1 /*
   2  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   3  *
   4  * This code is free software; you can redistribute it and/or modify it
   5  * under the terms of the GNU General Public License version 2 only, as
   6  * published by the Free Software Foundation.  Oracle designates this
   7  * particular file as subject to the "Classpath" exception as provided
   8  * by Oracle in the LICENSE file that accompanied this code.
   9  *
  10  * This code is distributed in the hope that it will be useful, but WITHOUT
  11  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  12  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  13  * version 2 for more details (a copy is included in the LICENSE file that
  14  * accompanied this code).
  15  *
  16  * You should have received a copy of the GNU General Public License version
  17  * 2 along with this work; if not, write to the Free Software Foundation,
  18  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  19  *
  20  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  21  * or visit www.oracle.com if you need additional information or have any
  22  * questions.
  23  */
  24 
  25 /*
  26  * This file is available under and governed by the GNU General Public
  27  * License version 2 only, as published by the Free Software Foundation.
  28  * However, the following notice accompanied the original version of this
  29  * file:
  30  *
  31  * Written by Doug Lea with assistance from members of JCP JSR-166
  32  * Expert Group and released to the public domain, as explained at
  33  * http://creativecommons.org/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 &quot;understand&quot; 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  * &quot;spin&quot; 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          * waitStatus value to indicate the next acquireShared should
 394          * unconditionally propagate
 395          */
 396         static final int PROPAGATE = -3;
 397 
 398         /**
 399          * Status field, taking on only the values:
 400          *   SIGNAL:     The successor of this node is (or will soon be)
 401          *               blocked (via park), so the current node must
 402          *               unpark its successor when it releases or
 403          *               cancels. To avoid races, acquire methods must
 404          *               first indicate they need a signal,
 405          *               then retry the atomic acquire, and then,
 406          *               on failure, block.
 407          *   CANCELLED:  This node is cancelled due to timeout or interrupt.
 408          *               Nodes never leave this state. In particular,
 409          *               a thread with cancelled node never again blocks.
 410          *   CONDITION:  This node is currently on a condition queue.
 411          *               It will not be used as a sync queue node
 412          *               until transferred, at which time the status
 413          *               will be set to 0. (Use of this value here has
 414          *               nothing to do with the other uses of the
 415          *               field, but simplifies mechanics.)
 416          *   PROPAGATE:  A releaseShared should be propagated to other
 417          *               nodes. This is set (for head node only) in
 418          *               doReleaseShared to ensure propagation
 419          *               continues, even if other operations have
 420          *               since intervened.
 421          *   0:          None of the above
 422          *
 423          * The values are arranged numerically to simplify use.
 424          * Non-negative values mean that a node doesn't need to
 425          * signal. So, most code doesn't need to check for particular
 426          * values, just for sign.
 427          *
 428          * The field is initialized to 0 for normal sync nodes, and
 429          * CONDITION for condition nodes.  It is modified using CAS
 430          * (or when possible, unconditional volatile writes).
 431          */
 432         volatile int waitStatus;
 433 
 434         /**
 435          * Link to predecessor node that current node/thread relies on
 436          * for checking waitStatus. Assigned during enqueing, and nulled
 437          * out (for sake of GC) only upon dequeuing.  Also, upon
 438          * cancellation of a predecessor, we short-circuit while
 439          * finding a non-cancelled one, which will always exist
 440          * because the head node is never cancelled: A node becomes
 441          * head only as a result of successful acquire. A
 442          * cancelled thread never succeeds in acquiring, and a thread only
 443          * cancels itself, not any other node.
 444          */
 445         volatile Node prev;
 446 
 447         /**
 448          * Link to the successor node that the current node/thread
 449          * unparks upon release. Assigned during enqueuing, adjusted
 450          * when bypassing cancelled predecessors, and nulled out (for
 451          * sake of GC) when dequeued.  The enq operation does not
 452          * assign next field of a predecessor until after attachment,
 453          * so seeing a null next field does not necessarily mean that
 454          * node is at end of queue. However, if a next field appears
 455          * to be null, we can scan prev's from the tail to
 456          * double-check.  The next field of cancelled nodes is set to
 457          * point to the node itself instead of null, to make life
 458          * easier for isOnSyncQueue.
 459          */
 460         volatile Node next;
 461 
 462         /**
 463          * The thread that enqueued this node.  Initialized on
 464          * construction and nulled out after use.
 465          */
 466         volatile Thread thread;
 467 
 468         /**
 469          * Link to next node waiting on condition, or the special
 470          * value SHARED.  Because condition queues are accessed only
 471          * when holding in exclusive mode, we just need a simple
 472          * linked queue to hold nodes while they are waiting on
 473          * conditions. They are then transferred to the queue to
 474          * re-acquire. And because conditions can only be exclusive,
 475          * we save a field by using special value to indicate shared
 476          * mode.
 477          */
 478         Node nextWaiter;
 479 
 480         /**
 481          * Returns true if node is waiting in shared mode
 482          */
 483         final boolean isShared() {
 484             return nextWaiter == SHARED;
 485         }
 486 
 487         /**
 488          * Returns previous node, or throws NullPointerException if null.
 489          * Use when predecessor cannot be null.  The null check could
 490          * be elided, but is present to help the VM.
 491          *
 492          * @return the predecessor of this node
 493          */
 494         final Node predecessor() throws NullPointerException {
 495             Node p = prev;
 496             if (p == null)
 497                 throw new NullPointerException();
 498             else
 499                 return p;
 500         }
 501 
 502         Node() {    // Used to establish initial head or SHARED marker
 503         }
 504 
 505         Node(Thread thread, Node mode) {     // Used by addWaiter
 506             this.nextWaiter = mode;
 507             this.thread = thread;
 508         }
 509 
 510         Node(Thread thread, int waitStatus) { // Used by Condition
 511             this.waitStatus = waitStatus;
 512             this.thread = thread;
 513         }
 514     }
 515 
 516     /**
 517      * Head of the wait queue, lazily initialized.  Except for
 518      * initialization, it is modified only via method setHead.  Note:
 519      * If head exists, its waitStatus is guaranteed not to be
 520      * CANCELLED.
 521      */
 522     private transient volatile Node head;
 523 
 524     /**
 525      * Tail of the wait queue, lazily initialized.  Modified only via
 526      * method enq to add new wait node.
 527      */
 528     private transient volatile Node tail;
 529 
 530     /**
 531      * The synchronization state.
 532      */
 533     private volatile int state;
 534 
 535     /**
 536      * Returns the current value of synchronization state.
 537      * This operation has memory semantics of a <tt>volatile</tt> read.
 538      * @return current state value
 539      */
 540     protected final int getState() {
 541         return state;
 542     }
 543 
 544     /**
 545      * Sets the value of synchronization state.
 546      * This operation has memory semantics of a <tt>volatile</tt> write.
 547      * @param newState the new state value
 548      */
 549     protected final void setState(int newState) {
 550         state = newState;
 551     }
 552 
 553     /**
 554      * Atomically sets synchronization state to the given updated
 555      * value if the current state value equals the expected value.
 556      * This operation has memory semantics of a <tt>volatile</tt> read
 557      * and write.
 558      *
 559      * @param expect the expected value
 560      * @param update the new value
 561      * @return true if successful. False return indicates that the actual
 562      *         value was not equal to the expected value.
 563      */
 564     protected final boolean compareAndSetState(int expect, int update) {
 565         // See below for intrinsics setup to support this
 566         return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
 567     }
 568 
 569     // Queuing utilities
 570 
 571     /**
 572      * The number of nanoseconds for which it is faster to spin
 573      * rather than to use timed park. A rough estimate suffices
 574      * to improve responsiveness with very short timeouts.
 575      */
 576     static final long spinForTimeoutThreshold = 1000L;
 577 
 578     /**
 579      * Inserts node into queue, initializing if necessary. See picture above.
 580      * @param node the node to insert
 581      * @return node's predecessor
 582      */
 583     private Node enq(final Node node) {
 584         for (;;) {
 585             Node t = tail;
 586             if (t == null) { // Must initialize
 587                 if (compareAndSetHead(new Node()))
 588                     tail = head;
 589             } else {
 590                 node.prev = t;
 591                 if (compareAndSetTail(t, node)) {
 592                     t.next = node;
 593                     return t;
 594                 }
 595             }
 596         }
 597     }
 598 
 599     /**
 600      * Creates and enqueues node for current thread and given mode.
 601      *
 602      * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
 603      * @return the new node
 604      */
 605     private Node addWaiter(Node mode) {
 606         Node node = new Node(Thread.currentThread(), mode);
 607         // Try the fast path of enq; backup to full enq on failure
 608         Node pred = tail;
 609         if (pred != null) {
 610             node.prev = pred;
 611             if (compareAndSetTail(pred, node)) {
 612                 pred.next = node;
 613                 return node;
 614             }
 615         }
 616         enq(node);
 617         return node;
 618     }
 619 
 620     /**
 621      * Sets head of queue to be node, thus dequeuing. Called only by
 622      * acquire methods.  Also nulls out unused fields for sake of GC
 623      * and to suppress unnecessary signals and traversals.
