FutureTask是怎样获取到异步执行结果的？

    // AbstractExecutorService
    public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        // 包装一层结果，RunnableFuture, 也实现了 Runnable 接口
        // 实际上就是 FutureTask 
        RunnableFuture<T> ftask = newTaskFor(task);
        // 然后交由 线程池进行调用任务了，即由 jvm 调用执行 Thread
        // 具体执行逻辑，在我之前的文章中也已经阐述，自行搜索
        execute(ftask);
        // 最后，把包装对象返回即可
        return ftask;
    }

    /**
     * Returns a {@code RunnableFuture} for the given callable task.
     *
     * @param callable the callable task being wrapped
     * @param <T> the type of the callable‘s result
     * @return a {@code RunnableFuture} which, when run, will call the
     * underlying callable and which, as a {@code Future}, will yield
     * the callable‘s result as its result and provide for
     * cancellation of the underlying task
     * @since 1.6
     */
    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
        return new FutureTask<T>(callable);
    }
    
    // FutureTask 实例化
    /**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Callable}.
     *
     * @param  callable the callable task
     * @throws NullPointerException if the callable is null
     */
    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

    // FutureTask.run()
    public void run() {
        // 不允许多次执行
        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 {
                    // 直接调用 call() 方法，获取返回结果
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    // 执行异常，包装异常信息
                    setException(ex);
                }
                // 将结果设置到当前的 FutureTask 实例变量 outcome 中，这样当前线程就可以获取了
                // 设置结果时，会将 state 同时变更
                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);
        }
    }
    
    /**
     * Sets the result of this future to the given value unless
     * this future has already been set or has been cancelled.
     *
     * <p>This method is invoked internally by the {@link #run} method
     * upon successful completion of the computation.
     *
     * @param v the value
     */
    protected void set(V v) {
        // 设置结果时，还不代表可以直接获取了，还有后续工作，所以设置为 COMPLETING 中间态
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            // 通知线程执行完成等后续工作
            finishCompletion();
        }
    }
    
    /**
     * Removes and signals all waiting threads, invokes done(), and
     * nulls out callable.
     */
    private void finishCompletion() {
        // assert state > COMPLETING;
        // 外部看起来是一个 for, 实际上只会执行一次, 目的是为了保证内部的锁获取成功
        // 如果有其他线程成功后, waiters也就会为null, 从而自身也一起退出了
        for (WaitNode q; (q = waiters) != null;) {
            // 保证更新的线程安全性
            // 只要锁获取成功，就会一次性更新完成，不会失败
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    // 依次唤醒等待的线程
                    if (t != null) {
                        q.thread = null;
                        LockSupport.unpark(t);
                    }
                    WaitNode next = q.next;
                    // 只有把所有 wait 线程都通知完后，才可以退出
                    if (next == null)
                        break;
                    q.next = null; // unlink to help gc
                    q = next;
                }
                break;
            }
        }

        // 完成后钩子方法，默认为空，如果需要做特殊操作可以自行复写即可
        done();

        callable = null;        // to reduce footprint
    }
    // 简单看一下异常信息的包装，与 正常结束方法类似，只是将 outcome 设置为了异常信息，完成状态设置为 EXCEPTIONAL
    /**
     * Causes this future to report an {@link ExecutionException}
     * with the given throwable as its cause, unless this future has
     * already been set or has been cancelled.
     *
     * <p>This method is invoked internally by the {@link #run} method
     * upon failure of the computation.
     *
     * @param t the cause of failure
     */
    protected void setException(Throwable t) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = t;
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }

    // Future.get()
    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        // 只要状态值小于 COMPLETING, 就说明任务还未完成, 去等待完成
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        // 只要等待完成, 再去把结果取回即可
        return report(s);
    }
    // 我们先看一下结果的取回逻辑 report(), 果然不出意外的简单, 只管取 outcome 即可
    /**
     * Returns result or throws exception for completed task.
     *
     * @param s completed state value
     */
    @SuppressWarnings("unchecked")
    private V report(int s) throws ExecutionException {
        Object x = outcome;
        // 正常执行完成, 直接返回
        if (s == NORMAL)
            return (V)x;
        // 此处会包含 CANCELLED/INTERRUPTING/INTERRUPTED
        if (s >= CANCELLED)
            throw new CancellationException();
        // 业务异常则会被包装成 ExecutionException
        throw new ExecutionException((Throwable)x);
    }
    // 看到取结果这么简单，那么 等待结束的逻辑的呢？看起来好像没那么简单了
    /**
     * 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 (;;) {
            // 中断则退出
            if (Thread.interrupted()) {
                // 因 q 是链表的头，所以会移除所有的等待队列，即中断是对所有线程的
                removeWaiter(q);
                throw new InterruptedException();
            }

