 | setHead(node); |
| 661 | if (propagate > 0 && node.waitStatus != 0) { |
| 662 | /* |
| 663 | * Don't bother fully figuring out successor. If it |
| 664 | * looks null, call unparkSuccessor anyway to be safe. |
| 665 | */ |
| 666 | Node s = node.next; |
| 667 | if (s == null || s.isShared()) |
| 668 | unparkSuccessor(node); |
| 669 | } |
| 670 | } |
| 671 | |
| 672 | // Utilities for various versions of acquire |
| 673 | |
| 674 | /** |
| 675 | * Cancels an ongoing attempt to acquire. |
| 676 | * |
| 677 | * @param node the node |
| 678 | */ |
| 679 | private void cancelAcquire(Node node) { |
| 680 | // Ignore if node doesn't exist |
| 681 | if (node == null) |
| 682 | return; |
| 683 | |
| 684 | node.thread = null; |
| 685 | |
| 686 | // Skip cancelled predecessors |
| 687 | Node pred = node.prev; |
| 688 | while (pred.waitStatus > 0) |
| 689 | node.prev = pred = pred.prev; |
| 690 | |
| 691 | // Getting this before setting waitStatus ensures staleness |
| 692 | Node predNext = pred.next; |
| 693 | |
| 694 | // Can use unconditional write instead of CAS here |
| 695 | node.waitStatus = Node.CANCELLED; |
| 696 | |
| 697 | // If we are the tail, remove ourselves |
| 698 | if (node == tail && compareAndSetTail(node, pred)) { |
| 699 | compareAndSetNext(pred, predNext, null); |
| 700 | } else { |
| 701 | // If "active" predecessor found... |
| 702 | if (pred != head |
| 703 | && (pred.waitStatus == Node.SIGNAL |
| 704 | || compareAndSetWaitStatus(pred, 0, Node.SIGNAL)) |
| 705 | && pred.thread != null) { |
| 706 | |
| 707 | // If successor is active, set predecessor's next link |
| 708 | Node next = node.next; |
| 709 | if (next != null && next.waitStatus <= 0) |
| 710 | compareAndSetNext(pred, predNext, next); |
| 711 | } else { |
| 712 | unparkSuccessor(node); |
| 713 | } |
| 714 | |
| 715 | node.next = node; // help GC |
| 716 | } |
| 717 | } |
| 718 | |
| 719 | /** |
| 720 | * Checks and updates status for a node that failed to acquire. |
| 721 | * Returns true if thread should block. This is the main signal |
| 722 | * control in all acquire loops. Requires that pred == node.prev |
| 723 | * |
| 724 | * @param pred node's predecessor holding status |
| 725 | * @param node the node |
| 726 | * @return {@code true} if thread should block |
| 727 | */ |
| 728 | private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) { |
| 729 | int s = pred.waitStatus; |
| 730 | if (s < 0) |
| 731 | /* |
| 732 | * This node has already set status asking a release |
| 733 | * to signal it, so it can safely park. |
| 734 | */ |
| 735 | return true; |
| 736 | if (s > 0) { |
| 737 | /* |
| 738 | * Predecessor was cancelled. Skip over predecessors and |
| 739 | * indicate retry. |
| 740 | */ |
| 741 | do { |
| 742 | node.prev = pred = pred.prev; |
| 743 | } while (pred.waitStatus > 0); |
| 744 | pred.next = node; |
| 745 | } |
| 746 | else |
| 747 | /* |
| 748 | * Indicate that we need a signal, but don't park yet. Caller |
| 749 | * will need to retry to make sure it cannot acquire before |
| 750 | * parking. |
| 751 | */ |
| 752 | compareAndSetWaitStatus(pred, 0, Node.SIGNAL); |
| 753 | return false; |
| 754 | } |
| 755 | |
| 756 | /** |
| 757 | * Convenience method to interrupt current thread. |
| 758 | */ |
| 759 | private static void selfInterrupt() { |
| 760 | Thread.currentThread().interrupt(); |
| 761 | } |
| 762 | |
| 763 | /** |
| 764 | * Convenience method to park and then check if interrupted |
| 765 | * |
| 766 | * @return {@code true} if interrupted |
| 767 | */ |
| 768 | private final boolean parkAndCheckInterrupt() { |
| 769 | LockSupport.park(this); |
| 770 | return Thread.interrupted(); |
| 771 | } |
| 772 | |
| 773 | /* |
| 774 | * Various flavors of acquire, varying in exclusive/shared and |
| 775 | * control modes. Each is mostly the same, but annoyingly |
| 776 | * different. Only a little bit of factoring is possible due to |
| 777 | * interactions of exception mechanics (including ensuring that we |
| 778 | * cancel if tryAcquire throws exception) and other control, at |
| 779 | * least not without hurting performance too much. |
| 780 | */ |
| 781 | |
| 782 | /** |
| 783 | * Acquires in exclusive uninterruptible mode for thread already in |
| 784 | * queue. Used by condition wait methods as well as acquire. |
| 785 | * |
| 786 | * @param node the node |
| 787 | * @param arg the acquire argument |
| 788 | * @return {@code true} if interrupted while waiting |
| 789 | */ |
| 790 | final boolean acquireQueued(final Node node, int arg) { |
| 791 | boolean failed = true; |
| 792 | try { |
| 793 | boolean interrupted = false; |
| 794 | for (;;) { |
| 795 | final Node p = node.predecessor(); |
| 796 | if (p == head && tryAcquire(arg)) { |
| 797 | setHead(node); |
| 798 | p.next = null; // help GC |
| 799 | failed = false; |
| 800 | return interrupted; |
| 801 | } |
| 802 | if (shouldParkAfterFailedAcquire(p, node) && |
| 803 | parkAndCheckInterrupt()) |
| 804 | interrupted = true; |
| 805 | } |
| 806 | } finally { |
| 807 | if (failed) |
| 808 | cancelAcquire(node); |
| 809 | } |
| 810 | } |
| 811 | |
| 812 | /** |
| 813 | * Acquires in exclusive interruptible mode. |
| 814 | * @param arg the acquire argument |
| 815 | */ |
| 816 | private void doAcquireInterruptibly(int arg) |
| 817 | throws InterruptedException { |
| 818 | final Node node = addWaiter(Node.EXCLUSIVE); |
| 819 | boolean failed = true; |
| 820 | try { |
| 821 | for (;;) { |
| 822 | final Node p = node.predecessor(); |
| 823 | if (p == head && tryAcquire(arg)) { |
| 824 | setHead(node); |
| 825 | p.next = null; // help GC |
| 826 | failed = false; |
| 827 | return; |
| 828 | } |
| 829 | if (shouldParkAfterFailedAcquire(p, node) && |
| 830 | parkAndCheckInterrupt()) |
| 831 | throw new InterruptedException(); |
| 832 | } |
| 833 | } finally { |
| 834 | if (failed) |
| 835 | cancelAcquire(node); |
| 836 | } |
| 837 | } |
| 838 | |
| 839 | /** |
| 840 | * Acquires in exclusive timed mode. |
| 841 | * |
| 842 | * @param arg the acquire argument |
| 843 | * @param nanosTimeout max wait time |
| 844 | * @return {@code true} if acquired |
| 845 | */ |
| 846 | private boolean doAcquireNanos(int arg, long nanosTimeout) |
| 847 | throws InterruptedException { |
| 848 | long lastTime = System.nanoTime(); |
| 849 | final Node node = addWaiter(Node.EXCLUSIVE); |
| 850 | boolean failed = true; |
| 851 | try { |
| 852 | for (;;) { |
| 853 | final Node p = node.predecessor(); |
| 854 | if (p == head && tryAcquire(arg)) { |
| 855 | setHead(node); |
| 856 | p.next = null; // help GC |
| 857 | failed = false; |
| 858 | return true; |
| 859 | } |
| 860 | if (nanosTimeout <= 0) |
| 861 | return false; |
| 862 | if (shouldParkAfterFailedAcquire(p, node) && |
| 863 | nanosTimeout > spinForTimeoutThreshold) |
| 864 | LockSupport.parkNanos(this, nanosTimeout); |
| 865 | long now = System.nanoTime(); |
| 866 | nanosTimeout -= now - lastTime; |
| 867 | lastTime = now; |
| 868 | if (Thread.interrupted()) |
| 869 | throw new InterruptedException(); |
| 870 | } |
| 871 | } finally { |
| 872 | if (failed) |
| 873 | cancelAcquire(node); |
| 874 | } |
| 875 | } |
| 876 | |
| 877 | /** |
| 878 | * Acquires in shared uninterruptible mode. |
| 879 | * @param arg the acquire argument |
| 880 | */ |
| 881 | private void doAcquireShared(int arg) { |
| 882 | final Node node = addWaiter(Node.SHARED); |
| 883 | boolean failed = true; |
| 884 | try { |
| 885 | boolean interrupted = false; |
| 886 | for (;;) { |
| 887 | final Node p = node.predecessor(); |
| 888 | if (p == head) { |
| 889 | int r = tryAcquireShared(arg); |
| 890 | if (r >= 0) { |
| 891 | setHeadAndPropagate(node, r); |
| 892 | p.next = null; // help GC |
| 893 | if (interrupted) |
| 894 | selfInterrupt(); |
| 895 | failed = false; |
| 896 | return; |
| 897 | } |
| 898 | } |
| 899 | if (shouldParkAfterFailedAcquire(p, node) && |
| 900 | parkAndCheckInterrupt()) |
| 901 | interrupted = true; |
| 902 | } |
| 903 | } finally { |
| 904 | if (failed) |
| 905 | cancelAcquire(node); |
| 906 | } |
| 907 | } |
| 908 | |
| 909 | /** |