 624      *
 625      * @param node the node
 626      */
 627     private void setHead(Node node) {
 628         head = node;
 629         node.thread = null;
 630         node.prev = null;
 631     }
 632 
 633     /**
 634      * Wakes up node's successor, if one exists.
 635      *
 636      * @param node the node
 637      */
 638     private void unparkSuccessor(Node node) {
 639         /*
 640          * If status is negative (i.e., possibly needing signal) try
 641          * to clear in anticipation of signalling.  It is OK if this
 642          * fails or if status is changed by waiting thread.
 643          */
 644         int ws = node.waitStatus;
 645         if (ws < 0)
 646             compareAndSetWaitStatus(node, ws, 0);
 647 
 648         /*
 649          * Thread to unpark is held in successor, which is normally
 650          * just the next node.  But if cancelled or apparently null,
 651          * traverse backwards from tail to find the actual
 652          * non-cancelled successor.
 653          */
 654         Node s = node.next;
 655         if (s == null || s.waitStatus > 0) {
 656             s = null;
 657             for (Node t = tail; t != null && t != node; t = t.prev)
 658                 if (t.waitStatus <= 0)
 659                     s = t;
 660         }
 661         if (s != null)
 662             LockSupport.unpark(s.thread);
 663     }
 664 
 665     /**
 666      * Release action for shared mode -- signal successor and ensure
 667      * propagation. (Note: For exclusive mode, release just amounts
 668      * to calling unparkSuccessor of head if it needs signal.)
 669      */
 670     private void doReleaseShared() {
 671         /*
 672          * Ensure that a release propagates, even if there are other
 673          * in-progress acquires/releases.  This proceeds in the usual
 674          * way of trying to unparkSuccessor of head if it needs
 675          * signal. But if it does not, status is set to PROPAGATE to
 676          * ensure that upon release, propagation continues.
 677          * Additionally, we must loop in case a new node is added
 678          * while we are doing this. Also, unlike other uses of
 679          * unparkSuccessor, we need to know if CAS to reset status
 680          * fails, if so rechecking.
 681          */
 682         for (;;) {
 683             Node h = head;
 684             if (h != null && h != tail) {
 685                 int ws = h.waitStatus;
 686                 if (ws == Node.SIGNAL) {
 687                     if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
 688                         continue;            // loop to recheck cases
 689                     unparkSuccessor(h);
 690                 }
 691                 else if (ws == 0 &&
 692                          !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
 693                     continue;                // loop on failed CAS
 694             }
 695             if (h == head)                   // loop if head changed
 696                 break;
 697         }
 698     }
 699 
 700     /**
 701      * Sets head of queue, and checks if successor may be waiting
 702      * in shared mode, if so propagating if either propagate > 0 or
 703      * PROPAGATE status was set.
 704      *
 705      * @param node the node
 706      * @param propagate the return value from a tryAcquireShared
 707      */
 708     private void setHeadAndPropagate(Node node, int propagate) {
 709         Node h = head; // Record old head for check below
 710         setHead(node);
 711         /*
 712          * Try to signal next queued node if:
 713          *   Propagation was indicated by caller,
 714          *     or was recorded (as h.waitStatus) by a previous operation
 715          *     (note: this uses sign-check of waitStatus because
 716          *      PROPAGATE status may transition to SIGNAL.)
 717          * and
 718          *   The next node is waiting in shared mode,
 719          *     or we don't know, because it appears null
 720          *
 721          * The conservatism in both of these checks may cause
 722          * unnecessary wake-ups, but only when there are multiple
 723          * racing acquires/releases, so most need signals now or soon
 724          * anyway.
 725          */
 726         if (propagate > 0 || h == null || h.waitStatus < 0) {
 727             Node s = node.next;
 728             if (s == null || s.isShared())
 729                 doReleaseShared();
 730         }
 731     }
 732 
 733     // Utilities for various versions of acquire
 734 
 735     /**
 736      * Cancels an ongoing attempt to acquire.
 737      *
 738      * @param node the node
 739      */
 740     private void cancelAcquire(Node node) {
 741         // Ignore if node doesn't exist
 742         if (node == null)
 743             return;
 744 
 745         node.thread = null;
 746 
 747         // Skip cancelled predecessors
 748         Node pred = node.prev;
 749         while (pred.waitStatus > 0)
 750             node.prev = pred = pred.prev;
 751 
 752         // predNext is the apparent node to unsplice. CASes below will
 753         // fail if not, in which case, we lost race vs another cancel
 754         // or signal, so no further action is necessary.
 755         Node predNext = pred.next;
 756 
 757         // Can use unconditional write instead of CAS here.
 758         // After this atomic step, other Nodes can skip past us.
 759         // Before, we are free of interference from other threads.
 760         node.waitStatus = Node.CANCELLED;
 761 
 762         // If we are the tail, remove ourselves.
 763         if (node == tail && compareAndSetTail(node, pred)) {
 764             compareAndSetNext(pred, predNext, null);
 765         } else {
 766             // If successor needs signal, try to set pred's next-link
 767             // so it will get one. Otherwise wake it up to propagate.
 768             int ws;
 769             if (pred != head &&
 770                 ((ws = pred.waitStatus) == Node.SIGNAL ||
 771                  (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
 772                 pred.thread != null) {
 773                 Node next = node.next;
 774                 if (next != null && next.waitStatus <= 0)
 775                     compareAndSetNext(pred, predNext, next);
 776             } else {
 777                 unparkSuccessor(node);
 778             }
 779 
 780             node.next = node; // help GC
 781         }
 782     }
 783 
 784     /**
 785      * Checks and updates status for a node that failed to acquire.
 786      * Returns true if thread should block. This is the main signal
 787      * control in all acquire loops.  Requires that pred == node.prev
 788      *
 789      * @param pred node's predecessor holding status
 790      * @param node the node
 791      * @return {@code true} if thread should block
 792      */
 793     private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
 794         int ws = pred.waitStatus;
 795         if (ws == Node.SIGNAL)
 796             /*
 797              * This node has already set status asking a release
 798              * to signal it, so it can safely park.
 799              */
 800             return true;
 801         if (ws > 0) {
 802             /*
 803              * Predecessor was cancelled. Skip over predecessors and
 804              * indicate retry.
 805              */
 806             do {
 807                 node.prev = pred = pred.prev;
 808             } while (pred.waitStatus > 0);
 809             pred.next = node;
 810         } else {
 811             /*
 812              * waitStatus must be 0 or PROPAGATE.  Indicate that we
 813              * need a signal, but don't park yet.  Caller will need to
 814              * retry to make sure it cannot acquire before parking.
 815              */
 816             compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
 817         }
 818         return false;
 819     }
 820 
 821     /**
 822      * Convenience method to interrupt current thread.
 823      */
 824     private static void selfInterrupt() {
 825         Thread.currentThread().interrupt();
 826     }
 827 
 828     /**
 829      * Convenience method to park and then check if interrupted
 830      *
 831      * @return {@code true} if interrupted
 832      */
 833     private final boolean parkAndCheckInterrupt() {
 834         LockSupport.park(this);
 835         return Thread.interrupted();
 836     }
 837 
 838     /*
 839      * Various flavors of acquire, varying in exclusive/shared and
 840      * control modes.  Each is mostly the same, but annoyingly
 841      * different.  Only a little bit of factoring is possible due to
 842      * interactions of exception mechanics (including ensuring that we
 843      * cancel if tryAcquire throws exception) and other control, at
 844      * least not without hurting performance too much.
 845      */
 846 
 847     /**
 848      * Acquires in exclusive uninterruptible mode for thread already in
 849      * queue. Used by condition wait methods as well as acquire.
 850      *
 851      * @param node the node
 852      * @param arg the acquire argument
 853      * @return {@code true} if interrupted while waiting
 854      */
 855     final boolean acquireQueued(final Node node, int arg) {
 856         boolean failed = true;
 857         try {
 858             boolean interrupted = false;
 859             for (;;) {
 860                 final Node p = node.predecessor();
 861                 if (p == head && tryAcquire(arg)) {
 862                     setHead(node);
 863                     p.next = null; // help GC
 864                     failed = false;
 865                     return interrupted;
 866                 }
 867                 if (shouldParkAfterFailedAcquire(p, node) &&
 868                     parkAndCheckInterrupt())
 869                     interrupted = true;
 870             }
 871         } finally {
 872             if (failed)
 873                 cancelAcquire(node);
 874         }
 875     }
 876 
 877     /**
 878      * Acquires in exclusive interruptible mode.
