AQS源码解读----AbstractQueuedSynchronizer

  36 package cn.com.pep;
  37 import java.util.concurrent.TimeUnit;
  38 import java.util.concurrent.locks.AbstractOwnableSynchronizer;
  39 import java.util.concurrent.locks.Condition;
  40 import java.util.concurrent.locks.LockSupport;
  41 import java.util.ArrayList;
  42 import java.util.Collection;
  43 import java.util.Date;
  44 
  45 import sun.misc.Unsafe;
  46 
  47 /**
  48  * Provides a framework for implementing blocking locks and related
  49  * synchronizers (semaphores, events, etc) that rely on
  50  * first-in-first-out (FIFO) wait queues.  This class is designed to
  51  * be a useful basis for most kinds of synchronizers that rely on a
  52  * single atomic {@code int} value to represent state. Subclasses
  53  * must define the protected methods that change this state, and which
  54  * define what that state means in terms of this object being acquired
  55  * or released.  Given these, the other methods in this class carry
  56  * out all queuing and blocking mechanics. Subclasses can maintain
  57  * other state fields, but only the atomically updated {@code int}
  58  * value manipulated using methods {@link #getState}, {@link
  59  * #setState} and {@link #compareAndSetState} is tracked with respect
  60  * to synchronization.
  61  *
  62  * <p>Subclasses should be defined as non-public internal helper
  63  * classes that are used to implement the synchronization properties
  64  * of their enclosing class.  Class
  65  * {@code AbstractQueuedSynchronizer} does not implement any
  66  * synchronization interface.  Instead it defines methods such as
  67  * {@link #acquireInterruptibly} that can be invoked as
  68  * appropriate by concrete locks and related synchronizers to
  69  * implement their public methods.
  70  *
  71  * <p>This class supports either or both a default <em>exclusive</em>
  72  * mode and a <em>shared</em> mode. When acquired in exclusive mode,
  73  * attempted acquires by other threads cannot succeed. Shared mode
  74  * acquires by multiple threads may (but need not) succeed. This class
  75  * does not &quot;understand&quot; these differences except in the
  76  * mechanical sense that when a shared mode acquire succeeds, the next
  77  * waiting thread (if one exists) must also determine whether it can
  78  * acquire as well. Threads waiting in the different modes share the
  79  * same FIFO queue. Usually, implementation subclasses support only
  80  * one of these modes, but both can come into play for example in a
  81  * {@link ReadWriteLock}. Subclasses that support only exclusive or
  82  * only shared modes need not define the methods supporting the unused mode.
  83  *
  84  * <p>This class defines a nested {@link ConditionObject} class that
  85  * can be used as a {@link Condition} implementation by subclasses
  86  * supporting exclusive mode for which method {@link
  87  * #isHeldExclusively} reports whether synchronization is exclusively
  88  * held with respect to the current thread, method {@link #release}
  89  * invoked with the current {@link #getState} value fully releases
  90  * this object, and {@link #acquire}, given this saved state value,
  91  * eventually restores this object to its previous acquired state.  No
  92  * {@code AbstractQueuedSynchronizer} method otherwise creates such a
  93  * condition, so if this constraint cannot be met, do not use it.  The
  94  * behavior of {@link ConditionObject} depends of course on the
  95  * semantics of its synchronizer implementation.
  96  *
  97  * <p>This class provides inspection, instrumentation, and monitoring
  98  * methods for the internal queue, as well as similar methods for
  99  * condition objects. These can be exported as desired into classes
 100  * using an {@code AbstractQueuedSynchronizer} for their
 101  * synchronization mechanics.
 102  *
 103  * <p>Serialization of this class stores only the underlying atomic
 104  * integer maintaining state, so deserialized objects have empty
 105  * thread queues. Typical subclasses requiring serializability will
 106  * define a {@code readObject} method that restores this to a known
 107  * initial state upon deserialization.
 108  *
 109  * <h3>Usage</h3>
 110  *
 111  * <p>To use this class as the basis of a synchronizer, redefine the
 112  * following methods, as applicable, by inspecting and/or modifying
 113  * the synchronization state using {@link #getState}, {@link
 114  * #setState} and/or {@link #compareAndSetState}:
 115  *
 116  * <ul>
 117  * <li> {@link #tryAcquire}
 118  * <li> {@link #tryRelease}
 119  * <li> {@link #tryAcquireShared}
 120  * <li> {@link #tryReleaseShared}
 121  * <li> {@link #isHeldExclusively}
 122  * </ul>
 123  *
 124  * Each of these methods by default throws {@link
 125  * UnsupportedOperationException}.  Implementations of these methods
 126  * must be internally thread-safe, and should in general be short and
 127  * not block. Defining these methods is the <em>only</em> supported
 128  * means of using this class. All other methods are declared
 129  * {@code final} because they cannot be independently varied.
 130  *
 131  * <p>You may also find the inherited methods from {@link
 132  * AbstractOwnableSynchronizer} useful to keep track of the thread
 133  * owning an exclusive synchronizer.  You are encouraged to use them
 134  * -- this enables monitoring and diagnostic tools to assist users in
 135  * determining which threads hold locks.
 136  *
 137  * <p>Even though this class is based on an internal FIFO queue, it
 138  * does not automatically enforce FIFO acquisition policies.  The core
 139  * of exclusive synchronization takes the form:
 140  *
 141  * <pre>
 142  * Acquire:
 143  *     while (!tryAcquire(arg)) {
 144  *        <em>enqueue thread if it is not already queued</em>;
 145  *        <em>possibly block current thread</em>;
 146  *     }
 147  *
 148  * Release:
 149  *     if (tryRelease(arg))
 150  *        <em>unblock the first queued thread</em>;
 151  * </pre>
 152  *
 153  * (Shared mode is similar but may involve cascading signals.)
 154  *
 155  * <p id="barging">Because checks in acquire are invoked before
 156  * enqueuing, a newly acquiring thread may <em>barge</em> ahead of
 157  * others that are blocked and queued.  However, you can, if desired,
 158  * define {@code tryAcquire} and/or {@code tryAcquireShared} to
 159  * disable barging by internally invoking one or more of the inspection
 160  * methods, thereby providing a <em>fair</em> FIFO acquisition order.
 161  * In particular, most fair synchronizers can define {@code tryAcquire}
 162  * to return {@code false} if {@link #hasQueuedPredecessors} (a method
 163  * specifically designed to be used by fair synchronizers) returns
 164  * {@code true}.  Other variations are possible.
 165  *
 166  * <p>Throughput and scalability are generally highest for the
 167  * default barging (also known as <em>greedy</em>,
 168  * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy.
 169  * While this is not guaranteed to be fair or starvation-free, earlier
 170  * queued threads are allowed to recontend before later queued
 171  * threads, and each recontention has an unbiased chance to succeed
 172  * against incoming threads.  Also, while acquires do not
 173  * &quot;spin&quot; in the usual sense, they may perform multiple
 174  * invocations of {@code tryAcquire} interspersed with other
 175  * computations before blocking.  This gives most of the benefits of
 176  * spins when exclusive synchronization is only briefly held, without
 177  * most of the liabilities when it isn't. If so desired, you can
 178  * augment this by preceding calls to acquire methods with
 179  * "fast-path" checks, possibly prechecking {@link #hasContended}
 180  * and/or {@link #hasQueuedThreads} to only do so if the synchronizer
 181  * is likely not to be contended.
 182  *
 183  * <p>This class provides an efficient and scalable basis for
 184  * synchronization in part by specializing its range of use to
 185  * synchronizers that can rely on {@code int} state, acquire, and
 186  * release parameters, and an internal FIFO wait queue. When this does
 187  * not suffice, you can build synchronizers from a lower level using
 188  * {@link java.util.concurrent.atomic atomic} classes, your own custom
 189  * {@link java.util.Queue} classes, and {@link LockSupport} blocking
 190  * support.
 191  *
 192  * <h3>Usage Examples</h3>
 193  *
 194  * <p>Here is a non-reentrant mutual exclusion lock class that uses
 195  * the value zero to represent the unlocked state, and one to
 196  * represent the locked state. While a non-reentrant lock
 197  * does not strictly require recording of the current owner
 198  * thread, this class does so anyway to make usage easier to monitor.
 199  * It also supports conditions and exposes
 200  * one of the instrumentation methods:
 201  *
 202  *  <pre> {@code
 203  * class Mutex implements Lock, java.io.Serializable {
 204  *
 205  *   // Our internal helper class
 206  *   private static class Sync extends AbstractQueuedSynchronizer {
 207  *     // Reports whether in locked state
 208  *     protected boolean isHeldExclusively() {
 209  *       return getState() == 1;
 210  *     }
 211  *
 212  *     // Acquires the lock if state is zero
 213  *     public boolean tryAcquire(int acquires) {
 214  *       assert acquires == 1; // Otherwise unused
 215  *       if (compareAndSetState(0, 1)) {
 216  *         setExclusiveOwnerThread(Thread.currentThread());
 217  *         return true;
 218  *       }
 219  *       return false;
 220  *     }
 221  *
 222  *     // Releases the lock by setting state to zero
 223  *     protected boolean tryRelease(int releases) {
 224  *       assert releases == 1; // Otherwise unused
 225  *       if (getState() == 0) throw new IllegalMonitorStateException();
 226  *       setExclusiveOwnerThread(null);
 227  *       setState(0);
 228  *       return true;
 229  *     }
 230  *
 231  *     // Provides a Condition
 232  *     Condition newCondition() { return new ConditionObject(); }
 233  *
 234  *     // Deserializes properly
 235  *     private void readObject(ObjectInputStream s)
 236  *         throws IOException, ClassNotFoundException {
 237  *       s.defaultReadObject();
 238  *       setState(0); // reset to unlocked state
 239  *     }
 240  *   }
 241  *
 242  *   // The sync object does all the hard work. We just forward to it.
 243  *   private final Sync sync = new Sync();
 244  *
 245  *   public void lock()                { sync.acquire(1); }
 246  *   public boolean tryLock()          { return sync.tryAcquire(1); }
 247  *   public void unlock()              { sync.release(1); }
 248  *   public Condition newCondition()   { return sync.newCondition(); }
 249  *   public boolean isLocked()         { return sync.isHeldExclusively(); }
 250  *   public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
 251  *   public void lockInterruptibly() throws InterruptedException {
 252  *     sync.acquireInterruptibly(1);
 253  *   }
 254  *   public boolean tryLock(long timeout, TimeUnit unit)
 255  *       throws InterruptedException {
 256  *     return sync.tryAcquireNanos(1, unit.toNanos(timeout));
 257  *   }
 258  * }}</pre>
 259  *
 260  * <p>Here is a latch class that is like a
 261  * {@link java.util.concurrent.CountDownLatch CountDownLatch}
 262  * except that it only requires a single {@code signal} to
 263  * fire. Because a latch is non-exclusive, it uses the {@code shared}
 264  * acquire and release methods.
 265  *
 266  *  <pre> {@code
 267  * class BooleanLatch {
 268  *
 269  *   private static class Sync extends AbstractQueuedSynchronizer {
 270  *     boolean isSignalled() { return getState() != 0; }
 271  *
 272  *     protected int tryAcquireShared(int ignore) {
 273  *       return isSignalled() ? 1 : -1;
 274  *     }
 275  *
 276  *     protected boolean tryReleaseShared(int ignore) {
 277  *       setState(1);
 278  *       return true;
 279  *     }
 280  *   }
 281  *
 282  *   private final Sync sync = new Sync();
 283  *   public boolean isSignalled() { return sync.isSignalled(); }
 284  *   public void signal()         { sync.releaseShared(1); }
 285  *   public void await() throws InterruptedException {
 286  *     sync.acquireSharedInterruptibly(1);
 287  *   }
 288  * }}</pre>
 289  *
 290  * @since 1.5
 291  * @author Doug Lea
 292  */
 293 public abstract class AbstractQueuedSynchronizer
 294     extends AbstractOwnableSynchronizer
 295     implements java.io.Serializable {
 296 
 297     private static final long serialVersionUID = 7373984972572414691L;
 298 
 299     /**
 300      * Creates a new {@code AbstractQueuedSynchronizer} instance
 301      * with initial synchronization state of zero.
 302      */
 303     protected AbstractQueuedSynchronizer() { }
 304 
 305     /**
 306      * Wait queue node class.
 307      *
 308      * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
 309      * Hagersten) lock queue. CLH locks are normally used for
 310      * spinlocks.  We instead use them for blocking synchronizers, but
 311      * use the same basic tactic of holding some of the control
 312      * information about a thread in the predecessor of its node.  A
 313      * "status" field in each node keeps track of whether a thread
 314      * should block.  A node is signalled when its predecessor
 315      * releases.  Each node of the queue otherwise serves as a
 316      * specific-notification-style monitor holding a single waiting
 317      * thread. The status field does NOT control whether threads are
 318      * granted locks etc though.  A thread may try to acquire if it is
 319      * first in the queue. But being first does not guarantee success;
 320      * it only gives the right to contend.  So the currently released
 321      * contender thread may need to rewait.
 322      *
 323      * <p>To enqueue into a CLH lock, you atomically splice it in as new
 324      * tail. To dequeue, you just set the head field.
