参考自Java同步框架AbstractQueuedSynchronizer
1,
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AbstractOwnableSynchronizer的类如下
public abstract class AbstractOwnableSynchronizer
implements java.io.Serializable {
/** Use serial ID even though all fields transient. */
private static final long serialVersionUID = 3737899427754241961L;
/**
* Empty constructor for use by subclasses.
*/
protected AbstractOwnableSynchronizer() { }
/**
* The current owner of exclusive mode synchronization.
*/
private transient Thread exclusiveOwnerThread;
/**
* Sets the thread that currently owns exclusive access.
* A {@code null} argument indicates that no thread owns access.
* This method does not otherwise impose any synchronization or
* {@code volatile} field accesses.
* @param thread the owner thread
*/
protected final void setExclusiveOwnerThread(Thread thread) {
exclusiveOwnerThread = thread;
}
/**
* Returns the thread last set by {@code setExclusiveOwnerThread},
* or {@code null} if never set. This method does not otherwise
* impose any synchronization or {@code volatile} field accesses.
* @return the owner thread
*/
protected final Thread getExclusiveOwnerThread() {
return exclusiveOwnerThread;
}
}
exclusiveOwnerThread代表的是当前获得同步的线程，因为是独占模式，在exclusiveOwnerThread持有同步的过程中其他的线程的任何同步获取请求将不能得到满足。
/**
* A synchronizer that may be exclusively owned by a thread.
一同同步器可以由线程独占
This
* class provides a basis for creating locks and related synchronizers
* that may entail a notion of ownership.
这个类为创建锁和相关同步器(包含所有权概念的)提供了基础
The
* {@code AbstractOwnableSynchronizer} class itself does not manage or
* use this information. However, subclasses and tools may use
* appropriately maintained values to help control and monitor access
* and provide diagnostics.
本身不能管理这些信息
*子类和工具可以使用适当维护的值来帮助控制和监视访问。
并提供诊断。
* @since 1.6
* @author Doug Lea
*/
至此，需要说明的是，AbstractQueuedSynchronizer不仅支持独占模式下的同步实现，还支持共享模式下的同步实现。在java的锁的实现上就有共享锁和独占锁的区别，而这些实现都是基于AbstractQueuedSynchronizer对于共享同步和独占同步的支持。
3,AQS对独占模式的支持
3.1基本过程
AbstractQueuedSynchronizer使用一个volatile类型的int来作为同步变量，任何想要获得锁的线程都需要来竞争该变量，获得锁的线程可以继续业务流程的执行，而没有获得锁的线程会被放到一个FIFO的队列中去，等待再次竞争同步变量来获得锁。
3.2node的数据结构
AbstractQueuedSynchronizer为每个没有获得锁的线程封装成一个Node再放到队列中去，下面先来分析一下Node这个数据结构：
/** waitStatus value to indicate thread has cancelled */
static final int CANCELLED = 1;
/** waitStatus value to indicate successor's thread needs unparking */
后续线程不停歇
static final int SIGNAL = -1;
/** waitStatus value to indicate thread is waiting on condition */
static final int CONDITION = -2;
/**
* waitStatus value to indicate the next acquireShared should
* unconditionally propagate
*/
static final int PROPAGATE = -3;
/** Marker to indicate a node is waiting in shared mode */
static final Node SHARED = new Node();共享模式
/** Marker to indicate a node is waiting in exclusive mode */
static final Node EXCLUSIVE = null;独占模式
Link to next node waiting on condition, or the special value SHARED.
Because condition queues are accessed only when holding in exclusive
mode, we just need a simple linked queue to hold nodes while they are
waiting on conditions. They are then transferred to the queue to re-acquire.
And because conditions can only be exclusive,we save a field by using
special value to indicate shared mode.
链接到下一个等待条件的节点(独占模式下)，或共享的特殊值(共享模式下)。因为条件队列只有在独占
模式下才被访问，所以当节点等待条件时，我们只需要一个简单的链接队列
来保存节点。然后将它们转移到队列以重新获取。由于条件只能是排他的，
所以我们使用特殊值来表示共享模式来保存字段。
AbstractQueuedSynchronizer使用双向链表来管理请求同步的Node，保存了链表的head和tail，新的Node将会被插到链表的尾端，而链表的head总是代表着获得锁的线程，链表头的线程释放了锁之后会通知后面的线程来竞争共享变量
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截取自acquireQueued
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {如果前驱结点为头结点,并且获取锁成功,就跳出自旋
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
/**
* Checks and updates status for a node that failed to acquire.
* Returns true if thread should block. This is the main signal
* control in all acquire loops. Requires that pred == node.prev.
*查看获取锁失败的node,如果阻塞了就返回true
* @param pred node's predecessor holding status
* @param node the node
* @return {@code true} if thread should block
*/
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
*/
return true;
if (ws > 0) {
/*
* Predecessor was cancelled. Skip over predecessors and
* indicate retry.
*/
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
} else {
/*
* waitStatus must be 0 or PROPAGATE. Indicate that we
* need a signal, but don't park yet. Caller will need to
* retry to make sure it cannot acquire before parking.
*/
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
只有当Node的前驱节点的状态为Node.SIGNAL的时候才会返回true，也就是说，只有当前驱节点的状态变为了Node.SIGNAL，才会去通知当前节点，所以如果前驱节点是Node.SIGNAL的，那么当前节点就可以放心的park就好了，前驱节点在完成工作之后在释放资源的时候会unpark它的后继节点
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
private void unparkSuccessor(Node node) {
/*
* If status is negative (i.e., possibly needing signal) try
* to clear in anticipation of signalling. It is OK if this
* fails or if status is changed by waiting thread.
*/
int ws = node.waitStatus;
if (ws < 0)
compareAndSetWaitStatus(node, ws, 0);
/*
* Thread to unpark is held in successor, which is normally
* just the next node. But if cancelled or apparently null,
* traverse backwards from tail to find the actual
* non-cancelled successor.
*/
Node s = node.next;
if (s == null || s.waitStatus > 0) {
s = null;
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
if (s != null)
LockSupport.unpark(s.thread);
}
首先通过tryRelease方法来保证资源安全完整的释放了之后，如果发现节点的状态小于0，会变为0。0代表的是初始化的状态，当前的线程已经完成了工作，释放了锁，就要恢复原来的样子。然后会获取该节点的后继节点，如果没有后续节点了，或者后继节点已经被取消了，那么从尾部开始向前找第一个符合要求的节点，然后unpark它。
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首先尝试使用tryReleaseShared方法来释放资源，如果释放失败，则返回false，如果释放成功了，那么继续执行doReleaseShared方法唤醒后续节点来竞争资源。需要注意的是，共享模式和独占模式的区别在于，独占模式只允许一个线程获得资源，而共享模式允许多个线程获得资源。所以在独占模式下只有当tryAcquire返回true的时候我们才能确定获得资源了，而在共享模式下，只要tryAcquireShared返回值大于等于0就可以说明获得资源了，所以你要确保你需要实现的需求和AbstractQueuedSynchronizer希望的是一致的。