| 910 | * Acquires in shared interruptible mode. |
| 911 | * @param arg the acquire argument |
| 912 | */ |
| 913 | private void doAcquireSharedInterruptibly(int arg) |
| 914 | throws InterruptedException { |
| 915 | final Node node = addWaiter(Node.SHARED); |
| 916 | boolean failed = true; |
| 917 | try { |
| 918 | for (;;) { |
| 919 | final Node p = node.predecessor(); |
| 920 | if (p == head) { |
| 921 | int r = tryAcquireShared(arg); |
| 922 | if (r >= 0) { |
| 923 | setHeadAndPropagate(node, r); |
| 924 | p.next = null; // help GC |
| 925 | failed = false; |
| 926 | return; |
| 927 | } |
| 928 | } |
| 929 | if (shouldParkAfterFailedAcquire(p, node) && |
| 930 | parkAndCheckInterrupt()) |
| 931 | throw new InterruptedException(); |
| 932 | } |
| 933 | } finally { |
| 934 | if (failed) |
| 935 | cancelAcquire(node); |
| 936 | } |
| 937 | } |
| 938 | |
| 939 | /** |
| 940 | * Acquires in shared timed mode. |
| 941 | * |
| 942 | * @param arg the acquire argument |
| 943 | * @param nanosTimeout max wait time |
| 944 | * @return {@code true} if acquired |
| 945 | */ |
| 946 | private boolean doAcquireSharedNanos(int arg, long nanosTimeout) |
| 947 | throws InterruptedException { |
| 948 | |
| 949 | long lastTime = System.nanoTime(); |
| 950 | final Node node = addWaiter(Node.SHARED); |
| 951 | boolean failed = true; |
| 952 | try { |
| 953 | for (;;) { |
| 954 | final Node p = node.predecessor(); |
| 955 | if (p == head) { |
| 956 | int r = tryAcquireShared(arg); |
| 957 | if (r >= 0) { |
| 958 | setHeadAndPropagate(node, r); |
| 959 | p.next = null; // help GC |
| 960 | failed = false; |
| 961 | return true; |
| 962 | } |
| 963 | } |
| 964 | if (nanosTimeout <= 0) |
| 965 | return false; |
| 966 | if (shouldParkAfterFailedAcquire(p, node) && |
| 967 | nanosTimeout > spinForTimeoutThreshold) |
| 968 | LockSupport.parkNanos(this, nanosTimeout); |
| 969 | long now = System.nanoTime(); |
| 970 | nanosTimeout -= now - lastTime; |
| 971 | lastTime = now; |
| 972 | if (Thread.interrupted()) |
| 973 | throw new InterruptedException(); |
| 974 | } |
| 975 | } finally { |
| 976 | if (failed) |
| 977 | cancelAcquire(node); |
| 978 | } |
| 979 | } |
| 980 | |
| 981 | // Main exported methods |
| 982 | |
| 983 | /** |
| 984 | * Attempts to acquire in exclusive mode. This method should query |
| 985 | * if the state of the object permits it to be acquired in the |
| 986 | * exclusive mode, and if so to acquire it. |
| 987 | * |
| 988 | * <p>This method is always invoked by the thread performing |
| 989 | * acquire. If this method reports failure, the acquire method |
| 990 | * may queue the thread, if it is not already queued, until it is |
| 991 | * signalled by a release from some other thread. This can be used |
| 992 | * to implement method {@link Lock#tryLock()}. |
| 993 | * |
| 994 | * <p>The default |
| 995 | * implementation throws {@link UnsupportedOperationException}. |
| 996 | * |
| 997 | * @param arg the acquire argument. This value is always the one |
| 998 | * passed to an acquire method, or is the value saved on entry |
| 999 | * to a condition wait. The value is otherwise uninterpreted |
| 1000 | * and can represent anything you like. |
| 1001 | * @return {@code true} if successful. Upon success, this object has |
| 1002 | * been acquired. |
| 1003 | * @throws IllegalMonitorStateException if acquiring would place this |
| 1004 | * synchronizer in an illegal state. This exception must be |
| 1005 | * thrown in a consistent fashion for synchronization to work |
| 1006 | * correctly. |
| 1007 | * @throws UnsupportedOperationException if exclusive mode is not supported |
| 1008 | */ |
| 1009 | protected boolean tryAcquire(int arg) { |
| 1010 | throw new UnsupportedOperationException(); |
| 1011 | } |
| 1012 | |
| 1013 | /** |
| 1014 | * Attempts to set the state to reflect a release in exclusive |
| 1015 | * mode. |
| 1016 | * |
| 1017 | * <p>This method is always invoked by the thread performing release. |
| 1018 | * |
| 1019 | * <p>The default implementation throws |
| 1020 | * {@link UnsupportedOperationException}. |
| 1021 | * |
| 1022 | * @param arg the release argument. This value is always the one |
| 1023 | * passed to a release method, or the current state value upon |
| 1024 | * entry to a condition wait. The value is otherwise |
| 1025 | * uninterpreted and can represent anything you like. |
| 1026 | * @return {@code true} if this object is now in a fully released |
| 1027 | * state, so that any waiting threads may attempt to acquire; |
| 1028 | * and {@code false} otherwise. |
| 1029 | * @throws IllegalMonitorStateException if releasing would place this |
| 1030 | * synchronizer in an illegal state. This exception must be |
| 1031 | * thrown in a consistent fashion for synchronization to work |
| 1032 | * correctly. |
| 1033 | * @throws UnsupportedOperationException if exclusive mode is not supported |
| 1034 | */ |
| 1035 | protected boolean tryRelease(int arg) { |
| 1036 | throw new UnsupportedOperationException(); |
| 1037 | } |
| 1038 | |
| 1039 | /** |
| 1040 | * Attempts to acquire in shared mode. This method should query if |
| 1041 | * the state of the object permits it to be acquired in the shared |
| 1042 | * mode, and if so to acquire it. |
| 1043 | * |
| 1044 | * <p>This method is always invoked by the thread performing |
| 1045 | * acquire. If this method reports failure, the acquire method |
| 1046 | * may queue the thread, if it is not already queued, until it is |
| 1047 | * signalled by a release from some other thread. |
| 1048 | * |
| 1049 | * <p>The default implementation throws {@link |
| 1050 | * UnsupportedOperationException}. |
| 1051 | * |
| 1052 | * @param arg the acquire argument. This value is always the one |
| 1053 | * passed to an acquire method, or is the value saved on entry |
| 1054 | * to a condition wait. The value is otherwise uninterpreted |
| 1055 | * and can represent anything you like. |
| 1056 | * @return a negative value on failure; zero if acquisition in shared |
| 1057 | * mode succeeded but no subsequent shared-mode acquire can |
| 1058 | * succeed; and a positive value if acquisition in shared |
| 1059 | * mode succeeded and subsequent shared-mode acquires might |
| 1060 | * also succeed, in which case a subsequent waiting thread |
| 1061 | * must check availability. (Support for three different |
| 1062 | * return values enables this method to be used in contexts |
| 1063 | * where acquires only sometimes act exclusively.) Upon |
| 1064 | * success, this object has been acquired. |
| 1065 | * @throws IllegalMonitorStateException if acquiring would place this |
| 1066 | * synchronizer in an illegal state. This exception must be |
| 1067 | * thrown in a consistent fashion for synchronization to work |
| 1068 | * correctly. |
| 1069 | * @throws UnsupportedOperationException if shared mode is not supported |
| 1070 | */ |
| 1071 | protected int tryAcquireShared(int arg) { |
| 1072 | throw new UnsupportedOperationException(); |
| 1073 | } |
| 1074 | |
| 1075 | /** |
| 1076 | * Attempts to set the state to reflect a release in shared mode. |
| 1077 | * |
| 1078 | * <p>This method is always invoked by the thread performing release. |
| 1079 | * |
| 1080 | * <p>The default implementation throws |
| 1081 | * {@link UnsupportedOperationException}. |
| 1082 | * |
| 1083 | * @param arg the release argument. This value is always the one |
| 1084 | * passed to a release method, or the current state value upon |
| 1085 | * entry to a condition wait. The value is otherwise |
| 1086 | * uninterpreted and can represent anything you like. |
| 1087 | * @return {@code true} if this release of shared mode may permit a |
| 1088 | * waiting acquire (shared or exclusive) to succeed; and |
| 1089 | * {@code false} otherwise |
| 1090 | * @throws IllegalMonitorStateException if releasing would place this |
| 1091 | * synchronizer in an illegal state. This exception must be |
| 1092 | * thrown in a consistent fashion for synchronization to work |
| 1093 | * correctly. |
| 1094 | * @throws UnsupportedOperationException if shared mode is not supported |
