哈希轮

本文分析 netty 的 HashedWheelTimer 主要流程

示例：

public static void main(String[] args) {
    // 指定 tick 的间隔时间，tick 的数量（规格化成 2 的幂之后，哈希数组的长度）
    HashedWheelTimer timer = new HashedWheelTimer(Executors.defaultThreadFactory(), 100,
            TimeUnit.MILLISECONDS, 32, true, 2);

    timer.newTimeout(new TimerTask() {
        @Override
        public void run(Timeout timeout) throws Exception {
            System.out.println("zhang");
        }
    }, 10, TimeUnit.SECONDS);

//        timer.stop();
}

结论：

一个哈希数组，数组元素是双向链表，链表元素是定时任务
一个队列，提交的定时任务先放到队列中
一个线程，一个 tick 一个 tick 地走，把队列中的任务挂到哈希槽上，触发过期任务，remainingRounds 减一
参数：tick 时间间隔，tick 总数。

提交定时任务，先把任务放入队列中

// 提交的任务首先加入到 queue 中
private final Queue<HashedWheelTimeout> timeouts = PlatformDependent.newMpscQueue();

// io.netty.util.HashedWheelTimer#newTimeout
public Timeout newTimeout(TimerTask task, long delay, TimeUnit unit) {
    long pendingTimeoutsCount = pendingTimeouts.incrementAndGet();

    if (maxPendingTimeouts > 0 && pendingTimeoutsCount > maxPendingTimeouts) {
        pendingTimeouts.decrementAndGet();
        throw new RejectedExecutionException("Number of pending timeouts ("
            + pendingTimeoutsCount + ") is greater than or equal to maximum allowed pending "
            + "timeouts (" + maxPendingTimeouts + ")");
    }

    start();

    long deadline = System.nanoTime() + unit.toNanos(delay) - startTime;
    // Guard against overflow.
    if (delay > 0 && deadline < 0) {
        deadline = Long.MAX_VALUE;
    }
    HashedWheelTimeout timeout = new HashedWheelTimeout(this, task, deadline);
    timeouts.add(timeout);
    return timeout;
}

线程死循环

// io.netty.util.HashedWheelTimer.Worker#run
public void run() {
    startTime = System.nanoTime();
    if (startTime == 0) {
        startTime = 1;
    }
    startTimeInitialized.countDown();
    do {
        // 等待下一个 tick
        final long deadline = waitForNextTick();
        if (deadline > 0) {
            // 当前 tick 数取模
            int idx = (int) (tick & mask);
            // 处理取消的 task
            processCancelledTasks();
            // 根据 idx 定位到 bucket
            HashedWheelBucket bucket = wheel[idx];
            // 把 queue 中任务转移到 bucket 的链表中
            transferTimeoutsToBuckets();
            // 遍历 bucket 上的定时任务，过期则触发，否则 remainingRounds 减一
            bucket.expireTimeouts(deadline);
            tick++;
        }
    } while (WORKER_STATE_UPDATER.get(HashedWheelTimer.this) == WORKER_STATE_STARTED);

    // Fill the unprocessedTimeouts so we can return them from stop() method.
    for (HashedWheelBucket bucket: wheel) {
        bucket.clearTimeouts(unprocessedTimeouts);
    }
    for (;;) {
        HashedWheelTimeout timeout = timeouts.poll();
        if (timeout == null) {
            break;
        }
        if (!timeout.isCancelled()) {
            unprocessedTimeouts.add(timeout);
        }
    }
    processCancelledTasks();
}

// io.netty.util.HashedWheelTimer.Worker#transferTimeoutsToBuckets
private void transferTimeoutsToBuckets() {
    // 最多转移十万个任务
    for (int i = 0; i < 100000; i++) {
        HashedWheelTimeout timeout = timeouts.poll();
        if (timeout == null) {
            // 队列为空，退出
            break;
        }
        if (timeout.state() == HashedWheelTimeout.ST_CANCELLED) {
            // 当前任务被取消，跳过
            continue;
        }
        // 任务截止时间 / tick 间隔
        long calculated = timeout.deadline / tickDuration;
        // 任务需要等待多少轮
        timeout.remainingRounds = (calculated - tick) / wheel.length;
        // 取 calculated 和当前 tick 数的较大值
        final long ticks = Math.max(calculated, tick); // Ensure we don't schedule for past.
        // 取模决定 bucket 位置
        int stopIndex = (int) (ticks & mask);

        HashedWheelBucket bucket = wheel[stopIndex];
        // 加入到 bucket 的链表
        bucket.addTimeout(timeout);
    }
}

触发定时任务，后者 remainingRounds 减一

// io.netty.util.HashedWheelTimer.HashedWheelBucket#expireTimeouts
public void expireTimeouts(long deadline) {
    HashedWheelTimeout timeout = head;

    // process all timeouts
    while (timeout != null) {
        HashedWheelTimeout next = timeout.next;
        if (timeout.remainingRounds <= 0) {
            // 从链表中删除任务
            next = remove(timeout);
            if (timeout.deadline <= deadline) {
                // 触发定时任务
                timeout.expire();
            } else {
                // The timeout was placed into a wrong slot. This should never happen.
                throw new IllegalStateException(String.format(
                        "timeout.deadline (%d) > deadline (%d)", timeout.deadline, deadline));
            }
        } else if (timeout.isCancelled()) {
            // 任务被取消，从链表中删除
            next = remove(timeout);
        } else {
            // 等待的轮数减一
            timeout.remainingRounds --;
        }
        timeout = next;
    }
}