 325      * <pre>
 326      *      +------+  prev +-----+       +-----+
 327      * head |      | <---- |     | <---- |     |  tail
 328      *      +------+       +-----+       +-----+
 329      * </pre>
 330      *
 331      * <p>Insertion into a CLH queue requires only a single atomic
 332      * operation on "tail", so there is a simple atomic point of
 333      * demarcation from unqueued to queued. Similarly, dequeuing
 334      * involves only updating the "head". However, it takes a bit
 335      * more work for nodes to determine who their successors are,
 336      * in part to deal with possible cancellation due to timeouts
 337      * and interrupts.
 338      *
 339      * <p>The "prev" links (not used in original CLH locks), are mainly
 340      * needed to handle cancellation. If a node is cancelled, its
 341      * successor is (normally) relinked to a non-cancelled
 342      * predecessor. For explanation of similar mechanics in the case
 343      * of spin locks, see the papers by Scott and Scherer at
 344      * http://www.cs.rochester.edu/u/scott/synchronization/
 345      *
 346      * <p>We also use "next" links to implement blocking mechanics.
 347      * The thread id for each node is kept in its own node, so a
 348      * predecessor signals the next node to wake up by traversing
 349      * next link to determine which thread it is.  Determination of
 350      * successor must avoid races with newly queued nodes to set
 351      * the "next" fields of their predecessors.  This is solved
 352      * when necessary by checking backwards from the atomically
 353      * updated "tail" when a node's successor appears to be null.
 354      * (Or, said differently, the next-links are an optimization
 355      * so that we don't usually need a backward scan.)
 356      *
 357      * <p>Cancellation introduces some conservatism to the basic
 358      * algorithms.  Since we must poll for cancellation of other
 359      * nodes, we can miss noticing whether a cancelled node is
 360      * ahead or behind us. This is dealt with by always unparking
 361      * successors upon cancellation, allowing them to stabilize on
 362      * a new predecessor, unless we can identify an uncancelled
 363      * predecessor who will carry this responsibility.
 364      *
 365      * <p>CLH queues need a dummy header node to get started. But
 366      * we don't create them on construction, because it would be wasted
 367      * effort if there is never contention. Instead, the node
 368      * is constructed and head and tail pointers are set upon first
 369      * contention.
 370      *
 371      * <p>Threads waiting on Conditions use the same nodes, but
 372      * use an additional link. Conditions only need to link nodes
 373      * in simple (non-concurrent) linked queues because they are
 374      * only accessed when exclusively held.  Upon await, a node is
 375      * inserted into a condition queue.  Upon signal, the node is
 376      * transferred to the main queue.  A special value of status
 377      * field is used to mark which queue a node is on.
 378      *
 379      * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
 380      * Scherer and Michael Scott, along with members of JSR-166
 381      * expert group, for helpful ideas, discussions, and critiques
 382      * on the design of this class.
 383      */
 384     static final class Node {
 385         /** Marker to indicate a node is waiting in shared mode */
 386         static final Node SHARED = new Node();
 387         /** Marker to indicate a node is waiting in exclusive mode */
 388         static final Node EXCLUSIVE = null;
 389 
 390         /** waitStatus value to indicate thread has cancelled */
 391         static final int CANCELLED =  1;
 392         /** waitStatus value to indicate successor's thread needs unparking */
 393         static final int SIGNAL    = -1;
 394         /** waitStatus value to indicate thread is waiting on condition */
 395         static final int CONDITION = -2;
 396         /**
 397          * waitStatus value to indicate the next acquireShared should
 398          * unconditionally propagate
 399          */
 400         static final int PROPAGATE = -3;
 401 
 402         /**
 403          * Status field, taking on only the values:
 404          *   SIGNAL:     The successor of this node is (or will soon be)
 405          *               blocked (via park), so the current node must
 406          *               unpark its successor when it releases or
 407          *               cancels. To avoid races, acquire methods must
 408          *               first indicate they need a signal,
 409          *               then retry the atomic acquire, and then,
 410          *               on failure, block.
 411          *   CANCELLED:  This node is cancelled due to timeout or interrupt.
 412          *               Nodes never leave this state. In particular,
 413          *               a thread with cancelled node never again blocks.
 414          *   CONDITION:  This node is currently on a condition queue.
 415          *               It will not be used as a sync queue node
 416          *               until transferred, at which time the status
 417          *               will be set to 0. (Use of this value here has
 418          *               nothing to do with the other uses of the
 419          *               field, but simplifies mechanics.)
 420          *   PROPAGATE:  A releaseShared should be propagated to other
 421          *               nodes. This is set (for head node only) in
 422          *               doReleaseShared to ensure propagation
 423          *               continues, even if other operations have
 424          *               since intervened.
 425          *   0:          None of the above
 426          *
 427          * The values are arranged numerically to simplify use.
 428          * Non-negative values mean that a node doesn't need to
 429          * signal. So, most code doesn't need to check for particular
 430          * values, just for sign.
 431          *
 432          * The field is initialized to 0 for normal sync nodes, and
 433          * CONDITION for condition nodes.  It is modified using CAS
 434          * (or when possible, unconditional volatile writes).
 435          */
 436         volatile int waitStatus;
 437 
 438         /**
 439          * Link to predecessor node that current node/thread relies on
 440          * for checking waitStatus. Assigned during enqueuing, and nulled
 441          * out (for sake of GC) only upon dequeuing.  Also, upon
 442          * cancellation of a predecessor, we short-circuit while
 443          * finding a non-cancelled one, which will always exist
 444          * because the head node is never cancelled: A node becomes
 445          * head only as a result of successful acquire. A
 446          * cancelled thread never succeeds in acquiring, and a thread only
 447          * cancels itself, not any other node.
 448          */
 449         volatile Node prev;
 450 
 451         /**
 452          * Link to the successor node that the current node/thread
 453          * unparks upon release. Assigned during enqueuing, adjusted
 454          * when bypassing cancelled predecessors, and nulled out (for
 455          * sake of GC) when dequeued.  The enq operation does not
 456          * assign next field of a predecessor until after attachment,
 457          * so seeing a null next field does not necessarily mean that
 458          * node is at end of queue. However, if a next field appears
 459          * to be null, we can scan prev's from the tail to
 460          * double-check.  The next field of cancelled nodes is set to
 461          * point to the node itself instead of null, to make life
 462          * easier for isOnSyncQueue.
 463          */
 464         volatile Node next;
 465 
 466         /**
 467          * The thread that enqueued this node.  Initialized on
 468          * construction and nulled out after use.
 469          */
 470         volatile Thread thread;
 471 
 472         /**
 473          * Link to next node waiting on condition, or the special
 474          * value SHARED.  Because condition queues are accessed only
 475          * when holding in exclusive mode, we just need a simple
 476          * linked queue to hold nodes while they are waiting on
 477          * conditions. They are then transferred to the queue to
 478          * re-acquire. And because conditions can only be exclusive,
 479          * we save a field by using special value to indicate shared
 480          * mode.
 481          */
 482         Node nextWaiter;
 483 
 484         /**
 485          * Returns true if node is waiting in shared mode.
 486          */
 487         final boolean isShared() {
 488             return nextWaiter == SHARED;
 489         }
 490 
 491         /**
 492          * Returns previous node, or throws NullPointerException if null.
 493          * Use when predecessor cannot be null.  The null check could
 494          * be elided, but is present to help the VM.
 495          *
 496          * @return the predecessor of this node
 497          */
 498         final Node predecessor() throws NullPointerException {
 499             Node p = prev;
 500             if (p == null)
 501                 throw new NullPointerException();
 502             else
 503                 return p;
 504         }
 505 
 506         Node() {    // Used to establish initial head or SHARED marker
 507         }
 508 
 509         Node(Thread thread, Node mode) {     // Used by addWaiter
 510             this.nextWaiter = mode;
 511             this.thread = thread;
 512         }
 513 
 514         Node(Thread thread, int waitStatus) { // Used by Condition
 515             this.waitStatus = waitStatus;
 516             this.thread = thread;
 517         }
 518     }
 519 
 520     /**
 521      * Head of the wait queue, lazily initialized.  Except for
 522      * initialization, it is modified only via method setHead.  Note:
 523      * If head exists, its waitStatus is guaranteed not to be
 524      * CANCELLED.
 525      */
 526     private transient volatile Node head;
 527 
 528     /**
 529      * Tail of the wait queue, lazily initialized.  Modified only via
 530      * method enq to add new wait node.
 531      */
 532     private transient volatile Node tail;
 533 
 534     /**
 535      * The synchronization state.
 536      */
 537     private volatile int state;
 538 
 539     /**
 540      * Returns the current value of synchronization state.
 541      * This operation has memory semantics of a {@code volatile} read.
 542      * @return current state value
 543      */
 544     protected final int getState() {
 545         return state;
 546     }
 547 
 548     /**
 549      * Sets the value of synchronization state.
 550      * This operation has memory semantics of a {@code volatile} write.
 551      * @param newState the new state value
 552      */
 553     protected final void setState(int newState) {
 554         state = newState;
 555     }
 556 
 557     /**
 558      * Atomically sets synchronization state to the given updated
 559      * value if the current state value equals the expected value.
 560      * This operation has memory semantics of a {@code volatile} read
 561      * and write.
 562      *
 563      * @param expect the expected value
 564      * @param update the new value
 565      * @return {@code true} if successful. False return indicates that the actual
 566      *         value was not equal to the expected value.
 567      */
 568     protected final boolean compareAndSetState(int expect, int update) {
 569         // See below for intrinsics setup to support this
 570         return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
 571     }
 572 
 573     // Queuing utilities
 574 
 575     /**
 576      * The number of nanoseconds for which it is faster to spin
 577      * rather than to use timed park. A rough estimate suffices
 578      * to improve responsiveness with very short timeouts.
 579      */
 580     static final long spinForTimeoutThreshold = 1000L;
 581 
 582     /**
 583      * Inserts node into queue, initializing if necessary. See picture above.
 584      * @param node the node to insert
 585      * @return node's predecessor
 586      */
 587     private Node enq(final Node node) {
 588         /*"自旋"，将给定的节点插入到同步队列的尾部*/
 589         for (;;) {
 590             Node t = tail;
 591             /*同步队列为空，则dummy哑节点作为同步队列的头结点head，并且将尾节点tail也指向头结点head*/
 592             if (t == null) {
 593                 /*CAS操作，设置同步队列的头结点*/
 594                 if (compareAndSetHead(new Node())) {
 595                     /*将尾节点设置为头结点，进入下次"自旋"*/
 596                     tail = head;
 597                 }
 598             }else {
 599                 /*尾部节点不为空，则进行正常添加动作*/
 600                 node.prev = t;
 601                 /*CAS操作，设置同步队列的头结点*/
 602                 if (compareAndSetTail(t, node)) {
 603                     t.next = node;
 604                     return t;
 605                 }
 606             }
 607         }
 608     }
 609 
 610     /**
 611      * Creates and enqueues node for current thread and given mode.
 612      * 以给定的模式包装当前线程节点，将当前节点加入到阻塞队列的队尾
 613      * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
 614      * @return the new node
 615      */
 616     private Node addWaiter(Node mode) {
 617         /*以给定的模式将当前线程包装成node节点*/
 618         Node node = new Node(Thread.currentThread(), mode);
 619         
 620         /*快速采用尾插法，将当前节点插入到同步队列的队尾*/
 621         Node predNode = tail;
 622         if (predNode != null) {
 623             /*preNode <-- node*/
 624             node.prev = predNode;
 625             /*采用CAS将node设置阻塞队列的尾节点，设置成功，说明没有并发*/
 626             if (compareAndSetTail(tail, node)) {
 627                 predNode.next = node;
 628                 /*尾插法插入成功，则直接返回当前节点*/
 629                 return node;
 630             }
 631         }
 632         /*"自旋"将节点加入到队列的尾部，直到成功为止*/
 633         enq(node);
 634         return node;
 635     }
 636 
 637     /**
 638      * Sets head of queue to be node, thus dequeuing. Called only by
 639      * acquire methods.  Also nulls out unused fields for sake of GC
 640      * and to suppress unnecessary signals and traversals.
 641      *
 642      * @param node the node
 643      */
 644     private void setHead(Node node) {
 645         head = node;
 646         node.thread = null;
 647         node.prev = null;
 648     }
 649 
 650     /**
 651      * Wakes up node's successor, if one exists.
 652      *
 653      * @param node the node
 654      */
 655     private void unparkSuccessor(Node node) {
 656         /*
 657          * If status is negative (i.e., possibly needing signal) try
 658          * to clear in anticipation of signalling.  It is OK if this
 659          * fails or if status is changed by waiting thread.
 660          */
 661         //在这里，这个节点其实是同步队列的头结点，头结点唤醒后继节点之后，使命就完成了，所以应该将其状态置为0
 662         int ws = node.waitStatus;
 663         if (ws < 0)
 664             compareAndSetWaitStatus(node, ws, 0);
 665 
 666         /*
 667          * Thread to unpark is held in successor, which is normally
 668          * just the next node.  But if cancelled or apparently null,
 669          * traverse backwards from tail to find the actual
 670          * non-cancelled successor.
