参考：https://www.cnblogs.com/cz123/p/7693064.html

Future模式异步取得返回值也要阻塞，属于异步（多线程）阻塞的情形。
Future仅仅是个接口：

public interface Future<V> {
    boolean cancel(boolean mayInterruptIfRunning); 
    boolean isCancelled();   
    boolean isDone();
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

而FutureTask实现了RunnableFuture<V>(RunnableFuture extends Runnable,Future<V>)

FutureTask实现的run方法：

public void run() {
        if (state != NEW ||
            !U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    set(result);
            }
        } finally {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }


protected void set(V v) {
        if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
            outcome = v;
            U.putOrderedInt(this, STATE, NORMAL); // final state
            finishCompletion();
        }
    }

将result赋值给outcome;

如果还没有执行完则线程阻塞，直到取得返回值。。LockSupport这个不太了解。

private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        // The code below is very delicate, to achieve these goals:
        // - call nanoTime exactly once for each call to park
        // - if nanos <= 0L, return promptly without allocation or nanoTime
        // - if nanos == Long.MIN_VALUE, don't underflow
        // - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic
        //   and we suffer a spurious wakeup, we will do no worse than
        //   to park-spin for a while
        long startTime = 0L;    // Special value 0L means not yet parked
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            int s = state;
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (s == COMPLETING)
                // We may have already promised (via isDone) that we are done
                // so never return empty-handed or throw InterruptedException
                Thread.yield();
            else if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }
            else if (q == null) {
                if (timed && nanos <= 0L)
                    return s;
                q = new WaitNode();
            }
            else if (!queued)
                queued = U.compareAndSwapObject(this, WAITERS,
                                                q.next = waiters, q);
            else if (timed) {
                final long parkNanos;
                if (startTime == 0L) { // first time
                    startTime = System.nanoTime();
                    if (startTime == 0L)
                        startTime = 1L;
                    parkNanos = nanos;
                } else {
                    long elapsed = System.nanoTime() - startTime;
                    if (elapsed >= nanos) {
                        removeWaiter(q);
                        return state;
                    }
                    parkNanos = nanos - elapsed;
                }
                // nanoTime may be slow; recheck before parking
                if (state < COMPLETING)
                    LockSupport.parkNanos(this, parkNanos);
            }
            else
                LockSupport.park(this);
        }
    }

确保取得返回值后，就调用report(int s); 返回执行结果。

private V report(int s) throws ExecutionException {
        Object x = outcome;
        if (s == NORMAL)
            return (V)x;
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }

因为FutureTask实现了Runable，实际上还是回到了常见的多线程。只不过可以在没有手动获取执行结果之前可以干点其他事情（异步），需要取得结果时若还没有执行完，则阻塞。