- 你的猜测是正确的,你完全正确。正如你可以在“实现概述”中所读到的。
* Joining Tasks
* =============
*
* Any of several actions may be taken when one worker is waiting
* to join a task stolen (or always held) by another. Because we
* are multiplexing many tasks on to a pool of workers, we can't
* just let them block (as in Thread.join). We also cannot just
* reassign the joiner's run-time stack with another and replace
* it later, which would be a form of "continuation", that even if
* possible is not necessarily a good idea since we may need both
* an unblocked task and its continuation to progress. Instead we
* combine two tactics:
*
* Helping: Arranging for the joiner to execute some task that it
* would be running if the steal had not occurred.
*
* Compensating: Unless there are already enough live threads,
* method tryCompensate() may create or re-activate a spare
* thread to compensate for blocked joiners until they unblock.
2. ForkJoinPool.invoke和ForkJoinPool.join在任务提交的方式上完全相同。您可以在代码中看到
public <T> T invoke(ForkJoinTask<T> task) {
if (task == null)
throw new NullPointerException();
externalPush(task);
return task.join();
}
public void execute(ForkJoinTask<?> task) {
if (task == null)
throw new NullPointerException();
externalPush(task);
}
在externalPush中,可以看到任务使用ThreadLocalRandom添加到随机选择的工作队列中。此外,它使用push方法进入队列头部。
final void externalPush(ForkJoinTask<?> task) {
WorkQueue[] ws; WorkQueue q; int m;
int r = ThreadLocalRandom.getProbe();
int rs = runState;
if ((ws = workQueues) != null && (m = (ws.length - 1)) >= 0 &&
(q = ws[m & r & SQMASK]) != null && r != 0 && rs > 0 &&
U.compareAndSwapInt(q, QLOCK, 0, 1)) {
ForkJoinTask<?>[] a; int am, n, s;
if ((a = q.array) != null &&
(am = a.length - 1) > (n = (s = q.top) - q.base)) {
int j = ((am & s) << ASHIFT) + ABASE;
U.putOrderedObject(a, j, task);
U.putOrderedInt(q, QTOP, s + 1);
U.putIntVolatile(q, QLOCK, 0);
if (n <= 1)
signalWork(ws, q);
return;
}
U.compareAndSwapInt(q, QLOCK, 1, 0);
}
externalSubmit(task);
}
我不确定您的意思是什么:
并且这是否取决于调用pool.execute(task)或pool.invoke(task)的线程是外部线程还是该 fork-join 池中的线程?