关于java:SpringBoot定时任务-经典定时任务结构设计时间轮Timing-Wheel案例和实现原理

Timer和ScheduledExecutorService是JDK内置的定时工作计划，而业内还有一个经典的定时工作的设计叫工夫轮(Timing Wheel), Netty外部基于工夫轮实现了一个HashedWheelTimer来优化百万量级I/O超时的检测，它是一个高性能，低消耗的数据结构，它适宜用非准实时，提早的短平快工作，例如心跳检测。本文次要介绍工夫轮(Timing Wheel)及其应用。@pdai

SpringBoot定时工作 - Netty HashedWheelTimer形式
- 常识筹备
  - 什么是工夫轮(Timing Wheel)
  - Netty的HashedWheelTimer要解决什么问题
  - HashedWheelTimer的应用形式
- 实现案例
  - Pom依赖
  - 2个简略例子
- 进一步了解
  - HashedWheelTimer是如何实现的?
  - 什么是多级Timing Wheel?
- 示例源码

常识筹备

须要对工夫轮(Timing Wheel)，以及Netty的HashedWheelTimer要解决什么问题有初步的意识。

什么是工夫轮(Timing Wheel)

工夫轮(Timing Wheel)是George Varghese和Tony Lauck在1996年的论文'Hashed and Hierarchical Timing Wheels: data structures to efficiently implement a timer facility'实现的，它在Linux内核中应用宽泛，是Linux内核定时器的实现办法和根底之一。

工夫轮(Timing Wheel)是一种环形的数据结构，就像一个时钟能够分成很多格子（Tick)，每个格子代表工夫的距离，它指向存储的具体任务（timerTask）的一个链表。

以上述在论文中的图片例子，这里一个轮子蕴含8个格子（Tick), 每个tick是一秒钟；

工作的增加：如果一个工作要在17秒后执行，那么它须要转2轮，最终加到Tick=1地位的链表中。

工作的执行：在时钟转2Round到Tick=1的地位，开始执行这个地位指向的链表中的这个工作。（# 这里示意残余须要转几轮再执行这个工作）

Netty的HashedWheelTimer要解决什么问题

HashedWheelTimer是Netty依据工夫轮(Timing Wheel)开发的工具类，它要解决什么问题呢？这外面有两个要点：提早工作 + 低时效性。@pdai

在Netty中的一个典型利用场景是判断某个连贯是否idle，如果idle（如客户端因为网络起因导致到服务器的心跳无奈送达），则服务器会被动断开连接，开释资源。判断连贯是否idle是通过定时工作实现的，然而Netty可能维持数百万级别的长连贯，对每个连贯去定义一个定时工作是不可行的，所以如何晋升I/O超时调度的效率呢？

Netty依据工夫轮(Timing Wheel)开发了HashedWheelTimer工具类，用来优化I/O超时调度(实质上是提早工作）；之所以采纳工夫轮(Timing Wheel)的构造还有一个很重要的起因是I/O超时这种类型的工作对时效性不须要十分精准。

HashedWheelTimer的应用形式

在理解工夫轮（Timing Wheel)和Netty的HashedWheelTimer要解决的问题后，咱们看下HashedWheelTimer的应用形式

通过构造函数看主要参数

public HashedWheelTimer(        ThreadFactory threadFactory,        long tickDuration, TimeUnit unit, int ticksPerWheel, boolean leakDetection,        long maxPendingTimeouts, Executor taskExecutor) {}

具体参数阐明如下：

threadFactory：线程工厂，用于创立工作线程，默认是Executors.defaultThreadFactory()
tickDuration：tick的周期，即多久tick一次
unit: tick周期的单位
ticksPerWheel：工夫轮的长度，一圈下来有多少格
leakDetection：是否开启内存透露检测，默认是true
maxPendingTimeouts：最多执行的工作数，默认是-1，即不限度。在高并发量状况下才会设置这个参数。

实现案例

这里展现下HashedWheelTimer的根本应用案例。@pdai

Pom依赖

引入pom的依赖

<dependency>    <groupId>io.netty</groupId>    <artifactId>netty-all</artifactId>    <version>4.1.77.Final</version></dependency>

2个简略例子

例子1：5秒后执行TimerTask

@SneakyThrowspublic static void simpleHashedWheelTimer() {    log.info("init task 1...");        HashedWheelTimer timer = new HashedWheelTimer(1, TimeUnit.SECONDS, 8);    // add a new timeout    timer.newTimeout(timeout -> {        log.info("running task 1...");    }, 5, TimeUnit.SECONDS);}

执行后果如下：

23:32:21.364 [main] INFO tech.pdai.springboot.schedule.timer.netty.HashedWheelTimerTester - init task 1......23:32:27.454 [pool-1-thread-1] INFO tech.pdai.springboot.schedule.timer.netty.HashedWheelTimerTester - running task 1...

