FutureTask源碼淺析

###　FutureTask
用Runnable提交異步任務是沒有返回值的，如果需要返回值的異步任務,使用Callable接口。
例子:

public static class CallAbleTask implements Callable<String> {

        @Override
        public String call() throws Exception {
            String ret = "this is callable task test";
            return ret;
        }
        
}

上面有返回值的實現原理是什麼？

答案是使用FutureTask。
FutureTask的構造函數:將Callable實現類封裝爲FutureTask，它實現了Runnable接口，之後調用
execute(futureTask)即可執行，下面分析futureTask源碼。

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

任務有成員變量state記錄狀態：

• 任務的初始狀態都是NEW, 這一點是構造函數保證的，我們後面分析構造函數的時候再講；
• 任務的終止狀態有4種：
  • NORMAL：任務正常執行完畢
  • EXCEPTIONAL：任務執行過程中發生異常
  • CANCELLED：任務被取消
  • INTERRUPTED：任務被中斷
• 任務的中間狀態有2種：

  • COMPLETING 正在設置任務結果

  • INTERRUPTING 正在中斷運行任務的線程

FutureTaskd的run代碼,用來運行callable實現類的call。該方法將cal的返回結果設置到對象變量result裏，當運行結束後將task的狀態設置爲NORMAL(final state)，喚醒因提早獲取結果而阻塞的線程。

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 {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    //調完call方法,開始把結果放入
                    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);
        }
    }

調完call方法,開始把得到的result存入對象變量裏

protected void set(V v) {
        //更新對象的任務狀態
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            //結果放入outcome
            outcome = v;
            //更新對象的任務狀態
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            //遍歷整個等待隊列鏈表,喚醒線程,gc掉這些等待線程node
            finishCompletion();
        }
}

遍歷整個等待隊列鏈表,喚醒線程,gc掉這些等待線程node

private void finishCompletion() {
        // assert state > COMPLETING;
        //遍歷等待隊列
        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;
            }
        }
        //子類可以覆蓋此函數,callable任務執行完就會執行done();
        done();

        callable = null;        // to reduce footprint
}

獲取結果

public V get() throws InterruptedException, ExecutionException {
    int s = state;
    if (s <= COMPLETING)
    //s<=completing即任務還沒運行完,調用awaitDone，掛起線程
        s = awaitDone(false, 0L);
    return report(s);
}

等待任務完成(運行態進入阻塞狀態)

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
            //s == COMPLETING即任務剛剛運行完,讓出cpu等一會
            //“運行狀態”進入到“就緒狀態”。然後再在for循環內
                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);
        }
}