Future接口和实现Future接口的FutureTask，代表异步计算的结果，Future使用示例如下：
ThreadPoolExecutor executor = new ThreadPoolExecutor(5, 10,
60, TimeUnit.SECONDS, new LinkedBlockingQueue<>());
Future future = executor.submit(() -> {
System.out.println("hello world");
return "hello world";
});
System.out.println(future.get());
Future接口声明如下：
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FutureTask除了实现Future接口外，还实现了Runnable接口。因此，FutureTask可以交给Executor执行，也可以由调用线程直接执行（FutureTask.run()）。根据FutureTask.run()方法被执行的时机，FutureTask可以处于以下3种状态：未启动、运行中、已完成。
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当FutureTask处于未启动或已启动状态时，执行FutureTask.get()方法将导致调用线程阻塞；当FutureTask处于已完成状态时，执行FutureTask.get()方法将导致调用线程立即返回结果或抛出异常。
当FutureTask处于未启动状态时，执行FutureTask.cancel()方法将导致此任务永远不会被执行；
当FutureTask处于已启动状态时，执行FutureTask.cancel（true）方法将以中断执行此任务线程的方式来试图停止任务；
当FutureTask处于已启动状态时，执行FutureTask.cancel（false）方法将不会对正在执行此任务的线程产生影响（让正在执行的任务运行完成）；
当FutureTask处于已完成状态时，执行FutureTask.cancel（…）方法将返回false。
FutureTask的生命周期如下：
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Future.get() 阻塞/唤醒原理
执行future.get()时，如果对应线程还未执行完，则会阻塞当前线程，以FutureTask为例，FutureTask中有一个int型的状态标志，表示future对应线程的运行状态。
/**
* Possible state transitions:
* NEW -> COMPLETING -> NORMAL
* NEW -> COMPLETING -> EXCEPTIONAL
* NEW -> CANCELLED
* NEW -> INTERRUPTING -> INTERRUPTED
*/
private volatile int state;
private static final int NEW = 0;
private static final int COMPLETING = 1;
private static final int NORMAL = 2;
private static final int EXCEPTIONAL = 3;
private static final int CANCELLED = 4;
private static final int INTERRUPTING = 5;
private static final int INTERRUPTED = 6;
当调用FutureTask.get()时，如果Future对应的任务已完成（正常执行完成或者抛出异常），执行返回；如果Future对应的任务未执行完成，则会将当前线程封装成一个NodeWait，以CAS方式添加到FutureTask.waiters链表上（单向链表，新节点都会作为head node添加上），然后会阻塞当前线程（包括超时阻塞）。
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING) // 线程未执行完成
s = awaitDone(false, 0L);
return report(s);
}
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) { // 线程已运行完成
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING) // cannot time out yet
Thread.yield(); // future task已完成，正在赋值outcome，get()返回的值就是outcome，这时不用加入WaitNode即可
else if (q == null)
q = new WaitNode(); // 生成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);
}
}
private V report(int s) throws ExecutionException {
Object x = outcome;
if (s == NORMAL) // 正常执行结束
return (V)x;
if (s >= CANCELLED) // 已取消
throw new CancellationException();
throw new ExecutionException((Throwable)x); // 抛出异常
}
在任务执行（run()方法）中，调用result = callable.call方法，正常执行完毕后调用set(result)设置Future结果；出现异常则调用setException(ex)。最后会调用finishCompletion()来唤醒阻塞在Future的所有线程。
设置完数据之后（不管是正常数据还是对应异常），当等待数据的线程来get时，就会返回或者直接给它抛异常；如果当线程已经get过并阻塞在这里时，FutureTask需要将这些线程唤醒起来。
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
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 (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
protected void setException(Throwable t) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}
// 唤醒所有等待线程
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
LockSupport.unpark(t);
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}
小结
FutureTask中的waiters是一个单向链表，如果多个线程阻塞在该Future上，最新阻塞的线程排列在链表前面，唤醒线程时依次从前到后遍历链表唤醒线程，这样处理貌似对最开始阻塞在Future上的线程不太公平哈，因为最开始阻塞的线程是到最后才被唤醒的。
