HashedWheelTimer定时任务算法解析

1、原理

HashedWheelTimer是采用一种定时轮的方式来管理和维护大量的Timer调度算法.Linux 内核中的定时器采用的就是这个方案。
一个HashedWheelTimer是环形结构，类似一个时钟，分为很多槽，一个槽代表一个时间间隔，每个槽又对应一个类似Map结构的对象，使用双向链表存储定时任务，指针周期性的跳动，跳动到一个槽位，就执行该槽位的定时任务。
环形结构可以根据超时时间的 hash 值(这个 hash 值实际上就是ticks & mask)将 task 分布到不同的槽位中, 当 tick 到那个槽位时, 只需要遍历那个槽位的 task 即可知道哪些任务会超时(而使用线性结构, 你每次 tick 都需要遍历所有 task), 所以, 我们任务量大的时候, 相应的增加 wheel 的 ticksPerWheel 值, 可以减少 tick 时遍历任务的个数.

2、结构图

3、效率

3.1优点

1. 可以添加、删除、取消定时任务
2. 能高效的处理大批定时任务

3.2缺点

1. 对内存要求较高，占用较高的内存
2. 时间精度要求不高

4、结合源码分析

首先来看HashedWheelTimer的构造函数，HashedWheelTimer有很多构造方法，但是最后都是调用一个：

    public HashedWheelTimer(
            ThreadFactory threadFactory,
            long tickDuration, TimeUnit unit, int ticksPerWheel,
            long maxPendingTimeouts) {

        if (threadFactory == null) {
            throw new NullPointerException("threadFactory");
        }
        if (unit == null) {
            throw new NullPointerException("unit");
        }
        if (tickDuration <= 0) {
            throw new IllegalArgumentException("tickDuration must be greater than 0: " + tickDuration);
        }
        if (ticksPerWheel <= 0) {
            throw new IllegalArgumentException("ticksPerWheel must be greater than 0: " + ticksPerWheel);
        }

        // Normalize ticksPerWheel to power of two and initialize the wheel.
        // 构造时间轮的槽位数，槽位数只能是2的幂次方
        wheel = createWheel(ticksPerWheel);
        // 时间轮槽位数
        mask = wheel.length - 1;

        // Convert tickDuration to nanos.
        // 初始化时间周期
        this.tickDuration = unit.toNanos(tickDuration);

        // Prevent overflow.
        if (this.tickDuration >= Long.MAX_VALUE / wheel.length) {
            throw new IllegalArgumentException(String.format(
                    "tickDuration: %d (expected: 0 < tickDuration in nanos < %d",
                    tickDuration, Long.MAX_VALUE / wheel.length));
        }
        // 初始化轮询时间轮的线程，使用这个线程周期性的轮询时间轮
        workerThread = threadFactory.newThread(worker);

        this.maxPendingTimeouts = maxPendingTimeouts;

        if (INSTANCE_COUNTER.incrementAndGet() > INSTANCE_COUNT_LIMIT &&
                WARNED_TOO_MANY_INSTANCES.compareAndSet(false, true)) {
            reportTooManyInstances();
        }
    }

时间轮实际就是一个HashedWeelBucket数组，上面这个构造方法就是在初始化这个数组，槽位数就是数组长度，tickDuration是时间周期，workerThread线程用来轮询数组；

    private static HashedWheelBucket[] createWheel(int ticksPerWheel) {
        if (ticksPerWheel <= 0) {
            throw new IllegalArgumentException(
                    "ticksPerWheel must be greater than 0: " + ticksPerWheel);
        }
        if (ticksPerWheel > 1073741824) {
            throw new IllegalArgumentException(
                    "ticksPerWheel may not be greater than 2^30: " + ticksPerWheel);
        }
        // HashedWheelBucket数组长度是2的幂次方，获取<=ticksPerWheel最大的2的幂次方
        ticksPerWheel = normalizeTicksPerWheel(ticksPerWheel);
        HashedWheelBucket[] wheel = new HashedWheelBucket[ticksPerWheel];
        for (int i = 0; i < wheel.length; i++) {
            wheel[i] = new HashedWheelBucket();
        }
        return wheel;
    }

