一、前言
在锁框架中，AbstractQueuedSynchronizer抽象类可以毫不夸张的说，占据着核心地位，它提供了一个基于FIFO队列，可以用于构建锁或者其他相关同步装置的基础框架。所以很有必要好好分析。
二、AbstractQueuedSynchronizer数据结构
分析类，首先就要分析底层采用了何种数据结构，抓住核心点进行分析，经过分析可知，AbstractQueuedSynchronizer类的数据结构如下
说明：AbstractQueuedSynchronizer类底层的数据结构是使用双向链表，是队列的一种实现，故也可看成是队列，其中Sync queue，即同步队列，是双向链表，包括head结点和tail结点，head结点主要用作后续的调度。而Condition queue不是必须的，其是一个单向链表，只有当使用Condition时，才会存在此单向链表。并且可能会有多个Condition queue。
三、AbstractQueuedSynchronizer源码分析
3.1 类的继承关系
public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer implements java.io.Serializable
说明：从类继承关系可知，AbstractQueuedSynchronizer继承自AbstractOwnableSynchronizer抽象类，并且实现了Serializable接口，可以进行序列化。而AbstractOwnableSynchronizer抽象类的源码如下
public abstract class AbstractOwnableSynchronizer implements java.io.Serializable { // 版本序列号 private static final long serialVersionUID = 3737899427754241961L; // 构造函数 protected AbstractOwnableSynchronizer() { } // 独占模式下的线程 private transient Thread exclusiveOwnerThread; // 设置独占线程 protected final void setExclusiveOwnerThread(Thread thread) { exclusiveOwnerThread = thread; } // 获取独占线程 protected final Thread getExclusiveOwnerThread() { return exclusiveOwnerThread; } }
说明：AbstractOwnableSynchronizer抽象类中，可以设置独占资源线程和获取独占资源线程。分别为setExclusiveOwnerThread与getExclusiveOwnerThread方法，这两个方法会被子类调用。
3.2 类的内部类
AbstractQueuedSynchronizer类有两个内部类，分别为Node类与ConditionObject类。下面分别做介绍。
1. Node类
static final class Node { // 模式，分为共享与独占 // 共享模式 static final Node SHARED = new Node(); // 独占模式 static final Node EXCLUSIVE = null; // 结点状态 // CANCELLED，值为1，表示当前的线程被取消 // SIGNAL，值为-1，表示当前节点的后继节点包含的线程需要运行，也就是unpark // CONDITION，值为-2，表示当前节点在等待condition，也就是在condition队列中 // PROPAGATE，值为-3，表示当前场景下后续的acquireShared能够得以执行 // 值为0，表示当前节点在sync队列中，等待着获取锁 static final int CANCELLED = 1; static final int SIGNAL = -1; static final int CONDITION = -2; static final int PROPAGATE = -3; // 结点状态 volatile int waitStatus; // 前驱结点 volatile Node prev; // 后继结点 volatile Node next; // 结点所对应的线程 volatile Thread thread; // 下一个等待者 Node nextWaiter; // 结点是否在共享模式下等待 final boolean isShared() { return nextWaiter == SHARED; } // 获取前驱结点，若前驱结点为空，抛出异常 final Node predecessor() throws NullPointerException { // 保存前驱结点 Node p = prev; if (p == null) // 前驱结点为空，抛出异常 throw new NullPointerException(); else // 前驱结点不为空，返回 return p; } // 无参构造函数 Node() { // Used to establish initial head or SHARED marker } // 构造函数 Node(Thread thread, Node mode) { // Used by addWaiter this.nextWaiter = mode; this.thread = thread; } // 构造函数 Node(Thread thread, int waitStatus) { // Used by Condition this.waitStatus = waitStatus; this.thread = thread; } }
说明：每个线程被阻塞的线程都会被封装成一个Node结点，放入队列。每个节点包含了一个Thread类型的引用，并且每个节点都存在一个状态，具体状态如下。
① CANCELLED，值为1，表示当前的线程被取消。
② SIGNAL，值为-1，表示当前节点的后继节点包含的线程需要运行，需要进行unpark操作。
③ CONDITION，值为-2，表示当前节点在等待condition，也就是在condition queue中。
④ PROPAGATE，值为-3，表示当前场景下后续的acquireShared能够得以执行。
⑤ 值为0，表示当前节点在sync queue中，等待着获取锁。
2. ConditionObject类
// 内部类 public class ConditionObject implements Condition, java.io.Serializable { // 版本号 private static final long serialVersionUID = 1173984872572414699L; /** First node of condition queue. */ // condition队列的头结点 private transient Node firstWaiter; /** Last node of condition queue. */ // condition队列的尾结点 private transient Node lastWaiter; /** * Creates a new {@code ConditionObject} instance. */ // 构造函数 public ConditionObject() { } // Internal methods /** * Adds a new waiter to wait queue. * @return its new wait node */ // 添加新的waiter到wait队列 private Node addConditionWaiter() { // 保存尾结点 Node t = lastWaiter; // If lastWaiter is cancelled, clean out. if (t != null && t.waitStatus != Node.CONDITION) { // 尾结点不为空，并且尾结点的状态不为CONDITION // 清除状态为CONDITION的结点 unlinkCancelledWaiters(); // 将最后一个结点重新赋值给t t = lastWaiter; } // 新建一个结点 Node node = new Node(Thread.currentThread(), Node.CONDITION); if (t == null) // 尾结点为空 // 设置condition队列的头结点 firstWaiter = node; else // 尾结点不为空 // 设置为节点的nextWaiter域为node结点 t.nextWaiter = node; // 更新condition队列的尾结点 lastWaiter = node; return node; } /** * Removes and transfers nodes until hit non-cancelled one or * null. Split out from signal in part to encourage compilers * to inline the case of no waiters. * @param first (non-null) the first node on condition queue */ private void doSignal(Node first) { // 循环 do { if ( (firstWaiter = first.nextWaiter) == null) // 该节点的nextWaiter为空 // 设置尾结点为空 lastWaiter = null; // 设置first结点的nextWaiter域 first.nextWaiter = null; } while (!transferForSignal(first) && (first = firstWaiter) != null); // 将结点从condition队列转移到sync队列失败并且condition队列中的头结点不为空，一直循环 } /** * Removes and transfers all nodes. * @param first (non-null) the first node on condition queue */ private void doSignalAll(Node first) { // condition队列的头结点尾结点都设置为空 lastWaiter = firstWaiter = null; // 循环 do { // 获取first结点的nextWaiter域结点 Node next = first.nextWaiter; // 设置first结点的nextWaiter域为空 first.nextWaiter = null; // 将first结点从condition队列转移到sync队列 transferForSignal(first); // 重新设置first first = next; } while (first != null); } /** * Unlinks cancelled waiter nodes from condition queue. * Called only while holding lock. This is called when * cancellation occurred during condition wait, and upon * insertion of a new waiter when lastWaiter is seen to have * been cancelled. This method is needed to avoid garbage * retention in the absence of signals. So even though it may * require a full traversal, it comes into play only when * timeouts or cancellations occur in the absence of * signals. It traverses all nodes rather than stopping at a * particular target to unlink all pointers to garbage nodes * without requiring many re-traversals during cancellation * storms. */ // 从condition队列中清除状态为CANCEL的结点 private void unlinkCancelledWaiters() { // 保存condition队列头结点 Node t = firstWaiter; Node trail = null; while (t != null) { // t不为空 // 下一个结点 Node next = t.nextWaiter; if (t.waitStatus != Node.CONDITION) { // t结点的状态不为CONDTION状态 // 设置t节点的额nextWaiter域为空 t.nextWaiter = null; if (trail == null) // trail为空 // 重新设置condition队列的头结点 firstWaiter = next; else // trail不为空 // 设置trail结点的nextWaiter域为next结点 trail.nextWaiter = next; if (next == null) // next结点为空 // 设置condition队列的尾结点 lastWaiter = trail; } else // t结点的状态为CONDTION状态 // 设置trail结点 trail = t; // 设置t结点 t = next; } } // public methods /** * Moves the longest-waiting thread, if one exists, from the * wait queue for this condition to the wait queue for the * owning lock. * * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ // 唤醒一个等待线程。如果所有的线程都在等待此条件，则选择其中的一个唤醒。在从 await 返回之前，该线程必须重新获取锁。 public final void signal() { if (!isHeldExclusively()) // 不被当前线程独占，抛出异常 throw new IllegalMonitorStateException(); // 保存condition队列头结点 Node first = firstWaiter; if (first != null) // 头结点不为空 // 唤醒一个等待线程 doSignal(first); } /** * Moves all threads from the wait queue for this condition to * the wait queue for the owning lock. * * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ // 唤醒所有等待线程。如果所有的线程都在等待此条件，则唤醒所有线程。在从 await 返回之前，每个线程都必须重新获取锁。 public final void signalAll() { if (!isHeldExclusively()) // 不被当前线程独占，抛出异常 throw new IllegalMonitorStateException(); // 保存condition队列头结点 Node first = firstWaiter; if (first != null) // 头结点不为空 // 唤醒所有等待线程 doSignalAll(first); } /** * Implements uninterruptible condition wait. * <ol> * <li> Save lock state returned by {@link #getState}. * <li> Invoke {@link #release} with saved state as argument, * throwing IllegalMonitorStateException if it fails. * <li> Block until signalled. