Java并发之Future

public void run() {
    // 状态不为new 或者 执行线程不为空，结束 run
    // CAS 更新
    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)
                // call 执行成功，设置结果
                set(result);
        }
    } finally {
        // runner must be non-null until state is settled to prevent concurrent calls to run()
        // CAS的条件
        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) {
    // cas 状态更新 new -> completing
    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        // future的结果
        outcome = v;
        // cas 状态更新 completing -> normal
        UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
        // 唤醒阻塞在get（） 方法上面的线程
        finishCompletion();
    }
}
protected void setException(Throwable t) {
    // 如果抛出异常，设置到当前任务的执行状态为正在运行
    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        // 将异常值赋值给outcome
        outcome = t;
        // 将当前线程的最终状态设置为执行异常
        UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
        // 完成任务之后，做清理，详细源码分析，参考： 5.1.3、 finishCompletion()
        finishCompletion();
    }
}

protected boolean runAndReset() {
    if (state != NEW ||
        !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                        null, Thread.currentThread()))
        return false;
    boolean ran = false;
    int s = state;
    try {
        Callable<V> c = callable;
        if (c != null && s == NEW) {
            try {
                c.call(); // don't set result
                ran = true;
            } catch (Throwable ex) {
                setException(ex);
            }
        }
    } finally {
        runner = null;
        s = state;
        if (s >= INTERRUPTING)
            handlePossibleCancellationInterrupt(s);
    }
    return ran && s == NEW;
}

public V get() throws InterruptedException, ExecutionException {
    int s = state;
    // 未完成，阻塞等待
    if (s <= COMPLETING)
        s = awaitDone(false, 0L);
    return report(s);
}
public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
    if (unit == null)
        throw new NullPointerException();
    int s = state;
    if (s <= COMPLETING &&
        (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
        throw new TimeoutException();
    return report(s);
}
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);
}

// ***********************************************************************************
 private int awaitDone(boolean timed, long nanos) throws InterruptedException {
    // 是否有时间限制
    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();
        // 下面的情况 s == new
        else if (q == null)
            q = new WaitNode();
        else if (!queued)
        // 将当前线程加入等待队列
            queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                    q.next = waiters, q);
        else if (timed) {
            nanos = deadline - System.nanoTime();
            if (nanos <= 0L) {
                removeWaiter(q);
                return state;
            }
            // 挂起
            LockSupport.parkNanos(this, nanos);
        }
        else
            // 挂起
            LockSupport.park(this);
    }
}
// =========================================================================
static final class WaitNode {
    volatile Thread thread;
    volatile WaitNode next;
    WaitNode() {
        thread = Thread.currentThread();
    }
}

// 终止任务
public boolean cancel(boolean mayInterruptIfRunning) {
    // 状态不为 new 或者修改状态失败，cas
    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; // runner -- 执行callable 的线程
                if (t != null)
                    t.interrupt();
            } finally { // final state
                UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
            }
        }
    } finally {
        //将阻塞在等待这个任务完成的线程唤醒，具体操作是LockSupport.unpark(t)
        finishCompletion();
    }
    return true;
}
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
}