 879      * @param arg the acquire argument
 880      */
 881     private void doAcquireInterruptibly(int arg)
 882         throws InterruptedException {
 883         final Node node = addWaiter(Node.EXCLUSIVE);
 884         boolean failed = true;
 885         try {
 886             for (;;) {
 887                 final Node p = node.predecessor();
 888                 if (p == head && tryAcquire(arg)) {
 889                     setHead(node);
 890                     p.next = null; // help GC
 891                     failed = false;
 892                     return;
 893                 }
 894                 if (shouldParkAfterFailedAcquire(p, node) &&
 895                     parkAndCheckInterrupt())
 896                     throw new InterruptedException();
 897             }
 898         } finally {
 899             if (failed)
 900                 cancelAcquire(node);
 901         }
 902     }
 903 
 904     /**
 905      * Acquires in exclusive timed mode.
 906      *
 907      * @param arg the acquire argument
 908      * @param nanosTimeout max wait time
 909      * @return {@code true} if acquired
 910      */
 911     private boolean doAcquireNanos(int arg, long nanosTimeout)
 912         throws InterruptedException {
 913         long lastTime = System.nanoTime();
 914         final Node node = addWaiter(Node.EXCLUSIVE);
 915         boolean failed = true;
 916         try {
 917             for (;;) {
 918                 final Node p = node.predecessor();
 919                 if (p == head && tryAcquire(arg)) {
 920                     setHead(node);
 921                     p.next = null; // help GC
 922                     failed = false;
 923                     return true;
 924                 }
 925                 if (nanosTimeout <= 0)
 926                     return false;
 927                 if (shouldParkAfterFailedAcquire(p, node) &&
 928                     nanosTimeout > spinForTimeoutThreshold)
 929                     LockSupport.parkNanos(this, nanosTimeout);
 930                 long now = System.nanoTime();
 931                 nanosTimeout -= now - lastTime;
 932                 lastTime = now;
 933                 if (Thread.interrupted())
 934                     throw new InterruptedException();
 935             }
 936         } finally {
 937             if (failed)
 938                 cancelAcquire(node);
 939         }
 940     }
 941 
 942     /**
 943      * Acquires in shared uninterruptible mode.
 944      * @param arg the acquire argument
 945      */
 946     private void doAcquireShared(int arg) {
 947         final Node node = addWaiter(Node.SHARED);
 948         boolean failed = true;
 949         try {
 950             boolean interrupted = false;
 951             for (;;) {
 952                 final Node p = node.predecessor();
 953                 if (p == head) {
 954                     int r = tryAcquireShared(arg);
 955                     if (r >= 0) {
 956                         setHeadAndPropagate(node, r);
 957                         p.next = null; // help GC
 958                         if (interrupted)
 959                             selfInterrupt();
 960                         failed = false;
 961                         return;
 962                     }
 963                 }
 964                 if (shouldParkAfterFailedAcquire(p, node) &&
 965                     parkAndCheckInterrupt())
 966                     interrupted = true;
 967             }
 968         } finally {
 969             if (failed)
 970                 cancelAcquire(node);
 971         }
 972     }
 973 
 974     /**
 975      * Acquires in shared interruptible mode.
 976      * @param arg the acquire argument
 977      */
 978     private void doAcquireSharedInterruptibly(int arg)
 979         throws InterruptedException {
 980         final Node node = addWaiter(Node.SHARED);
 981         boolean failed = true;
 982         try {
 983             for (;;) {
 984                 final Node p = node.predecessor();
 985                 if (p == head) {
 986                     int r = tryAcquireShared(arg);
 987                     if (r >= 0) {
 988                         setHeadAndPropagate(node, r);
 989                         p.next = null; // help GC
 990                         failed = false;
 991                         return;
 992                     }
 993                 }
 994                 if (shouldParkAfterFailedAcquire(p, node) &&
 995                     parkAndCheckInterrupt())
 996                     throw new InterruptedException();
 997             }
 998         } finally {
 999             if (failed)
1000                 cancelAcquire(node);
1001         }
1002     }
1003 
1004     /**
1005      * Acquires in shared timed mode.
1006      *
1007      * @param arg the acquire argument
1008      * @param nanosTimeout max wait time
1009      * @return {@code true} if acquired
1010      */
1011     private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
1012         throws InterruptedException {
1013 
1014         long lastTime = System.nanoTime();
1015         final Node node = addWaiter(Node.SHARED);
1016         boolean failed = true;
1017         try {
1018             for (;;) {
1019                 final Node p = node.predecessor();
1020                 if (p == head) {
1021                     int r = tryAcquireShared(arg);
1022                     if (r >= 0) {
1023                         setHeadAndPropagate(node, r);
1024                         p.next = null; // help GC
1025                         failed = false;
1026                         return true;
1027                     }
1028                 }
1029                 if (nanosTimeout <= 0)
1030                     return false;
1031                 if (shouldParkAfterFailedAcquire(p, node) &&
1032                     nanosTimeout > spinForTimeoutThreshold)
1033                     LockSupport.parkNanos(this, nanosTimeout);
1034                 long now = System.nanoTime();
1035                 nanosTimeout -= now - lastTime;
1036                 lastTime = now;
1037                 if (Thread.interrupted())
1038                     throw new InterruptedException();
1039             }
1040         } finally {
1041             if (failed)
1042                 cancelAcquire(node);
1043         }
1044     }
1045 
1046     // Main exported methods
1047 
1048     /**
1049      * Attempts to acquire in exclusive mode. This method should query
1050      * if the state of the object permits it to be acquired in the
1051      * exclusive mode, and if so to acquire it.
1052      *
1053      * <p>This method is always invoked by the thread performing
1054      * acquire.  If this method reports failure, the acquire method
1055      * may queue the thread, if it is not already queued, until it is
1056      * signalled by a release from some other thread. This can be used
1057      * to implement method {@link Lock#tryLock()}.
1058      *
1059      * <p>The default
1060      * implementation throws {@link UnsupportedOperationException}.
1061      *
1062      * @param arg the acquire argument. This value is always the one
1063      *        passed to an acquire method, or is the value saved on entry
1064      *        to a condition wait.  The value is otherwise uninterpreted
1065      *        and can represent anything you like.
1066      * @return {@code true} if successful. Upon success, this object has
1067      *         been acquired.
1068      * @throws IllegalMonitorStateException if acquiring would place this
1069      *         synchronizer in an illegal state. This exception must be
1070      *         thrown in a consistent fashion for synchronization to work
1071      *         correctly.
1072      * @throws UnsupportedOperationException if exclusive mode is not supported
1073      */
1074     protected boolean tryAcquire(int arg) {
1075         throw new UnsupportedOperationException();
1076     }
1077 
1078     /**
1079      * Attempts to set the state to reflect a release in exclusive
1080      * mode.
1081      *
1082      * <p>This method is always invoked by the thread performing release.
1083      *
1084      * <p>The default implementation throws
1085      * {@link UnsupportedOperationException}.
1086      *
1087      * @param arg the release argument. This value is always the one
1088      *        passed to a release method, or the current state value upon
1089      *        entry to a condition wait.  The value is otherwise
1090      *        uninterpreted and can represent anything you like.
1091      * @return {@code true} if this object is now in a fully released
1092      *         state, so that any waiting threads may attempt to acquire;
1093      *         and {@code false} otherwise.
1094      * @throws IllegalMonitorStateException if releasing would place this
1095      *         synchronizer in an illegal state. This exception must be
1096      *         thrown in a consistent fashion for synchronization to work
1097      *         correctly.
1098      * @throws UnsupportedOperationException if exclusive mode is not supported
1099      */
1100     protected boolean tryRelease(int arg) {
1101         throw new UnsupportedOperationException();
1102     }
1103 
1104     /**
1105      * Attempts to acquire in shared mode. This method should query if
1106      * the state of the object permits it to be acquired in the shared
1107      * mode, and if so to acquire it.
1108      *
1109      * <p>This method is always invoked by the thread performing
1110      * acquire.  If this method reports failure, the acquire method
1111      * may queue the thread, if it is not already queued, until it is
1112      * signalled by a release from some other thread.
1113      *
1114      * <p>The default implementation throws {@link
1115      * UnsupportedOperationException}.
1116      *
1117      * @param arg the acquire argument. This value is always the one
1118      *        passed to an acquire method, or is the value saved on entry
1119      *        to a condition wait.  The value is otherwise uninterpreted
1120      *        and can represent anything you like.
1121      * @return a negative value on failure; zero if acquisition in shared
1122      *         mode succeeded but no subsequent shared-mode acquire can
1123      *         succeed; and a positive value if acquisition in shared
1124      *         mode succeeded and subsequent shared-mode acquires might
1125      *         also succeed, in which case a subsequent waiting thread
1126      *         must check availability. (Support for three different
1127      *         return values enables this method to be used in contexts
1128      *         where acquires only sometimes act exclusively.)  Upon
1129      *         success, this object has been acquired.