 671          */
 672         Node s = node.next;
 673         //因为s.next相当于从同步队列的头部遍历所以可能会出现s == null的情况，上面分析过原因，不再赘述了。
 674         if (s == null || s.waitStatus > 0) {
 675             s = null;
 676             //从同步队列的尾部向前遍历，找到当前node节点(头结点)的最近的有效后继节点
 677             for (Node t = tail; t != null && t != node; t = t.prev)
 678                 if (t.waitStatus <= 0)
 679                     s = t;
 680         }
 681         
 682         /**
 683          * 找到最近的有效后继节点，则唤醒后继节点中的线程在parkAndCheckInterrupt()方法上的阻塞，去尝试竞争共享资源，
 684          * 这就体现了线程之间的协作，而在这个竞争的过程中也会忽略这个Node.CANCELLED状态的节点，这当前node节点也就放弃了竞争共享资源的机会，相当于出队了。
 685          */
 686         if (s != null)
 687             LockSupport.unpark(s.thread);
 688     }
 689 
 690     /**
 691      * Release action for shared mode -- signals successor and ensures
 692      * propagation. (Note: For exclusive mode, release just amounts
 693      * to calling unparkSuccessor of head if it needs signal.)
 694      */
 695     private void doReleaseShared() {
 696         /*
 697          * Ensure that a release propagates, even if there are other
 698          * in-progress acquires/releases.  This proceeds in the usual
 699          * way of trying to unparkSuccessor of head if it needs
 700          * signal. But if it does not, status is set to PROPAGATE to
 701          * ensure that upon release, propagation continues.
 702          * Additionally, we must loop in case a new node is added
 703          * while we are doing this. Also, unlike other uses of
 704          * unparkSuccessor, we need to know if CAS to reset status
 705          * fails, if so rechecking.
 706          */
 707         for (;;) {
 708             Node h = head;
 709             if (h != null && h != tail) {
 710                 int ws = h.waitStatus;
 711                 if (ws == Node.SIGNAL) {
 712                     if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
 713                         continue;            // loop to recheck cases
 714                     unparkSuccessor(h);
 715                 }
 716                 else if (ws == 0 &&
 717                          !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
 718                     continue;                // loop on failed CAS
 719             }
 720             if (h == head)                   // loop if head changed
 721                 break;
 722         }
 723     }
 724 
 725     /**
 726      * Sets head of queue, and checks if successor may be waiting
 727      * in shared mode, if so propagating if either propagate > 0 or
 728      * PROPAGATE status was set.
 729      *
 730      * @param node the node
 731      * @param propagate the return value from a tryAcquireShared
 732      */
 733     private void setHeadAndPropagate(Node node, int propagate) {
 734         //后继节点成功获取了共享锁，队列的"旧head"还没有改变，将其保存下来，锁定到方法的局部变量做后序的判断使用；
 735         Node h = head; // Record old head for check below
 736         /**
 737          * 将这个获取共享锁成功的后继节点设置为同步队列的“新head”,此时同步队列的head发生变化， 此线程还未唤起任何线程。
 738          */
 739         setHead(node);
 740         /**
 741          * 1、h == null这个条件什么时候成立呢？仔细翻了下AQS中的源码发现：
 742          * 这个setHeadAndPropagate()方法只在共享锁模式下，同步队列head的后继节点成功获取了共享锁才会调用。
 743          * 获取到共享锁的当前线程是同步队列的头结点的后继节点，"旧head"有后继节点，说明同步队列不为空,那么"旧head"也必定不为空，
 744          * 此方法中第一行通过h == head,在执行setHead(node)方法之前将"旧head"保存了下来，所以h == null必定不会成立,
 745          * 至于为什么这么写呢? 查阅了下资料网上说"发现这个是防止空指针异常发生的标准写法（既如果要取一个对象的某个属性进行判断的时候，首先对这个对象进行null判断）。"
 746          * 这说的过去吧?
 747          * 
 748          * 2、(h = head) == null这个条件什么时候成立呢？
 749          * 这个条件也是不可能成立的，下面这种情况应该是最常见的：
 750          *     （1）、例如有个Semaphore实例s初始化了2个许可，线程A首先调用s.acquire(2)申请了两个许可，成功申请到了许可；
 751          *     （2）、线程B调用了s.acquire()方法申请一个许可，申请失败，加入到同步队列；
 752          *     （3）、线程C调用了s.acquire()方法申请一个许可，申请失败，加入到同步队列；
 753          *  （4）、线程A调用了s.releaseShared(2)方法释放了两个许可，再调用doReleaseShared()方法，进行同步队列唤醒；
 754          *  （5）、首先唤醒了同步队列中的线程B，B线程获取到共享锁：
 755          *      a)、如果此时线程B还未setHead(Node)方法，还未改变同步队列的head头结点，那么线程A的唤醒工作就结束，也仅仅只是唤醒了同步队列中的线程B，
 756          *              则必定有（h = head） == Node(C) ！= null成立，线程C的唤醒工作仍然需要线程B去执行；
 757          *      b)、如果此时线程B执行了SetHead(Node)方法，改变了同步队列的head头结点，那么线程A同时也会唤醒线程C，相当于线程A同时唤醒了线程B和线程C：
 758          *         1）、如果线程C中的setHeadAndPropagate()在线程B前调用完毕（即线程C执行了setHead()方法改变了同步队列的head），那么 (h = head) == Node(C);
 759          *         2）、如果线程C中的setHeadAndPropagate()在线程B之后才调用(即线程C此时还未执行setHead()方法，未改变同步队列的head)，那么 (h = head) == Node(B)
 760          *  所以综上所述，只要执行过addWaiter()方法，向同步队列中添加过线程，那么（h = head）== null必定不成立。只能理解为“防止空指针的标准写法”。 
 761          */
 762         if (propagate > 0 || h == null || h.waitStatus < 0 ||
 763             (h = head) == null || h.waitStatus < 0) {
 764             Node s = node.next;
 765             /**
 766              * s == null这种情况是可能存在的，如果当前唤醒的这个node节点是同步队列的尾节点就可能出现node.next == null;
 767              * s.isShared()指定是共享锁模式，当前线程获取共享锁之后，是需要尝试唤醒同步队列中的其它线程的。
 768              */
 769             if (s == null || s.isShared())
 770                 doReleaseShared();
 771         }
 772     }
 773 
 774     // Utilities for various versions of acquire
 775 
 776     /**
 777      * Cancels an ongoing attempt to acquire.
 778      * @param node the node
 779      */
 780     private void cancelAcquire(Node node) {
 781         //当前节点为空，则说明当前线程永远不会被调度到了，所以直接返回
 782         if (node == null) {
 783             return;
 784         }
 785         
 786         /**
 787          * 接下来将点前Node节点从同步队列出队，主要做以下几件事：
 788          * 1、将当前节点不与任何线程绑定，设置当前节点为Node.CANCELLED状态；
 789          * 2、将当前取消节点的前置非取消节点和后置非取消节点"链接"起来；
 790          * 3、如果前置节点释放了锁，那么当前取消节点承担起后续节点的唤醒职责。
 791          */
 792         
 793         //1、取消当前节点与线程的绑定
 794         node.thread = null;
 795         
 796         //2、找到当前节点的有效前继节点pred
 797         Node pred = node.prev;
 798         while (pred.waitStatus > 0) {
 799             //为什么双向链表从后往前遍历呢？而不是从前往后遍历呢？
 800             node.prev = pred = pred.prev;
 801         }
 802         
 803         //用作CAS操作时候的条件判断需要使用的值
 804         Node predNext = pred.next;
 805         
 806         //3、将当前节点设置为取消状态
 807         node.waitStatus = Node.CANCELLED;
 808         
 809         /**
 810          * 接下来就需要将当前取消节点的前后两个有效节点"链接"起来了，"达成让当前node节点出队的目的"。
 811          * 这里按照node节点在同步队列中的不同位置分了三种情况：
 812          * 1、node节点是同步队列的尾节点tail;
 813          * 2、node节点既不是同步队列头结点head的后继节点，也不是尾节点tail;
 814          * 3、node节点是同步队列头结点head的后继节点；
 815          */
 816         
 817         //1、node是尾节点，并且执行过程中没有并发，直接将pred设置为同步队列的tail
 818         if (node == tail && compareAndSetTail(node, pred)) {
 819             /*
 820              * 此时pred已经设置为同步队列的tail，需要通过CAS操作，将pred的next指向null,没有节点再引用node，就完成了node节点的出队
 821              * 可以看出出队操作会破坏这个同步队列的next指针，这应该“向链表从后往前遍历呢？而不是从前往后遍历呢”的原因吧？
 822              */
 823             compareAndSetNext(pred, predNext, null);
 824         }else {
 825             /*
 826              * 2、node不是尾节点，也不是头结点head的后继节点,那么当前节点node出队以后，node的有效前继结点pred，
 827              *  就有义务在它自身释放资源的时候，唤醒node的有效后继节点successor,即将pred的状态设置为Node.SIGNAL;
 828              */
 829             int ws;
 830             //能执行到这里，说明当前node节点不是head的后继节点，也不是同步队列tail节点
 831             if (pred != head &&
 832                     ((ws = pred.waitStatus) == Node.SIGNAL ||
 833                     //前继节点状态虽然有效但不是SIGNAL,采用CAS操作设置为SIGNAL确保后继有效节点可以被唤醒
 834                     (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) && 
 835                     pred.thread != null) {
 836                 Node next = node.next;
 837                 //只负责唤醒有效后继节点
 838                 if (next != null && next.waitStatus <= 0) {
 839                     /**
 840                      * 下面这段代码相当于将pred-->next，我们提到这个同步队列是个双向队列，那么pred<--next这是谁执行的呢？
 841                      * 答案是其他线程：其它线程在获取共享资源在同步队列中阻塞的时候，调用shouldParkAfterFailedAcquire()方法，
 842                      * 从后向前遍历队列，寻找能唤醒它的有效前继节点，当找到node的时候，因为它的状态已经是Node.CANCELLED,所以会忽略node节点，
 843                      * 直到遍历到有效前继节点pred,将next.prev执行pred,即next--->pred，没有节点再引用node节点，所以node节点至此才完成出队。
 844                      */
 845                     compareAndSetNext(pred, predNext, next);
 846                 }
 847             }else {
 848                 //3、说明node节点是同步队列head的后继节点，调用unparkSuccessor(Node)"出队"。
 849                 unparkSuccessor(node);
 850             }
 851             
 852             node.next = node;//help GC
 853         }
 854     }
 855 
 856     /**
 857      * Checks and updates status for a node that failed to acquire.
 858      * Returns true if thread should block. This is the main signal
 859      * control in all acquire loops.  Requires that pred == node.prev.
 860      *
 861      * @param pred node's predecessor holding status
 862      * @param node the node
 863      * @return {@code true} if thread should block
 864      */
 865     private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
 866         int ws = pred.waitStatus;
 867         /*判断前驱结点的状态，只有前驱结点的状态为SIGNAL,后继节点才能被唤醒，所以其可以安心地挂起来了*/
 868         if (ws == node.waitStatus) {
 869             return true;
 870         }
 871         
 872         /*ws>0表示前驱结点中的线程已经被取消调度了，则认为其是无效节点，继续向前查找，直至找到有效状态的节点*/
 873         if (ws > 0) {
 874             do {
 875                 //前驱结点已经被取消，则将前驱结点设置为pred = pred.prev
 876                 node.prev = pred = pred.prev;
 877                 //不断遍历，直到找到第一个不是取消状态的节点
 878             } while (pred.waitStatus > 0);
 879             //
 880             pred.next = node;
 881         }else {
 882             /*前驱结点状态正常，将前驱结点状态设置为SIGNAL,则前驱结点释放资源的时候，就可以尝试唤醒它的后继节点了*/
 883             compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
 884         }
 885         return false;
 886     }
 887 
 888     /**
 889      * Convenience method to interrupt current thread.
 890      */
 891     static void selfInterrupt() {
 892         Thread.currentThread().interrupt();
 893     }
 894 
 895     /**
 896      * Convenience method to park and then check if interrupted
 897      *
 898      * @return {@code true} if interrupted
 899      */
 900     private final boolean parkAndCheckInterrupt() {
 901         //将当前线程挂起
 902         LockSupport.park(this);
 903         //检测当前线程的中断状态，并且会清除线程的中断标记
 904         return Thread.interrupted();
 905     }
 906 
 907     /*
 908      * Various flavors of acquire, varying in exclusive/shared and
 909      * control modes.  Each is mostly the same, but annoyingly
 910      * different.  Only a little bit of factoring is possible due to
 911      * interactions of exception mechanics (including ensuring that we
 912      * cancel if tryAcquire throws exception) and other control, at
 913      * least not without hurting performance too much.
 914      */
 915 
 916     /**
 917      * Acquires in exclusive uninterruptible mode for thread already in
 918      * queue. Used by condition wait methods as well as acquire.
