源码

  1.     public void unlock() {
  2.         sync.release(1);
  3.     }
  1.     public final boolean release(int arg) {
  2.         if (tryRelease(arg)) {
  3.             Node h = head;
  4.             //waitStatus =-1 表示自己有义务去唤醒别人
  5.             if (h != null && h.waitStatus != 0)
  6.                 unparkSuccessor(h);
  7.             return true;
  8.         }
  9.         return false;
  10.     }
  1.         protected final boolean tryRelease(int releases) {
  2.             int c = getState() - releases;
  3.             if (Thread.currentThread() != getExclusiveOwnerThread())
  4.                 //基本不会发生
  5.                 throw new IllegalMonitorStateException();
  6.             boolean free = false;
  7.             if (c == 0) {
  8.                 free = true;
  9.                 setExclusiveOwnerThread(null);
  10.             }
  11.             setState(c);
  12.             return free;
  13.         }

unparkSuccessor

  1.     private void unparkSuccessor(Node node) {
  3.         int ws = node.waitStatus;
  4.         if (ws < 0)
  5.             //将waitstatus由-1改成0，防止多线程情况下的访问导致需要额外唤醒别的线程
  6.             com 
  8.         Node s = node.next;
  9.         //非正常情况 下一个节点被中断
  10.         if (s == null || s.waitStatus > 0) {
  11.             s = null;
  12.             for (Node t = tail; t != null && t != node; t = t.prev)
  13.                 if (t.waitStatus <= 0)
  14.                     s = t;
  15.         }
  16.          //正常情况
  17.         if (s != null)
  18.             LockSupport.unpark(s.thread);
  19.     }

解锁流程图

解锁之前

image.png

解锁之后

image.png