            int s = state;
            // 执行完成后，将线程置空即可，删除工作会有其他地方完成
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            // 正在处理结果，稍作等待即可
            else if (s == COMPLETING) // cannot time out yet
                Thread.yield();
            // 其他情况，先创建自己的等待标识，以便在下一次循环中进行入队等待
            else if (q == null)
                q = new WaitNode();
            // 进行一次入队等待，将 q 作为头节点
            else if (!queued)
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            // 限时的等待，等待超时后，直接返回当前状态即可
            else if (timed) {
                nanos = deadline - System.nanoTime();
                if (nanos <= 0L) {
                    removeWaiter(q);
                    return state;
                }
                // 最长等待预定时间
                LockSupport.parkNanos(this, nanos);
            }
            // 此处进行无限期等待，但当被唤醒时，一定有状态变更的时候，应该会在下一个周期结束循环
            else
                LockSupport.park(this);
        }
    }

    // Thread.join(), 通过 isAlive() 判断是否完成
    /**
     * Waits for this thread to die.
     *
     * <p> An invocation of this method behaves in exactly the same
     * way as the invocation
     *
     * <blockquote>
     * {@linkplain #join(long) join}{@code (0)}
     * </blockquote>
     *
     * @throws  InterruptedException
     *          if any thread has interrupted the current thread. The
     *          <i>interrupted status</i> of the current thread is
     *          cleared when this exception is thrown.
     */
    public final void join() throws InterruptedException {
        join(0);
    }

    /**
     * Waits at most {@code millis} milliseconds for this thread to
     * die. A timeout of {@code 0} means to wait forever.
     *
     * <p> This implementation uses a loop of {@code this.wait} calls
     * conditioned on {@code this.isAlive}. As a thread terminates the
     * {@code this.notifyAll} method is invoked. It is recommended that
     * applications not use {@code wait}, {@code notify}, or
     * {@code notifyAll} on {@code Thread} instances.
     *
     * @param  millis
     *         the time to wait in milliseconds
     *
     * @throws  IllegalArgumentException
     *          if the value of {@code millis} is negative
     *
     * @throws  InterruptedException
     *          if any thread has interrupted the current thread. The
     *          <i>interrupted status</i> of the current thread is
     *          cleared when this exception is thrown.
     */
    public final synchronized void join(long millis)
    throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        // 无限期等待
        if (millis == 0) {
            while (isAlive()) {
                // 这是个 native 方法，即由jvm进行控制
                // Thread 任务执行完成后，将进行 notifyAll()
                // 同理下面的限时等待
                wait(0);
            }
        } else {
            // 限时等待
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

    // 不指定返回值的 Runnable, 此处的返回值一定 void
    public Future<?> submit(Runnable task);
    // 指定返回值的 Runnable, 由 T 进行返回值接收
    public <T> Future<T> submit(Runnable task, T result);

    // AbstractExecutorService.submit()
    public Future<?> submit(Runnable task) {
        if (task == null) throw new NullPointerException();
        // 明确返回值为 Void
        RunnableFuture<Void> ftask = newTaskFor(task, null);
        execute(ftask);
        return ftask;
    }
    // 同样使用 FutureTask 进行封装，只是调用了不同的构造器
    protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
        return new FutureTask<T>(runnable, value);
    }
    // FutureTask, 使用 Executors 工具类生成一个 callable, 屏蔽掉 Callable 与 Runnable 的差异 
    public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // ensure visibility of callable
    }
    // Executors 使用一个适合器类将 Runnable 封装成 Callable
    /**
     * Returns a {@link Callable} object that, when
     * called, runs the given task and returns the given result.  This
     * can be useful when applying methods requiring a
     * {@code Callable} to an otherwise resultless action.
     * @param task the task to run
     * @param result the result to return
     * @param <T> the type of the result
     * @return a callable object
     * @throws NullPointerException if task null
     */
    public static <T> Callable<T> callable(Runnable task, T result) {
        if (task == null)
            throw new NullPointerException();
        return new RunnableAdapter<T>(task, result);
    }
    // 而 RunnableAdapter 也是很简单, 仅将 call() 转而调用 run() 方法即可
    /**
     * A callable that runs given task and returns given result
     */
    static final class RunnableAdapter<T> implements Callable<T> {
        final Runnable task;
        final T result;
        RunnableAdapter(Runnable task, T result) {
            this.task = task;
            this.result = result;
        }
        public T call() {
            task.run();
            return result;
        }
    }