 919      * 使线程阻塞在同步队列中等待获取资源，直到获取资源成功才返回，此过程中线程发生了中断就返回true,否则就返回false
 920      * @param node the node
 921      * @param arg the acquire argument
 922      * @return {@code true} if interrupted while waiting
 923      */
 924     final boolean acquireQueued(final Node node, int arg) {
 925         /*标记等待过程中是否发生了异常*/
 926         boolean failed = true;
 927         
 928         try {
 929             /*标记线程阻塞的过程中是否发生了中断*/
 930             boolean interrupted = false;
 931             /*线程自旋阻塞*/
 932             for(;;){
 933                 /*获取当前节点的前驱结点*/
 934                 final Node p = node.predecessor();
 935                 /*前驱结点是头结点，则说明当前线程有资格获取共享资源，尝试获取，获取成功，将当前节点设置为头结点*/
 936                 if (p == head && tryAcquire(arg)) {
 937                     /*将当前节点设置为头结点*/
 938                     setHead(node);
 939                     p.next = null; //help GC
 940                     failed = false;
 941                     return interrupted;
 942                 }
 943                 
 944                 /*判断当前线程是否可以挂起*/
 945                 if (shouldParkAfterFailedAcquire(p, node)
 946                         /*当前线程可以挂起，则挂起线程，并且线程被unpark()唤醒，检查线程的状态*/
 947                         && parkAndCheckInterrupt()) {
 948                     interrupted = true;
 949                 }
 950             }
 951         }finally{
 952             if (failed) {
 953                 /*阻塞获取同步资源的时候，发生了异常，取消当前线程的在同步队列中的排队*/
 954                 cancelAcquire(node);
 955             }
 956         }
 957     }
 958 
 959     /**
 960      * Acquires in exclusive interruptible mode.
 961      * @param arg the acquire argument
 962      */
 963     //尝试获取锁，并响应中断
 964     private void doAcquireInterruptibly(int arg)
 965         throws InterruptedException {
 966         //将当前线程包装成Node节点，加入到同步队列sync queue中
 967         final Node node = addWaiter(Node.EXCLUSIVE);
 968         //标记是否获取锁的过程中是否发生了异常
 969         boolean failed = true;
 970         try {
 971             //自旋
 972             for (;;) {
 973                 //获取当前节点的前驱结点
 974                 final Node p = node.predecessor();
 975                 //如果当前节点的前驱结点是头结点head，尝试获取锁，当前线程才有资格获取排他锁（头结点head是个哑结点，不代表任何线程）
 976                 if (p == head && tryAcquire(arg)) {
 977                     //当前节点获取锁成功，将当前节点设置为头结点head
 978                     setHead(node);
 979                     //将原来的头结点从同步队列sync queue中移除
 980                     p.next = null; // help GC
 981                     failed = false;
 982                     return;
 983                 }
 984                 //尝试获取锁失败，判断是否可以将当前线程安全地挂起，只有当前线程有效的前驱结点状态为Node.SIGNAL，当前线程才可以安全地被挂起，后续也会被及时地唤醒
 985                 if (shouldParkAfterFailedAcquire(p, node) &&
 986                         //将当前线程挂起，并等他唤醒的时候判断是否发生了线程中断，发生了线程中断，则进入finally块中，取消当前线程对锁的尝试获取
 987                     parkAndCheckInterrupt())
 988                     throw new InterruptedException();
 989             }
 990         } finally {
 991             if (failed)
 992                 cancelAcquire(node);
 993         }
 994     }
 995 
 996     /**
 997      * Acquires in exclusive timed mode.
 998      *
 999      * @param arg the acquire argument
1000      * @param nanosTimeout max wait time
1001      * @return {@code true} if acquired
1002      */
1003     private boolean doAcquireNanos(int arg, long nanosTimeout)
1004             throws InterruptedException {
1005         //指定的时间<=0则直接返回false
1006         if (nanosTimeout <= 0L)
1007             return false;
1008         //计算当前线程阻塞的截至时间
1009         final long deadline = System.nanoTime() + nanosTimeout;
1010         //将当前线程包装成Node节点，加入到同步队列sync queue中
1011         final Node node = addWaiter(Node.EXCLUSIVE);
1012         boolean failed = true;
1013         try {
1014             //自旋尝试获取锁
1015             for (;;) {
1016                 //获取当前节点的前驱结点
1017                 final Node p = node.predecessor();
1018                 //前驱结点是头结点，则当前节点有资格尝试获取锁，则尝试获取锁
1019                 if (p == head && tryAcquire(arg)) {
1020                     //获取锁成功，则将当前节点设置为头结点head
1021                     setHead(node);
1022                     //将同步队列sync queue中原来的头结点移除队列
1023                     p.next = null; // help GC
1024                     //表示获取锁的过程没有发生异常
1025                     failed = false;
1026                     return true;
1027                 }
1028                 //计算剩余的等待时间
1029                 nanosTimeout = deadline - System.nanoTime();
1030                 //剩余的等待时间<=0，直接返回false
1031                 if (nanosTimeout <= 0L)
1032                     return false;
1033                 //当前线程获取锁失败，判断是否可以将当前线程安全地挂起
1034                 if (shouldParkAfterFailedAcquire(p, node) &&
1035                         //如果剩余的等待时间<= spinForTimeoutThreshold，则不用将当前线程挂起，进行自旋即可
1036                     nanosTimeout > spinForTimeoutThreshold)
1037                     LockSupport.parkNanos(this, nanosTimeout);
1038                 //获取锁的过程中发生了中断，则直接抛出InterruptedException异常
1039                 if (Thread.interrupted())
1040                     throw new InterruptedException();
1041             }
1042         } finally {
1043             //获取锁的过程中发生了异常，则将当前线程取消
1044             if (failed)
1045                 cancelAcquire(node);
1046         }
1047     }
1048 
1049     /**
1050      * Acquires in shared uninterruptible mode.
1051      * @param arg the acquire argument
1052      */
1053     private void doAcquireShared(int arg) {
1054         //将竞争共享资源失败的线程加入到同步队列中，并标记为共享模式
1055         final Node node = addWaiter(Node.SHARED);
1056         //标记阻塞获取资源的过程中是否发生了异常
1057         boolean failed = true;
1058         
1059         try {
1060             //标记阻塞获取资源的过程中，是否发生了线程中断请求
1061             boolean interrupted = false;
1062             //线程阻塞等待获取资源，被有效前继节点唤醒后，尝试竞争共享资源
1063             for(;;){
1064                 /**
1065                  * 当前线程被唤醒之后，什么时候有资格竞争共享资源呢？
1066                  * 之后当它的前继节点是头结点（头结点是当前持有共享资源的线程），在唤醒后继节点的过程中，可能释放了资源，所以后继节点尝试获取一次共享资源。
1067                  */
1068                 final Node p = node.predecessor();
1069                 if (p == head) {
1070                     int r = tryAcquireShared(arg);
1071                     if (r >= 0) {
1072                         setHeadAndPropagate(node, r);
1073                         p.next = null;//help GC
1074                         if (interrupted) {
1075                             selfInterrupt();
1076                         }
1077                         failed = false;
1078                         return;
1079                     }
1080                 }
1081                 
1082                 if (shouldParkAfterFailedAcquire(p, node) &&
1083                         parkAndCheckInterrupt()) {
1084                     interrupted = true;
1085                 }
1086             }
1087         } finally{
1088             if (failed) {
1089                 cancelAcquire(node);
1090             }
1091         }
1092     }
1093 
1094     /**
1095      * Acquires in shared interruptible mode.
1096      * @param arg the acquire argument
1097      */
1098     private void doAcquireSharedInterruptibly(int arg)
1099         throws InterruptedException {
1100         final Node node = addWaiter(Node.SHARED);
1101         boolean failed = true;
1102         try {
1103             for (;;) {
1104                 final Node p = node.predecessor();
1105                 if (p == head) {
1106                     int r = tryAcquireShared(arg);
1107                     if (r >= 0) {
1108                         setHeadAndPropagate(node, r);
1109                         p.next = null; // help GC
1110                         failed = false;
1111                         return;
1112                     }
1113                 }
1114                 if (shouldParkAfterFailedAcquire(p, node) &&
1115                     parkAndCheckInterrupt())
1116                     throw new InterruptedException();
1117             }
1118         } finally {
1119             if (failed)
1120                 cancelAcquire(node);
1121         }
1122     }
1123 
1124     /**
1125      * Acquires in shared timed mode.
1126      *
1127      * @param arg the acquire argument
1128      * @param nanosTimeout max wait time
1129      * @return {@code true} if acquired
1130      */
1131     private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
1132             throws InterruptedException {
1133         if (nanosTimeout <= 0L)
1134             return false;
1135         final long deadline = System.nanoTime() + nanosTimeout;
1136         final Node node = addWaiter(Node.SHARED);
1137         boolean failed = true;
1138         try {
1139             for (;;) {
1140                 final Node p = node.predecessor();
1141                 if (p == head) {
1142                     int r = tryAcquireShared(arg);
1143                     if (r >= 0) {
1144                         setHeadAndPropagate(node, r);
1145                         p.next = null; // help GC
1146                         failed = false;
1147                         return true;
1148                     }
1149                 }
1150                 nanosTimeout = deadline - System.nanoTime();
1151                 if (nanosTimeout <= 0L)
1152                     return false;
1153                 if (shouldParkAfterFailedAcquire(p, node) &&
1154                     nanosTimeout > spinForTimeoutThreshold)
1155                     LockSupport.parkNanos(this, nanosTimeout);
1156                 if (Thread.interrupted())
1157                     throw new InterruptedException();
1158             }
1159         } finally {
1160             if (failed)
1161                 cancelAcquire(node);
1162         }
1163     }
1164 
1165     // Main exported methods
1166 
1167     /**
1168      * Attempts to acquire in exclusive mode. This method should query
1169      * if the state of the object permits it to be acquired in the
1170      * exclusive mode, and if so to acquire it.
1171      *
1172      * <p>This method is always invoked by the thread performing
1173      * acquire.  If this method reports failure, the acquire method
1174      * may queue the thread, if it is not already queued, until it is
1175      * signalled by a release from some other thread. This can be used
1176      * to implement method {@link Lock#tryLock()}.
1177      *
1178      * <p>The default
1179      * implementation throws {@link UnsupportedOperationException}.
1180      *
1181      * @param arg the acquire argument. This value is always the one
1182      *        passed to an acquire method, or is the value saved on entry
1183      *        to a condition wait.  The value is otherwise uninterpreted
1184      *        and can represent anything you like.
1185      * @return {@code true} if successful. Upon success, this object has
1186      *         been acquired.
1187      * @throws IllegalMonitorStateException if acquiring would place this
1188      *         synchronizer in an illegal state. This exception must be
1189      *         thrown in a consistent fashion for synchronization to work
1190      *         correctly.
1191      * @throws UnsupportedOperationException if exclusive mode is not supported
1192      */
1193     protected boolean tryAcquire(int arg) {
1194         throw new UnsupportedOperationException();
1195     }
1196 
1197     /**
1198      * Attempts to set the state to reflect a release in exclusive
1199      * mode.
1200      *
1201      * <p>This method is always invoked by the thread performing release.
1202      *
1203      * <p>The default implementation throws
1204      * {@link UnsupportedOperationException}.
1205      *
1206      * @param arg the release argument. This value is always the one
1207      *        passed to a release method, or the current state value upon
1208      *        entry to a condition wait.  The value is otherwise
1209      *        uninterpreted and can represent anything you like.
1210      * @return {@code true} if this object is now in a fully released
1211      *         state, so that any waiting threads may attempt to acquire;
1212      *         and {@code false} otherwise.
1213      * @throws IllegalMonitorStateException if releasing would place this
1214      *         synchronizer in an illegal state. This exception must be
1215      *         thrown in a consistent fashion for synchronization to work
1216      *         correctly.
1217      * @throws UnsupportedOperationException if exclusive mode is not supported
1218      */
1219     protected boolean tryRelease(int arg) {
1220         throw new UnsupportedOperationException();
1221     }
1222 
1223     /**
1224      * Attempts to acquire in shared mode. This method should query if
1225      * the state of the object permits it to be acquired in the shared
1226      * mode, and if so to acquire it.
1227      *
1228      * <p>This method is always invoked by the thread performing
1229      * acquire.  If this method reports failure, the acquire method
1230      * may queue the thread, if it is not already queued, until it is
1231      * signalled by a release from some other thread.
1232      *
1233      * <p>The default implementation throws {@link
1234      * UnsupportedOperationException}.
1235      *
1236      * @param arg the acquire argument. This value is always the one
1237      *        passed to an acquire method, or is the value saved on entry
1238      *        to a condition wait.  The value is otherwise uninterpreted
1239      *        and can represent anything you like.
1240      * @return a negative value on failure; zero if acquisition in shared
1241      *         mode succeeded but no subsequent shared-mode acquire can
1242      *         succeed; and a positive value if acquisition in shared
1243      *         mode succeeded and subsequent shared-mode acquires might
1244      *         also succeed, in which case a subsequent waiting thread
1245      *         must check availability. (Support for three different
1246      *         return values enables this method to be used in contexts
1247      *         where acquires only sometimes act exclusively.)  Upon
1248      *         success, this object has been acquired.
