并发工具类：Future获取异步执行结果

有了Runnable为什么要搞个Callable?

public interface Future<V> {

	// 取消任务的执行
    boolean cancel(boolean mayInterruptIfRunning);

	// 任务是否已经取消
    boolean isCancelled();

	// 任务是否已经完成
    boolean isDone();

	// 获取任务执行结果，会阻塞线程
    V get() throws InterruptedException, ExecutionException;

	// 超时获取任务执行结果，会阻塞线程
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

public void run() {
	// 状态不是 NEW 说明任务没被执行过，或者已经被取消
    if (state != NEW ||
        !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                     null, Thread.currentThread()))
        return;
    try {
        Callable<V> c = callable;
        if (c != null && state == NEW) {
            V result;
            // 是否正常结束
            boolean ran;
            try {
            	// 调用 Callable 的 call 方法
                result = c.call();
                ran = true;
            } catch (Throwable ex) {
                result = null;
                ran = false;
                // 保存 call 方法抛出的异常
                setException(ex);
            }
            if (ran)
            	// 保存 call 方法的执行结果
                set(result);
        }
    } finally {
        runner = null;
        int s = state;
        // 任务被中断，则执行中断处理
        if (s >= INTERRUPTING)
            handlePossibleCancellationInterrupt(s);
    }
}

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

// 保存异常结果
protected void setException(Throwable t) {
    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        outcome = t;
        UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
        finishCompletion();
    }
}

因为有可能有多个线程来get，所以封装成WaitNode，放到阻塞队列中

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 {
    // 计算等待截止时间
    final long deadline = timed ? System.nanoTime() + nanos : 0L;
    WaitNode q = null;
    boolean queued = false;
    for (;;) {
    	// 如果当前线程被中断，则在等待队列中删除该节点，并抛出 InterruptedException
        if (Thread.interrupted()) {
            removeWaiter(q);
            throw new InterruptedException();
        }

        int s = state;
        // 状态大于 COMPLETING 说明已经达到某个最终状态（正常结束/异常结束/取消）
        // 把 Thread 置为空，并返回结果
        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) {
        	// 如果设置超时时间
        	// 时间到，则不再等待结果
            nanos = deadline - System.nanoTime();
            if (nanos <= 0L) {
                removeWaiter(q);
                return state;
            }
            // 阻塞等待特定时间
            LockSupport.parkNanos(this, nanos);
        }
        else
        	// 挂起当前线程，直到被其他线程唤醒
            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
        // 需要中断任务执行线程
        if (mayInterruptIfRunning) {
            try {
                Thread t = runner;
                if (t != null)
                    t.interrupt();
            } finally { // final state
                // 修改最终状态为 INTERRUPTED
                UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
            }
        }
    } finally {
        finishCompletion();
    }
    return true;
}

参考博客

好文
[0]https://www.pdai.tech/md/java/thread/java-thread-x-juc-executor-FutureTask.html
[1]https://www.cnblogs.com/dolphin0520/p/3949310.html
[2]https://jishuin.proginn.com/p/763bfbd29dbd