初始化的HashedWheelBucket数组的长度必须是2的幂次方。HashedWheelTimer初始化完了，记下来就是如何向时间轮里添加定时任务，其实很简单，只要调用newTimeOut()方法即可

    public Timeout newTimeout(TimerTask task, long delay, TimeUnit unit) {
        if (task == null) {
            throw new NullPointerException("task");
        }
        if (unit == null) {
            throw new NullPointerException("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 + ")");
        }

        // 开启时间轮轮询
        start();

        // Add the timeout to the timeout queue which will be processed on the next tick.
        // During processing all the queued HashedWheelTimeouts will be added to the correct HashedWheelBucket.
        long deadline = System.nanoTime() + unit.toNanos(delay) - startTime;

        // Guard against overflow.
        if (delay > 0 && deadline < 0) {
            deadline = Long.MAX_VALUE;
        }
        // 将定时任务封装成HashedWheelTimeout
        HashedWheelTimeout timeout = new HashedWheelTimeout(this, task, deadline);
        // 将定时任务存储到任务链表中
        timeouts.add(timeout);
        return timeout;
    }

在newTimeOut()方法中会去开启轮询时间轮的线程(即workerThread)，接下来在看如何轮询：

    public void start() {
        // 判断HashedWheelTimer状态，如果状态开启，则开启轮询线程
        switch (WORKER_STATE_UPDATER.get(this)) {
            case WORKER_STATE_INIT:
                if (WORKER_STATE_UPDATER.compareAndSet(this, WORKER_STATE_INIT, WORKER_STATE_STARTED)) {
                    workerThread.start();
                }
                break;
            case WORKER_STATE_STARTED:
                break;
            case WORKER_STATE_SHUTDOWN:
                throw new IllegalStateException("cannot be started once stopped");
            default:
                throw new Error("Invalid WorkerState");
        }

        // Wait until the startTime is initialized by the worker.
        while (startTime == 0) {
            try {
                // 阻塞当前线程，目的是保证轮询线程workerThread开启
                startTimeInitialized.await();
            } catch (InterruptedException ignore) {
                // Ignore - it will be ready very soon.
            }
        }
    }

在这个方法中会去开启workerThread线程，执行workerThread线程中run()方法

        public void run() {
            // Initialize the startTime.
            // 初始化开始时间
            startTime = System.nanoTime();
            if (startTime == 0) {
                // We use 0 as an indicator for the uninitialized value here, so make sure it's not 0 when initialized.
                startTime = 1;
            }

            // Notify the other threads waiting for the initialization at start().
            // 唤醒阻塞的线程
            startTimeInitialized.countDown();

            do {
                // 根据周期时间tickDuration，进行周期性的tick下一个槽位
                final long deadline = waitForNextTick();
                if (deadline > 0) {
                    // 获取下一个槽位的角标
                    int idx = (int) (tick & mask);
                    processCancelledTasks();
                    // 获取该角标对应的HashedWheelBucket对象
                    HashedWheelBucket bucket =
                            wheel[idx];
                    // 将存储在链表timeOuts中的定时任务存储到对应的槽位的HashedWheelBucket对象中        
                    transferTimeoutsToBuckets();
                    // 执行槽位中定时任务
                    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();
        }

在上面方法中，轮询时间轮，执行对应槽位的定时任务，在执行之前，会先将存储在链表中任务按照各自的时间放入对应的槽位中，接下来咱们来看如何根据周期时间进行tick

        private long waitForNextTick() {
            // 获取下一个槽位的等待时间
            long deadline = tickDuration * (tick + 1);

            for (; ; ) {
                // 获取当前时间间隔
                final long currentTime = System.nanoTime() - startTime;
                // 计算tick到下一个槽位需要等待的时间
                long sleepTimeMs = (deadline - currentTime + 999999) / 1000000;