1249      * @throws IllegalMonitorStateException if acquiring would place this
1250      *         synchronizer in an illegal state. This exception must be
1251      *         thrown in a consistent fashion for synchronization to work
1252      *         correctly.
1253      * @throws UnsupportedOperationException if shared mode is not supported
1254      */
1255     protected int tryAcquireShared(int arg) {
1256         for(;;){
1257             //计算可用资源量
1258             int available = getState();
1259             //计算剩余资源量
1260             int remaining = available - arg;
1261             //计算资源数量，不够直接放回，加入CAS是为了保证state状态一定能够更新成功
1262             if (remaining < 0 ||
1263                     compareAndSetState(available, remaining)) {
1264                     return remaining;
1265             }
1266         }
1267         
1268         //throw new UnsupportedOperationException();
1269     }
1270 
1271     /**
1272      * Attempts to set the state to reflect a release in shared mode.
1273      *
1274      * <p>This method is always invoked by the thread performing release.
1275      *
1276      * <p>The default implementation throws
1277      * {@link UnsupportedOperationException}.
1278      *
1279      * @param arg the release argument. This value is always the one
1280      *        passed to a release method, or the current state value upon
1281      *        entry to a condition wait.  The value is otherwise
1282      *        uninterpreted and can represent anything you like.
1283      * @return {@code true} if this release of shared mode may permit a
1284      *         waiting acquire (shared or exclusive) to succeed; and
1285      *         {@code false} otherwise
1286      * @throws IllegalMonitorStateException if releasing would place this
1287      *         synchronizer in an illegal state. This exception must be
1288      *         thrown in a consistent fashion for synchronization to work
1289      *         correctly.
1290      * @throws UnsupportedOperationException if shared mode is not supported
1291      */
1292     protected boolean tryReleaseShared(int arg) {
1293         throw new UnsupportedOperationException();
1294     }
1295 
1296     /**
1297      * Returns {@code true} if synchronization is held exclusively with
1298      * respect to the current (calling) thread.  This method is invoked
1299      * upon each call to a non-waiting {@link ConditionObject} method.
1300      * (Waiting methods instead invoke {@link #release}.)
1301      *
1302      * <p>The default implementation throws {@link
1303      * UnsupportedOperationException}. This method is invoked
1304      * internally only within {@link ConditionObject} methods, so need
1305      * not be defined if conditions are not used.
1306      *
1307      * @return {@code true} if synchronization is held exclusively;
1308      *         {@code false} otherwise
1309      * @throws UnsupportedOperationException if conditions are not supported
1310      */
1311     protected boolean isHeldExclusively() {
1312         throw new UnsupportedOperationException();
1313     }
1314 
1315     /**
1316      * Acquires in exclusive mode, ignoring interrupts.  Implemented
1317      * by invoking at least once {@link #tryAcquire},
1318      * returning on success.  Otherwise the thread is queued, possibly
1319      * repeatedly blocking and unblocking, invoking {@link
1320      * #tryAcquire} until success.  This method can be used
1321      * to implement method {@link Lock#lock}.
1322      *
1323      * @param arg the acquire argument.  This value is conveyed to
1324      *        {@link #tryAcquire} but is otherwise uninterpreted and
1325      *        can represent anything you like.
1326      */
1327     public final void acquire(int arg) {
1328         /*加塞抢占共享资源（因为同步队列中可能还有其它节点等待），获取成功则直接返回*/
1329         if (!tryAcquire(arg)  
1330                 /* 1、抢占共享资源失败，则将当前节点放入到同步队列的尾部，并标记为独占模式；
1331                  * 2、使线程阻塞在同步队列中获取资源，直到获取成功才返回；如果整个过程中被中断过就返回true，否则就返回false；*/
1332                 && acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) {
1333             /*阻塞获取资源的过程中是不响应线程中断的，内部进行了中断检测，所以这块进行线程中断*/
1334             selfInterrupt();
1335         }
1336     }
1337 
1338     /**
1339      * Acquires in exclusive mode, aborting if interrupted.
1340      * Implemented by first checking interrupt status, then invoking
1341      * at least once {@link #tryAcquire}, returning on
1342      * success.  Otherwise the thread is queued, possibly repeatedly
1343      * blocking and unblocking, invoking {@link #tryAcquire}
1344      * until success or the thread is interrupted.  This method can be
1345      * used to implement method {@link Lock#lockInterruptibly}.
1346      *
1347      * @param arg the acquire argument.  This value is conveyed to
1348      *        {@link #tryAcquire} but is otherwise uninterpreted and
1349      *        can represent anything you like.
1350      * @throws InterruptedException if the current thread is interrupted
1351      */
1352     public final void acquireInterruptibly(int arg)
1353             throws InterruptedException {
1354         if (Thread.interrupted())
1355             throw new InterruptedException();
1356         if (!tryAcquire(arg))
1357             doAcquireInterruptibly(arg);
1358     }
1359 
1360     /**
1361      * Attempts to acquire in exclusive mode, aborting if interrupted,
1362      * and failing if the given timeout elapses.  Implemented by first
1363      * checking interrupt status, then invoking at least once {@link
1364      * #tryAcquire}, returning on success.  Otherwise, the thread is
1365      * queued, possibly repeatedly blocking and unblocking, invoking
1366      * {@link #tryAcquire} until success or the thread is interrupted
1367      * or the timeout elapses.  This method can be used to implement
1368      * method {@link Lock#tryLock(long, TimeUnit)}.
1369      *
1370      * @param arg the acquire argument.  This value is conveyed to
1371      *        {@link #tryAcquire} but is otherwise uninterpreted and
1372      *        can represent anything you like.
1373      * @param nanosTimeout the maximum number of nanoseconds to wait
1374      * @return {@code true} if acquired; {@code false} if timed out
1375      * @throws InterruptedException if the current thread is interrupted
1376      */
1377     public final boolean tryAcquireNanos(int arg, long nanosTimeout)
1378             throws InterruptedException {
1379         //当前线程被中断，直接抛出异常
1380         if (Thread.interrupted())
1381             throw new InterruptedException();
1382         //尝试直接获取锁，获取成功就直接返回
1383         return tryAcquire(arg) ||
1384                 //否则就“阻塞”指定时间，再尝试获取锁
1385             doAcquireNanos(arg, nanosTimeout);
1386     }
1387 
1388     /**
1389      * Releases in exclusive mode.  Implemented by unblocking one or
1390      * more threads if {@link #tryRelease} returns true.
1391      * This method can be used to implement method {@link Lock#unlock}.
1392      *
1393      * @param arg the release argument.  This value is conveyed to
1394      *        {@link #tryRelease} but is otherwise uninterpreted and
1395      *        can represent anything you like.
1396      * @return the value returned from {@link #tryRelease}
1397      */
1398     public final boolean release(int arg) {
1399         if (tryRelease(arg)) {
1400             Node h = head;
1401             //当前线程已经释放锁，但是sync queue同步队列不为空，并且头结点head未被取消
1402             if (h != null && h.waitStatus != 0)
1403                 //说明还有其它线程等待竞争锁，则尝试唤醒sync queue队列中等待锁的线程
1404                 unparkSuccessor(h);
1405             return true;
1406         }
1407         return false;
1408     }
1409 
1410     /**
1411      * Acquires in shared mode, ignoring interrupts.  Implemented by
1412      * first invoking at least once {@link #tryAcquireShared},
1413      * returning on success.  Otherwise the thread is queued, possibly
1414      * repeatedly blocking and unblocking, invoking {@link
1415      * #tryAcquireShared} until success.
1416      *
1417      * @param arg the acquire argument.  This value is conveyed to
1418      *        {@link #tryAcquireShared} but is otherwise uninterpreted
1419      *        and can represent anything you like.
1420      */
1421     public final void acquireShared(int arg) {
1422         if (tryAcquireShared(arg) < 0)
1423             doAcquireShared(arg);
1424     }
1425 
1426     /**
1427      * Acquires in shared mode, aborting if interrupted.  Implemented
1428      * by first checking interrupt status, then invoking at least once
1429      * {@link #tryAcquireShared}, returning on success.  Otherwise the
1430      * thread is queued, possibly repeatedly blocking and unblocking,
1431      * invoking {@link #tryAcquireShared} until success or the thread
1432      * is interrupted.
1433      * @param arg the acquire argument.
1434      * This value is conveyed to {@link #tryAcquireShared} but is
1435      * otherwise uninterpreted and can represent anything
1436      * you like.
1437      * @throws InterruptedException if the current thread is interrupted
1438      */
1439     public final void acquireSharedInterruptibly(int arg)
1440             throws InterruptedException {
1441         //当前线程已经被中断，则直接抛出InterruptedException异常
1442         if (Thread.interrupted())
1443             throw new InterruptedException();
1444         if (tryAcquireShared(arg) < 0)
1445             doAcquireSharedInterruptibly(arg);
1446     }
1447 
1448     /**
1449      * Attempts to acquire in shared mode, aborting if interrupted, and
1450      * failing if the given timeout elapses.  Implemented by first
1451      * checking interrupt status, then invoking at least once {@link
1452      * #tryAcquireShared}, returning on success.  Otherwise, the
1453      * thread is queued, possibly repeatedly blocking and unblocking,
1454      * invoking {@link #tryAcquireShared} until success or the thread
1455      * is interrupted or the timeout elapses.
1456      *
1457      * @param arg the acquire argument.  This value is conveyed to
1458      *        {@link #tryAcquireShared} but is otherwise uninterpreted
1459      *        and can represent anything you like.
1460      * @param nanosTimeout the maximum number of nanoseconds to wait
1461      * @return {@code true} if acquired; {@code false} if timed out
1462      * @throws InterruptedException if the current thread is interrupted
1463      */
1464     public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
1465             throws InterruptedException {
1466         if (Thread.interrupted())
1467             throw new InterruptedException();
1468         return tryAcquireShared(arg) >= 0 ||
1469             doAcquireSharedNanos(arg, nanosTimeout);
1470     }
1471 
1472     /**
1473      * Releases in shared mode.  Implemented by unblocking one or more
1474      * threads if {@link #tryReleaseShared} returns true.
1475      *
1476      * @param arg the release argument.  This value is conveyed to
1477      *        {@link #tryReleaseShared} but is otherwise uninterpreted
1478      *        and can represent anything you like.
1479      * @return the value returned from {@link #tryReleaseShared}
1480      */
1481     public final boolean releaseShared(int arg) {
1482         //尝试释放锁，释放成功，则返回true
1483         if (tryReleaseShared(arg)) {
1484             //唤醒同步队列中，等待获取锁的线程
1485             doReleaseShared();
1486             return true;
1487         }
1488         return false;
1489     }
1490 
1491     // Queue inspection methods
1492 
1493     /**
1494      * Queries whether any threads are waiting to acquire. Note that
1495      * because cancellations due to interrupts and timeouts may occur
1496      * at any time, a {@code true} return does not guarantee that any
1497      * other thread will ever acquire.
1498      *
1499      * <p>In this implementation, this operation returns in
1500      * constant time.
1501      *
1502      * @return {@code true} if there may be other threads waiting to acquire
1503      */
1504     public final boolean hasQueuedThreads() {
1505         return head != tail;
1506     }
1507 
1508     /**
1509      * Queries whether any threads have ever contended to acquire this
1510      * synchronizer; that is if an acquire method has ever blocked.
1511      *
1512      * <p>In this implementation, this operation returns in
1513      * constant time.
1514      *
1515      * @return {@code true} if there has ever been contention
1516      */
1517     public final boolean hasContended() {
1518         return head != null;
1519     }
1520 
1521     /**
1522      * Returns the first (longest-waiting) thread in the queue, or
1523      * {@code null} if no threads are currently queued.
1524      *
1525      * <p>In this implementation, this operation normally returns in
1526      * constant time, but may iterate upon contention if other threads are
1527      * concurrently modifying the queue.
1528      *
1529      * @return the first (longest-waiting) thread in the queue, or
1530      *         {@code null} if no threads are currently queued
1531      */
1532     public final Thread getFirstQueuedThread() {
1533         // handle only fast path, else relay
1534         return (head == tail) ? null : fullGetFirstQueuedThread();
1535     }
1536 
1537     /**
1538      * Version of getFirstQueuedThread called when fastpath fails
1539      */
1540     private Thread fullGetFirstQueuedThread() {
1541         /*
1542          * The first node is normally head.next. Try to get its
1543          * thread field, ensuring consistent reads: If thread
1544          * field is nulled out or s.prev is no longer head, then
1545          * some other thread(s) concurrently performed setHead in
1546          * between some of our reads. We try this twice before
1547          * resorting to traversal.
1548          */
1549         Node h, s;
1550         Thread st;
1551         if (((h = head) != null && (s = h.next) != null &&
1552              s.prev == head && (st = s.thread) != null) ||
1553             ((h = head) != null && (s = h.next) != null &&
1554              s.prev == head && (st = s.thread) != null))
1555             return st;
1556 
1557         /*
1558          * Head's next field might not have been set yet, or may have
1559          * been unset after setHead. So we must check to see if tail
1560          * is actually first node. If not, we continue on, safely
1561          * traversing from tail back to head to find first,
1562          * guaranteeing termination.
