JAVA并发之ReentrantLock源码（二）

 1     private Node addWaiter(Node mode) {
 2         Node node = new Node(Thread.currentThread(), mode);
 3         // Try the fast path of enq; backup to full enq on failure
 4         Node pred = tail;
 5         if (pred != null) {//如果原来有头有尾
 6             node.prev = pred;//设置当前节点的前置节点
 7             if (compareAndSetTail(pred, node)) {//CAS尝试将其放入末尾
 8                 pred.next = node;
 9                 return node;
10             }
11         }
12         enq(node);//当原来没头没尾的时候要同时CAS设置头尾节点为当前节点
13         return node;
14     }
15 
16     private Node enq(final Node node) {
17         for (;;) {//自旋不断的用cas设置头尾节点
18             Node t = tail;
19             if (t == null) { // Must initialize
20                 if (compareAndSetHead(new Node()))
21                     tail = head;
22             } else {
23                 node.prev = t;
24                 if (compareAndSetTail(t, node)) {
25                     t.next = node;
26                     return t;
27                 }
28             }
29         }
30     }

 1     final boolean acquireQueued(final Node node, int arg) {
 2         boolean failed = true;
 3         try {
 4             boolean interrupted = false;
 5             for (;;) {//不断自旋
 6                 final Node p = node.predecessor();
 7                 if (p == head && tryAcquire(arg)) {//如果当前节点的前一个节点排队排到了头的位置并且可以获取到资源
 8                     setHead(node);
 9                     p.next = null; // help GC
10                     failed = false;
11                     return interrupted;
12                 }
13                 if (shouldParkAfterFailedAcquire(p, node) &&
14                     parkAndCheckInterrupt())//判断是否需要去休息（如果是的话），就执行parkAndCheckInterrupt（）方法去休息
15                     interrupted = true;
16             }
17         } finally {
18             if (failed)
19                 cancelAcquire(node);
20         }
21     }
22 
23     private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
24         int ws = pred.waitStatus;
25         if (ws == Node.SIGNAL)
26             /*
27              * This node has already set status asking a release
28              * to signal it, so it can safely park.
29              * 如果前一个节点已经变成 SIGNAL（会提醒下一个节点）状态，当前节点的线程就可以去“睡觉”了
30              */
31             return true;
32         if (ws > 0) {
33             /*
34              * Predecessor was cancelled. Skip over predecessors and
35              * indicate retry.
36              * 如果前一个线程已经被取消了，那么将他们跳过不断重复
37              */
38             do {
39                 node.prev = pred = pred.prev;
40             } while (pred.waitStatus > 0);
41             pred.next = node;
42         } else {
43             /*
44              * waitStatus must be 0 or PROPAGATE.  Indicate that we
45              * need a signal, but don‘t park yet.  Caller will need to
46              * retry to make sure it cannot acquire before parking.
47              * 如果前一个节点没有变成SIGNAL状态，就把它置为signal状态
48              */
49             compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
50         }
51         return false;
52     }
53 
54     private final boolean parkAndCheckInterrupt() {
55         LockSupport.park(this);//通过park方法让当前线程阻塞
56         return Thread.interrupted();//返回当前线程的中断情况
57     }