FutureTask类中get方法阻塞的问题:
get方法的实现:
/** * @throws CancellationException {@inheritDoc} */ public V get() throws InterruptedException, ExecutionException { int s = state; if (s <= COMPLETING) s = awaitDone(false, 0L); return report(s); }
这里的state是线程完成的标志,线程完成之后该state为2。线程启动之前该值为1。
COMPLETING的值为1。
s <= COMPLETING表示当前线程还未执行完。就调用awaitDone方法。
awaitDone方法中:
第一个参数:timed true if use timed waits,为false表示不使用timed waits,意思是不设置超时时间,如果线程未完成,会一直阻塞。
第二个参数:nanos time ,等待的时间。
awaitDone方法的实现:
/** * Awaits completion or aborts on interrupt or timeout. * * @param timed true if use timed waits * @param nanos time to wait, if timed * @return state upon completion */ private int awaitDone(boolean timed, long nanos) throws InterruptedException { final long deadline = timed ? System.nanoTime() + nanos : 0L; WaitNode q = null; boolean queued = false; for (;;) { if (Thread.interrupted()) { removeWaiter(q); throw new InterruptedException(); } int s = state; if (s > COMPLETING) { if (q != null) q.thread = null; return s; } else if (s == COMPLETING) // cannot time out yet Thread.yield(); else if (q == null) q = new WaitNode(); else if (!queued) queued = UNSAFE.compareAndSwapObject(this, waitersOffset, q.next = waiters, q); else if (timed) { nanos = deadline - System.nanoTime(); if (nanos <= 0L) { removeWaiter(q); return state; } LockSupport.parkNanos(this, nanos); } else LockSupport.park(this); } }