| 1095 | */ |
| 1096 | protected boolean tryReleaseShared(int arg) { |
| 1097 | throw new UnsupportedOperationException(); |
| 1098 | } |
| 1099 | |
| 1100 | /** |
| 1101 | * Returns {@code true} if synchronization is held exclusively with |
| 1102 | * respect to the current (calling) thread. This method is invoked |
| 1103 | * upon each call to a non-waiting {@link ConditionObject} method. |
| 1104 | * (Waiting methods instead invoke {@link #release}.) |
| 1105 | * |
| 1106 | * <p>The default implementation throws {@link |
| 1107 | * UnsupportedOperationException}. This method is invoked |
| 1108 | * internally only within {@link ConditionObject} methods, so need |
| 1109 | * not be defined if conditions are not used. |
| 1110 | * |
| 1111 | * @return {@code true} if synchronization is held exclusively; |
| 1112 | * {@code false} otherwise |
| 1113 | * @throws UnsupportedOperationException if conditions are not supported |
| 1114 | */ |
| 1115 | protected boolean isHeldExclusively() { |
| 1116 | throw new UnsupportedOperationException(); |
| 1117 | } |
| 1118 | |
| 1119 | /** |
| 1120 | * Acquires in exclusive mode, ignoring interrupts. Implemented |
| 1121 | * by invoking at least once {@link #tryAcquire}, |
| 1122 | * returning on success. Otherwise the thread is queued, possibly |
| 1123 | * repeatedly blocking and unblocking, invoking {@link |
| 1124 | * #tryAcquire} until success. This method can be used |
| 1125 | * to implement method {@link Lock#lock}. |
| 1126 | * |
| 1127 | * @param arg the acquire argument. This value is conveyed to |
| 1128 | * {@link #tryAcquire} but is otherwise uninterpreted and |
| 1129 | * can represent anything you like. |
| 1130 | */ |
| 1131 | public final void acquire(int arg) { |
| 1132 | if (!tryAcquire(arg) && |
| 1133 | acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) |
| 1134 | selfInterrupt(); |
| 1135 | } |
| 1136 | |
| 1137 | /** |
| 1138 | * Acquires in exclusive mode, aborting if interrupted. |
| 1139 | * Implemented by first checking interrupt status, then invoking |
| 1140 | * at least once {@link #tryAcquire}, returning on |
| 1141 | * success. Otherwise the thread is queued, possibly repeatedly |
| 1142 | * blocking and unblocking, invoking {@link #tryAcquire} |
| 1143 | * until success or the thread is interrupted. This method can be |
| 1144 | * used to implement method {@link Lock#lockInterruptibly}. |
| 1145 | * |
| 1146 | * @param arg the acquire argument. This value is conveyed to |
| 1147 | * {@link #tryAcquire} but is otherwise uninterpreted and |
| 1148 | * can represent anything you like. |
| 1149 | * @throws InterruptedException if the current thread is interrupted |
| 1150 | */ |
| 1151 | public final void acquireInterruptibly(int arg) throws InterruptedException { |
| 1152 | if (Thread.interrupted()) |
| 1153 | throw new InterruptedException(); |
| 1154 | if (!tryAcquire(arg)) |
| 1155 | doAcquireInterruptibly(arg); |
| 1156 | } |
| 1157 | |
| 1158 | /** |
| 1159 | * Attempts to acquire in exclusive mode, aborting if interrupted, |
| 1160 | * and failing if the given timeout elapses. Implemented by first |
| 1161 | * checking interrupt status, then invoking at least once {@link |
| 1162 | * #tryAcquire}, returning on success. Otherwise, the thread is |
| 1163 | * queued, possibly repeatedly blocking and unblocking, invoking |
| 1164 | * {@link #tryAcquire} until success or the thread is interrupted |
| 1165 | * or the timeout elapses. This method can be used to implement |
| 1166 | * method {@link Lock#tryLock(long, TimeUnit)}. |
| 1167 | * |
| 1168 | * @param arg the acquire argument. This value is conveyed to |
| 1169 | * {@link #tryAcquire} but is otherwise uninterpreted and |
| 1170 | * can represent anything you like. |
| 1171 | * @param nanosTimeout the maximum number of nanoseconds to wait |
| 1172 | * @return {@code true} if acquired; {@code false} if timed out |
| 1173 | * @throws InterruptedException if the current thread is interrupted |
| 1174 | */ |
| 1175 | public final boolean tryAcquireNanos(int arg, long nanosTimeout) throws InterruptedException { |
| 1176 | if (Thread.interrupted()) |
| 1177 | throw new InterruptedException(); |
| 1178 | return tryAcquire(arg) || |
| 1179 | doAcquireNanos(arg, nanosTimeout); |
| 1180 | } |
| 1181 | |
| 1182 | /** |
| 1183 | * Releases in exclusive mode. Implemented by unblocking one or |
| 1184 | * more threads if {@link #tryRelease} returns true. |
| 1185 | * This method can be used to implement method {@link Lock#unlock}. |
| 1186 | * |
| 1187 | * @param arg the release argument. This value is conveyed to |
| 1188 | * {@link #tryRelease} but is otherwise uninterpreted and |
| 1189 | * can represent anything you like. |
| 1190 | * @return the value returned from {@link #tryRelease} |
| 1191 | */ |
| 1192 | public final boolean release(int arg) { |
| 1193 | if (tryRelease(arg)) { |
| 1194 | Node h = head; |
| 1195 | if (h != null && h.waitStatus != 0) |
| 1196 | unparkSuccessor(h); |
| 1197 | return true; |
| 1198 | } |
| 1199 | return false; |
| 1200 | } |
| 1201 | |
| 1202 | /** |
| 1203 | * Acquires in shared mode, ignoring interrupts. Implemented by |
| 1204 | * first invoking at least once {@link #tryAcquireShared}, |
| 1205 | * returning on success. Otherwise the thread is queued, possibly |
| 1206 | * repeatedly blocking and unblocking, invoking {@link |
| 1207 | * #tryAcquireShared} until success. |
| 1208 | * |
| 1209 | * @param arg the acquire argument. This value is conveyed to |
| 1210 | * {@link #tryAcquireShared} but is otherwise uninterpreted |
| 1211 | * and can represent anything you like. |
| 1212 | */ |
| 1213 | public final void acquireShared(int arg) { |
| 1214 | if (tryAcquireShared(arg) < 0) |
| 1215 | doAcquireShared(arg); |
| 1216 | } |
| 1217 | |
| 1218 | /** |
| 1219 | * Acquires in shared mode, aborting if interrupted. Implemented |
| 1220 | * by first checking interrupt status, then invoking at least once |
| 1221 | * {@link #tryAcquireShared}, returning on success. Otherwise the |
| 1222 | * thread is queued, possibly repeatedly blocking and unblocking, |
| 1223 | * invoking {@link #tryAcquireShared} until success or the thread |
| 1224 | * is interrupted. |
| 1225 | * @param arg the acquire argument |
| 1226 | * This value is conveyed to {@link #tryAcquireShared} but is |
| 1227 | * otherwise uninterpreted and can represent anything |
| 1228 | * you like. |
| 1229 | * @throws InterruptedException if the current thread is interrupted |
| 1230 | */ |
| 1231 | public final void acquireSharedInterruptibly(int arg) throws InterruptedException { |
| 1232 | if (Thread.interrupted()) |
| 1233 | throw new InterruptedException(); |
| 1234 | if (tryAcquireShared(arg) < 0) |
| 1235 | doAcquireSharedInterruptibly(arg); |
| 1236 | } |
| 1237 | |
| 1238 | /** |
| 1239 | * Attempts to acquire in shared mode, aborting if interrupted, and |
| 1240 | * failing if the given timeout elapses. Implemented by first |
| 1241 | * checking interrupt status, then invoking at least once {@link |
| 1242 | * #tryAcquireShared}, returning on success. Otherwise, the |
| 1243 | * thread is queued, possibly repeatedly blocking and unblocking, |
| 1244 | * invoking {@link #tryAcquireShared} until success or the thread |
| 1245 | * is interrupted or the timeout elapses. |
| 1246 | * |
| 1247 | * @param arg the acquire argument. This value is conveyed to |
| 1248 | * {@link #tryAcquireShared} but is otherwise uninterpreted |
| 1249 | * and can represent anything you like. |
| 1250 | * @param nanosTimeout the maximum number of nanoseconds to wait |
| 1251 | * @return {@code true} if acquired; {@code false} if timed out |
| 1252 | * @throws InterruptedException if the current thread is interrupted |
| 1253 | */ |
| 1254 | public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException { |
| 1255 | if (Thread.interrupted()) |
| 1256 | throw new InterruptedException(); |
| 1257 | return tryAcquireShared(arg) >= 0 || |
| 1258 | doAcquireSharedNanos(arg, nanosTimeout); |
| 1259 | } |
| 1260 | |
