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();
        }
    }