• FutureTask如何实现的阻塞返回的

    futureTask在使用get()函数的时候会阻塞等待返回

    1. /**
    2.      * @throws CancellationException {@inheritDoc}
    3.      */
    4.     public V get() throws InterruptedException, ExecutionException {
    5.         int s = state;
    6.         if (s <= COMPLETING)
    7.             s = awaitDone(false, 0L);
    8.         return report(s);
    9.     }

    可以看到这里使用的是awaitDone 进行阻塞，report 返回结果

    1. /**
    2.      * Awaits completion or aborts on interrupt or timeout.
    3.      *
    4.      * @param timed true if use timed waits
    5.      * @param nanos time to wait, if timed
    6.      * @return state upon completion
    7.      */
    8.     private int awaitDone(boolean timed, long nanos)
    9.         throws InterruptedException {
    10.         final long deadline = timed ? System.nanoTime() + nanos : 0L;
    11.         WaitNode q = null;
    12.         boolean queued = false;
    13.         for (;;) {
    14.             if (Thread.interrupted()) {
    15.                 removeWaiter(q);
    16.                 throw new InterruptedException();
    17.             }
    19.             int s = state;
    20.             if (s > COMPLETING) {
    21.                 if (q != null)
    22.                     q.thread = null;
    23.                 return s;
    24.             }
    25.             else if (s == COMPLETING) // cannot time out yet
    26.                 Thread.yield();
    27.             else if (q == null)
    28.                 q = new WaitNode();
    29.             else if (!queued)
    30.                 queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
    31.                                                      q.next = waiters, q);
    32.             else if (timed) {
    33.                 nanos = deadline - System.nanoTime();
    34.                 if (nanos <= 0L) {
    35.                     removeWaiter(q);
    36.                     return state;
    37.                 }
    38.                 LockSupport.parkNanos(this, nanos);
    39.             }
    40.             else
    41.                 LockSupport.park(this);
    42.         }
    43.     }