并發工具類：Future擷取異步執行結果

有了Runnable為什麼要搞個Callable?

public interface Future<V> {

    // 取消任務的執行
    boolean cancel(boolean mayInterruptIfRunning);

    // 任務是否已經取消
    boolean isCancelled();

    // 任務是否已經完成
    boolean isDone();

    // 擷取任務執行結果，會阻塞線程
    V get() throws InterruptedException, ExecutionException;

    // 逾時擷取任務執行結果，會阻塞線程
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}      

public void run() {
    // 狀态不是 NEW 說明任務沒被執行過，或者已經被取消
    if (state != NEW ||
        !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                     null, Thread.currentThread()))
        return;
    try {
        Callable<V> c = callable;
        if (c != null && state == NEW) {
            V result;
            // 是否正常結束
            boolean ran;
            try {
                // 調用 Callable 的 call 方法
                result = c.call();
                ran = true;
            } catch (Throwable ex) {
                result = null;
                ran = false;
                // 儲存 call 方法抛出的異常
                setException(ex);
            }
            if (ran)
                // 儲存 call 方法的執行結果
                set(result);
        }
    } finally {
        runner = null;
        int s = state;
        // 任務被中斷，則執行中斷處理
        if (s >= INTERRUPTING)
            handlePossibleCancellationInterrupt(s);
    }
}      

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

// 儲存異常結果
protected void setException(Throwable t) {
    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        outcome = t;
        UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
        finishCompletion();
    }
}      

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 {
    // 計算等待截止時間
    final long deadline = timed ? System.nanoTime() + nanos : 0L;
    WaitNode q = null;
    boolean queued = false;
    for (;;) {
        // 如果目前線程被中斷，則在等待隊列中删除該節點，并抛出 InterruptedException
        if (Thread.interrupted()) {
            removeWaiter(q);
            throw new InterruptedException();
        }

        int s = state;
        // 狀态大于 COMPLETING 說明已經達到某個最終狀态（正常結束/異常結束/取消）
        // 把 Thread 置為空，并傳回結果
        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) {
            // 如果設定逾時時間
            // 時間到，則不再等待結果
            nanos = deadline - System.nanoTime();
            if (nanos <= 0L) {
                removeWaiter(q);
                return state;
            }
            // 阻塞等待特定時間
            LockSupport.parkNanos(this, nanos);
        }
        else
            // 挂起目前線程，直到被其他線程喚醒
            LockSupport.park(this);
    }
}      

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
}      

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
        // 需要中斷任務執行線程
        if (mayInterruptIfRunning) {
            try {
                Thread t = runner;
                if (t != null)
                    t.interrupt();
            } finally { // final state
                // 修改最終狀态為 INTERRUPTED
                UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
            }
        }
    } finally {
        finishCompletion();
    }
    return true;
}      

參考部落格