例子2：工作生效后cancel并让它从新在3秒后执行。

@SneakyThrowspublic static void reScheduleHashedWheelTimer() {    log.info("init task 2...");    HashedWheelTimer timer = new HashedWheelTimer(1, TimeUnit.SECONDS, 8);    Thread.sleep(5000);    // add a new timeout    Timeout tm = timer.newTimeout(timeout -> {        log.info("running task 2...");    }, 5, TimeUnit.SECONDS);    // cancel    if (!tm.isExpired()) {        log.info("cancel task 2...");        tm.cancel();    }    // reschedule    timer.newTimeout(tm.task(), 3, TimeUnit.SECONDS);}

23:28:36.408 [main] INFO tech.pdai.springboot.schedule.timer.netty.HashedWheelTimerTester - init task 2...23:28:41.412 [main] INFO tech.pdai.springboot.schedule.timer.netty.HashedWheelTimerTester - cancel task 2...23:28:45.414 [pool-2-thread-1] INFO tech.pdai.springboot.schedule.timer.netty.HashedWheelTimerTester - running task 2...

进一步了解

咱们通过如下问题进一步了解HashedWheelTimer。@pdai

HashedWheelTimer是如何实现的?

简略看下HashedWheelTimer是如何实现的

Worker：worker工作线程次要负责任务调度触发，单线程运行。
HashedWheelBucket：工夫轮下面的格子，外部持有HashedWheelTimeout组成的链表构造的头尾节点，多个格子组成的工夫轮造成一圈又一圈的工作环
HashedWheelTimeout：往工夫轮外面提交的工作会被封装成HashedWheelTimeout

构造函数

public HashedWheelTimer(        ThreadFactory threadFactory,        long tickDuration, TimeUnit unit, int ticksPerWheel, boolean leakDetection,        long maxPendingTimeouts, Executor taskExecutor) {    checkNotNull(threadFactory, "threadFactory");    checkNotNull(unit, "unit");    checkPositive(tickDuration, "tickDuration");    checkPositive(ticksPerWheel, "ticksPerWheel");    this.taskExecutor = checkNotNull(taskExecutor, "taskExecutor");    // Normalize ticksPerWheel to power of two and initialize the wheel.    wheel = createWheel(ticksPerWheel);    mask = wheel.length - 1;    // Convert tickDuration to nanos.    long duration = unit.toNanos(tickDuration);    // Prevent overflow.    if (duration >= Long.MAX_VALUE / wheel.length) {        throw new IllegalArgumentException(String.format(                "tickDuration: %d (expected: 0 < tickDuration in nanos < %d",                tickDuration, Long.MAX_VALUE / wheel.length));    }    if (duration < MILLISECOND_NANOS) {        logger.warn("Configured tickDuration {} smaller than {}, using 1ms.",                    tickDuration, MILLISECOND_NANOS);        this.tickDuration = MILLISECOND_NANOS;    } else {        this.tickDuration = duration;    }    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();    }}

创立wheel

private static HashedWheelBucket[] createWheel(int ticksPerWheel) {    //ticksPerWheel may not be greater than 2^30    checkInRange(ticksPerWheel, 1, 1073741824, "ticksPerWheel");    ticksPerWheel = normalizeTicksPerWheel(ticksPerWheel);    HashedWheelBucket[] wheel = new HashedWheelBucket[ticksPerWheel];    for (int i = 0; i < wheel.length; i ++) {        wheel[i] = new HashedWheelBucket();    }    return wheel;}private static int normalizeTicksPerWheel(int ticksPerWheel) {    int normalizedTicksPerWheel = 1;    while (normalizedTicksPerWheel < ticksPerWheel) {        normalizedTicksPerWheel <<= 1;    }    return normalizedTicksPerWheel;}

工作的增加

@Overridepublic Timeout newTimeout(TimerTask task, long delay, TimeUnit unit) {    checkNotNull(task, "task");    checkNotNull(unit, "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();    // 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;}

执行办法

/**    * 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.        }    }}

进行办法

@Overridepublic Set<Timeout> stop() {    if (Thread.currentThread() == workerThread) {        throw new IllegalStateException(                HashedWheelTimer.class.getSimpleName() +                        ".stop() cannot be called from " +                        TimerTask.class.getSimpleName());    }    if (!WORKER_STATE_UPDATER.compareAndSet(this, WORKER_STATE_STARTED, WORKER_STATE_SHUTDOWN)) {        // workerState can be 0 or 2 at this moment - let it always be 2.        if (WORKER_STATE_UPDATER.getAndSet(this, WORKER_STATE_SHUTDOWN) != WORKER_STATE_SHUTDOWN) {            INSTANCE_COUNTER.decrementAndGet();            if (leak != null) {                boolean closed = leak.close(this);                assert closed;            }        }        return Collections.emptySet();    }    try {        boolean interrupted = false;        while (workerThread.isAlive()) {            workerThread.interrupt();            try {                workerThread.join(100);            } catch (InterruptedException ignored) {                interrupted = true;            }        }        if (interrupted) {            Thread.currentThread().interrupt();        }    } finally {        INSTANCE_COUNTER.decrementAndGet();        if (leak != null) {            boolean closed = leak.close(this);            assert closed;        }    }    return worker.unprocessedTimeouts();}

什么是多级Timing Wheel?

多级的工夫轮是比拟好了解的，时钟是有小时，分钟，秒的，秒转一圈(Round)分钟就转一个格（Tick）, 分钟转一圈(Round)小时就转一格（Tick）。

PS：显然HashedWheelTimer是一层工夫轮。

示例源码

https://github.com/realpdai/t...