1130      * @throws IllegalMonitorStateException if acquiring would place this
1131      *         synchronizer in an illegal state. This exception must be
1132      *         thrown in a consistent fashion for synchronization to work
1133      *         correctly.
1134      * @throws UnsupportedOperationException if shared mode is not supported
1135      */
1136     protected int tryAcquireShared(int arg) {
1137         throw new UnsupportedOperationException();
1138     }
1139 
1140     /**
1141      * Attempts to set the state to reflect a release in shared mode.
1142      *
1143      * <p>This method is always invoked by the thread performing release.
1144      *
1145      * <p>The default implementation throws
1146      * {@link UnsupportedOperationException}.
1147      *
1148      * @param arg the release argument. This value is always the one
1149      *        passed to a release method, or the current state value upon
1150      *        entry to a condition wait.  The value is otherwise
1151      *        uninterpreted and can represent anything you like.
1152      * @return {@code true} if this release of shared mode may permit a
1153      *         waiting acquire (shared or exclusive) to succeed; and
1154      *         {@code false} otherwise
1155      * @throws IllegalMonitorStateException if releasing would place this
1156      *         synchronizer in an illegal state. This exception must be
1157      *         thrown in a consistent fashion for synchronization to work
1158      *         correctly.
1159      * @throws UnsupportedOperationException if shared mode is not supported
1160      */
1161     protected boolean tryReleaseShared(int arg) {
1162         throw new UnsupportedOperationException();
1163     }
1164 
1165     /**
1166      * Returns {@code true} if synchronization is held exclusively with
1167      * respect to the current (calling) thread.  This method is invoked
1168      * upon each call to a non-waiting {@link ConditionObject} method.
1169      * (Waiting methods instead invoke {@link #release}.)
1170      *
1171      * <p>The default implementation throws {@link
1172      * UnsupportedOperationException}. This method is invoked
1173      * internally only within {@link ConditionObject} methods, so need
1174      * not be defined if conditions are not used.
1175      *
1176      * @return {@code true} if synchronization is held exclusively;
1177      *         {@code false} otherwise
1178      * @throws UnsupportedOperationException if conditions are not supported
1179      */
1180     protected boolean isHeldExclusively() {
1181         throw new UnsupportedOperationException();
1182     }
1183 
1184     /**
1185      * Acquires in exclusive mode, ignoring interrupts.  Implemented
1186      * by invoking at least once {@link #tryAcquire},
1187      * returning on success.  Otherwise the thread is queued, possibly
1188      * repeatedly blocking and unblocking, invoking {@link
1189      * #tryAcquire} until success.  This method can be used
1190      * to implement method {@link Lock#lock}.
1191      *
1192      * @param arg the acquire argument.  This value is conveyed to
1193      *        {@link #tryAcquire} but is otherwise uninterpreted and
1194      *        can represent anything you like.
1195      */
1196     public final void acquire(int arg) {
1197         if (!tryAcquire(arg) &&
1198             acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
1199             selfInterrupt();
1200     }
1201 
1202     /**
1203      * Acquires in exclusive mode, aborting if interrupted.
1204      * Implemented by first checking interrupt status, then invoking
1205      * at least once {@link #tryAcquire}, returning on
1206      * success.  Otherwise the thread is queued, possibly repeatedly
1207      * blocking and unblocking, invoking {@link #tryAcquire}
1208      * until success or the thread is interrupted.  This method can be
1209      * used to implement method {@link Lock#lockInterruptibly}.
1210      *
1211      * @param arg the acquire argument.  This value is conveyed to
1212      *        {@link #tryAcquire} but is otherwise uninterpreted and
1213      *        can represent anything you like.
1214      * @throws InterruptedException if the current thread is interrupted
1215      */
1216     public final void acquireInterruptibly(int arg) throws InterruptedException {
1217         if (Thread.interrupted())
1218             throw new InterruptedException();
1219         if (!tryAcquire(arg))
1220             doAcquireInterruptibly(arg);
1221     }
1222 
1223     /**
1224      * Attempts to acquire in exclusive mode, aborting if interrupted,
1225      * and failing if the given timeout elapses.  Implemented by first
1226      * checking interrupt status, then invoking at least once {@link
1227      * #tryAcquire}, returning on success.  Otherwise, the thread is
1228      * queued, possibly repeatedly blocking and unblocking, invoking
1229      * {@link #tryAcquire} until success or the thread is interrupted
1230      * or the timeout elapses.  This method can be used to implement
1231      * method {@link Lock#tryLock(long, TimeUnit)}.
1232      *
1233      * @param arg the acquire argument.  This value is conveyed to
1234      *        {@link #tryAcquire} but is otherwise uninterpreted and
1235      *        can represent anything you like.
1236      * @param nanosTimeout the maximum number of nanoseconds to wait
1237      * @return {@code true} if acquired; {@code false} if timed out
1238      * @throws InterruptedException if the current thread is interrupted
1239      */
1240     public final boolean tryAcquireNanos(int arg, long nanosTimeout) throws InterruptedException {
1241         if (Thread.interrupted())
1242             throw new InterruptedException();
1243         return tryAcquire(arg) ||
1244             doAcquireNanos(arg, nanosTimeout);
1245     }
1246 
1247     /**
1248      * Releases in exclusive mode.  Implemented by unblocking one or
1249      * more threads if {@link #tryRelease} returns true.
1250      * This method can be used to implement method {@link Lock#unlock}.
1251      *
1252      * @param arg the release argument.  This value is conveyed to
1253      *        {@link #tryRelease} but is otherwise uninterpreted and
1254      *        can represent anything you like.
1255      * @return the value returned from {@link #tryRelease}
1256      */
1257     public final boolean release(int arg) {
1258         if (tryRelease(arg)) {
1259             Node h = head;
1260             if (h != null && h.waitStatus != 0)
1261                 unparkSuccessor(h);
1262             return true;
1263         }
1264         return false;
1265     }
1266 
1267     /**
1268      * Acquires in shared mode, ignoring interrupts.  Implemented by
1269      * first invoking at least once {@link #tryAcquireShared},
1270      * returning on success.  Otherwise the thread is queued, possibly
1271      * repeatedly blocking and unblocking, invoking {@link
1272      * #tryAcquireShared} until success.
1273      *
1274      * @param arg the acquire argument.  This value is conveyed to
1275      *        {@link #tryAcquireShared} but is otherwise uninterpreted
1276      *        and can represent anything you like.
1277      */
1278     public final void acquireShared(int arg) {
1279         if (tryAcquireShared(arg) < 0)
1280             doAcquireShared(arg);
1281     }
1282 
1283     /**
1284      * Acquires in shared mode, aborting if interrupted.  Implemented
1285      * by first checking interrupt status, then invoking at least once
1286      * {@link #tryAcquireShared}, returning on success.  Otherwise the
1287      * thread is queued, possibly repeatedly blocking and unblocking,
1288      * invoking {@link #tryAcquireShared} until success or the thread
1289      * is interrupted.
1290      * @param arg the acquire argument
1291      * This value is conveyed to {@link #tryAcquireShared} but is
1292      * otherwise uninterpreted and can represent anything
1293      * you like.
1294      * @throws InterruptedException if the current thread is interrupted
1295      */
1296     public final void acquireSharedInterruptibly(int arg) throws InterruptedException {
1297         if (Thread.interrupted())
1298             throw new InterruptedException();
1299         if (tryAcquireShared(arg) < 0)
1300             doAcquireSharedInterruptibly(arg);
1301     }
1302 
1303     /**
1304      * Attempts to acquire in shared mode, aborting if interrupted, and
1305      * failing if the given timeout elapses.  Implemented by first
1306      * checking interrupt status, then invoking at least once {@link
1307      * #tryAcquireShared}, returning on success.  Otherwise, the
1308      * thread is queued, possibly repeatedly blocking and unblocking,
1309      * invoking {@link #tryAcquireShared} until success or the thread
1310      * is interrupted or the timeout elapses.
1311      *
1312      * @param arg the acquire argument.  This value is conveyed to
1313      *        {@link #tryAcquireShared} but is otherwise uninterpreted
1314      *        and can represent anything you like.
1315      * @param nanosTimeout the maximum number of nanoseconds to wait
1316      * @return {@code true} if acquired; {@code false} if timed out
1317      * @throws InterruptedException if the current thread is interrupted
1318      */
1319     public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException {
1320         if (Thread.interrupted())
1321             throw new InterruptedException();
1322         return tryAcquireShared(arg) >= 0 ||
1323             doAcquireSharedNanos(arg, nanosTimeout);
1324     }
1325 
1326     /**
1327      * Releases in shared mode.  Implemented by unblocking one or more
1328      * threads if {@link #tryReleaseShared} returns true.
