二十、FutureTask 原理

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20.1 一个例子

    static class Task implements Callable<Integer> {
        @Override
        public Integer call() throws Exception {
            System.out.println("子线程在进行计算");
            Thread.sleep(1000);
            int sum = 0;
            for (int i = 0; i < 100; i++)
                sum += i;
            return sum;
        }
    }

    public static void main(String[] args) throws InterruptedException {

        ExecutorService executor = Executors.newCachedThreadPool();
        Task task = new Task();
        FutureTask<Integer> futureTask = new FutureTask<Integer>(task);
        executor.submit(futureTask);

        System.out.println("主线程在执行任务");

        try {
            System.out.println("task运行结果" + futureTask.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

        System.out.println("所有任务执行完毕");
        executor.shutdown();

    }

如上代码主线程会在futureTask.get()出阻塞直到task任务执行完毕，并且会返回结果。

20.1 原理

先看下类图结构

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FutureTask 内部有一个state用来展示任务的状态,并且是volatile修饰的：

/** Possible state transitions:
 * NEW -> COMPLETING -> NORMAL 正常的状态转移
 * NEW -> COMPLETING -> EXCEPTIONAL 异常
 * NEW -> CANCELLED 取消
 * NEW -> INTERRUPTING -> INTERRUPTED 中断
 */
 
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;

其中构造FutureTask实例时候状态为new

    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;      
    }

把FutureTask提交到线程池或者线程执行start时候会调用run方法：

public void run() {

    //如果当前不是new状态，或者当前cas设置当前线程失败则返回，只有一个线程可以成功。
    if (state != NEW ||
        !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                     null, Thread.currentThread()))
        return;
    try {
        //当前状态为new 则调用任务的call方法执行任务
        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);完成NEW -> COMPLETING -> EXCEPTIONAL 状态转移
            }

            //执行任务成功则保存结果更新状态，unpark所有等待线程。
            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) {
    //状态从new->COMPLETING
    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        outcome = v;
        //状态从COMPLETING-》NORMAL
        UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
        //unpark所有等待线程。
        finishCompletion();
    }
}

任务提交后，会调用 get方法获取结果，这个get方法是阻塞的。

   public V get() throws InterruptedException, ExecutionException {
        int s = state;
        //如果当前状态是new或者COMPLETING则等待，因为位normal或者exceptional时候才说明数据计算完成了。
        if (s <= COMPLETING)
            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 (;;) {

        //如果被中断，则抛异常
        if (Thread.interrupted()) {
            removeWaiter(q);
            throw new InterruptedException();
        }

        //组建单列表
        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)
            q = new WaitNode();
        else if (!queued)
            queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                 q.next = waiters, q);
        else if (timed) {


            nanos = deadline - System.nanoTime();
            //超时则返回
            if (nanos <= 0L) {
                removeWaiter(q);
                return state;
            }
            //否者设置park超时时间
            LockSupport.parkNanos(this, nanos);
        }
        else
            //直接挂起当前线程
            LockSupport.park(this);
    }
}

    private V report(int s) throws ExecutionException {
        Object x = outcome;
        if (s == NORMAL)
            return (V)x;
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }

在submit任务后还可以调用futuretask的cancel来取消任务：

    public boolean cancel(boolean mayInterruptIfRunning) {
        //只有任务是new的才能取消
        if (state != NEW)
            return false;
       //运行时允许中断
        if (mayInterruptIfRunning) {
           //完成new->INTERRUPTING
            if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING))
                return false;
            Thread t = runner;
            if (t != null)
                t.interrupt();
            //完成INTERRUPTING->INTERRUPTED
            UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state
        }
       //不允许中断则直接new->CANCELLED
        else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED))
            return false;
        finishCompletion();
        return true;
    }

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