1563          */
1564 
1565         Node t = tail;
1566         Thread firstThread = null;
1567         while (t != null && t != head) {
1568             Thread tt = t.thread;
1569             if (tt != null)
1570                 firstThread = tt;
1571             t = t.prev;
1572         }
1573         return firstThread;
1574     }
1575 
1576     /**
1577      * Returns true if the given thread is currently queued.
1578      *
1579      * <p>This implementation traverses the queue to determine
1580      * presence of the given thread.
1581      *
1582      * @param thread the thread
1583      * @return {@code true} if the given thread is on the queue
1584      * @throws NullPointerException if the thread is null
1585      */
1586     public final boolean isQueued(Thread thread) {
1587         if (thread == null)
1588             throw new NullPointerException();
1589         for (Node p = tail; p != null; p = p.prev)
1590             if (p.thread == thread)
1591                 return true;
1592         return false;
1593     }
1594 
1595     /**
1596      * Returns {@code true} if the apparent first queued thread, if one
1597      * exists, is waiting in exclusive mode.  If this method returns
1598      * {@code true}, and the current thread is attempting to acquire in
1599      * shared mode (that is, this method is invoked from {@link
1600      * #tryAcquireShared}) then it is guaranteed that the current thread
1601      * is not the first queued thread.  Used only as a heuristic in
1602      * ReentrantReadWriteLock.
1603      */
1604     final boolean apparentlyFirstQueuedIsExclusive() {
1605         Node h, s;
1606         return (h = head) != null &&
1607             (s = h.next)  != null &&
1608             !s.isShared()         &&
1609             s.thread != null;
1610     }
1611 
1612     /**
1613      * Queries whether any threads have been waiting to acquire longer
1614      * than the current thread.
1615      *
1616      * <p>An invocation of this method is equivalent to (but may be
1617      * more efficient than):
1618      *  <pre> {@code
1619      * getFirstQueuedThread() != Thread.currentThread() &&
1620      * hasQueuedThreads()}</pre>
1621      *
1622      * <p>Note that because cancellations due to interrupts and
1623      * timeouts may occur at any time, a {@code true} return does not
1624      * guarantee that some other thread will acquire before the current
1625      * thread.  Likewise, it is possible for another thread to win a
1626      * race to enqueue after this method has returned {@code false},
1627      * due to the queue being empty.
1628      *
1629      * <p>This method is designed to be used by a fair synchronizer to
1630      * avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>.
1631      * Such a synchronizer's {@link #tryAcquire} method should return
1632      * {@code false}, and its {@link #tryAcquireShared} method should
1633      * return a negative value, if this method returns {@code true}
1634      * (unless this is a reentrant acquire).  For example, the {@code
1635      * tryAcquire} method for a fair, reentrant, exclusive mode
1636      * synchronizer might look like this:
1637      *
1638      *  <pre> {@code
1639      * protected boolean tryAcquire(int arg) {
1640      *   if (isHeldExclusively()) {
1641      *     // A reentrant acquire; increment hold count
1642      *     return true;
1643      *   } else if (hasQueuedPredecessors()) {
1644      *     return false;
1645      *   } else {
1646      *     // try to acquire normally
1647      *   }
1648      * }}</pre>
1649      *
1650      * @return {@code true} if there is a queued thread preceding the
1651      *         current thread, and {@code false} if the current thread
1652      *         is at the head of the queue or the queue is empty
1653      * @since 1.7
1654      */
1655     public final boolean hasQueuedPredecessors() {
1656         // The correctness of this depends on head being initialized
1657         // before tail and on head.next being accurate if the current
1658         // thread is first in queue.
1659         Node t = tail; // Read fields in reverse initialization order
1660         Node h = head;
1661         Node s;
1662         return h != t &&
1663             ((s = h.next) == null || s.thread != Thread.currentThread());
1664     }
1665 
1666 
1667     // Instrumentation and monitoring methods
1668 
1669     /**
1670      * Returns an estimate of the number of threads waiting to
1671      * acquire.  The value is only an estimate because the number of
1672      * threads may change dynamically while this method traverses
1673      * internal data structures.  This method is designed for use in
1674      * monitoring system state, not for synchronization
1675      * control.
1676      *
1677      * @return the estimated number of threads waiting to acquire
1678      */
1679     public final int getQueueLength() {
1680         int n = 0;
1681         for (Node p = tail; p != null; p = p.prev) {
1682             if (p.thread != null)
1683                 ++n;
1684         }
1685         return n;
1686     }
1687 
1688     /**
1689      * Returns a collection containing threads that may be waiting to
1690      * acquire.  Because the actual set of threads may change
1691      * dynamically while constructing this result, the returned
1692      * collection is only a best-effort estimate.  The elements of the
1693      * returned collection are in no particular order.  This method is
1694      * designed to facilitate construction of subclasses that provide
1695      * more extensive monitoring facilities.
1696      *
1697      * @return the collection of threads
1698      */
1699     public final Collection<Thread> getQueuedThreads() {
1700         ArrayList<Thread> list = new ArrayList<Thread>();
1701         for (Node p = tail; p != null; p = p.prev) {
1702             Thread t = p.thread;
1703             if (t != null)
1704                 list.add(t);
1705         }
1706         return list;
1707     }
1708 
1709     /**
1710      * Returns a collection containing threads that may be waiting to
1711      * acquire in exclusive mode. This has the same properties
1712      * as {@link #getQueuedThreads} except that it only returns
1713      * those threads waiting due to an exclusive acquire.
1714      *
1715      * @return the collection of threads
1716      */
1717     public final Collection<Thread> getExclusiveQueuedThreads() {
1718         ArrayList<Thread> list = new ArrayList<Thread>();
1719         for (Node p = tail; p != null; p = p.prev) {
1720             if (!p.isShared()) {
1721                 Thread t = p.thread;
1722                 if (t != null)
1723                     list.add(t);
1724             }
1725         }
1726         return list;
1727     }
1728 
1729     /**
1730      * Returns a collection containing threads that may be waiting to
1731      * acquire in shared mode. This has the same properties
1732      * as {@link #getQueuedThreads} except that it only returns
1733      * those threads waiting due to a shared acquire.
1734      *
1735      * @return the collection of threads
1736      */
1737     public final Collection<Thread> getSharedQueuedThreads() {
1738         ArrayList<Thread> list = new ArrayList<Thread>();
1739         for (Node p = tail; p != null; p = p.prev) {
1740             if (p.isShared()) {
1741                 Thread t = p.thread;
1742                 if (t != null)
1743                     list.add(t);
1744             }
1745         }
1746         return list;
1747     }
1748 
1749     /**
1750      * Returns a string identifying this synchronizer, as well as its state.
1751      * The state, in brackets, includes the String {@code "State ="}
1752      * followed by the current value of {@link #getState}, and either
1753      * {@code "nonempty"} or {@code "empty"} depending on whether the
1754      * queue is empty.
1755      *
1756      * @return a string identifying this synchronizer, as well as its state
1757      */
1758     public String toString() {
1759         int s = getState();
1760         String q  = hasQueuedThreads() ? "non" : "";
1761         return super.toString() +
1762             "[State = " + s + ", " + q + "empty queue]";
1763     }
1764 
1765 
1766     // Internal support methods for Conditions
1767 
1768     /**
1769      * Returns true if a node, always one that was initially placed on
1770      * a condition queue, is now waiting to reacquire on sync queue.
1771      * @param node the node
1772      * @return true if is reacquiring
1773      */
1774     //判断当前线程是否在sync queue同步队列中。
1775     final boolean isOnSyncQueue(Node node) {
1776         /**
1777          * 1、当前节点node的状态waitStatus为Node.CONDITION，则其必定还在条件队列condition中；
1778          * 2、如果当前节点node的node.prev == null，说明当前节点还未被加入到sync queue队列中，因为node节点要加入到sync queue中（无论本次加入是否成功），则本次尝试必有node.prev != null；
1779          */
1780         if (node.waitStatus == Node.CONDITION || node.prev == null)
1781             return false;
1782         //如果当前节点node.next != null，说明其已经有后继节点了，则其必定在sync queue同步队列中
1783         if (node.next != null) // If has successor, it must be on queue
1784             return true;
1785         /*
1786          * node.prev can be non-null, but not yet on queue because
1787          * the CAS to place it on queue can fail. So we have to
1788          * traverse from tail to make sure it actually made it.  It
1789          * will always be near the tail in calls to this method, and
1790          * unless the CAS failed (which is unlikely), it will be
1791          * there, so we hardly ever traverse much.
1792          */
1793         /**
1794          * 能走到这里来，说明当前节点node.waitStatus != Node.CONDITION && node.prev != null && node.next == null
1795          * ，说明已经开始尝试将当前节点node加入到sync queue同步队列中，已经完成了node.prev = tail,但是进行接下来的CAS操作设置sync queue的尾节点，有可能失败
1796          * 导致当前节点入队失败，所以从sync queue的尾节点开始遍历，进一步确认当前节点是否成功加入到了sync queue中。
1797          */
1798         return findNodeFromTail(node);
1799     }
1800 
1801     /**
1802      * Returns true if node is on sync queue by searching backwards from tail.
1803      * Called only when needed by isOnSyncQueue.
1804      * @return true if present
1805      */
1806     //判断指定的node节点是否在sync queue同步队列中
1807     private boolean findNodeFromTail(Node node) {
1808         Node t = tail;
1809         //从链表的尾部开始遍历
1810         for (;;) {
1811             //遍历到的当前节点等于指定的节点，则返回true
1812             if (t == node)
1813                 return true;
1814             //遍历到的当前节点已经为空，说明链表已经遍历结束，未找到指定的节点，则返回false
1815             if (t == null)
1816                 return false;
1817             //移动指针，进入下一次自旋遍历
1818             t = t.prev;
1819         }
1820     }
1821 
1822     /**
1823      * Transfers a node from a condition queue onto sync queue.
1824      * Returns true if successful.
1825      * @param node the node
1826      * @return true if successfully transferred (else the node was
1827      * cancelled before signal)
1828      */
1829     final boolean transferForSignal(Node node) {
1830         /*
1831          * If cannot change waitStatus, the node has been cancelled.
1832          */
1833         //调用signal方法的时候，当前node节点已经取消了等待，则忽略这个节点
1834         if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
1835             return false;
1836 
1837         /*
1838          * Splice onto queue and try to set waitStatus of predecessor to
1839          * indicate that thread is (probably) waiting. If cancelled or
1840          * attempt to set waitStatus fails, wake up to resync (in which
1841          * case the waitStatus can be transiently and harmlessly wrong).
1842          */
1843         //如果这个节点node在条件队列condition中正常等待，则将node加入到sync queue同步队列中，并返回这个新结点的前驱结点
1844         Node p = enq(node);
1845         int ws = p.waitStatus;
1846         /**
1847          * 同步队列sync queue中的节点是通过前驱结点来唤醒的，前驱结点已经被取消，或者前驱结点的waitStatus设置Node.SIGNAL失败，
1848          * 后继节点不能通过前驱结点正常唤醒，则直接唤醒这个刚刚加入到同步队列sync queue中的节点
1849          */
1850         if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
1851             LockSupport.unpark(node.thread);
1852         return true;
1853     }
1854 
1855     /**
1856      * Transfers node, if necessary, to sync queue after a cancelled wait.
1857      * Returns true if thread was cancelled before being signalled.
1858      *
1859      * @param node the node
1860      * @return true if cancelled before the node was signalled
1861      */
1862     final boolean transferAfterCancelledWait(Node node) {
1863         /**
1864          * 判断一个node是否被signal()过，最简单有效的方式就是是否离开了condition条件队列，进入到了sync queue同步队列中
1865          */
1866         //如果一个节点的状态还是Node.CONDITION，说明它还未被signal过，是因为中断导致了它被唤醒（从condition条件队列转移到sync queue同步队列中）
1867         if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
1868             //将当前被唤醒的节点加入到同步队列sync queue中，准备竞争锁，但是还没有将这个节点从条件队列中移除，所以后面的处理一下
1869             enq(node);
1870             return true;
1871         }
1872         /*
1873          * If we lost out to a signal(), then we can't proceed
1874          * until it finishes its enq().  Cancelling during an
1875          * incomplete transfer is both rare and transient, so just
1876          * spin.
1877          */
1878         while (!isOnSyncQueue(node))
1879             Thread.yield();
1880         return false;
1881     }
1882 
1883     /**
1884      * Invokes release with current state value; returns saved state.
1885      * Cancels node and throws exception on failure.
1886      * @param node the condition node for this wait
1887      * @return previous sync state
1888      */
1889     //完全释放线程所占用的锁
1890     final int fullyRelease(Node node) {
1891         boolean failed = true;
1892         try {
1893             //获取当前锁状态
1894             int savedState = getState();
1895             //完全释放锁，对于重入锁而言，无论重入了几次都一次释放；同时持有锁的线程不是当前线程时候，也会直接抛出IllegalMonitorStateException异常，直接进入finally块
1896             if (release(savedState)) {
1897                 failed = false;
1898                 return savedState;
1899             } else {
1900                 throw new IllegalMonitorStateException();
1901             }
1902         } finally {
1903             /**
1904              * 如果释放锁失败，则将刚才新加入到条件队列尾节点的node节点的waitStatus设置为Node.CANCELLED，
1905              * 后序其它线程被包装成Node节点加入到条件队列中时候，会将这些已经被取消的节点移除
1906              */
1907             if (failed)
1908                 node.waitStatus = Node.CANCELLED;
1909         }
1910     }
1911 
1912     // Instrumentation methods for conditions
1913 
1914     /**
1915      * Queries whether the given ConditionObject
1916      * uses this synchronizer as its lock.