1329      *
1330      * @param arg the release argument.  This value is conveyed to
1331      *        {@link #tryReleaseShared} but is otherwise uninterpreted
1332      *        and can represent anything you like.
1333      * @return the value returned from {@link #tryReleaseShared}
1334      */
1335     public final boolean releaseShared(int arg) {
1336         if (tryReleaseShared(arg)) {
1337             doReleaseShared();
1338             return true;
1339         }
1340         return false;
1341     }
1342 
1343     // Queue inspection methods
1344 
1345     /**
1346      * Queries whether any threads are waiting to acquire. Note that
1347      * because cancellations due to interrupts and timeouts may occur
1348      * at any time, a {@code true} return does not guarantee that any
1349      * other thread will ever acquire.
1350      *
1351      * <p>In this implementation, this operation returns in
1352      * constant time.
1353      *
1354      * @return {@code true} if there may be other threads waiting to acquire
1355      */
1356     public final boolean hasQueuedThreads() {
1357         return head != tail;
1358     }
1359 
1360     /**
1361      * Queries whether any threads have ever contended to acquire this
1362      * synchronizer; that is if an acquire method has ever blocked.
1363      *
1364      * <p>In this implementation, this operation returns in
1365      * constant time.
1366      *
1367      * @return {@code true} if there has ever been contention
1368      */
1369     public final boolean hasContended() {
1370         return head != null;
1371     }
1372 
1373     /**
1374      * Returns the first (longest-waiting) thread in the queue, or
1375      * {@code null} if no threads are currently queued.
1376      *
1377      * <p>In this implementation, this operation normally returns in
1378      * constant time, but may iterate upon contention if other threads are
1379      * concurrently modifying the queue.
1380      *
1381      * @return the first (longest-waiting) thread in the queue, or
1382      *         {@code null} if no threads are currently queued
1383      */
1384     public final Thread getFirstQueuedThread() {
1385         // handle only fast path, else relay
1386         return (head == tail) ? null : fullGetFirstQueuedThread();
1387     }
1388 
1389     /**
1390      * Version of getFirstQueuedThread called when fastpath fails
1391      */
1392     private Thread fullGetFirstQueuedThread() {
1393         /*
1394          * The first node is normally head.next. Try to get its
1395          * thread field, ensuring consistent reads: If thread
1396          * field is nulled out or s.prev is no longer head, then
1397          * some other thread(s) concurrently performed setHead in
1398          * between some of our reads. We try this twice before
1399          * resorting to traversal.
1400          */
1401         Node h, s;
1402         Thread st;
1403         if (((h = head) != null && (s = h.next) != null &&
1404              s.prev == head && (st = s.thread) != null) ||
1405             ((h = head) != null && (s = h.next) != null &&
1406              s.prev == head && (st = s.thread) != null))
1407             return st;
1408 
1409         /*
1410          * Head's next field might not have been set yet, or may have
1411          * been unset after setHead. So we must check to see if tail
1412          * is actually first node. If not, we continue on, safely
1413          * traversing from tail back to head to find first,
1414          * guaranteeing termination.
1415          */
1416 
1417         Node t = tail;
1418         Thread firstThread = null;
1419         while (t != null && t != head) {
1420             Thread tt = t.thread;
1421             if (tt != null)
1422                 firstThread = tt;
1423             t = t.prev;
1424         }
1425         return firstThread;
1426     }
1427 
1428     /**
1429      * Returns true if the given thread is currently queued.
1430      *
1431      * <p>This implementation traverses the queue to determine
1432      * presence of the given thread.
1433      *
1434      * @param thread the thread
1435      * @return {@code true} if the given thread is on the queue
1436      * @throws NullPointerException if the thread is null
1437      */
1438     public final boolean isQueued(Thread thread) {
1439         if (thread == null)
1440             throw new NullPointerException();
1441         for (Node p = tail; p != null; p = p.prev)
1442             if (p.thread == thread)
1443                 return true;
1444         return false;
1445     }
1446 
1447     /**
1448      * Returns {@code true} if the apparent first queued thread, if one
1449      * exists, is waiting in exclusive mode.  If this method returns
1450      * {@code true}, and the current thread is attempting to acquire in
1451      * shared mode (that is, this method is invoked from {@link
1452      * #tryAcquireShared}) then it is guaranteed that the current thread
1453      * is not the first queued thread.  Used only as a heuristic in
1454      * ReentrantReadWriteLock.
1455      */
1456     final boolean apparentlyFirstQueuedIsExclusive() {
1457         Node h, s;
1458         return (h = head) != null &&
1459             (s = h.next)  != null &&
1460             !s.isShared()         &&
1461             s.thread != null;
1462     }
1463 
1464     /**
1465      * Queries whether any threads have been waiting to acquire longer
1466      * than the current thread.
1467      *
1468      * <p>An invocation of this method is equivalent to (but may be
1469      * more efficient than):
1470      *  <pre> {@code
1471      * getFirstQueuedThread() != Thread.currentThread() &&
1472      * hasQueuedThreads()}</pre>
1473      *
1474      * <p>Note that because cancellations due to interrupts and
1475      * timeouts may occur at any time, a {@code true} return does not
1476      * guarantee that some other thread will acquire before the current
1477      * thread.  Likewise, it is possible for another thread to win a
1478      * race to enqueue after this method has returned {@code false},
1479      * due to the queue being empty.
1480      *
1481      * <p>This method is designed to be used by a fair synchronizer to
1482      * avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>.
1483      * Such a synchronizer's {@link #tryAcquire} method should return
1484      * {@code false}, and its {@link #tryAcquireShared} method should
1485      * return a negative value, if this method returns {@code true}
1486      * (unless this is a reentrant acquire).  For example, the {@code
1487      * tryAcquire} method for a fair, reentrant, exclusive mode
1488      * synchronizer might look like this:
1489      *
1490      *  <pre> {@code
1491      * protected boolean tryAcquire(int arg) {
1492      *   if (isHeldExclusively()) {
1493      *     // A reentrant acquire; increment hold count
1494      *     return true;
1495      *   } else if (hasQueuedPredecessors()) {
1496      *     return false;
1497      *   } else {
1498      *     // try to acquire normally
1499      *   }
1500      * }}</pre>
1501      *
1502      * @return {@code true} if there is a queued thread preceding the
1503      *         current thread, and {@code false} if the current thread
1504      *         is at the head of the queue or the queue is empty
1505      * @since 1.7
1506      */
1507     public final boolean hasQueuedPredecessors() {
1508         // The correctness of this depends on head being initialized
1509         // before tail and on head.next being accurate if the current
1510         // thread is first in queue.
1511         Node t = tail; // Read fields in reverse initialization order
1512         Node h = head;
1513         Node s;
1514         return h != t &&
1515             ((s = h.next) == null || s.thread != Thread.currentThread());
1516     }
1517 
1518 
1519     // Instrumentation and monitoring methods
1520 
1521     /**
1522      * Returns an estimate of the number of threads waiting to
1523      * acquire.  The value is only an estimate because the number of
1524      * threads may change dynamically while this method traverses
1525      * internal data structures.  This method is designed for use in
1526      * monitoring system state, not for synchronization
1527      * control.
1528      *
1529      * @return the estimated number of threads waiting to acquire
1530      */
1531     public final int getQueueLength() {
1532         int n = 0;
1533         for (Node p = tail; p != null; p = p.prev) {
1534             if (p.thread != null)
1535                 ++n;
1536         }
1537         return n;
1538     }
1539 
1540     /**
1541      * Returns a collection containing threads that may be waiting to
1542      * acquire.  Because the actual set of threads may change
1543      * dynamically while constructing this result, the returned
1544      * collection is only a best-effort estimate.  The elements of the
1545      * returned collection are in no particular order.  This method is
1546      * designed to facilitate construction of subclasses that provide
1547      * more extensive monitoring facilities.
1548      *
1549      * @return the collection of threads
1550      */
1551     public final Collection<Thread> getQueuedThreads() {
1552         ArrayList<Thread> list = new ArrayList<Thread>();
1553         for (Node p = tail; p != null; p = p.prev) {
1554             Thread t = p.thread;
1555             if (t != null)
1556                 list.add(t);
1557         }
1558         return list;
1559     }
1560 
1561     /**
1562      * Returns a collection containing threads that may be waiting to
1563      * acquire in exclusive mode. This has the same properties
1564      * as {@link #getQueuedThreads} except that it only returns
1565      * those threads waiting due to an exclusive acquire.
