如果该队列已经有node即tail!=null，则将新节点的前驱节点置为tail，再通过CAS将tail指向当前节点，前驱节点的后继节点指向当前节点，然后返回当前节点；
如果队列为空或者CAS失败，则通过enq入队：
/** * Inserts node into queue, initializing if necessary. See picture above. * @param node the node to insert * @return node's predecessor */ 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; } } } }
进队的时候，要么是第一个入队并且设置head节点并且循环设置tail，要么是add tail，如果CAS不成功，则会无限循环，直到设置成功，即使高并发的场景，也最终能够保证设置成功，然后返回包装好的node节点；
acquireQueued/** * Acquires in exclusive uninterruptible mode for thread already in * queue. Used by condition wait methods as well as acquire. * * @param node the node * @param arg the acquire argument * @return {@code true} if interrupted while waiting */ 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); } }
该方法的主要作用就是将已经进入虚拟队列的节点进行阻塞，我们看到，如果当前节点的前驱节点是head并且尝试获取锁的时候成功了，则直接返回，不需要阻塞；
如果前驱节点不是头节点或者获取锁的时候失败了，则进行判定是否需要阻塞：
/** * 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. * * @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; }
这段代码对该节点的前驱节点的状态进行判断，如果前驱节点已经处于signal状态，则返回true，表明当前节点可以进入阻塞状态；
否则，将前驱节点状态CAS置为signal状态，然后通过上层的for循环进入parkAndCheckInterrupt代码块park：
/** * Convenience method to park and then check if interrupted * * @return {@code true} if interrupted */ private final boolean parkAndCheckInterrupt() { LockSupport.park(this); return Thread.interrupted(); }
这个时候将该线程交给操作系统内核进行阻塞；