1917      *
1918      * @param condition the condition
1919      * @return {@code true} if owned
1920      * @throws NullPointerException if the condition is null
1921      */
1922     public final boolean owns(ConditionObject condition) {
1923         return condition.isOwnedBy(this);
1924     }
1925 
1926     /**
1927      * Queries whether any threads are waiting on the given condition
1928      * associated with this synchronizer. Note that because timeouts
1929      * and interrupts may occur at any time, a {@code true} return
1930      * does not guarantee that a future {@code signal} will awaken
1931      * any threads.  This method is designed primarily for use in
1932      * monitoring of the system state.
1933      *
1934      * @param condition the condition
1935      * @return {@code true} if there are any waiting threads
1936      * @throws IllegalMonitorStateException if exclusive synchronization
1937      *         is not held
1938      * @throws IllegalArgumentException if the given condition is
1939      *         not associated with this synchronizer
1940      * @throws NullPointerException if the condition is null
1941      */
1942     public final boolean hasWaiters(ConditionObject condition) {
1943         if (!owns(condition))
1944             throw new IllegalArgumentException("Not owner");
1945         return condition.hasWaiters();
1946     }
1947 
1948     /**
1949      * Returns an estimate of the number of threads waiting on the
1950      * given condition associated with this synchronizer. Note that
1951      * because timeouts and interrupts may occur at any time, the
1952      * estimate serves only as an upper bound on the actual number of
1953      * waiters.  This method is designed for use in monitoring of the
1954      * system state, not for synchronization control.
1955      *
1956      * @param condition the condition
1957      * @return the estimated number of waiting threads
1958      * @throws IllegalMonitorStateException if exclusive synchronization
1959      *         is not held
1960      * @throws IllegalArgumentException if the given condition is
1961      *         not associated with this synchronizer
1962      * @throws NullPointerException if the condition is null
1963      */
1964     public final int getWaitQueueLength(ConditionObject condition) {
1965         if (!owns(condition))
1966             throw new IllegalArgumentException("Not owner");
1967         return condition.getWaitQueueLength();
1968     }
1969 
1970     /**
1971      * Returns a collection containing those threads that may be
1972      * waiting on the given condition associated with this
1973      * synchronizer.  Because the actual set of threads may change
1974      * dynamically while constructing this result, the returned
1975      * collection is only a best-effort estimate. The elements of the
1976      * returned collection are in no particular order.
1977      *
1978      * @param condition the condition
1979      * @return the collection of threads
1980      * @throws IllegalMonitorStateException if exclusive synchronization
1981      *         is not held
1982      * @throws IllegalArgumentException if the given condition is
1983      *         not associated with this synchronizer
1984      * @throws NullPointerException if the condition is null
1985      */
1986     public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
1987         if (!owns(condition))
1988             throw new IllegalArgumentException("Not owner");
1989         return condition.getWaitingThreads();
1990     }
1991 
1992     /**
1993      * Condition implementation for a {@link
1994      * AbstractQueuedSynchronizer} serving as the basis of a {@link
1995      * Lock} implementation.
1996      *
1997      * <p>Method documentation for this class describes mechanics,
1998      * not behavioral specifications from the point of view of Lock
1999      * and Condition users. Exported versions of this class will in
2000      * general need to be accompanied by documentation describing
2001      * condition semantics that rely on those of the associated
2002      * {@code AbstractQueuedSynchronizer}.
2003      *
2004      * <p>This class is Serializable, but all fields are transient,
2005      * so deserialized conditions have no waiters.
2006      */
2007     public class ConditionObject implements Condition, java.io.Serializable {
2008         private static final long serialVersionUID = 1173984872572414699L;
2009         /** First node of condition queue. */
2010         private transient Node firstWaiter;
2011         /** Last node of condition queue. */
2012         private transient Node lastWaiter;
2013 
2014         /**
2015          * Creates a new {@code ConditionObject} instance.
2016          */
2017         public ConditionObject() { }
2018 
2019         // Internal methods
2020 
2021         /**
2022          * Adds a new waiter to wait queue.
2023          * @return its new wait node
2024          */
2025         //将当前线程包装成Node节点，加入到条件队列中
2026         private Node addConditionWaiter() {
2027             //条件队列的尾节点
2028             Node t = lastWaiter;
2029             // If lastWaiter is cancelled, clean out.
2030             //条件尾节点不为空，说明条件队列不为空，但是它的尾节点的waitStatus已经不是Node.CONDITION了，说明其已经取消了，则清除掉条件队列中已经取消的节点
2031             if (t != null && t.waitStatus != Node.CONDITION) {
2032                 //清除掉条件队列中已经取消的节点
2033                 unlinkCancelledWaiters();
2034                 t = lastWaiter;
2035             }
2036             //将当前线程包装成Node节点，并将新的node节点的状态设置为Node.CONDITION
2037             Node node = new Node(Thread.currentThread(), Node.CONDITION);
2038             //如果条件队列为空，则将条件队列的头结点firstWaiter指向刚创建出来的node节点
2039             if (t == null)
2040                 firstWaiter = node;
2041             else
2042                 //否则， 将条件队列旧的尾节点的nextWaiter指向新创建的node节点
2043                 t.nextWaiter = node;
2044             //将新创建的node节点设置为条件队列的尾节点
2045             lastWaiter = node;
2046             return node;
2047         }
2048 
2049         /**
2050          * Removes and transfers nodes until hit non-cancelled one or
2051          * null. Split out from signal in part to encourage compilers
2052          * to inline the case of no waiters.
2053          * @param first (non-null) the first node on condition queue
2054          */
2055         //唤醒条件队列中第一个未被取消的节点
2056         private void doSignal(Node first) {
2057             do {
2058                 //如果条件队列中只有一个节点，则将条件队列清空
2059                 if ( (firstWaiter = first.nextWaiter) == null)
2060                     //则将条件队列的尾节点lastWaiter也置为null
2061                     lastWaiter = null;
2062                 //将当前要被唤醒的节点从条件队列中移除
2063                 first.nextWaiter = null;
2064                 //当前节点已经被取消了，并且条件队列中还有节点，则进行下一次循环
2065             } while (!transferForSignal(first) &&
2066                      (first = firstWaiter) != null);
2067         }
2068 
2069         /**
2070          * Removes and transfers all nodes.
2071          * @param first (non-null) the first node on condition queue
2072          */
2073         private void doSignalAll(Node first) {
2074             //清空条件队列
2075             lastWaiter = firstWaiter = null;
2076             do {
2077                 //获取条件队列头结点的下一个结点
2078                 Node next = first.nextWaiter;
2079                 //将条件队列的头结点从链表上移除
2080                 first.nextWaiter = null;
2081                 //将这个刚从条件队列condition中移除的节点转移到sync queue同步队列中
2082                 transferForSignal(first);
2083                 //修改指针，准备继续下一次的遍历
2084                 first = next;
2085                 //first ！= null表示链表还未遍历完成，继续下次遍历
2086             } while (first != null);
2087         }
2088 
2089         /**
2090          * Unlinks cancelled waiter nodes from condition queue.
2091          * Called only while holding lock. This is called when
2092          * cancellation occurred during condition wait, and upon
2093          * insertion of a new waiter when lastWaiter is seen to have
2094          * been cancelled. This method is needed to avoid garbage
2095          * retention in the absence of signals. So even though it may
2096          * require a full traversal, it comes into play only when
2097          * timeouts or cancellations occur in the absence of
2098          * signals. It traverses all nodes rather than stopping at a
2099          * particular target to unlink all pointers to garbage nodes
2100          * without requiring many re-traversals during cancellation
2101          * storms.
2102          */
2103         private void unlinkCancelledWaiters() {
2104             //当前节点,从表示条件队列链表的头结点firstWaiter开始遍历
2105             Node t = firstWaiter;
2106             //表示条件队列中最后一个waitStatus不为Node.CANCELLED的节点
2107             Node trail = null;
2108             //条件队列不为空
2109             while (t != null) {
2110                 //当前节点的后继节点
2111                 Node next = t.nextWaiter;
2112                 //如果当前节点t已经取消了正常等待，则将节点t从条件队列中移除
2113                 if (t.waitStatus != Node.CONDITION) {
2114                     //将当前节点从条件队列移除
2115                     t.nextWaiter = null;
2116                     //trail == null表示条件队列当前节点之前的都被取消了，所以将条件队列的头结点firstWaiter重置指向next
2117                     if (trail == null)
2118                         firstWaiter = next;
2119                     else
2120                         //当前节点已经被取消，trail != null,则需要将trail与当前节点断开，指向next;
2121                         trail.nextWaiter = next;
2122                     //如果next == null说明队列已经完成遍历，将原来的条件队列中最后一个正常等待的节点赋值给lastWaiter；
2123                     if (next == null)
2124                         lastWaiter = trail;
2125                 }
2126                 else{
2127                     //如果当前节点未被取消，就将当前节点记为队列中最后一个未被取消的节点，令trail = t;
2128                     trail = t;
2129                 }
2130                 //修改当前节点的指针，指向下一个节点next
2131                 t = next;
2132             }
2133         }
2134 
2135         // public methods
2136 
2137         /**
2138          * Moves the longest-waiting thread, if one exists, from the
2139          * wait queue for this condition to the wait queue for the
2140          * owning lock.
2141          *
2142          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2143          *         returns {@code false}
2144          */
2145         public final void signal() {
2146             //判断当前线程是否是持有锁的线程，如果不是则直接抛出IllegalMonitorStateException异常
2147             if (!isHeldExclusively())
2148                 throw new IllegalMonitorStateException();
2149             Node first = firstWaiter;
2150             //条件队列condition不为空
2151             if (first != null)
2152                 //唤醒条件队列condition中的头结点
2153                 doSignal(first);
2154         }
2155 
2156         /**
2157          * Moves all threads from the wait queue for this condition to
2158          * the wait queue for the owning lock.
2159          *
2160          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2161          *         returns {@code false}
2162          */
2163         public final void signalAll() {
2164             //判断当前线程是否是持有锁的线程，如果不是，则直接抛出IllegalMonitorStateException异常
2165             if (!isHeldExclusively())
2166                 throw new IllegalMonitorStateException();
2167             Node first = firstWaiter;
2168             //判断条件队列是否为空
2169             if (first != null)
2170                 //唤醒条件队列中进行等待的节点
2171                 doSignalAll(first);
2172         }
2173 
2174         /**
2175          * Implements uninterruptible condition wait.
2176          * <ol>
2177          * <li> Save lock state returned by {@link #getState}.
2178          * <li> Invoke {@link #release} with saved state as argument,
2179          *      throwing IllegalMonitorStateException if it fails.
2180          * <li> Block until signalled.
2181          * <li> Reacquire by invoking specialized version of
2182          *      {@link #acquire} with saved state as argument.
2183          * </ol>
2184          */
2185         /**
2186          * await()方法中，中断和signal()起到了相同的效果，将当前线程唤醒，但是中断一个正常等待的线程，线程被唤醒后，
2187          * 抢到了锁，但是发现等待的条件还未满足，线程会重新被挂起，所以我们希望有一个方法，在等待的时候，不要响应线程中断，
2188          * 所以下面awaitUnitertuptibly()方法就实现了这个功能。
2189          */
2190         public final void awaitUninterruptibly() {
2191             //将当前线程包装成Node节点，加入到条件队列condition中
2192             Node node = addConditionWaiter();
2193             //释放当前线程所持有的锁
2194             int savedState = fullyRelease(node);
2195             boolean interrupted = false;
2196             //判断当前线程是否已经在sync queue同步队列
2197             while (!isOnSyncQueue(node)) {
2198                 //当前线程未在sync queue同步队列中，说明其还没有资格竞争独占锁，将当前线程挂起
2199                 LockSupport.park(this);
2200                 /**
2201                  * 判断当前线程是否是由于被其他线程中断唤醒的，如果是则只是简单记录下其中断标记即可，
2202                  * 当前线程要跳出此while循环则满足的唯一条件是，被其他线程signal()，进入到同步队列
2203                  */
2204                 if (Thread.interrupted())
2205                     interrupted = true;
2206             }
2207             
2208             /**
2209              * 这也是一个阻塞操作，当前线程已经加入到同步队列中，尝试竞争锁，竞争失败，找一个合适的地方，
2210              * 将当前线程挂起，等待其他线程signal()，如果在竞争锁的过程中，发生了异常，则需要记录一下
2211              */
2212             if (acquireQueued(node, savedState) || interrupted)
2213                 selfInterrupt();
2214         }
2215 
2216         /*
2217          * For interruptible waits, we need to track whether to throw
2218          * InterruptedException, if interrupted while blocked on
2219          * condition, versus reinterrupt current thread, if
2220          * interrupted while blocked waiting to re-acquire.
