FutureTask实现原理

执行过程

提交的Runnable任务在内部统一被转换为Callable任务。查看submit方法的返回值，为一个Future，实际上这个Futrue为FutureTask实例，通过此实例，调用get方法，可以阻塞当前线程，直到任务运行完毕，返回结果。

整个调用链条如下所示：

worker thread -> futureTask.run() -> callable.call() -> task.run()

如果提交的是Callable任务，则只有前面三个调用。

FutureTask run 执行

    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 {
                    // 调用callable call 方法
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    // 异常处理
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    // 结果处理
                    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);
        }
    }

异常处理&结果处理

     /**
     * 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)) {
            // 执行结果
            outcome = t;
            // 设置Task 执行状态
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            // 清理waiters栈
            finishCompletion();
        }
    }

    /**
     * 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) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            // 执行结果
            outcome = v;
            // 设置Task 执行状态
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            // 清理waiters栈
            finishCompletion();
        }
    }

FutureTask get 阻塞获取结果

多线程获取结果

执行 callable.call() 过程中有多个线程调用了 同个FutureTask实例的get方法，这时候，这些线程会被阻塞，存于一个栈中。

如上图所示：线程1，2，3调用FutureTask.get方法，由于任务未执行结束，这时候，三个线程都将被阻塞休眠，FutureTask中有一个栈，用于存放等待线程，栈顶指针为 FutureTask.waiters引用。

当任务执行完毕后，会调用finishCompletion方法，迭代唤醒整个栈中的线程，这时候，各个线程都将被唤醒，并且可以顺利拿到任务的执行结果(执行结果存于 FutureTask.outcome)。

代码分析

    /**
     * 设置超时时间等待FutureTask执行结果
     * @throws CancellationException {@inheritDoc}
     */
    public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        if (unit == null)
            throw new NullPointerException();
        int s = state;
        if (s <= COMPLETING &&
            // 加入等待栈并阻塞
            (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
            throw new TimeoutException();
        return report(s);
    }


    /**
     * 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 (;;) {
            if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }

            int s = state;
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (s == COMPLETING) // cannot time out yet
                Thread.yield();
            else if (q == null)
                // 等待Node
                q = new WaitNode();
            else if (!queued)
                // 加入栈顶
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            else if (timed) {
                nanos = deadline - System.nanoTime();
                if (nanos <= 0L) {
                    // 判断休眠结束返回
                    removeWaiter(q);
                    return state;
                }
                // 阻塞休眠
                LockSupport.parkNanos(this, nanos);
            }
            else
                // 阻塞
                LockSupport.park(this);
        }
    }