1566      *
1567      * @return the collection of threads
1568      */
1569     public final Collection<Thread> getExclusiveQueuedThreads() {
1570         ArrayList<Thread> list = new ArrayList<Thread>();
1571         for (Node p = tail; p != null; p = p.prev) {
1572             if (!p.isShared()) {
1573                 Thread t = p.thread;
1574                 if (t != null)
1575                     list.add(t);
1576             }
1577         }
1578         return list;
1579     }
1580 
1581     /**
1582      * Returns a collection containing threads that may be waiting to
1583      * acquire in shared mode. This has the same properties
1584      * as {@link #getQueuedThreads} except that it only returns
1585      * those threads waiting due to a shared acquire.
1586      *
1587      * @return the collection of threads
1588      */
1589     public final Collection<Thread> getSharedQueuedThreads() {
1590         ArrayList<Thread> list = new ArrayList<Thread>();
1591         for (Node p = tail; p != null; p = p.prev) {
1592             if (p.isShared()) {
1593                 Thread t = p.thread;
1594                 if (t != null)
1595                     list.add(t);
1596             }
1597         }
1598         return list;
1599     }
1600 
1601     /**
1602      * Returns a string identifying this synchronizer, as well as its state.
1603      * The state, in brackets, includes the String {@code "State ="}
1604      * followed by the current value of {@link #getState}, and either
1605      * {@code "nonempty"} or {@code "empty"} depending on whether the
1606      * queue is empty.
1607      *
1608      * @return a string identifying this synchronizer, as well as its state
1609      */
1610     public String toString() {
1611         int s = getState();
1612         String q  = hasQueuedThreads() ? "non" : "";
1613         return super.toString() +
1614             "[State = " + s + ", " + q + "empty queue]";
1615     }
1616 
1617 
1618     // Internal support methods for Conditions
1619 
1620     /**
1621      * Returns true if a node, always one that was initially placed on
1622      * a condition queue, is now waiting to reacquire on sync queue.
1623      * @param node the node
1624      * @return true if is reacquiring
1625      */
1626     final boolean isOnSyncQueue(Node node) {
1627         if (node.waitStatus == Node.CONDITION || node.prev == null)
1628             return false;
1629         if (node.next != null) // If has successor, it must be on queue
1630             return true;
1631         /*
1632          * node.prev can be non-null, but not yet on queue because
1633          * the CAS to place it on queue can fail. So we have to
1634          * traverse from tail to make sure it actually made it.  It
1635          * will always be near the tail in calls to this method, and
1636          * unless the CAS failed (which is unlikely), it will be
1637          * there, so we hardly ever traverse much.
1638          */
1639         return findNodeFromTail(node);
1640     }
1641 
1642     /**
1643      * Returns true if node is on sync queue by searching backwards from tail.
1644      * Called only when needed by isOnSyncQueue.
1645      * @return true if present
1646      */
1647     private boolean findNodeFromTail(Node node) {
1648         Node t = tail;
1649         for (;;) {
1650             if (t == node)
1651                 return true;
1652             if (t == null)
1653                 return false;
1654             t = t.prev;
1655         }
1656     }
1657 
1658     /**
1659      * Transfers a node from a condition queue onto sync queue.
1660      * Returns true if successful.
1661      * @param node the node
1662      * @return true if successfully transferred (else the node was
1663      * cancelled before signal).
1664      */
1665     final boolean transferForSignal(Node node) {
1666         /*
1667          * If cannot change waitStatus, the node has been cancelled.
1668          */
1669         if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
1670             return false;
1671 
1672         /*
1673          * Splice onto queue and try to set waitStatus of predecessor to
1674          * indicate that thread is (probably) waiting. If cancelled or
1675          * attempt to set waitStatus fails, wake up to resync (in which
1676          * case the waitStatus can be transiently and harmlessly wrong).
1677          */
1678         Node p = enq(node);
1679         int ws = p.waitStatus;
1680         if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
1681             LockSupport.unpark(node.thread);
1682         return true;
1683     }
1684 
1685     /**
1686      * Transfers node, if necessary, to sync queue after a cancelled
1687      * wait. Returns true if thread was cancelled before being
1688      * signalled.
1689      * @param current the waiting thread
1690      * @param node its node
1691      * @return true if cancelled before the node was signalled
1692      */
1693     final boolean transferAfterCancelledWait(Node node) {
1694         if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
1695             enq(node);
1696             return true;
1697         }
1698         /*
1699          * If we lost out to a signal(), then we can't proceed
1700          * until it finishes its enq().  Cancelling during an
1701          * incomplete transfer is both rare and transient, so just
1702          * spin.
1703          */
1704         while (!isOnSyncQueue(node))
1705             Thread.yield();
1706         return false;
1707     }
1708 
1709     /**
1710      * Invokes release with current state value; returns saved state.
1711      * Cancels node and throws exception on failure.
1712      * @param node the condition node for this wait
1713      * @return previous sync state
1714      */
1715     final int fullyRelease(Node node) {
1716         boolean failed = true;
1717         try {
1718             int savedState = getState();
1719             if (release(savedState)) {
1720                 failed = false;
1721                 return savedState;
1722             } else {
1723                 throw new IllegalMonitorStateException();
1724             }
1725         } finally {
1726             if (failed)
1727                 node.waitStatus = Node.CANCELLED;
1728         }
1729     }
1730 
1731     // Instrumentation methods for conditions
1732 
1733     /**
1734      * Queries whether the given ConditionObject
1735      * uses this synchronizer as its lock.
1736      *
1737      * @param condition the condition
1738      * @return <tt>true</tt> if owned
1739      * @throws NullPointerException if the condition is null
1740      */
1741     public final boolean owns(ConditionObject condition) {
1742         if (condition == null)
1743             throw new NullPointerException();
1744         return condition.isOwnedBy(this);
1745     }
1746 
1747     /**
1748      * Queries whether any threads are waiting on the given condition
1749      * associated with this synchronizer. Note that because timeouts
1750      * and interrupts may occur at any time, a <tt>true</tt> return
1751      * does not guarantee that a future <tt>signal</tt> will awaken
1752      * any threads.  This method is designed primarily for use in
1753      * monitoring of the system state.
1754      *
1755      * @param condition the condition
1756      * @return <tt>true</tt> if there are any waiting threads
1757      * @throws IllegalMonitorStateException if exclusive synchronization
1758      *         is not held
1759      * @throws IllegalArgumentException if the given condition is
1760      *         not associated with this synchronizer
1761      * @throws NullPointerException if the condition is null
1762      */
1763     public final boolean hasWaiters(ConditionObject condition) {
1764         if (!owns(condition))
1765             throw new IllegalArgumentException("Not owner");
1766         return condition.hasWaiters();
1767     }
1768 
1769     /**
1770      * Returns an estimate of the number of threads waiting on the
1771      * given condition associated with this synchronizer. Note that
1772      * because timeouts and interrupts may occur at any time, the
1773      * estimate serves only as an upper bound on the actual number of
1774      * waiters.  This method is designed for use in monitoring of the
1775      * system state, not for synchronization control.
1776      *
1777      * @param condition the condition
1778      * @return the estimated number of waiting threads
1779      * @throws IllegalMonitorStateException if exclusive synchronization
1780      *         is not held
1781      * @throws IllegalArgumentException if the given condition is
1782      *         not associated with this synchronizer
1783      * @throws NullPointerException if the condition is null
1784      */
1785     public final int getWaitQueueLength(ConditionObject condition) {
1786         if (!owns(condition))
1787             throw new IllegalArgumentException("Not owner");
1788         return condition.getWaitQueueLength();
1789     }
1790 
1791     /**
1792      * Returns a collection containing those threads that may be
1793      * waiting on the given condition associated with this
1794      * synchronizer.  Because the actual set of threads may change
1795      * dynamically while constructing this result, the returned
1796      * collection is only a best-effort estimate. The elements of the
1797      * returned collection are in no particular order.
1798      *
1799      * @param condition the condition
1800      * @return the collection of threads
1801      * @throws IllegalMonitorStateException if exclusive synchronization
1802      *         is not held
1803      * @throws IllegalArgumentException if the given condition is
1804      *         not associated with this synchronizer
1805      * @throws NullPointerException if the condition is null
1806      */
1807     public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
1808         if (!owns(condition))
1809             throw new IllegalArgumentException("Not owner");
1810         return condition.getWaitingThreads();
1811     }
1812 
1813     /**
1814      * Condition implementation for a {@link
1815      * AbstractQueuedSynchronizer} serving as the basis of a {@link
1816      * Lock} implementation.
1817      *
1818      * <p>Method documentation for this class describes mechanics,
1819      * not behavioral specifications from the point of view of Lock
1820      * and Condition users. Exported versions of this class will in
1821      * general need to be accompanied by documentation describing
1822      * condition semantics that rely on those of the associated
1823      * <tt>AbstractQueuedSynchronizer</tt>.
