LockSupport

• LockSupport
• LockSupport

class FIFOMutex {
    private final AtomicBoolean locked = new AtomicBoolean(false);
    private final Queue<Thread> waiters
      = new ConcurrentLinkedQueue<Thread>();

    public void lock() {
      boolean wasInterrupted = false;
      Thread current = Thread.currentThread();
      waiters.add(current);

      // Block while not first in queue or cannot acquire lock
      while (waiters.peek() != current ||
             !locked.compareAndSet(false, true)) {
        LockSupport.park(this);
        if (Thread.interrupted()) // ignore interrupts while waiting
          wasInterrupted = true;
      }

      waiters.remove();
      if (wasInterrupted)          // reassert interrupt status on exit
        current.interrupt();
    }

    public void unlock() {
      locked.set(false);
      LockSupport.unpark(waiters.peek());
    }
  }