| 1261 | /** |
| 1262 | * Releases in shared mode. Implemented by unblocking one or more |
| 1263 | * threads if {@link #tryReleaseShared} returns true. |
| 1264 | * |
| 1265 | * @param arg the release argument. This value is conveyed to |
| 1266 | * {@link #tryReleaseShared} but is otherwise uninterpreted |
| 1267 | * and can represent anything you like. |
| 1268 | * @return the value returned from {@link #tryReleaseShared} |
| 1269 | */ |
| 1270 | public final boolean releaseShared(int arg) { |
| 1271 | if (tryReleaseShared(arg)) { |
| 1272 | Node h = head; |
| 1273 | if (h != null && h.waitStatus != 0) |
| 1274 | unparkSuccessor(h); |
| 1275 | return true; |
| 1276 | } |
| 1277 | return false; |
| 1278 | } |
| 1279 | |
| 1280 | // Queue inspection methods |
| 1281 | |
| 1282 | /** |
| 1283 | * Queries whether any threads are waiting to acquire. Note that |
| 1284 | * because cancellations due to interrupts and timeouts may occur |
| 1285 | * at any time, a {@code true} return does not guarantee that any |
| 1286 | * other thread will ever acquire. |
| 1287 | * |
| 1288 | * <p>In this implementation, this operation returns in |
| 1289 | * constant time. |
| 1290 | * |
| 1291 | * @return {@code true} if there may be other threads waiting to acquire |
| 1292 | */ |
| 1293 | public final boolean hasQueuedThreads() { |
| 1294 | return head != tail; |
| 1295 | } |
| 1296 | |
| 1297 | /** |
| 1298 | * Queries whether any threads have ever contended to acquire this |
| 1299 | * synchronizer; that is if an acquire method has ever blocked. |
| 1300 | * |
| 1301 | * <p>In this implementation, this operation returns in |
| 1302 | * constant time. |
| 1303 | * |
| 1304 | * @return {@code true} if there has ever been contention |
| 1305 | */ |
| 1306 | public final boolean hasContended() { |
| 1307 | return head != null; |
| 1308 | } |
| 1309 | |
| 1310 | /** |
| 1311 | * Returns the first (longest-waiting) thread in the queue, or |
| 1312 | * {@code null} if no threads are currently queued. |
| 1313 | * |
| 1314 | * <p>In this implementation, this operation normally returns in |
| 1315 | * constant time, but may iterate upon contention if other threads are |
| 1316 | * concurrently modifying the queue. |
| 1317 | * |
| 1318 | * @return the first (longest-waiting) thread in the queue, or |
| 1319 | * {@code null} if no threads are currently queued |
| 1320 | */ |
| 1321 | public final Thread getFirstQueuedThread() { |
| 1322 | // handle only fast path, else relay |
| 1323 | return (head == tail) ? null : fullGetFirstQueuedThread(); |
| 1324 | } |
| 1325 | |
| 1326 | /** |
| 1327 | * Version of getFirstQueuedThread called when fastpath fails |
| 1328 | */ |
| 1329 | private Thread fullGetFirstQueuedThread() { |
| 1330 | /* |
| 1331 | * The first node is normally head.next. Try to get its |
| 1332 | * thread field, ensuring consistent reads: If thread |
| 1333 | * field is nulled out or s.prev is no longer head, then |
| 1334 | * some other thread(s) concurrently performed setHead in |
| 1335 | * between some of our reads. We try this twice before |
| 1336 | * resorting to traversal. |
| 1337 | */ |
| 1338 | Node h, s; |
| 1339 | Thread st; |
| 1340 | if (((h = head) != null && (s = h.next) != null && |
| 1341 | s.prev == head && (st = s.thread) != null) || |
| 1342 | ((h = head) != null && (s = h.next) != null && |
| 1343 | s.prev == head && (st = s.thread) != null)) |
| 1344 | return st; |
| 1345 | |
| 1346 | /* |
| 1347 | * Head's next field might not have been set yet, or may have |
| 1348 | * been unset after setHead. So we must check to see if tail |
| 1349 | * is actually first node. If not, we continue on, safely |
| 1350 | * traversing from tail back to head to find first, |
| 1351 | * guaranteeing termination. |
| 1352 | */ |
| 1353 | |
| 1354 | Node t = tail; |
| 1355 | Thread firstThread = null; |
| 1356 | while (t != null && t != head) { |
| 1357 | Thread tt = t.thread; |
| 1358 | if (tt != null) |
| 1359 | firstThread = tt; |
| 1360 | t = t.prev; |
| 1361 | } |
| 1362 | return firstThread; |
| 1363 | } |
| 1364 | |
| 1365 | /** |
| 1366 | * Returns true if the given thread is currently queued. |
| 1367 | * |
| 1368 | * <p>This implementation traverses the queue to determine |
| 1369 | * presence of the given thread. |
| 1370 | * |
| 1371 | * @param thread the thread |
| 1372 | * @return {@code true} if the given thread is on the queue |
| 1373 | * @throws NullPointerException if the thread is null |
| 1374 | */ |
| 1375 | public final boolean isQueued(Thread thread) { |
| 1376 | if (thread == null) |
| 1377 | throw new NullPointerException(); |
| 1378 | for (Node p = tail; p != null; p = p.prev) |
| 1379 | if (p.thread == thread) |
| 1380 | return true; |
| 1381 | return false; |
| 1382 | } |
| 1383 | |
| 1384 | /** |
| 1385 | * Returns {@code true} if the apparent first queued thread, if one |
| 1386 | * exists, is waiting in exclusive mode. If this method returns |
| 1387 | * {@code true}, and the current thread is attempting to acquire in |
| 1388 | * shared mode (that is, this method is invoked from {@link |
| 1389 | * #tryAcquireShared}) then it is guaranteed that the current thread |
| 1390 | * is not the first queued thread. Used only as a heuristic in |
| 1391 | * ReentrantReadWriteLock. |
| 1392 | */ |
| 1393 | final boolean apparentlyFirstQueuedIsExclusive() { |
| 1394 | Node h, s; |
| 1395 | return (h = head) != null && |
| 1396 | (s = h.next) != null && |
| 1397 | !s.isShared() && |
| 1398 | s.thread != null; |
| 1399 | } |
| 1400 | |
| 1401 | /** |
| 1402 | * Queries whether any threads have been waiting to acquire longer |
| 1403 | * than the current thread. |
| 1404 | * |
| 1405 | * <p>An invocation of this method is equivalent to (but may be |
| 1406 | * more efficient than): |
| 1407 | * <pre> {@code |
| 1408 | * getFirstQueuedThread() != Thread.currentThread() && |
| 1409 | * hasQueuedThreads()}</pre> |
| 1410 | * |
| 1411 | * <p>Note that because cancellations due to interrupts and |
| 1412 | * timeouts may occur at any time, a {@code true} return does not |
| 1413 | * guarantee that some other thread will acquire before the current |
| 1414 | * thread. Likewise, it is possible for another thread to win a |
| 1415 | * race to enqueue after this method has returned {@code false}, |
| 1416 | * due to the queue being empty. |
| 1417 | * |
| 1418 | * <p>This method is designed to be used by a fair synchronizer to |
| 1419 | * avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>. |
| 1420 | * Such a synchronizer's {@link #tryAcquire} method should return |
| 1421 | * {@code false}, and its {@link #tryAcquireShared} method should |
| 1422 | * return a negative value, if this method returns {@code true} |
| 1423 | * (unless this is a reentrant acquire). For example, the {@code |
| 1424 | * tryAcquire} method for a fair, reentrant, exclusive mode |
| 1425 | * synchronizer might look like this: |
| 1426 | * |
| 1427 | * <pre> {@code |
| 1428 | * protected boolean tryAcquire(int arg) { |
| 1429 | * if (isHeldExclusively()) { |
| 1430 | * // A reentrant acquire; increment hold count |
| 1431 | * return true; |
| 1432 | * } else if (hasQueuedPredecessors()) { |
| 1433 | * return false; |
| 1434 | * } else { |
| 1435 | * // try to acquire normally |
| 1436 | * } |
| 1437 | * }}</pre> |
| 1438 | * |
| 1439 | * @return {@code true} if there is a queued thread preceding the |
| 1440 | * current thread, and {@code false} if the current thread |
| 1441 | * is at the head of the queue or the queue is empty |
| 1442 | * @since 1.7 |
| 1443 | */ |
| 1444 | public final boolean hasQueuedPredecessors() { |
| 1445 | // The correctness of this depends on head being initialized |
| 1446 | // before tail and on head.next being accurate if the current |
| 1447 | // thread is first in queue. |
| 1448 | Node h, s; |
| 1449 | return (h = head) != tail && |
| 1450 | ((s = h.next) == null || s.thread != Thread.currentThread()); |
| 1451 | } |
| 1452 | |
| 1453 | |
| 1454 | // Instrumentation and monitoring methods |
| 1455 | |
| 1456 | /** |