1824      *
1825      * <p>This class is Serializable, but all fields are transient,
1826      * so deserialized conditions have no waiters.
1827      */
1828     public class ConditionObject implements Condition, java.io.Serializable {
1829         private static final long serialVersionUID = 1173984872572414699L;
1830         /** First node of condition queue. */
1831         private transient Node firstWaiter;
1832         /** Last node of condition queue. */
1833         private transient Node lastWaiter;
1834 
1835         /**
1836          * Creates a new <tt>ConditionObject</tt> instance.
1837          */
1838         public ConditionObject() { }
1839 
1840         // Internal methods
1841 
1842         /**
1843          * Adds a new waiter to wait queue.
1844          * @return its new wait node
1845          */
1846         private Node addConditionWaiter() {
1847             Node t = lastWaiter;
1848             // If lastWaiter is cancelled, clean out.
1849             if (t != null && t.waitStatus != Node.CONDITION) {
1850                 unlinkCancelledWaiters();
1851                 t = lastWaiter;
1852             }
1853             Node node = new Node(Thread.currentThread(), Node.CONDITION);
1854             if (t == null)
1855                 firstWaiter = node;
1856             else
1857                 t.nextWaiter = node;
1858             lastWaiter = node;
1859             return node;
1860         }
1861 
1862         /**
1863          * Removes and transfers nodes until hit non-cancelled one or
1864          * null. Split out from signal in part to encourage compilers
1865          * to inline the case of no waiters.
1866          * @param first (non-null) the first node on condition queue
1867          */
1868         private void doSignal(Node first) {
1869             do {
1870                 if ( (firstWaiter = first.nextWaiter) == null)
1871                     lastWaiter = null;
1872                 first.nextWaiter = null;
1873             } while (!transferForSignal(first) &&
1874                      (first = firstWaiter) != null);
1875         }
1876 
1877         /**
1878          * Removes and transfers all nodes.
1879          * @param first (non-null) the first node on condition queue
1880          */
1881         private void doSignalAll(Node first) {
1882             lastWaiter = firstWaiter = null;
1883             do {
1884                 Node next = first.nextWaiter;
1885                 first.nextWaiter = null;
1886                 transferForSignal(first);
1887                 first = next;
1888             } while (first != null);
1889         }
1890 
1891         /**
1892          * Unlinks cancelled waiter nodes from condition queue.
1893          * Called only while holding lock. This is called when
1894          * cancellation occurred during condition wait, and upon
1895          * insertion of a new waiter when lastWaiter is seen to have
1896          * been cancelled. This method is needed to avoid garbage
1897          * retention in the absence of signals. So even though it may
1898          * require a full traversal, it comes into play only when
1899          * timeouts or cancellations occur in the absence of
1900          * signals. It traverses all nodes rather than stopping at a
1901          * particular target to unlink all pointers to garbage nodes
1902          * without requiring many re-traversals during cancellation
1903          * storms.
1904          */
1905         private void unlinkCancelledWaiters() {
1906             Node t = firstWaiter;
1907             Node trail = null;
1908             while (t != null) {
1909                 Node next = t.nextWaiter;
1910                 if (t.waitStatus != Node.CONDITION) {
1911                     t.nextWaiter = null;
1912                     if (trail == null)
1913                         firstWaiter = next;
1914                     else
1915                         trail.nextWaiter = next;
1916                     if (next == null)
1917                         lastWaiter = trail;
1918                 }
1919                 else
1920                     trail = t;
1921                 t = next;
1922             }
1923         }
1924 
1925         // public methods
1926 
1927         /**
1928          * Moves the longest-waiting thread, if one exists, from the
1929          * wait queue for this condition to the wait queue for the
1930          * owning lock.
1931          *
1932          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
1933          *         returns {@code false}
1934          */
1935         public final void signal() {
1936             if (!isHeldExclusively())
1937                 throw new IllegalMonitorStateException();
1938             Node first = firstWaiter;
1939             if (first != null)
1940                 doSignal(first);
1941         }
1942 
1943         /**
1944          * Moves all threads from the wait queue for this condition to
1945          * the wait queue for the owning lock.
1946          *
1947          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
1948          *         returns {@code false}
1949          */
1950         public final void signalAll() {
1951             if (!isHeldExclusively())
1952                 throw new IllegalMonitorStateException();
1953             Node first = firstWaiter;
1954             if (first != null)
1955                 doSignalAll(first);
1956         }
1957 
1958         /**
1959          * Implements uninterruptible condition wait.
1960          * <ol>
1961          * <li> Save lock state returned by {@link #getState}.
1962          * <li> Invoke {@link #release} with
1963          *      saved state as argument, throwing
1964          *      IllegalMonitorStateException if it fails.
1965          * <li> Block until signalled.
1966          * <li> Reacquire by invoking specialized version of
1967          *      {@link #acquire} with saved state as argument.
1968          * </ol>
1969          */
1970         public final void awaitUninterruptibly() {
1971             Node node = addConditionWaiter();
1972             int savedState = fullyRelease(node);
1973             boolean interrupted = false;
1974             while (!isOnSyncQueue(node)) {
1975                 LockSupport.park(this);
1976                 if (Thread.interrupted())
1977                     interrupted = true;
1978             }
1979             if (acquireQueued(node, savedState) || interrupted)
1980                 selfInterrupt();
1981         }
1982 
1983         /*
1984          * For interruptible waits, we need to track whether to throw
1985          * InterruptedException, if interrupted while blocked on
1986          * condition, versus reinterrupt current thread, if
1987          * interrupted while blocked waiting to re-acquire.
1988          */
1989 
1990         /** Mode meaning to reinterrupt on exit from wait */
1991         private static final int REINTERRUPT =  1;
1992         /** Mode meaning to throw InterruptedException on exit from wait */
1993         private static final int THROW_IE    = -1;
1994 
1995         /**
1996          * Checks for interrupt, returning THROW_IE if interrupted
1997          * before signalled, REINTERRUPT if after signalled, or
1998          * 0 if not interrupted.
1999          */
2000         private int checkInterruptWhileWaiting(Node node) {
2001             return Thread.interrupted() ?
2002                 (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
2003                 0;
2004         }
2005 
2006         /**
2007          * Throws InterruptedException, reinterrupts current thread, or
2008          * does nothing, depending on mode.
2009          */
2010         private void reportInterruptAfterWait(int interruptMode)
2011             throws InterruptedException {
2012             if (interruptMode == THROW_IE)
2013                 throw new InterruptedException();
2014             else if (interruptMode == REINTERRUPT)
2015                 selfInterrupt();
2016         }
2017 
2018         /**
2019          * Implements interruptible condition wait.
2020          * <ol>
2021          * <li> If current thread is interrupted, throw InterruptedException.
2022          * <li> Save lock state returned by {@link #getState}.
2023          * <li> Invoke {@link #release} with
2024          *      saved state as argument, throwing
2025          *      IllegalMonitorStateException if it fails.
2026          * <li> Block until signalled or interrupted.
2027          * <li> Reacquire by invoking specialized version of
2028          *      {@link #acquire} with saved state as argument.
2029          * <li> If interrupted while blocked in step 4, throw InterruptedException.
2030          * </ol>
2031          */
2032         public final void await() throws InterruptedException {
2033             if (Thread.interrupted())
2034                 throw new InterruptedException();
2035             Node node = addConditionWaiter();
2036             int savedState = fullyRelease(node);
2037             int interruptMode = 0;
2038             while (!isOnSyncQueue(node)) {
2039                 LockSupport.park(this);
2040                 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2041                     break;
2042             }
2043             if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2044                 interruptMode = REINTERRUPT;
2045             if (node.nextWaiter != null) // clean up if cancelled
2046                 unlinkCancelledWaiters();
2047             if (interruptMode != 0)
2048                 reportInterruptAfterWait(interruptMode);
2049         }
2050 
2051         /**
2052          * Implements timed condition wait.
2053          * <ol>
2054          * <li> If current thread is interrupted, throw InterruptedException.
2055          * <li> Save lock state returned by {@link #getState}.
2056          * <li> Invoke {@link #release} with
2057          *      saved state as argument, throwing
2058          *      IllegalMonitorStateException if it fails.
2059          * <li> Block until signalled, interrupted, or timed out.
2060          * <li> Reacquire by invoking specialized version of
2061          *      {@link #acquire} with saved state as argument.
2062          * <li> If interrupted while blocked in step 4, throw InterruptedException.
