Java并发编程（五）--异步计算
表征异步计算的Future：

Future接口有一个get方法，这个方法会执行到计算结束才返回，它可以被中断取消。从它的实现类来看FutureTask来看，其内部也有一个Sync的同步控制类，任务提交的时候会执行

void innerRun() {            if (!compareAndSetState(0, RUNNING))                return;            try {                runner = Thread.currentThread();                if (getState() == RUNNING) // recheck after setting thread                    innerSet(callable.call());                else                    releaseShared(0); // cancel            } catch (Throwable ex) {                innerSetException(ex);            }        }

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调用innerSet的代码：

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void innerSet(V v) {    for (;;) {int s = getState();if (s == RAN)    return;                if (s == CANCELLED) {    // aggressively release to set runner to null,    // in case we are racing with a cancel request    // that will try to interrupt runner                    releaseShared(0);                    return;                }if (compareAndSetState(s, RAN)) {                    result = v;                    releaseShared(0);                    done();    return;                }            }        }

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一直循环执行，直到完毕后将状态设置为RAN，同时释放锁。

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用户调用Future.get()方法时，会尝试加锁

V innerGet() throws InterruptedException, ExecutionException {            acquireSharedInterruptibly(0);            if (getState() == CANCELLED)                throw new CancellationException();            if (exception != null)                throw new ExecutionException(exception);            return result;        }

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任务执行完毕或者被取消时，释放锁，此时返回result。通过这种方式实现异步计算。