| 1457 | * Returns an estimate of the number of threads waiting to |
| 1458 | * acquire. The value is only an estimate because the number of |
| 1459 | * threads may change dynamically while this method traverses |
| 1460 | * internal data structures. This method is designed for use in |
| 1461 | * monitoring system state, not for synchronization |
| 1462 | * control. |
| 1463 | * |
| 1464 | * @return the estimated number of threads waiting to acquire |
| 1465 | */ |
| 1466 | public final int getQueueLength() { |
| 1467 | int n = 0; |
| 1468 | for (Node p = tail; p != null; p = p.prev) { |
| 1469 | if (p.thread != null) |
| 1470 | ++n; |
| 1471 | } |
| 1472 | return n; |
| 1473 | } |
| 1474 | |
| 1475 | /** |
| 1476 | * Returns a collection containing threads that may be waiting to |
| 1477 | * acquire. Because the actual set of threads may change |
| 1478 | * dynamically while constructing this result, the returned |
| 1479 | * collection is only a best-effort estimate. The elements of the |
| 1480 | * returned collection are in no particular order. This method is |
| 1481 | * designed to facilitate construction of subclasses that provide |
| 1482 | * more extensive monitoring facilities. |
| 1483 | * |
| 1484 | * @return the collection of threads |
| 1485 | */ |
| 1486 | public final Collection<Thread> getQueuedThreads() { |
| 1487 | ArrayList<Thread> list = new ArrayList<Thread>(); |
| 1488 | for (Node p = tail; p != null; p = p.prev) { |
| 1489 | Thread t = p.thread; |
| 1490 | if (t != null) |
| 1491 | list.add(t); |
| 1492 | } |
| 1493 | return list; |
| 1494 | } |
| 1495 | |
| 1496 | /** |
| 1497 | * Returns a collection containing threads that may be waiting to |
| 1498 | * acquire in exclusive mode. This has the same properties |
| 1499 | * as {@link #getQueuedThreads} except that it only returns |
| 1500 | * those threads waiting due to an exclusive acquire. |
| 1501 | * |
| 1502 | * @return the collection of threads |
| 1503 | */ |
| 1504 | public final Collection<Thread> getExclusiveQueuedThreads() { |
| 1505 | ArrayList<Thread> list = new ArrayList<Thread>(); |
| 1506 | for (Node p = tail; p != null; p = p.prev) { |
| 1507 | if (!p.isShared()) { |
| 1508 | Thread t = p.thread; |
| 1509 | if (t != null) |
| 1510 | list.add(t); |
| 1511 | } |
| 1512 | } |
| 1513 | return list; |
| 1514 | } |
| 1515 | |
| 1516 | /** |
| 1517 | * Returns a collection containing threads that may be waiting to |
| 1518 | * acquire in shared mode. This has the same properties |
| 1519 | * as {@link #getQueuedThreads} except that it only returns |
| 1520 | * those threads waiting due to a shared acquire. |
| 1521 | * |
| 1522 | * @return the collection of threads |
| 1523 | */ |
| 1524 | public final Collection<Thread> getSharedQueuedThreads() { |
| 1525 | ArrayList<Thread> list = new ArrayList<Thread>(); |
| 1526 | for (Node p = tail; p != null; p = p.prev) { |
| 1527 | if (p.isShared()) { |
| 1528 | Thread t = p.thread; |
| 1529 | if (t != null) |
| 1530 | list.add(t); |
| 1531 | } |
| 1532 | } |
| 1533 | return list; |
| 1534 | } |
| 1535 | |
| 1536 | /** |
| 1537 | * Returns a string identifying this synchronizer, as well as its state. |
| 1538 | * The state, in brackets, includes the String {@code "State ="} |
| 1539 | * followed by the current value of {@link #getState}, and either |
| 1540 | * {@code "nonempty"} or {@code "empty"} depending on whether the |
| 1541 | * queue is empty. |
| 1542 | * |
| 1543 | * @return a string identifying this synchronizer, as well as its state |
| 1544 | */ |
| 1545 | public String toString() { |
| 1546 | int s = getState(); |
| 1547 | String q = hasQueuedThreads() ? "non" : ""; |
| 1548 | return super.toString() + |
| 1549 | "[State = " + s + ", " + q + "empty queue]"; |
| 1550 | } |
| 1551 | |
| 1552 | |
| 1553 | // Internal support methods for Conditions |
| 1554 | |
| 1555 | /** |
| 1556 | * Returns true if a node, always one that was initially placed on |
| 1557 | * a condition queue, is now waiting to reacquire on sync queue. |
| 1558 | * @param node the node |
| 1559 | * @return true if is reacquiring |
| 1560 | */ |
| 1561 | final boolean isOnSyncQueue(Node node) { |
| 1562 | if (node.waitStatus == Node.CONDITION || node.prev == null) |
| 1563 | return false; |
| 1564 | if (node.next != null) // If has successor, it must be on queue |
| 1565 | return true; |
| 1566 | /* |
| 1567 | * node.prev can be non-null, but not yet on queue because |
| 1568 | * the CAS to place it on queue can fail. So we have to |
| 1569 | * traverse from tail to make sure it actually made it. It |
| 1570 | * will always be near the tail in calls to this method, and |
| 1571 | * unless the CAS failed (which is unlikely), it will be |
| 1572 | * there, so we hardly ever traverse much. |
| 1573 | */ |
| 1574 | return findNodeFromTail(node); |
| 1575 | } |
| 1576 | |
| 1577 | /** |
| 1578 | * Returns true if node is on sync queue by searching backwards from tail. |
| 1579 | * Called only when needed by isOnSyncQueue. |
| 1580 | * @return true if present |
| 1581 | */ |
| 1582 | private boolean findNodeFromTail(Node node) { |
| 1583 | Node t = tail; |
| 1584 | for (;;) { |
| 1585 | if (t == node) |
| 1586 | return true; |
| 1587 | if (t == null) |
| 1588 | return false; |
| 1589 | t = t.prev; |
| 1590 | } |
| 1591 | } |
| 1592 | |
| 1593 | /** |
| 1594 | * Transfers a node from a condition queue onto sync queue. |
| 1595 | * Returns true if successful. |
| 1596 | * @param node the node |
| 1597 | * @return true if successfully transferred (else the node was |
| 1598 | * cancelled before signal). |
| 1599 | */ |
| 1600 | final boolean transferForSignal(Node node) { |
| 1601 | /* |
| 1602 | * If cannot change waitStatus, the node has been cancelled. |
| 1603 | */ |
| 1604 | if (!compareAndSetWaitStatus(node, Node.CONDITION, 0)) |
| 1605 | return false; |
| 1606 | |
| 1607 | /* |
| 1608 | * Splice onto queue and try to set waitStatus of predecessor to |
| 1609 | * indicate that thread is (probably) waiting. If cancelled or |
| 1610 | * attempt to set waitStatus fails, wake up to resync (in which |
| 1611 | * case the waitStatus can be transiently and harmlessly wrong). |
| 1612 | */ |
| 1613 | Node p = enq(node); |
| 1614 | int c = p.waitStatus; |
| 1615 | if (c > 0 || !compareAndSetWaitStatus(p, c, Node.SIGNAL)) |
| 1616 | LockSupport.unpark(node.thread); |
| 1617 | return true; |
| 1618 | } |
| 1619 | |
| 1620 | /** |
| 1621 | * Transfers node, if necessary, to sync queue after a cancelled |
| 1622 | * wait. Returns true if thread was cancelled before being |
| 1623 | * signalled. |
| 1624 | * @param current the waiting thread |
| 1625 | * @param node its node |
| 1626 | * @return true if cancelled before the node was signalled |
| 1627 | */ |
| 1628 | final boolean transferAfterCancelledWait(Node node) { |
| 1629 | if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) { |
| 1630 | enq(node); |
| 1631 | return true; |
| 1632 | } |
| 1633 | /* |
| 1634 | * If we lost out to a signal(), then we can't proceed |
| 1635 | * until it finishes its enq(). Cancelling during an |
| 1636 | * incomplete transfer is both rare and transient, so just |
| 1637 | * spin. |
| 1638 | */ |
| 1639 | while (!isOnSyncQueue(node)) |
| 1640 | Thread.yield(); |
| 1641 | return false; |
| 1642 | } |
| 1643 | |
| 1644 | /** |
| 1645 | * Invokes release with current state value; returns saved state. |
| 1646 | * Cancels node and throws exception on failure. |
| 1647 | * @param node the condition node for this wait |
| 1648 | * @return previous sync state |
| 1649 | */ |
| 1650 | final int fullyRelease(Node node) { |
| 1651 | boolean failed = true; |
| 1652 | try { |
| 1653 | int savedState = getState(); |
| 1654 | if (release(savedState)) { |
| 1655 | failed = false; |
| 1656 | return savedState; |
| 1657 | } else { |
| 1658 | throw new IllegalMonitorStateException(); |
| 1659 | } |
| 1660 | } finally { |
| 1661 | if (failed) |
| 1662 | node.waitStatus = Node.CANCELLED; |
| 1663 | } |
| 1664 | } |
| 1665 | |
| 1666 | // Instrumentation methods for conditions |
| 1667 | |
