FutureTask是java中一個(gè)用來實(shí)現(xiàn)可取消的同步計(jì)算的類?？扇∠且?yàn)檫@個(gè)類可以調(diào)用cancel方法取消計(jì)算(其實(shí)也是有條件的取消)，同步是因?yàn)檎{(diào)用get方法獲取計(jì)算結(jié)果的時(shí)候需要等待計(jì)算完成。

1.簡(jiǎn)單使用

Callable<Integer> callable = new Callable<Integer>()
{
    @Override
    public Integer call() throws Exception
    {
        int sum = 0;
        int value = 10;
        while (value-- >= 0)
        {
            sum += value;
            try
            {
                Thread.sleep(500);
            }
            catch (InterruptedException e)
            {
                e.printStackTrace();
            }
        }
        System.out.println("done");
        return sum;
    }
};


FutureTask<Integer> task = new FutureTask<Integer>(callable);
new Thread(task).start();
try
{
    System.out.println(task.get());
}
catch (InterruptedException e)
{
    e.printStackTrace();
}
catch (ExecutionException e)
{
    e.printStackTrace();
}

FutureTask的使用比較簡(jiǎn)單，通過構(gòu)造方法傳入Callable對(duì)象或者是Runnable對(duì)象和返回值，新開一個(gè)線程執(zhí)行這個(gè)Callable對(duì)象，最后通過get方法獲取計(jì)算結(jié)果。

2.原理

從FutureTask的繼承關(guān)系上來看，F(xiàn)utureTask實(shí)現(xiàn)了RunnableFuture接口，RunnbleFuture接口又繼承了Runnable和Future接口，所以FutureTask可以看做是一個(gè)Runnable和Future的組合體。

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

從構(gòu)造方法上看，僅僅是設(shè)置了兩個(gè)成員變量。當(dāng)放到線程中去執(zhí)行的時(shí)候，看看run方法。

public void run()
{
  //1.CAS設(shè)置runner為當(dāng)前的線程
    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
            {  
             //2.執(zhí)行call的內(nèi)容
                result = c.call();
                ran = true;
            }
            catch (Throwable ex)
            {
                result = null;
                ran = false;
                setException(ex);
            }
           //3.設(shè)置執(zhí)行結(jié)果
            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);
    }
}

代碼比較簡(jiǎn)單，首先通過CAS的方式設(shè)置runner變量為當(dāng)前的線程，然后執(zhí)行callable中的call方法，執(zhí)行完成之后調(diào)用set方法，并且在fianlly中重置runner為null。再看下set方法。

protected void set(V v)
{
    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING))
    {
        outcome = v;
        UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
        finishCompletion();
    }
}

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
}

set方法中，首先通過CAS的方式設(shè)置state為Completing，然后將結(jié)果設(shè)置給outcome，在此通過CAS將state設(shè)置為NORMAL.finishCompletion方法是將通過調(diào)用get方法而導(dǎo)致阻塞的線程喚醒。

調(diào)用get方法的時(shí)候會(huì)同步等待，看看get方法。

public V get() throws InterruptedException, ExecutionException
{
    int s = state;
    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 (;;)
    {
        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)
            q = new 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);
    }
}

調(diào)用get的時(shí)機(jī)，如果是在COMPLETING狀態(tài)之后，直接調(diào)用report方法返回。如果是在任務(wù)完成之前，則會(huì)調(diào)用awaitDone方法。awaitDone方法采用一個(gè)自旋的方式來確定state的狀態(tài)，調(diào)用步奏如下：

• 新建一個(gè)WaitNode對(duì)象
• 通過CAS的方式將waitNode放到等待鏈表中
• 掛起當(dāng)前的線程

當(dāng)當(dāng)前的任務(wù)執(zhí)行完成之后，也就是調(diào)用set方法之后，state狀態(tài)被重置，并且通過finishCompletion方法喚醒通過get方法而阻塞的線程，此時(shí)get方法繼續(xù)執(zhí)行，執(zhí)行report方法。

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);
    }

report方法直接將set方法中設(shè)置的outcome返回即可。

再來看下cancel方法

public boolean cancel(boolean mayInterruptIfRunning)
{
    if (!(state == NEW &&
            UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
                                     mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
        return false;
    try      // in case call to interrupt throws exception
    {
        if (mayInterruptIfRunning)
        {
            try
            {
                Thread t = runner;
                if (t != null)
                    t.interrupt();
            }
            finally     // final state
            {
                UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
            }
        }
    }
    finally
    {
        finishCompletion();
    }
    return true;
}

調(diào)用cancel的時(shí)機(jī)決定了它的行為，如果調(diào)用的時(shí)候，任務(wù)還沒有結(jié)束，首先通過CAS的方式設(shè)置state的狀態(tài)為INTERRUPTING或者是CANCELLED，然后通過finishCompletion方法喚醒所有掛起的線程，那么在上面介紹的awaitDone方法將會(huì)跳出循環(huán)，進(jìn)入report方法，拋出異常。如果任務(wù)已經(jīng)結(jié)束，那么cancel方法直接返回false，表示cancel失敗。所有cancel方法只是取消所有get方法的掛起，立刻返回當(dāng)前的值或者拋出異常，而不是關(guān)閉當(dāng)前的正在運(yùn)行的線程。