public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
fair为true是公平锁，false为false是非公平锁。
final void lock() { acquire(1); }
public final void acquire(int arg) { if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) selfInterrupt(); }
tryAcquire 尝试获取独占锁
protected final boolean tryAcquire(int acquires) { final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) { if (!hasQueuedPredecessors() && compareAndSetState(0, acquires)) { setExclusiveOwnerThread(current); return true; } } else if (current == getExclusiveOwnerThread()) { int nextc = c + acquires; if (nextc < 0) throw new Error("Maximum lock count exceeded"); setState(nextc); return true; } return false; }
先获取同步对象的状态getState();
如果状态是0，还没有被线程占用。
hasQueuedPredecessors()；看看同步队列中是否有等待的线程，如果有，获取锁失败(独占锁肯定失败，共享锁不一定)。
如果没有使用compareAndSetState(0, acquires);使用cas的方式把状态从0改为1；
把获取到资源的线程调用setExclusiveOwnerThread(current);也就是设置当前的独占线程设置为当前的线程；
else if (current == getExclusiveOwnerThread())
如果状态不是0，判断是否还是当前的线程(也就是ExclusiveOwnerThread)；如果是让state加1，int nextc = c + acquires;
如果tryAcquire(arg)失败，就会把当前的线程添加到CLH队列中。
private Node addWaiter(Node mode) { Node node = new Node(Thread.currentThread(), mode); // Try the fast path of enq; backup to full enq on failure Node pred = tail; if (pred != null) { node.prev = pred; if (compareAndSetTail(pred, node)) { pred.next = node; return node; } } enq(node); return node; }
node.prev = pred;
if (pred != null)如果CLH队列中有节点的话
把当前节点的前置节点指向好。
使用cas把当前节点放到CLH队列的尾部compareAndSetTail(pred, node)；
如果CLH队列中没有节点的话调用enq(node)方法；
private Node enq(final Node node) { for (;;) { Node t = tail; if (t == null) { // Must initialize if (compareAndSetHead(new Node())) tail = head; } else { node.prev = t; if (compareAndSetTail(t, node)) { t.next = node; return t; } } } }
先初始化一个空的Node节点作为head节点compareAndSetHead(new Node())；
然后再把新节点添加到空的节点的后面；
final boolean acquireQueued(final Node node, int arg) { boolean failed = true; try { boolean interrupted = false; for (;;) { final Node p = node.predecessor(); if (p == head && tryAcquire(arg)) { setHead(node); p.next = null; // help GC failed = false; return interrupted; } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) interrupted = true; } } finally { if (failed) cancelAcquire(node); } }
把刚添加到队列中的Thread节点，准备阻塞，然后等待获取锁。
拿到当前节点的前置节点final Node p = node.predecessor()；先判断if (p == head && tryAcquire(arg)) 前置节点是否已经变成head节点，如果变成head节点，这时候再去尝试获取锁。
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; }
if (ws == Node.SIGNAL)如果当前节点的前驱节点是signal，当前节点可以被阻塞；
if (ws > 0) 如果前驱节点大于0，这个前驱节点就会被移除队列，
当前驱节点的waitStatus的状态是0或者PROPAGATE时，使用cas将其设置为SIGNAL，然后当前节点才可以park。
private final boolean parkAndCheckInterrupt() { LockSupport.park(this); return Thread.interrupted(); }
阻塞当前节点，并返回当前节点的阻塞状态。
公平锁的unlock方法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; }
如果当前线程不等于获取独占锁的线程，那么抛出异常
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
如果c==0 把获取独占锁的线程设置为null；
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); }
如果后一个节点是null或者waitStatus大于0，那么就从尾部开始遍历，找到正常阻塞的一个节点。