| 1668 | /** |
| 1669 | * Queries whether the given ConditionObject |
| 1670 | * uses this synchronizer as its lock. |
| 1671 | * |
| 1672 | * @param condition the condition |
| 1673 | * @return <tt>true</tt> if owned |
| 1674 | * @throws NullPointerException if the condition is null |
| 1675 | */ |
| 1676 | public final boolean owns(ConditionObject condition) { |
| 1677 | if (condition == null) |
| 1678 | throw new NullPointerException(); |
| 1679 | return condition.isOwnedBy(this); |
| 1680 | } |
| 1681 | |
| 1682 | /** |
| 1683 | * Queries whether any threads are waiting on the given condition |
| 1684 | * associated with this synchronizer. Note that because timeouts |
| 1685 | * and interrupts may occur at any time, a <tt>true</tt> return |
| 1686 | * does not guarantee that a future <tt>signal</tt> will awaken |
| 1687 | * any threads. This method is designed primarily for use in |
| 1688 | * monitoring of the system state. |
| 1689 | * |
| 1690 | * @param condition the condition |
| 1691 | * @return <tt>true</tt> if there are any waiting threads |
| 1692 | * @throws IllegalMonitorStateException if exclusive synchronization |
| 1693 | * is not held |
| 1694 | * @throws IllegalArgumentException if the given condition is |
| 1695 | * not associated with this synchronizer |
| 1696 | * @throws NullPointerException if the condition is null |
| 1697 | */ |
| 1698 | public final boolean hasWaiters(ConditionObject condition) { |
| 1699 | if (!owns(condition)) |
| 1700 | throw new IllegalArgumentException("Not owner"); |
| 1701 | return condition.hasWaiters(); |
| 1702 | } |
| 1703 | |
| 1704 | /** |
| 1705 | * Returns an estimate of the number of threads waiting on the |
| 1706 | * given condition associated with this synchronizer. Note that |
| 1707 | * because timeouts and interrupts may occur at any time, the |
| 1708 | * estimate serves only as an upper bound on the actual number of |
| 1709 | * waiters. This method is designed for use in monitoring of the |
| 1710 | * system state, not for synchronization control. |
| 1711 | * |
| 1712 | * @param condition the condition |
| 1713 | * @return the estimated number of waiting threads |
| 1714 | * @throws IllegalMonitorStateException if exclusive synchronization |
| 1715 | * is not held |
| 1716 | * @throws IllegalArgumentException if the given condition is |
| 1717 | * not associated with this synchronizer |
| 1718 | * @throws NullPointerException if the condition is null |
| 1719 | */ |
| 1720 | public final int getWaitQueueLength(ConditionObject condition) { |
| 1721 | if (!owns(condition)) |
| 1722 | throw new IllegalArgumentException("Not owner"); |
| 1723 | return condition.getWaitQueueLength(); |
| 1724 | } |
| 1725 | |
| 1726 | /** |
| 1727 | * Returns a collection containing those threads that may be |
| 1728 | * waiting on the given condition associated with this |
| 1729 | * synchronizer. Because the actual set of threads may change |
| 1730 | * dynamically while constructing this result, the returned |
| 1731 | * collection is only a best-effort estimate. The elements of the |
| 1732 | * returned collection are in no particular order. |
| 1733 | * |
| 1734 | * @param condition the condition |
| 1735 | * @return the collection of threads |
| 1736 | * @throws IllegalMonitorStateException if exclusive synchronization |
| 1737 | * is not held |
| 1738 | * @throws IllegalArgumentException if the given condition is |
| 1739 | * not associated with this synchronizer |
| 1740 | * @throws NullPointerException if the condition is null |
| 1741 | */ |
| 1742 | public final Collection<Thread> getWaitingThreads(ConditionObject condition) { |
| 1743 | if (!owns(condition)) |
| 1744 | throw new IllegalArgumentException("Not owner"); |
| 1745 | return condition.getWaitingThreads(); |
| 1746 | } |
| 1747 | |
| 1748 | /** |
| 1749 | * Condition implementation for a {@link |
| 1750 | * AbstractQueuedSynchronizer} serving as the basis of a {@link |
| 1751 | * Lock} implementation. |
| 1752 | * |
| 1753 | * <p>Method documentation for this class describes mechanics, |
| 1754 | * not behavioral specifications from the point of view of Lock |
| 1755 | * and Condition users. Exported versions of this class will in |
| 1756 | * general need to be accompanied by documentation describing |
| 1757 | * condition semantics that rely on those of the associated |
| 1758 | * <tt>AbstractQueuedSynchronizer</tt>. |
| 1759 | * |
| 1760 | * <p>This class is Serializable, but all fields are transient, |
| 1761 | * so deserialized conditions have no waiters. |
| 1762 | */ |
| 1763 | public class ConditionObject implements Condition, java.io.Serializable { |
| 1764 | private static final long serialVersionUID = 1173984872572414699L; |
| 1765 | /** First node of condition queue. */ |
| 1766 | private transient Node firstWaiter; |
| 1767 | /** Last node of condition queue. */ |
| 1768 | private transient Node lastWaiter; |
| 1769 | |
| 1770 | /** |
| 1771 | * Creates a new <tt>ConditionObject</tt> instance. |
| 1772 | */ |
| 1773 | public ConditionObject() { } |
| 1774 | |
| 1775 | // Internal methods |
| 1776 | |
| 1777 | /** |
| 1778 | * Adds a new waiter to wait queue. |
| 1779 | * @return its new wait node |
| 1780 | */ |
| 1781 | private Node addConditionWaiter() { |
| 1782 | Node t = lastWaiter; |
| 1783 | // If lastWaiter is cancelled, clean out. |
| 1784 | if (t != null && t.waitStatus != Node.CONDITION) { |
| 1785 | unlinkCancelledWaiters(); |
| 1786 | t = lastWaiter; |
| 1787 | } |
| 1788 | Node node = new Node(Thread.currentThread(), Node.CONDITION); |
| 1789 | if (t == null) |
| 1790 | firstWaiter = node; |
| 1791 | else |
| 1792 | t.nextWaiter = node; |
| 1793 | lastWaiter = node; |
| 1794 | return node; |
| 1795 | } |
| 1796 | |
| 1797 | /** |
| 1798 | * Removes and transfers nodes until hit non-cancelled one or |
| 1799 | * null. Split out from signal in part to encourage compilers |
| 1800 | * to inline the case of no waiters. |
| 1801 | * @param first (non-null) the first node on condition queue |
| 1802 | */ |
| 1803 | private void doSignal(Node first) { |
| 1804 | do { |
| 1805 | if ( (firstWaiter = first.nextWaiter) == null) |
| 1806 | lastWaiter = null; |
| 1807 | first.nextWaiter = null; |
| 1808 | } while (!transferForSignal(first) && |
| 1809 | (first = firstWaiter) != null); |
| 1810 | } |
| 1811 | |
| 1812 | /** |
| 1813 | * Removes and transfers all nodes. |
| 1814 | * @param first (non-null) the first node on condition queue |
| 1815 | */ |
| 1816 | private void doSignalAll(Node first) { |
| 1817 | lastWaiter = firstWaiter = null; |
| 1818 | do { |
| 1819 | Node next = first.nextWaiter; |
| 1820 | first.nextWaiter = null; |
| 1821 | transferForSignal(first); |
| 1822 | first = next; |
| 1823 | } while (first != null); |
| 1824 | } |
| 1825 | |
| 1826 | /** |
| 1827 | * Unlinks cancelled waiter nodes from condition queue. |
| 1828 | * Called only while holding lock. This is called when |
| 1829 | * cancellation occurred during condition wait, and upon |
| 1830 | * insertion of a new waiter when lastWaiter is seen to have |
| 1831 | * been cancelled. This method is needed to avoid garbage |
| 1832 | * retention in the absence of signals. So even though it may |
| 1833 | * require a full traversal, it comes into play only when |
| 1834 | * timeouts or cancellations occur in the absence of |
| 1835 | * signals. It traverses all nodes rather than stopping at a |
| 1836 | * particular target to unlink all pointers to garbage nodes |
| 1837 | * without requiring many re-traversals during cancellation |
| 1838 | * storms. |
| 1839 | */ |
| 1840 | private void unlinkCancelledWaiters() { |
| 1841 | Node t = firstWaiter; |
| 1842 | Node trail = null; |
| 1843 | while (t != null) { |
| 1844 | Node next = t.nextWaiter; |
| 1845 | if (t.waitStatus != Node.CONDITION) { |
| 1846 | t.nextWaiter = null; |
| 1847 | if (trail == null) |
| 1848 | firstWaiter = next; |
| 1849 | else |
| 1850 | trail.nextWaiter = next; |
| 1851 | if (next == null) |
| 1852 | lastWaiter = trail; |
| 1853 | } |
| 1854 | else |