                // 当前时间间隔大于等于下一个槽位周期时间，不需要等待，直接返回(从这个地方就可以得出HashedWheelTimer对时间精度要求不高，并不是严格按照延迟时间来执行的)
                if (sleepTimeMs <= 0) {
                    if (currentTime == Long.MIN_VALUE) {
                        return -Long.MAX_VALUE;
                    } else {
                        return currentTime;
                    }
                }
                if (isWindows()) {
                    sleepTimeMs = sleepTimeMs / 10 * 10;
                }

                try {
                    // 当前时间间隔小于下一个槽位周期时间，则进行休眠
                    Thread.sleep(sleepTimeMs);
                } catch (InterruptedException ignored) {
                    if (WORKER_STATE_UPDATER.get(HashedWheelTimer.this) == WORKER_STATE_SHUTDOWN) {
                        return Long.MIN_VALUE;
                    }
                }
            }
        }

分析了如何实现时间间隔轮询，接下来分析如何将任务存储到HashedWheelBucket中

        private void transferTimeoutsToBuckets() {
            // transfer only max. 100000 timeouts per tick to prevent a thread to stale the workerThread when it just
            // adds new timeouts in a loop.
            // 遍历timeouts链表，默认遍历链表100000个任务
            for (int i = 0; i < 100000; i++) {
                HashedWheelTimeout timeout = timeouts.poll();
                if (timeout == null) {
                    // all processed
                    break;
                }
                // 任务的状态等于取消，直接跳过
                if (timeout.state() == HashedWheelTimeout.ST_CANCELLED) {
                    // Was cancelled in the meantime.
                    continue;
                }

                // 设置任务需要轮询的圈数，如：槽位=8，周期tickDuration=100ms，任务时间=900ms，则说明需要轮询一圈后，才能会执行到该任务，即remainingRounds= 1，槽位角标stopIndex=1
                long calculated = timeout.deadline / tickDuration;
                timeout.remainingRounds = (calculated - tick) / wheel.length;

                // Ensure we don't schedule for past.
                final long ticks = Math.max(calculated, tick);
                int stopIndex = (int) (ticks & mask);

                HashedWheelBucket bucket = wheel[stopIndex];
                // 将定时任务存储到对应的HashedWheelBucket槽位中
                bucket.addTimeout(timeout);
            }
        }

HashedWheelBucket是一个包含双向链表的对象，addTimeout将任务存储到链表的末端

        void expireTimeouts(long deadline) {
            // 获取链表表头任务
            HashedWheelTimeout timeout = head;

            // process all timeouts
            while (timeout != null) {
                // 获取表头的下一个任务
                HashedWheelTimeout next = timeout.next;
                if (timeout.remainingRounds <= 0) {
                    // 将要执行的任务从链表中删除
                    next = remove(timeout);
                    // 任务的时间小于间隔时间，执行任务
                    if (timeout.deadline <= deadline) {
                        // 执行任务
                        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()) {
                    next = remove(timeout);
                } else {
                    timeout.remainingRounds--;
                }
                timeout = next;
            }
        }

上面这个方法就是遍历槽位中链表中的任务进行执行

        public void expire() {
            if (!compareAndSetState(ST_INIT, ST_EXPIRED)) {
                return;
            }

            try {
                // **这个地方就是真正执行封装的task任务，执行具体的任务逻辑**
                task.run(this);
            } catch (Throwable t) {
                if (logger.isWarnEnabled()) {
                    logger.warn("An exception was thrown by " + TimerTask.class.getSimpleName() + '.', t);
                }
            }
        }

以上就是HashedWheelTimer执行的整个过程，在分析的过程中最好还是结合具体的实例去分析，这样会更有利于自己的理解。