2063          * </ol>
2064          */
2065         public final long awaitNanos(long nanosTimeout) throws InterruptedException {
2066             if (Thread.interrupted())
2067                 throw new InterruptedException();
2068             Node node = addConditionWaiter();
2069             int savedState = fullyRelease(node);
2070             long lastTime = System.nanoTime();
2071             int interruptMode = 0;
2072             while (!isOnSyncQueue(node)) {
2073                 if (nanosTimeout <= 0L) {
2074                     transferAfterCancelledWait(node);
2075                     break;
2076                 }
2077                 LockSupport.parkNanos(this, nanosTimeout);
2078                 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2079                     break;
2080 
2081                 long now = System.nanoTime();
2082                 nanosTimeout -= now - lastTime;
2083                 lastTime = now;
2084             }
2085             if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2086                 interruptMode = REINTERRUPT;
2087             if (node.nextWaiter != null)
2088                 unlinkCancelledWaiters();
2089             if (interruptMode != 0)
2090                 reportInterruptAfterWait(interruptMode);
2091             return nanosTimeout - (System.nanoTime() - lastTime);
2092         }
2093 
2094         /**
2095          * Implements absolute timed condition wait.
2096          * <ol>
2097          * <li> If current thread is interrupted, throw InterruptedException.
2098          * <li> Save lock state returned by {@link #getState}.
2099          * <li> Invoke {@link #release} with
2100          *      saved state as argument, throwing
2101          *      IllegalMonitorStateException if it fails.
2102          * <li> Block until signalled, interrupted, or timed out.
2103          * <li> Reacquire by invoking specialized version of
2104          *      {@link #acquire} with saved state as argument.
2105          * <li> If interrupted while blocked in step 4, throw InterruptedException.
2106          * <li> If timed out while blocked in step 4, return false, else true.
2107          * </ol>
2108          */
2109         public final boolean awaitUntil(Date deadline) throws InterruptedException {
2110             if (deadline == null)
2111                 throw new NullPointerException();
2112             long abstime = deadline.getTime();
2113             if (Thread.interrupted())
2114                 throw new InterruptedException();
2115             Node node = addConditionWaiter();
2116             int savedState = fullyRelease(node);
2117             boolean timedout = false;
2118             int interruptMode = 0;
2119             while (!isOnSyncQueue(node)) {
2120                 if (System.currentTimeMillis() > abstime) {
2121                     timedout = transferAfterCancelledWait(node);
2122                     break;
2123                 }
2124                 LockSupport.parkUntil(this, abstime);
2125                 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2126                     break;
2127             }
2128             if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2129                 interruptMode = REINTERRUPT;
2130             if (node.nextWaiter != null)
2131                 unlinkCancelledWaiters();
2132             if (interruptMode != 0)
2133                 reportInterruptAfterWait(interruptMode);
2134             return !timedout;
2135         }
2136 
2137         /**
2138          * Implements timed condition wait.
2139          * <ol>
2140          * <li> If current thread is interrupted, throw InterruptedException.
2141          * <li> Save lock state returned by {@link #getState}.
2142          * <li> Invoke {@link #release} with
2143          *      saved state as argument, throwing
2144          *      IllegalMonitorStateException if it fails.
2145          * <li> Block until signalled, interrupted, or timed out.
2146          * <li> Reacquire by invoking specialized version of
2147          *      {@link #acquire} with saved state as argument.
2148          * <li> If interrupted while blocked in step 4, throw InterruptedException.
2149          * <li> If timed out while blocked in step 4, return false, else true.
2150          * </ol>
2151          */
2152         public final boolean await(long time, TimeUnit unit) throws InterruptedException {
2153             if (unit == null)
2154                 throw new NullPointerException();
2155             long nanosTimeout = unit.toNanos(time);
2156             if (Thread.interrupted())
2157                 throw new InterruptedException();
2158             Node node = addConditionWaiter();
2159             int savedState = fullyRelease(node);
2160             long lastTime = System.nanoTime();
2161             boolean timedout = false;
2162             int interruptMode = 0;
2163             while (!isOnSyncQueue(node)) {
2164                 if (nanosTimeout <= 0L) {
2165                     timedout = transferAfterCancelledWait(node);
2166                     break;
2167                 }
2168                 if (nanosTimeout >= spinForTimeoutThreshold)
2169                     LockSupport.parkNanos(this, nanosTimeout);
2170                 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2171                     break;
2172                 long now = System.nanoTime();
2173                 nanosTimeout -= now - lastTime;
2174                 lastTime = now;
2175             }
2176             if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2177                 interruptMode = REINTERRUPT;
2178             if (node.nextWaiter != null)
2179                 unlinkCancelledWaiters();
2180             if (interruptMode != 0)
2181                 reportInterruptAfterWait(interruptMode);
2182             return !timedout;
2183         }
2184 
2185         //  support for instrumentation
2186 
2187         /**
2188          * Returns true if this condition was created by the given
2189          * synchronization object.
2190          *
2191          * @return {@code true} if owned
2192          */
2193         final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
2194             return sync == AbstractQueuedSynchronizer.this;
2195         }
2196 
2197         /**
2198          * Queries whether any threads are waiting on this condition.
2199          * Implements {@link AbstractQueuedSynchronizer#hasWaiters}.
2200          *
2201          * @return {@code true} if there are any waiting threads
2202          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2203          *         returns {@code false}
2204          */
2205         protected final boolean hasWaiters() {
2206             if (!isHeldExclusively())
2207                 throw new IllegalMonitorStateException();
2208             for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2209                 if (w.waitStatus == Node.CONDITION)
2210                     return true;
2211             }
2212             return false;
2213         }
2214 
2215         /**
2216          * Returns an estimate of the number of threads waiting on
2217          * this condition.
2218          * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength}.
2219          *
2220          * @return the estimated number of waiting threads
2221          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2222          *         returns {@code false}
2223          */
2224         protected final int getWaitQueueLength() {
2225             if (!isHeldExclusively())
2226                 throw new IllegalMonitorStateException();
2227             int n = 0;
2228             for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2229                 if (w.waitStatus == Node.CONDITION)
2230                     ++n;
2231             }
2232             return n;
2233         }
2234 
2235         /**
2236          * Returns a collection containing those threads that may be
2237          * waiting on this Condition.
2238          * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads}.
2239          *
2240          * @return the collection of threads
2241          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2242          *         returns {@code false}
2243          */
2244         protected final Collection<Thread> getWaitingThreads() {
2245             if (!isHeldExclusively())
2246                 throw new IllegalMonitorStateException();
2247             ArrayList<Thread> list = new ArrayList<Thread>();
2248             for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2249                 if (w.waitStatus == Node.CONDITION) {
2250                     Thread t = w.thread;
2251                     if (t != null)
2252                         list.add(t);
2253                 }
2254             }
2255             return list;
2256         }
2257     }
2258 
2259     /**
2260      * Setup to support compareAndSet. We need to natively implement
2261      * this here: For the sake of permitting future enhancements, we
2262      * cannot explicitly subclass AtomicInteger, which would be
2263      * efficient and useful otherwise. So, as the lesser of evils, we
2264      * natively implement using hotspot intrinsics API. And while we
2265      * are at it, we do the same for other CASable fields (which could
2266      * otherwise be done with atomic field updaters).
2267      */
2268     private static final Unsafe unsafe = Unsafe.getUnsafe();
2269     private static final long stateOffset;
2270     private static final long headOffset;
2271     private static final long tailOffset;
2272     private static final long waitStatusOffset;
2273     private static final long nextOffset;
2274 
2275     static {
2276         try {
2277             stateOffset = unsafe.objectFieldOffset
2278                 (AbstractQueuedSynchronizer.class.getDeclaredField("state"));
2279             headOffset = unsafe.objectFieldOffset
2280                 (AbstractQueuedSynchronizer.class.getDeclaredField("head"));
2281             tailOffset = unsafe.objectFieldOffset
2282                 (AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
2283             waitStatusOffset = unsafe.objectFieldOffset
2284                 (Node.class.getDeclaredField("waitStatus"));
2285             nextOffset = unsafe.objectFieldOffset
2286                 (Node.class.getDeclaredField("next"));
2287 
2288         } catch (Exception ex) { throw new Error(ex); }
2289     }
2290 
2291     /**
2292      * CAS head field. Used only by enq.
2293      */
2294     private final boolean compareAndSetHead(Node update) {
2295         return unsafe.compareAndSwapObject(this, headOffset, null, update);
2296     }
2297 
2298     /**
2299      * CAS tail field. Used only by enq.
2300      */
2301     private final boolean compareAndSetTail(Node expect, Node update) {
2302         return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
2303     }
2304 
2305     /**
2306      * CAS waitStatus field of a node.
2307      */
2308     private final static boolean compareAndSetWaitStatus(Node node,
2309                                                          int expect,
2310                                                          int update) {
2311         return unsafe.compareAndSwapInt(node, waitStatusOffset,
2312                                         expect, update);
2313     }
2314 
2315     /**
2316      * CAS next field of a node.
2317      */
2318     private final static boolean compareAndSetNext(Node node,
2319                                                    Node expect,
2320                                                    Node update) {
2321         return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
2322     }
2323 }