| 1855 | trail = t; |
| 1856 | t = next; |
| 1857 | } |
| 1858 | } |
| 1859 | |
| 1860 | // public methods |
| 1861 | |
| 1862 | /** |
| 1863 | * Moves the longest-waiting thread, if one exists, from the |
| 1864 | * wait queue for this condition to the wait queue for the |
| 1865 | * owning lock. |
| 1866 | * |
| 1867 | * @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
| 1868 | * returns {@code false} |
| 1869 | */ |
| 1870 | public final void signal() { |
| 1871 | if (!isHeldExclusively()) |
| 1872 | throw new IllegalMonitorStateException(); |
| 1873 | Node first = firstWaiter; |
| 1874 | if (first != null) |
| 1875 | doSignal(first); |
| 1876 | } |
| 1877 | |
| 1878 | /** |
| 1879 | * Moves all threads from the wait queue for this condition to |
| 1880 | * the wait queue for the owning lock. |
| 1881 | * |
| 1882 | * @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
| 1883 | * returns {@code false} |
| 1884 | */ |
| 1885 | public final void signalAll() { |
| 1886 | if (!isHeldExclusively()) |
| 1887 | throw new IllegalMonitorStateException(); |
| 1888 | Node first = firstWaiter; |
| 1889 | if (first != null) |
| 1890 | doSignalAll(first); |
| 1891 | } |
| 1892 | |
| 1893 | /** |
| 1894 | * Implements uninterruptible condition wait. |
| 1895 | * <ol> |
| 1896 | * <li> Save lock state returned by {@link #getState}. |
| 1897 | * <li> Invoke {@link #release} with |
| 1898 | * saved state as argument, throwing |
| 1899 | * IllegalMonitorStateException if it fails. |
| 1900 | * <li> Block until signalled. |
| 1901 | * <li> Reacquire by invoking specialized version of |
| 1902 | * {@link #acquire} with saved state as argument. |
| 1903 | * </ol> |
| 1904 | */ |
| 1905 | public final void awaitUninterruptibly() { |
| 1906 | Node node = addConditionWaiter(); |
| 1907 | int savedState = fullyRelease(node); |
| 1908 | boolean interrupted = false; |
| 1909 | while (!isOnSyncQueue(node)) { |
| 1910 | LockSupport.park(this); |
| 1911 | if (Thread.interrupted()) |
| 1912 | interrupted = true; |
| 1913 | } |
| 1914 | if (acquireQueued(node, savedState) || interrupted) |
| 1915 | selfInterrupt(); |
| 1916 | } |
| 1917 | |
| 1918 | /* |
| 1919 | * For interruptible waits, we need to track whether to throw |
| 1920 | * InterruptedException, if interrupted while blocked on |
| 1921 | * condition, versus reinterrupt current thread, if |
| 1922 | * interrupted while blocked waiting to re-acquire. |
| 1923 | */ |
| 1924 | |
| 1925 | /** Mode meaning to reinterrupt on exit from wait */ |
| 1926 | private static final int REINTERRUPT = 1; |
| 1927 | /** Mode meaning to throw InterruptedException on exit from wait */ |
| 1928 | private static final int THROW_IE = -1; |
| 1929 | |
| 1930 | /** |
| 1931 | * Checks for interrupt, returning THROW_IE if interrupted |
| 1932 | * before signalled, REINTERRUPT if after signalled, or |
| 1933 | * 0 if not interrupted. |
| 1934 | */ |
| 1935 | private int checkInterruptWhileWaiting(Node node) { |
| 1936 | return Thread.interrupted() ? |
| 1937 | (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) : |
| 1938 | 0; |
| 1939 | } |
| 1940 | |
| 1941 | /** |
| 1942 | * Throws InterruptedException, reinterrupts current thread, or |
| 1943 | * does nothing, depending on mode. |
| 1944 | */ |
| 1945 | private void reportInterruptAfterWait(int interruptMode) |
| 1946 | throws InterruptedException { |
| 1947 | if (interruptMode == THROW_IE) |
| 1948 | throw new InterruptedException(); |
| 1949 | else if (interruptMode == REINTERRUPT) |
| 1950 | selfInterrupt(); |
| 1951 | } |
| 1952 | |
| 1953 | /** |
| 1954 | * Implements interruptible condition wait. |
| 1955 | * <ol> |
| 1956 | * <li> If current thread is interrupted, throw InterruptedException. |
| 1957 | * <li> Save lock state returned by {@link #getState}. |
| 1958 | * <li> Invoke {@link #release} with |
| 1959 | * saved state as argument, throwing |
| 1960 | * IllegalMonitorStateException if it fails. |
| 1961 | * <li> Block until signalled or interrupted. |
| 1962 | * <li> Reacquire by invoking specialized version of |
| 1963 | * {@link #acquire} with saved state as argument. |
| 1964 | * <li> If interrupted while blocked in step 4, throw InterruptedException. |
| 1965 | * </ol> |
| 1966 | */ |
| 1967 | public final void await() throws InterruptedException { |
| 1968 | if (Thread.interrupted()) |
| 1969 | throw new InterruptedException(); |
| 1970 | Node node = addConditionWaiter(); |
| 1971 | int savedState = fullyRelease(node); |
| 1972 | int interruptMode = 0; |
| 1973 | while (!isOnSyncQueue(node)) { |
| 1974 | LockSupport.park(this); |
| 1975 | if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
| 1976 | break; |
| 1977 | } |
| 1978 | if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
| 1979 | interruptMode = REINTERRUPT; |
| 1980 | if (node.nextWaiter != null) // clean up if cancelled |
| 1981 | unlinkCancelledWaiters(); |
| 1982 | if (interruptMode != 0) |
| 1983 | reportInterruptAfterWait(interruptMode); |
| 1984 | } |
| 1985 | |
| 1986 | /** |
| 1987 | * Implements timed condition wait. |
| 1988 | * <ol> |
| 1989 | * <li> If current thread is interrupted, throw InterruptedException. |
| 1990 | * <li> Save lock state returned by {@link #getState}. |
| 1991 | * <li> Invoke {@link #release} with |
| 1992 | * saved state as argument, throwing |
| 1993 | * IllegalMonitorStateException if it fails. |
| 1994 | * <li> Block until signalled, interrupted, or timed out. |
| 1995 | * <li> Reacquire by invoking specialized version of |
| 1996 | * {@link #acquire} with saved state as argument. |
| 1997 | * <li> If interrupted while blocked in step 4, throw InterruptedException. |
| 1998 | * </ol> |
| 1999 | */ |
| 2000 | public final long awaitNanos(long nanosTimeout) throws InterruptedException { |
| 2001 | if (Thread.interrupted()) |
| 2002 | throw new InterruptedException(); |
| 2003 | Node node = addConditionWaiter(); |
| 2004 | int savedState = fullyRelease(node); |
| 2005 | long lastTime = System.nanoTime(); |
| 2006 | int interruptMode = 0; |
| 2007 | while (!isOnSyncQueue(node)) { |
| 2008 | if (nanosTimeout <= 0L) { |
| 2009 | transferAfterCancelledWait(node); |
| 2010 | break; |
| 2011 | } |
| 2012 | LockSupport.parkNanos(this, nanosTimeout); |
| 2013 | if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
| 2014 | break; |
| 2015 | |
| 2016 | long now = System.nanoTime(); |
| 2017 | nanosTimeout -= now - lastTime; |
| 2018 | lastTime = now; |
| 2019 | } |
| 2020 | if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
| 2021 | interruptMode = REINTERRUPT; |
| 2022 | if (node.nextWaiter != null) |
| 2023 | unlinkCancelledWaiters(); |
| 2024 | if (interruptMode != 0) |
| 2025 | reportInterruptAfterWait(interruptMode); |
| 2026 | return nanosTimeout - (System.nanoTime() - lastTime); |
| 2027 | } |
| 2028 | |
| 2029 | /** |
| 2030 | * Implements absolute timed condition wait. |
| 2031 | * <ol> |
| 2032 | * <li> If current thread is interrupted, throw InterruptedException. |
| 2033 | * <li> Save lock state returned by {@link #getState}. |
| 2034 | * <li> Invoke {@link #release} with |
| 2035 | * saved state as argument, throwing |
| 2036 | * IllegalMonitorStateException if it fails. |
| 2037 | * <li> Block until signalled, interrupted, or timed out. |
| 2038 | * <li> Reacquire by invoking specialized version of |
| 2039 | * {@link #acquire} with saved state as argument. |
| 2040 | * <li> If interrupted while blocked in step 4, throw InterruptedException. |
| 2041 | * <li> If timed out while blocked in step 4, return false, else true. |
| 2042 | * </ol> |
| 2043 | */ |
| 2044 | public final boolean awaitUntil(Date deadline) throws InterruptedException { |
| 2045 | if (deadline == null) |
| 2046 | throw new NullPointerException(); |
| 2047 | long abstime = deadline.getTime(); |
| 2048 | if (Thread.interrupted()) |
| 2049 | throw new InterruptedException(); |
| 2050 | Node node = addConditionWaiter(); |
| 2051 | int savedState = fullyRelease(node); |
| 2052 | boolean timedout = false; |
| 2053 | int interruptMode = 0; |
| 2054 | while (!isOnSyncQueue(node)) { |
