此类采用模板模式设计，此类为一个抽象类，但是没抽象方法，每个sync子类需要实现5个受保护的方法
这个5个方法在AbstractQueuedSynchronizer 都抛出throw new UnsupportedOperationException();
AbstractQueuedSynchronizer 中有3个属性：主要声明一个状态和一个wait queue,通过
wait queue中Node 为一个双向链表，需要去理解Node中几个静态字段值的意义，下面为他的源码：
static final class Node {
/** waitStatus value to indicate thread has cancelled */
static final int CANCELLED = 1;
/** waitStatus value to indicate successor's thread needs unparking */
static final int SIGNAL = -1;
/** waitStatus value to indicate thread is waiting on condition */
static final int CONDITION = -2;
/** Marker to indicate a node is waiting in shared mode */
static final Node SHARED = new Node();
/** Marker to indicate a node is waiting in exclusive mode */
static final Node EXCLUSIVE = null;
/**
* Status field, taking on only the values:
* SIGNAL: The successor of this node is (or will soon be)
* blocked (via park), so the current node must
* unpark its successor when it releases or
* cancels. To avoid races, acquire methods must
* first indicate they need a signal,
* then retry the atomic acquire, and then,
* on failure, block.
* CANCELLED: This node is cancelled due to timeout or interrupt.
* Nodes never leave this state. In particular,
* a thread with cancelled node never again blocks.
* CONDITION: This node is currently on a condition queue.
* It will not be used as a sync queue node until
* transferred. (Use of this value here
* has nothing to do with the other uses
* of the field, but simplifies mechanics.)
* 0: None of the above
*
* The values are arranged numerically to simplify use.
* Non-negative values mean that a node doesn't need to
* signal. So, most code doesn't need to check for particular
* values, just for sign.
*
* The field is initialized to 0 for normal sync nodes, and
* CONDITION for condition nodes. It is modified only using
* CAS.
*/
volatile int waitStatus;
/**
* Link to predecessor node that current node/thread relies on
* for checking waitStatus. Assigned during enqueing, and nulled
* out (for sake of GC) only upon dequeuing. Also, upon
* cancellation of a predecessor, we short-circuit while
* finding a non-cancelled one, which will always exist
* because the head node is never cancelled: A node becomes
* head only as a result of successful acquire. A
* cancelled thread never succeeds in acquiring, and a thread only
* cancels itself, not any other node.
*/
volatile Node prev;
/**
* Link to the successor node that the current node/thread
* unparks upon release. Assigned once during enqueuing, and
* nulled out (for sake of GC) when no longer needed. Upon
* cancellation, we cannot adjust this field, but can notice
* status and bypass the node if cancelled. The enq operation
* does not assign next field of a predecessor until after
* attachment, so seeing a null next field does not
* necessarily mean that node is at end of queue. However, if
* a next field appears to be null, we can scan prev's from
* the tail to double-check.
*/
volatile Node next;
/**
* The thread that enqueued this node. Initialized on
* construction and nulled out after use.
*/
volatile Thread thread;
/**
* 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.
*/
Node nextWaiter;
/**
* Returns true if node is waiting in shared mode
*/
final boolean isShared() {
return nextWaiter == SHARED;
}
/**
* Returns previous node, or throws NullPointerException if
* null. Use when predecessor cannot be null.
* @return the predecessor of this node
*/
final Node predecessor() throws NullPointerException {
Node p = prev;
if (p == null)
throw new NullPointerException();
else
return p;
}
Node() { // Used to establish initial head or SHARED marker
}
Node(Thread thread, Node mode) { // Used by addWaiter
this.nextWaiter = mode;
this.thread = thread;
}
Node(Thread thread, int waitStatus) { // Used by Condition
this.waitStatus = waitStatus;
this.thread = thread;
}
}
获取锁定调用的方法，其实这个方法是阻塞的：
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
如果获取不成功则调用如下方法：
final boolean acquireQueued(final Node node, int arg) {
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {//当节点是头节点且独占时才返回
setHead(node);
p.next = null; // help GC
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())//阻塞并判断是否打断，其实这个判断才是自旋锁真正的猥琐点，
意思是如果你的前继节点不是head,而且当你的前继节点状态是Node.SIGNAL时，你这个线程将被park()，直到另外的线程release时,发现head.next是你这个node时，才unpark，你才能继续循环并获取锁
interrupted = true;
}
shouldParkAfterFailedAcquire这个方法删除所有waitStatus>0也就是CANCELLED状态的Node,并设置前继节点为signal
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int s = pred.waitStatus;
if (s < 0)
/*
* This node has already set status asking a release
* to signal it, so it can safely park
*/
return true;
if (s > 0) {
/*
* Predecessor was cancelled. Skip over predecessors and
* indicate retry.
*/
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
}
else
/*
* 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, 0, Node.SIGNAL);
return false;
}
使用LockSupport.park(this)，禁用当前线程
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);//block
return Thread.interrupted();
}
释放锁：
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);//unblock
return true;
}
return false;
}
private void unparkSuccessor(Node node) {
/*
* Try to clear status in anticipation of signalling. It is
* OK if this fails or if status is changed by waiting thread.
*/
compareAndSetWaitStatus(node, Node.SIGNAL, 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);
}
} catch (RuntimeException ex) {
cancelAcquire(node);
throw ex;
}
}
看下ReentrantLock锁中sync的实现：
static abstract class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = -5179523762034025860L;
/**
* Performs {@link Lock#lock}. The main reason for subclassing
* is to allow fast path for nonfair version.
*/
abstract void lock();
/**
* Performs non-fair tryLock. tryAcquire is
* implemented in subclasses, but both need nonfair
* try for trylock method.
*/
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
protected final boolean tryRelease(int releases) {
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
setState(c);
return free;
}
protected final boolean isHeldExclusively() {
// While we must in general read state before owner,
// we don't need to do so to check if current thread is owner
return getExclusiveOwnerThread() == Thread.currentThread();
}
final ConditionObject newCondition() {
return new ConditionObject();
}
// Methods relayed from outer class
final Thread getOwner() {
return getState() == 0 ? null : getExclusiveOwnerThread();
}
final int getHoldCount() {
return isHeldExclusively() ? getState() : 0;
}
final boolean isLocked() {
return getState() != 0;
}
/**
* Reconstitutes this lock instance from a stream.
* @param s the stream
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
setState(0); // reset to unlocked state
}
}
非公平规则下nonfairTryAcquire，获取当前锁的state,通过CAS原子操作设置为1，并将当前线程设置为独占线程，如果当前线程已经拿了锁，则state增加1
公平锁中 有如下判断：
if (isFirst(current) &&//判断头元素
compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
在获取锁步骤：
1.调用tryAcquire来获取，如果失败，则进入2
2.调用addWaiter，以独占模式将node放到tail位置
3.调用acquireQueued方法，此方法阻塞，直到node的pre为head,并成功获取锁定，也可能存在阻塞并打断情况
释放锁的步骤：
1.放弃排他锁持有权
2.unpark 节点的下一个blocked节点
公平锁与非公平锁：从代码上看，非公平锁是让当前线程优先独占，而公平锁则是让等待时间最长的线程优先，非公平的可能让其他线程没机会执行，而公平的则可以让等待时间最长的先执行，但是性能上会差点