LockSupport源码探索

public class LockSupport
extends java.lang.Object
Basic thread blocking primitives for creating locks and other synchronization classes.
This class associates, with each thread that uses it, a permit (in the sense of the Semaphore class). A call to park will return immediately if the permit is available, consuming it in the process; otherwise it may block. A call to unpark makes the permit available, if it was not already available. (Unlike with Semaphores though, permits do not accumulate. There is at most one.)
Methods park and unpark provide efficient means of blocking and unblocking threads that do not encounter the problems that cause the deprecated methods Thread.suspend and Thread.resume to be unusable for such purposes: Races between one thread invoking park and another thread trying to unpark it will preserve liveness, due to the permit. Additionally, park will return if the caller's thread was interrupted, and timeout versions are supported. The park method may also return at any other time, for "no reason", so in general must be invoked within a loop that rechecks conditions upon return. In this sense park serves as an optimization of a "busy wait" that does not waste as much time spinning, but must be paired with an unpark to be effective.
The three forms of park each also support a blocker object parameter. This object is recorded while the thread is blocked to permit monitoring and diagnostic tools to identify the reasons that threads are blocked. (Such tools may access blockers using method getBlocker.) The use of these forms rather than the original forms without this parameter is strongly encouraged. The normal argument to supply as a blocker within a lock implementation is this.
These methods are designed to be used as tools for creating higher-level synchronization utilities, and are not in themselves useful for most concurrency control applications. The park method is designed for use only in constructions of the form:
while (!canProceed()) { ... LockSupport.park(this); }
where neither canProceed nor any other actions prior to the call to park entail locking or blocking. Because only one permit is associated with each thread, any intermediary uses of park could interfere with its intended effects.
Sample Usage. Here is a sketch of a first-in-first-out non-reentrant lock class:

  class FIFOMutex {
    private final AtomicBoolean locked = new AtomicBoolean(false);//锁，true代表已经被某个线程获取了锁
    private final Queue<Thread> waiters
      = new ConcurrentLinkedQueue<Thread>();

    public void lock() {
      boolean wasInterrupted = false;//记录线程是否曾经被中断过
      Thread current = Thread.currentThread();
      waiters.add(current);//把当前线程入队列，入队后当前线程可能在队头也可能不在队头

      // Block while not first in queue or cannot acquire lock//不是在队头或者无法获得锁(用!locked.compareAndSet(false, true)来实现)，则block当前线程
      while (waiters.peek() != current ||
             !locked.compareAndSet(false, true)) {
         LockSupport.park(this);//block当前线程
         if (Thread.interrupted()) // ignore interrupts while waiting//因为 LockSupport.park(this);可能会因为线程被中断而马上返回，而线程的中断需要被上层调用代码感知，所以要做这个判断。interrupted()方法判断完是否中断后，会马上重置中断状态，即将中断状态设置为false.
           wasInterrupted = true;//发现线程曾经被中断，则马上标志线程被中断，wasInterrupted后续还有用，用来重新恢复线程中断状态，即是让线程中断状态为true
      }

      waiters.remove();//删除队头元素，即在队列中删除当前线程自己
      if (wasInterrupted)          // reassert interrupt status on exit
         current.interrupt();//恢复中断状态，即让中断状态为true
    }

    public void unlock() {
      locked.set(false);//锁，为false，代表某个线程放弃了锁，当前锁可用
      LockSupport.unpark(waiters.peek());//peek()方法只是查看队头元素，并不删除。 LockSupport.unpark(waiters.peek());意思是将队头的线程解开阻塞状态，让它从lock()方法的LockSupport.park(this);这行继续执行下去
    }
  }