| 2055 | if (System.currentTimeMillis() > abstime) { |
| 2056 | timedout = transferAfterCancelledWait(node); |
| 2057 | break; |
| 2058 | } |
| 2059 | LockSupport.parkUntil(this, abstime); |
| 2060 | if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
| 2061 | break; |
| 2062 | } |
| 2063 | if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
| 2064 | interruptMode = REINTERRUPT; |
| 2065 | if (node.nextWaiter != null) |
| 2066 | unlinkCancelledWaiters(); |
| 2067 | if (interruptMode != 0) |
| 2068 | reportInterruptAfterWait(interruptMode); |
| 2069 | return !timedout; |
| 2070 | } |
| 2071 | |
| 2072 | /** |
| 2073 | * Implements timed condition wait. |
| 2074 | * <ol> |
| 2075 | * <li> If current thread is interrupted, throw InterruptedException. |
| 2076 | * <li> Save lock state returned by {@link #getState}. |
| 2077 | * <li> Invoke {@link #release} with |
| 2078 | * saved state as argument, throwing |
| 2079 | * IllegalMonitorStateException if it fails. |
| 2080 | * <li> Block until signalled, interrupted, or timed out. |
| 2081 | * <li> Reacquire by invoking specialized version of |
| 2082 | * {@link #acquire} with saved state as argument. |
| 2083 | * <li> If interrupted while blocked in step 4, throw InterruptedException. |
| 2084 | * <li> If timed out while blocked in step 4, return false, else true. |
| 2085 | * </ol> |
| 2086 | */ |
| 2087 | public final boolean await(long time, TimeUnit unit) throws InterruptedException { |
| 2088 | if (unit == null) |
| 2089 | throw new NullPointerException(); |
| 2090 | long nanosTimeout = unit.toNanos(time); |
| 2091 | if (Thread.interrupted()) |
| 2092 | throw new InterruptedException(); |
| 2093 | Node node = addConditionWaiter(); |
| 2094 | int savedState = fullyRelease(node); |
| 2095 | long lastTime = System.nanoTime(); |
| 2096 | boolean timedout = false; |
| 2097 | int interruptMode = 0; |
| 2098 | while (!isOnSyncQueue(node)) { |
| 2099 | if (nanosTimeout <= 0L) { |
| 2100 | timedout = transferAfterCancelledWait(node); |
| 2101 | break; |
| 2102 | } |
| 2103 | if (nanosTimeout >= spinForTimeoutThreshold) |
| 2104 | LockSupport.parkNanos(this, nanosTimeout); |
| 2105 | if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
| 2106 | break; |
| 2107 | long now = System.nanoTime(); |
| 2108 | nanosTimeout -= now - lastTime; |
| 2109 | lastTime = now; |
| 2110 | } |
| 2111 | if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
| 2112 | interruptMode = REINTERRUPT; |
| 2113 | if (node.nextWaiter != null) |
| 2114 | unlinkCancelledWaiters(); |
| 2115 | if (interruptMode != 0) |
| 2116 | reportInterruptAfterWait(interruptMode); |
| 2117 | return !timedout; |
| 2118 | } |
| 2119 | |
| 2120 | // support for instrumentation |
| 2121 | |
| 2122 | /** |
| 2123 | * Returns true if this condition was created by the given |
| 2124 | * synchronization object. |
| 2125 | * |
| 2126 | * @return {@code true} if owned |
| 2127 | */ |
| 2128 | final boolean isOwnedBy(AbstractQueuedSynchronizer sync) { |
| 2129 | return sync == AbstractQueuedSynchronizer.this; |
| 2130 | } |
| 2131 | |
| 2132 | /** |
| 2133 | * Queries whether any threads are waiting on this condition. |
| 2134 | * Implements {@link AbstractQueuedSynchronizer#hasWaiters}. |
| 2135 | * |
| 2136 | * @return {@code true} if there are any waiting threads |
| 2137 | * @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
| 2138 | * returns {@code false} |
| 2139 | */ |
| 2140 | protected final boolean hasWaiters() { |
| 2141 | if (!isHeldExclusively()) |
| 2142 | throw new IllegalMonitorStateException(); |
| 2143 | for (Node w = firstWaiter; w != null; w = w.nextWaiter) { |
| 2144 | if (w.waitStatus == Node.CONDITION) |
| 2145 | return true; |
| 2146 | } |
| 2147 | return false; |
| 2148 | } |
| 2149 | |
| 2150 | /** |
| 2151 | * Returns an estimate of the number of threads waiting on |
| 2152 | * this condition. |
| 2153 | * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength}. |
| 2154 | * |
| 2155 | * @return the estimated number of waiting threads |
| 2156 | * @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
| 2157 | * returns {@code false} |
| 2158 | */ |
| 2159 | protected final int getWaitQueueLength() { |
| 2160 | if (!isHeldExclusively()) |
| 2161 | throw new IllegalMonitorStateException(); |
| 2162 | int n = 0; |
| 2163 | for (Node w = firstWaiter; w != null; w = w.nextWaiter) { |
| 2164 | if (w.waitStatus == Node.CONDITION) |
| 2165 | ++n; |
| 2166 | } |
| 2167 | return n; |
| 2168 | } |
| 2169 | |
| 2170 | /** |
| 2171 | * Returns a collection containing those threads that may be |
| 2172 | * waiting on this Condition. |
| 2173 | * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads}. |
| 2174 | * |
| 2175 | * @return the collection of threads |
| 2176 | * @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
| 2177 | * returns {@code false} |
| 2178 | */ |
| 2179 | protected final Collection<Thread> getWaitingThreads() { |
| 2180 | if (!isHeldExclusively()) |
| 2181 | throw new IllegalMonitorStateException(); |
| 2182 | ArrayList<Thread> list = new ArrayList<Thread>(); |
| 2183 | for (Node w = firstWaiter; w != null; w = w.nextWaiter) { |
| 2184 | if (w.waitStatus == Node.CONDITION) { |
| 2185 | Thread t = w.thread; |
| 2186 | if (t != null) |
| 2187 | list.add(t); |
| 2188 | } |
| 2189 | } |
| 2190 | return list; |
| 2191 | } |
| 2192 | } |
| 2193 | |
| 2194 | /** |
| 2195 | * Setup to support compareAndSet. We need to natively implement |
| 2196 | * this here: For the sake of permitting future enhancements, we |
| 2197 | * cannot explicitly subclass AtomicInteger, which would be |
| 2198 | * efficient and useful otherwise. So, as the lesser of evils, we |
| 2199 | * natively implement using hotspot intrinsics API. And while we |
| 2200 | * are at it, we do the same for other CASable fields (which could |
| 2201 | * otherwise be done with atomic field updaters). |
| 2202 | */ |
| 2203 | private static final Unsafe unsafe = Unsafe.getUnsafe(); |
| 2204 | private static final long stateOffset; |
| 2205 | private static final long headOffset; |
| 2206 | private static final long tailOffset; |
| 2207 | private static final long waitStatusOffset; |
| 2208 | private static final long nextOffset; |
| 2209 | |
| 2210 | static { |
| 2211 | try { |
| 2212 | stateOffset = unsafe.objectFieldOffset |
| 2213 | (AbstractQueuedSynchronizer.class.getDeclaredField("state")); |
| 2214 | headOffset = unsafe.objectFieldOffset |
| 2215 | (AbstractQueuedSynchronizer.class.getDeclaredField("head")); |
| 2216 | tailOffset = unsafe.objectFieldOffset |
| 2217 | (AbstractQueuedSynchronizer.class.getDeclaredField("tail")); |
| 2218 | waitStatusOffset = unsafe.objectFieldOffset |
| 2219 | (Node.class.getDeclaredField("waitStatus")); |
| 2220 | nextOffset = unsafe.objectFieldOffset |
| 2221 | (Node.class.getDeclaredField("next")); |
| 2222 | |
| 2223 | } catch (Exception ex) { throw new Error(ex); } |
| 2224 | } |
| 2225 | |
| 2226 | /** |
| 2227 | * CAS head field. Used only by enq. |
| 2228 | */ |
| 2229 | private final boolean compareAndSetHead(Node update) { |
| 2230 | return unsafe.compareAndSwapObject(this, headOffset, null, update); |
| 2231 | } |
| 2232 | |
| 2233 | /** |
| 2234 | * CAS tail field. Used only by enq. |
| 2235 | */ |
| 2236 | private final boolean compareAndSetTail(Node expect, Node update) { |
| 2237 | return unsafe.compareAndSwapObject(this, tailOffset, expect, update); |
| 2238 | } |
| 2239 | |
| 2240 | /** |
| 2241 | * CAS waitStatus field of a node. |
| 2242 | */ |
| 2243 | private final static boolean compareAndSetWaitStatus(Node node, |
| 2244 | int expect, |
| 2245 | int update) { |
| 2246 | return unsafe.compareAndSwapInt(node, waitStatusOffset, |
| 2247 | expect, update); |
| 2248 | } |
| 2249 | |
| 2250 | /** |
| 2251 | * CAS next field of a node. |
| 2252 | */ |
| 2253 | private final static boolean compareAndSetNext(Node node, |
| 2254 | Node expect, |
| 2255 | Node update) { |
| 2256 | return unsafe.compareAndSwapObject(node, nextOffset, expect, update); |
| 2257 | } |
| 2258 | } |