時間輪演算法

時間輪是一種高效、低延遲的排程資料結構。其在Linux核心中廣泛使用，是Linux核心定時器的實現方法和基礎之一。按使用場景，大致可以分為兩種時間輪：原始時間輪和分層時間輪。分層時間輪是原始時間輪的升級版本，來應對時間“槽”數量比較大的情況，對記憶體和精度都有很高要求的情況。延遲任務的場景一般只需要用到原始時間輪就可以了。

程式碼案例

推薦使用Netty提供的HashedWheelTimer工具類來實現延遲任務。

引入依賴：

<dependency>
      <groupId>io.netty</groupId>
      <artifactId>netty-common</artifactId>
      <version>4.1.23.Final</version>
</dependency>
 

紅包過期佇列資訊：

/**
 * 紅包過期佇列資訊
 */
public class RedPacketTimerTask implements TimerTask {

    private static final DateTimeFormatter F = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.SSS");

    /**
     * 紅包 ID
     */
    private final long redPacketId;

    /**
     * 建立時間戳
     */
    private final long timestamp;

    public RedPacketTimerTask(long redPacketId) {
        this.redPacketId = redPacketId;
        this.timestamp = System.currentTimeMillis();
    }

    @Override
    public void run(Timeout timeout) {
        //非同步處理任務
        System.out.println(String.format("任務執行時間:%s,紅包建立時間:%s,紅包ID:%s",
                LocalDateTime.now().format(F), LocalDateTime.ofInstant(Instant.ofEpochMilli(timestamp), ZoneId.systemDefault()).format(F), redPacketId));
    }
}
 

測試用例：

/**
 * 基於 netty 的時間輪演算法 HashedWheelTimer 實現的延遲任務
 */
public class RedPacketHashedWheelTimer {

    private static final DateTimeFormatter F = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.SSS");

    public static void main(String[] args) throws Exception {
        ThreadFactory factory = r -> {
            Thread thread = new Thread(r);
            thread.setDaemon(true);
            thread.setName("RedPacketHashedWheelTimerWorker");
            return thread;
        };
        /**
         * @param tickDuration - 每tick一次的時間間隔
         * @param unit - tickDuration 的時間單位
         * @param ticksPerWheel - 時間輪中的槽數
         * @param leakDetection - 檢查記憶體溢位
         */
        Timer timer = new HashedWheelTimer(factory, 1,
                                           TimeUnit.SECONDS, 100,true);
        System.out.println(String.format("開始任務時間:%s",LocalDateTime.now().format(F)));
        for(int i=1;i<10;i++){
            TimerTask timerTask = new RedPacketTimerTask(i);
            timer.newTimeout(timerTask, i, TimeUnit.SECONDS);
        }
        Thread.sleep(Integer.MAX_VALUE);
    }
}
 

列印任務執行日誌：

開始任務時間:2020-02-12 15:22:23.404
任務執行時間:2020-02-12 15:22:25.410,紅包建立時間:2020-02-12 15:22:23.409,紅包ID:1
任務執行時間:2020-02-12 15:22:26.411,紅包建立時間:2020-02-12 15:22:23.414,紅包ID:2
任務執行時間:2020-02-12 15:22:27.424,紅包建立時間:2020-02-12 15:22:23.414,紅包ID:3
任務執行時間:2020-02-12 15:22:28.410,紅包建立時間:2020-02-12 15:22:23.414,紅包ID:4
任務執行時間:2020-02-12 15:22:29.411,紅包建立時間:2020-02-12 15:22:23.414,紅包ID:5
任務執行時間:2020-02-12 15:22:30.409,紅包建立時間:2020-02-12 15:22:23.414,紅包ID:6
任務執行時間:2020-02-12 15:22:31.411,紅包建立時間:2020-02-12 15:22:23.414,紅包ID:7
任務執行時間:2020-02-12 15:22:32.409,紅包建立時間:2020-02-12 15:22:23.414,紅包ID:8
任務執行時間:2020-02-12 15:22:33.411,紅包建立時間:2020-02-12 15:22:23.414,紅包ID:9

原始碼相關

其核心是workerThread執行緒，主要負責每過tickDuration時間就累加一次tick。同時也負責執行到期的timeout任務以及新增timeout任務到指定的wheel中。

構造方法：

public HashedWheelTimer(
            ThreadFactory threadFactory,
            long tickDuration, TimeUnit unit, int ticksPerWheel, boolean leakDetection,
            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.
        wheel = createWheel(ticksPerWheel);
        mask = wheel.length - 1;

        // Convert tickDuration to nanos.
        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));
        }
        //這裡-爪窪筆記
        workerThread = threadFactory.newThread(worker);

        leak = leakDetection || !workerThread.isDaemon() ? leakDetector.track(this) : null;

        this.maxPendingTimeouts = maxPendingTimeouts;

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

新增任務，建立即啟動：

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 + ")");
        }
        //這裡-爪窪筆記
        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;
        }
        HashedWheelTimeout timeout = new HashedWheelTimeout(this, task, deadline);
        timeouts.add(timeout);
        return timeout;
}

執行緒啟動：

   /**
     * Starts the background thread explicitly.  The background thread will
     * start automatically on demand even if you did not call this method.
     *
     * @throws IllegalStateException if this timer has been
     *                               {@linkplain #stop() stopped} already
     */
    public void start() {
        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 {
                startTimeInitialized.await();
            } catch (InterruptedException ignore) {
                // Ignore - it will be ready very soon.
            }
        }
    }

執行相關操作：

public void run() {
            // Initialize the startTime.
            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().
            startTimeInitialized.countDown();

            do {
                final long deadline = waitForNextTick();
                if (deadline > 0) {
                    int idx = (int) (tick & mask);
                    processCancelledTasks();
                    HashedWheelBucket bucket =
                            wheel[idx];
                    transferTimeoutsToBuckets();
                    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();
}

小結

以上方案並沒有實現持久化和分散式，生產環境可根據實際業務需求選擇使用。

原始碼

https://gitee.com/52itstyle/spring-boot-seck