2221          */
2222 
2223         /** Mode meaning to reinterrupt on exit from wait */
2224         private static final int REINTERRUPT =  1;
2225         /** Mode meaning to throw InterruptedException on exit from wait */
2226         private static final int THROW_IE    = -1;
2227 
2228         /**
2229          * Checks for interrupt, returning THROW_IE if interrupted
2230          * before signalled, REINTERRUPT if after signalled, or
2231          * 0 if not interrupted.
2232          */
2233         private int checkInterruptWhileWaiting(Node node) {
2234             //判断在等待的过程中，线程是否发生了中断
2235             return Thread.interrupted() ?
2236                     //判断线程中断时发生在signal()之前，还是signal()之后，THROW_IE表示中断发生在signal()之前，REINTERRUPT表示中断发生在signal()之后，中断来得太晚了
2237                 (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
2238                 0;
2239         }
2240 
2241         /**
2242          * Throws InterruptedException, reinterrupts current thread, or
2243          * does nothing, depending on mode.
2244          */
2245         private void reportInterruptAfterWait(int interruptMode)
2246             throws InterruptedException {
2247             //表示还未被其他线程signal()就发生了中断，当前线程是由于中断被唤醒的，所以抛出InterruptedException异常
2248             if (interruptMode == THROW_IE)
2249                 throw new InterruptedException();
2250             //表示被其它线程signal()过了之后才发生了线程中断，中断来得太迟了，所以补一下中断标记就行了
2251             else if (interruptMode == REINTERRUPT)
2252                 selfInterrupt();
2253         }
2254 
2255         /**
2256          * Implements interruptible condition wait.
2257          * <ol>
2258          * <li> If current thread is interrupted, throw InterruptedException.
2259          * <li> Save lock state returned by {@link #getState}.
2260          * <li> Invoke {@link #release} with saved state as argument,
2261          *      throwing IllegalMonitorStateException if it fails.
2262          * <li> Block until signalled or interrupted.
2263          * <li> Reacquire by invoking specialized version of
2264          *      {@link #acquire} with saved state as argument.
2265          * <li> If interrupted while blocked in step 4, throw InterruptedException.
2266          * </ol>
2267          */
2268         public final void await() throws InterruptedException {
2269             //调用await()方法时，当前线程已经被中断，则直接抛出中断异常
2270             if (Thread.interrupted())
2271                 throw new InterruptedException();
2272             //将当前线程包装成Node节点，加入到条件队列中
2273             Node node = addConditionWaiter();
2274             //释放当前线程所占用的锁
2275             int savedState = fullyRelease(node);
2276             int interruptMode = 0;
2277             //判断代表当前线程节点是否在同步队列中，如果不在，说明还没有其它线程调用signal()方法，将当前线程从条件队列Conditon种转移到同步队列中
2278             while (!isOnSyncQueue(node)) {
2279                 //当前线程还在条件队列中，没有资格竞争锁，则直接将当前线程挂起
2280                 LockSupport.park(this);
2281                 //能走到这里，说明当前线程被其他线程signal()或者被其它线程中断了
2282                 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2283                     break;
2284             }
2285             //interruptMode != THROW_IE表示当前线程不是由于线程中断而被唤醒的
2286             if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2287                 interruptMode = REINTERRUPT;
2288             //上面将从条件队列condition中唤醒的节点加入到sync queue条件队列中时候，并没有从条件队列中移除，这里需要清理一下
2289             if (node.nextWaiter != null) // clean up if cancelled
2290                 unlinkCancelledWaiters();
2291             //当前节点在等待的过程中发生了中断
2292             if (interruptMode != 0)
2293                 reportInterruptAfterWait(interruptMode);
2294         }
2295 
2296         /**
2297          * Implements timed condition wait.
2298          * <ol>
2299          * <li> If current thread is interrupted, throw InterruptedException.
2300          * <li> Save lock state returned by {@link #getState}.
2301          * <li> Invoke {@link #release} with saved state as argument,
2302          *      throwing IllegalMonitorStateException if it fails.
2303          * <li> Block until signalled, interrupted, or timed out.
2304          * <li> Reacquire by invoking specialized version of
2305          *      {@link #acquire} with saved state as argument.
2306          * <li> If interrupted while blocked in step 4, throw InterruptedException.
2307          * </ol>
2308          */
2309         public final long awaitNanos(long nanosTimeout)
2310                 throws InterruptedException {
2311             if (Thread.interrupted())
2312                 throw new InterruptedException();
2313             Node node = addConditionWaiter();
2314             int savedState = fullyRelease(node);
2315             final long deadline = System.nanoTime() + nanosTimeout;
2316             int interruptMode = 0;
2317             while (!isOnSyncQueue(node)) {
2318                 if (nanosTimeout <= 0L) {
2319                     transferAfterCancelledWait(node);
2320                     break;
2321                 }
2322                 if (nanosTimeout >= spinForTimeoutThreshold)
2323                     LockSupport.parkNanos(this, nanosTimeout);
2324                 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2325                     break;
2326                 nanosTimeout = deadline - System.nanoTime();
2327             }
2328             if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2329                 interruptMode = REINTERRUPT;
2330             if (node.nextWaiter != null)
2331                 unlinkCancelledWaiters();
2332             if (interruptMode != 0)
2333                 reportInterruptAfterWait(interruptMode);
2334             return deadline - System.nanoTime();
2335         }
2336 
2337         /**
2338          * Implements absolute timed condition wait.
2339          * <ol>
2340          * <li> If current thread is interrupted, throw InterruptedException.
2341          * <li> Save lock state returned by {@link #getState}.
2342          * <li> Invoke {@link #release} with saved state as argument,
2343          *      throwing IllegalMonitorStateException if it fails.
2344          * <li> Block until signalled, interrupted, or timed out.
2345          * <li> Reacquire by invoking specialized version of
2346          *      {@link #acquire} with saved state as argument.
2347          * <li> If interrupted while blocked in step 4, throw InterruptedException.
2348          * <li> If timed out while blocked in step 4, return false, else true.
2349          * </ol>
2350          */
2351         public final boolean awaitUntil(Date deadline)
2352                 throws InterruptedException {
2353             long abstime = deadline.getTime();
2354             if (Thread.interrupted())
2355                 throw new InterruptedException();
2356             Node node = addConditionWaiter();
2357             int savedState = fullyRelease(node);
2358             boolean timedout = false;
2359             int interruptMode = 0;
2360             while (!isOnSyncQueue(node)) {
2361                 if (System.currentTimeMillis() > abstime) {
2362                     timedout = transferAfterCancelledWait(node);
2363                     break;
2364                 }
2365                 LockSupport.parkUntil(this, abstime);
2366                 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2367                     break;
2368             }
2369             if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2370                 interruptMode = REINTERRUPT;
2371             if (node.nextWaiter != null)
2372                 unlinkCancelledWaiters();
2373             if (interruptMode != 0)
2374                 reportInterruptAfterWait(interruptMode);
2375             return !timedout;
2376         }
2377 
2378         /**
2379          * Implements timed condition wait.
2380          * <ol>
2381          * <li> If current thread is interrupted, throw InterruptedException.
2382          * <li> Save lock state returned by {@link #getState}.
2383          * <li> Invoke {@link #release} with saved state as argument,
2384          *      throwing IllegalMonitorStateException if it fails.
2385          * <li> Block until signalled, interrupted, or timed out.
2386          * <li> Reacquire by invoking specialized version of
2387          *      {@link #acquire} with saved state as argument.
2388          * <li> If interrupted while blocked in step 4, throw InterruptedException.
2389          * <li> If timed out while blocked in step 4, return false, else true.
2390          * </ol>
2391          */
2392         public final boolean await(long time, TimeUnit unit)
2393                 throws InterruptedException {
2394             long nanosTimeout = unit.toNanos(time);
2395             if (Thread.interrupted())
2396                 throw new InterruptedException();
2397             Node node = addConditionWaiter();
2398             int savedState = fullyRelease(node);
2399             final long deadline = System.nanoTime() + nanosTimeout;
2400             boolean timedout = false;
2401             int interruptMode = 0;
2402             while (!isOnSyncQueue(node)) {
2403                 if (nanosTimeout <= 0L) {
2404                     timedout = transferAfterCancelledWait(node);
2405                     break;
2406                 }
2407                 if (nanosTimeout >= spinForTimeoutThreshold)
2408                     LockSupport.parkNanos(this, nanosTimeout);
2409                 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2410                     break;
2411                 nanosTimeout = deadline - System.nanoTime();
2412             }
2413             if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2414                 interruptMode = REINTERRUPT;
2415             if (node.nextWaiter != null)
2416                 unlinkCancelledWaiters();
2417             if (interruptMode != 0)
2418                 reportInterruptAfterWait(interruptMode);
2419             return !timedout;
2420         }
2421 
2422         //  support for instrumentation
2423 
2424         /**
2425          * Returns true if this condition was created by the given
2426          * synchronization object.
2427          *
2428          * @return {@code true} if owned
2429          */
2430         final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
2431             return sync == AbstractQueuedSynchronizer.this;
2432         }
2433 
2434         /**
2435          * Queries whether any threads are waiting on this condition.
2436          * Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
2437          *
2438          * @return {@code true} if there are any waiting threads
2439          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2440          *         returns {@code false}
2441          */
2442         protected final boolean hasWaiters() {
2443             if (!isHeldExclusively())
2444                 throw new IllegalMonitorStateException();
2445             for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2446                 if (w.waitStatus == Node.CONDITION)
2447                     return true;
2448             }
2449             return false;
2450         }
2451 
2452         /**
2453          * Returns an estimate of the number of threads waiting on
2454          * this condition.
2455          * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
2456          *
2457          * @return the estimated number of waiting threads
2458          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2459          *         returns {@code false}
2460          */
2461         protected final int getWaitQueueLength() {
2462             if (!isHeldExclusively())
2463                 throw new IllegalMonitorStateException();
2464             int n = 0;
2465             for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2466                 if (w.waitStatus == Node.CONDITION)
2467                     ++n;
2468             }
2469             return n;
2470         }
2471 
2472         /**
2473          * Returns a collection containing those threads that may be
2474          * waiting on this Condition.
2475          * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
2476          *
2477          * @return the collection of threads
2478          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2479          *         returns {@code false}
2480          */
2481         protected final Collection<Thread> getWaitingThreads() {
2482             if (!isHeldExclusively())
2483                 throw new IllegalMonitorStateException();
2484             ArrayList<Thread> list = new ArrayList<Thread>();
2485             for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2486                 if (w.waitStatus == Node.CONDITION) {
2487                     Thread t = w.thread;
2488                     if (t != null)
2489                         list.add(t);
2490                 }
2491             }
2492             return list;
2493         }
2494     }
2495 
2496     /**
2497      * Setup to support compareAndSet. We need to natively implement
2498      * this here: For the sake of permitting future enhancements, we
2499      * cannot explicitly subclass AtomicInteger, which would be
2500      * efficient and useful otherwise. So, as the lesser of evils, we
2501      * natively implement using hotspot intrinsics API. And while we
2502      * are at it, we do the same for other CASable fields (which could
2503      * otherwise be done with atomic field updaters).
2504      */
2505     private static final Unsafe unsafe = Unsafe.getUnsafe();
2506     private static final long stateOffset;
2507     private static final long headOffset;
2508     private static final long tailOffset;
2509     private static final long waitStatusOffset;
2510     private static final long nextOffset;
2511 
2512     static {
2513         try {
2514             stateOffset = unsafe.objectFieldOffset
2515                 (AbstractQueuedSynchronizer.class.getDeclaredField("state"));
2516             headOffset = unsafe.objectFieldOffset
2517                 (AbstractQueuedSynchronizer.class.getDeclaredField("head"));
2518             tailOffset = unsafe.objectFieldOffset
2519                 (AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
2520             waitStatusOffset = unsafe.objectFieldOffset
2521                 (Node.class.getDeclaredField("waitStatus"));
2522             nextOffset = unsafe.objectFieldOffset
2523                 (Node.class.getDeclaredField("next"));
2524 
2525         } catch (Exception ex) { throw new Error(ex); }
2526     }
2527 
2528     /**
2529      * CAS head field. Used only by enq.
2530      */
2531     private final boolean compareAndSetHead(Node update) {
2532         return unsafe.compareAndSwapObject(this, headOffset, null, update);
2533     }
2534 
2535     /**
2536      * CAS tail field. Used only by enq.
2537      */
2538     private final boolean compareAndSetTail(Node expect, Node update) {
2539         return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
2540     }
2541 
2542     /**
2543      * CAS waitStatus field of a node.
2544      */
2545     private static final boolean compareAndSetWaitStatus(Node node,
2546                                                          int expect,
2547                                                          int update) {
2548         return unsafe.compareAndSwapInt(node, waitStatusOffset,
2549                                         expect, update);
2550     }
2551 
2552     /**
2553      * CAS next field of a node.
2554      */
2555     private static final boolean compareAndSetNext(Node node,
2556                                                    Node expect,
2557                                                    Node update) {
2558         return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
2559     }
2560 }