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * </ol> */ // 等待，当前线程在接到信号之前一直处于等待状态，不响应中断 public final void awaitUninterruptibly() { // 添加一个结点到等待队列 Node node = addConditionWaiter(); // 获取释放的状态 int savedState = fullyRelease(node); boolean interrupted = false; while (!isOnSyncQueue(node)) { // // 阻塞当前线程 LockSupport.park(this); if (Thread.interrupted()) // 当前线程被中断 // 设置interrupted状态 interrupted = true; } if (acquireQueued(node, savedState) || interrupted) // selfInterrupt(); } /* * For interruptible waits, we need to track whether to throw * InterruptedException, if interrupted while blocked on * condition, versus reinterrupt current thread, if * interrupted while blocked waiting to re-acquire. */ /** Mode meaning to reinterrupt on exit from wait */ private static final int REINTERRUPT = 1; /** Mode meaning to throw InterruptedException on exit from wait */ private static final int THROW_IE = -1; /** * Checks for interrupt, returning THROW_IE if interrupted * before signalled, REINTERRUPT if after signalled, or * 0 if not interrupted. */ private int checkInterruptWhileWaiting(Node node) { return Thread.interrupted() ? (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) : 0; } /** * Throws InterruptedException, reinterrupts current thread, or * does nothing, depending on mode. */ private void reportInterruptAfterWait(int interruptMode) throws InterruptedException { if (interruptMode == THROW_IE) throw new InterruptedException(); else if (interruptMode == REINTERRUPT) selfInterrupt(); } /** * Implements interruptible condition wait. * <ol> * <li> If current thread is interrupted, throw InterruptedException. * <li> Save lock state returned by {@link #getState}. * <li> Invoke {@link #release} with saved state as argument, * throwing IllegalMonitorStateException if it fails. * <li> Block until signalled or interrupted. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * <li> If interrupted while blocked in step 4, throw InterruptedException. * </ol> */ // // 等待，当前线程在接到信号或被中断之前一直处于等待状态 public final void await() throws InterruptedException { if (Thread.interrupted()) // 当前线程被中断，抛出异常 throw new InterruptedException(); // 在wait队列上添加一个结点 Node node = addConditionWaiter(); // int savedState = fullyRelease(node); int interruptMode = 0; while (!isOnSyncQueue(node)) { // 阻塞当前线程 LockSupport.park(this); if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) // 检查结点等待时的中断类型 break; } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; if (node.nextWaiter != null) // clean up if cancelled unlinkCancelledWaiters(); if (interruptMode != 0) reportInterruptAfterWait(interruptMode); } /** * Implements timed condition wait. * <ol> * <li> If current thread is interrupted, throw InterruptedException. * <li> Save lock state returned by {@link #getState}. * <li> Invoke {@link #release} with saved state as argument, * throwing IllegalMonitorStateException if it fails. * <li> Block until signalled, interrupted, or timed out. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * <li> If interrupted while blocked in step 4, throw InterruptedException. * </ol> */ // 等待，当前线程在接到信号、被中断或到达指定等待时间之前一直处于等待状态 public final long awaitNanos(long nanosTimeout) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); int savedState = fullyRelease(node); final long deadline = System.nanoTime() + nanosTimeout; int interruptMode = 0; while (!isOnSyncQueue(node)) { if (nanosTimeout <= 0L) { transferAfterCancelledWait(node); break; } if (nanosTimeout >= spinForTimeoutThreshold) LockSupport.parkNanos(this, nanosTimeout); if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) break; nanosTimeout = deadline - System.nanoTime(); } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; if (node.nextWaiter != null) unlinkCancelledWaiters(); if (interruptMode != 0) reportInterruptAfterWait(interruptMode); return deadline - System.nanoTime(); } /** * Implements absolute timed condition wait. * <ol> * <li> If current thread is interrupted, throw InterruptedException. * <li> Save lock state returned by {@link #getState}. * <li> Invoke {@link #release} with saved state as argument, * throwing IllegalMonitorStateException if it fails. * <li> Block until signalled, interrupted, or timed out. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * <li> If interrupted while blocked in step 4, throw InterruptedException. * <li> If timed out while blocked in step 4, return false, else true. * </ol> */ // 等待，当前线程在接到信号、被中断或到达指定最后期限之前一直处于等待状态 public final boolean awaitUntil(Date deadline) throws InterruptedException { long abstime = deadline.getTime(); if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); int savedState = fullyRelease(node); boolean timedout = false; int interruptMode = 0; while (!isOnSyncQueue(node)) { if (System.currentTimeMillis() > abstime) { timedout = transferAfterCancelledWait(node); break; } LockSupport.parkUntil(this, abstime); if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) break; } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; if (node.nextWaiter != null) unlinkCancelledWaiters(); if (interruptMode != 0) reportInterruptAfterWait(interruptMode); return !timedout; } /** * Implements timed condition wait. * <ol> * <li> If current thread is interrupted, throw InterruptedException. * <li> Save lock state returned by {@link #getState}. * <li> Invoke {@link #release} with saved state as argument, * throwing IllegalMonitorStateException if it fails. * <li> Block until signalled, interrupted, or timed out. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * <li> If interrupted while blocked in step 4, throw InterruptedException. * <li> If timed out while blocked in step 4, return false, else true. * </ol> */ // 等待，当前线程在接到信号、被中断或到达指定等待时间之前一直处于等待状态。此方法在行为上等效于：awaitNanos(unit.toNanos(time)) > 0 public final boolean await(long time, TimeUnit unit) throws InterruptedException { long nanosTimeout = unit.toNanos(time); if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); int savedState = fullyRelease(node); final long deadline = System.nanoTime() + nanosTimeout; boolean timedout = false; int interruptMode = 0; while (!isOnSyncQueue(node)) { if (nanosTimeout <= 0L) { timedout = transferAfterCancelledWait(node); break; } if (nanosTimeout >= spinForTimeoutThreshold) LockSupport.parkNanos(this, nanosTimeout); if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) break; nanosTimeout = deadline - System.nanoTime(); } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; if (node.nextWaiter != null) unlinkCancelledWaiters(); if (interruptMode != 0) reportInterruptAfterWait(interruptMode); return !timedout; } // support for instrumentation /** * Returns true if this condition was created by the given * synchronization object. * * @return {@code true} if owned */ final boolean isOwnedBy(AbstractQueuedSynchronizer sync) { return sync == AbstractQueuedSynchronizer.this; } /** * Queries whether any threads are waiting on this condition. * Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}. * * @return {@code true} if there are any waiting threads * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ // 查询是否有正在等待此条件的任何线程 protected final boolean hasWaiters() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); for (Node w = firstWaiter; w != null; w = w.nextWaiter) { if (w.waitStatus == Node.CONDITION) return true; } return false; } /** * Returns an estimate of the number of threads waiting on * this condition. * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}. * * @return the estimated number of waiting threads * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ // 返回正在等待此条件的线程数估计值 protected final int getWaitQueueLength() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); int n = 0; for (Node w = firstWaiter; w != null; w = w.nextWaiter) { if (w.waitStatus == Node.CONDITION) ++n; } return n; } /** * Returns a collection containing those threads that may be * waiting on this Condition. * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}. * * @return the collection of threads * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ // 返回包含那些可能正在等待此条件的线程集合 protected final Collection<Thread> getWaitingThreads() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); ArrayList<Thread> list = new ArrayList<Thread>(); for (Node w = firstWaiter; w != null; w = w.nextWaiter) { if (w.waitStatus == Node.CONDITION) { Thread t = w.thread; if (t != null) list.add(t); } } return list; } }
说明：此类实现了Condition接口，Condition接口定义了条件操作规范，具体如下
public interface Condition { // 等待，当前线程在接到信号或被中断之前一直处于等待状态 void await() throws InterruptedException; // 等待，当前线程在接到信号之前一直处于等待状态，不响应中断 void awaitUninterruptibly(); //等待，当前线程在接到信号、被中断或到达指定等待时间之前一直处于等待状态 long awaitNanos(long nanosTimeout) throws InterruptedException; // 等待，当前线程在接到信号、被中断或到达指定等待时间之前一直处于等待状态。此方法在行为上等效于：awaitNanos(unit.toNanos(time)) > 0 boolean await(long time, TimeUnit unit) throws InterruptedException; // 等待，当前线程在接到信号、被中断或到达指定最后期限之前一直处于等待状态 boolean awaitUntil(Date deadline) throws InterruptedException; // 唤醒一个等待线程。如果所有的线程都在等待此条件，则选择其中的一个唤醒。在从 await 返回之前，该线程必须重新获取锁。 void signal(); // 唤醒所有等待线程。如果所有的线程都在等待此条件，则唤醒所有线程。在从 await 返回之前，每个线程都必须重新获取锁。 void signalAll(); }
说明：Condition接口中定义了await、signal函数，用来等待条件、释放条件。之后会详细分析CondtionObject的源码。
3.3 类的属性
public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer implements java.io.Serializable { // 版本号 private static final long serialVersionUID = 7373984972572414691L; // 头结点 private transient volatile Node head; // 尾结点 private transient volatile Node tail; // 状态 private volatile int state; // 自旋时间 static final long spinForTimeoutThreshold = 1000L; // Unsafe类实例 private static final Unsafe unsafe = Unsafe.getUnsafe(); // state内存偏移地址 private static final long stateOffset; // head内存偏移地址 private static final long headOffset; // state内存偏移地址 private static final long tailOffset; // tail内存偏移地址 private static final long waitStatusOffset; // next内存偏移地址 private static final long nextOffset; // 静态初始化块 static { try { stateOffset = unsafe.objectFieldOffset (AbstractQueuedSynchronizer.class.getDeclaredField("state")); headOffset = unsafe.objectFieldOffset (AbstractQueuedSynchronizer.class.getDeclaredField("head")); tailOffset = unsafe.objectFieldOffset (AbstractQueuedSynchronizer.class.getDeclaredField("tail")); waitStatusOffset = unsafe.objectFieldOffset (Node.class.getDeclaredField("waitStatus")); nextOffset = unsafe.objectFieldOffset (Node.class.getDeclaredField("next")); } catch (Exception ex) { throw new Error(ex); } } }
说明：属性中包含了头结点head，尾结点tail，状态state、自旋时间spinForTimeoutThreshold，还有AbstractQueuedSynchronizer抽象的属性在内存中的偏移地址，通过该偏移地址，可以获取和设置该属性的值，同时还包括一个静态初始化块，用于加载内存偏移地址。
3.4 类的构造函数
protected AbstractQueuedSynchronizer() { }
说明：此类构造函数为从抽象构造函数，供子类调用。
