一览定时器

╰+哭是因爲堅強的太久メ 2022-02-25 19:58 178阅读 0赞

转自[https://mp.weixin.qq.com/s?\_\_biz=MzU0MzQ5MDA0Mw==&mid=2247485156&idx=2&sn=ae87e871a063a14d0f30551b0d4843ac&chksm=fb0bec70cc7c6566f66a9a6adf03488b2354ff9c4c2e42adeeacc2439e71ec0aa6cbf919ceaa&scene=0&xtrack=1\#rd][https_mp.weixin.qq.com_s_biz_MzU0MzQ5MDA0Mw_mid_2247485156_idx_2_sn_ae87e871a063a14d0f30551b0d4843ac_chksm_fb0bec70cc7c6566f66a9a6adf03488b2354ff9c4c2e42adeeacc2439e71ec0aa6cbf919ceaa_scene_0_xtrack_1_rd]

大概有Timer,Quartz，Spring Schedule等等

*  Timer源码读解
 *  ScheduleExecutorService周期调度
 *  Timer与ScheduleExecutorService区别

### Timer源码 ###

主要变量

/**
         * The timer task queue.  This data structure is shared with the timer
         * thread.  The timer produces tasks, via its various schedule calls,
         * and the timer thread consumes, executing timer tasks as appropriate,
         * and removing them from the queue when they're obsolete.
         */
        private final TaskQueue queue = new TaskQueue();
    
        /**
         * The timer thread.
         */
        private final TimerThread thread = new TimerThread(queue);

queue储存timertask，以及它们的优先级别，延迟时间。

TimerThread是继承了Thread线程，通过重写run方法

public void run() {
            try {
                mainLoop();
            } finally {
                // Someone killed this Thread, behave as if Timer cancelled
                synchronized(queue) {
                    newTasksMayBeScheduled = false;
                    queue.clear();  // Eliminate obsolete references
                }
            }
        }

在初始化的时候调用这个TimerThread,

public Timer(String name) {
            thread.setName(name);
            thread.start();
        }

只执行一次（delay是毫秒级别）

public void schedule(TimerTask task, long delay) {
            if (delay < 0)
                throw new IllegalArgumentException("Negative delay.");
            sched(task, System.currentTimeMillis()+delay, 0);
        }

多次执行

public void schedule(TimerTask task, long delay, long period) {
            if (delay < 0)
                throw new IllegalArgumentException("Negative delay.");
            if (period <= 0)
                throw new IllegalArgumentException("Non-positive period.");
            sched(task, System.currentTimeMillis()+delay, -period);
        }

轮询的处理方式

if (!taskFired) // Task hasn't yet fired; wait
                            queue.wait(executionTime - currentTime);
                    }
                    if (taskFired)  // Task fired; run it, holding no locks
                        task.run();

wait(executionTime - currentTime)让queue等待唤醒，执行run方法

### Timer涉及TimerTask ###

实现了Runnable接口

类变量

/**
         * This object is used to control access to the TimerTask internals.
         */
        final Object lock = new Object();
    
        /**
         * The state of this task, chosen from the constants below.
         */
        int state = VIRGIN;
    
        /**
         * This task has not yet been scheduled.
         */
        static final int VIRGIN = 0;
    
        /**
         * This task is scheduled for execution.  If it is a non-repeating task,
         * it has not yet been executed.
         */
        static final int SCHEDULED   = 1;
    
        /**
         * This non-repeating task has already executed (or is currently
         * executing) and has not been cancelled.
         */
        static final int EXECUTED    = 2;
    
        /**
         * This task has been cancelled (with a call to TimerTask.cancel).
         */
        static final int CANCELLED   = 3;
    
        /**
         * Next execution time for this task in the format returned by
         * System.currentTimeMillis, assuming this task is scheduled for execution.
         * For repeating tasks, this field is updated prior to each task execution.
         */
        long nextExecutionTime;
    
        /**
         * Period in milliseconds for repeating tasks.  A positive value indicates
         * fixed-rate execution.  A negative value indicates fixed-delay execution.
         * A value of 0 indicates a non-repeating task.
         */
        long period = 0;

可以看到包括task的状态，lock为锁，保证方法的执行安全。还有nextExecutionTime下次执行的时间。

### Timer总结: ###

可以看出它是由一个线程去执行的，把task加到queue里面而已。那么如果里面由一个报错了，后面都完蛋了。

举个栗子

Timer t=new Timer();
            t.schedule(new TimerTask() {
                @Override
                public void run() {
                    i=i/0;
                    System.out.println(i);
                }
            },0,1000);
            t.schedule(new TimerTask() {
                @Override
                public void run() {
                    i=i+1;
                    System.out.println(i);
                }
            },0,1000);

出错直接停止了。

### ScheduledExecutorService出场 ###

它由两种调度策略：ScheduleAtFixedRate 和 ScheduleWithFixedDelay

ScheduledExecutorService 提供了两种常用的周期调度方法 ScheduleAtFixedRate 和 ScheduleWithFixedDelay。
    
    ScheduleAtFixedRate 每次执行时间为上一次任务开始起向后推一个时间间隔，即每次执行时间为 : $initialDelay$, $initialDelay+period$, $initialDelay+2*period$, …
    
    ScheduleWithFixedDelay 每次执行时间为上一次任务结束起向后推一个时间间隔，即每次执行时间为：$initialDelay$, $initialDelay+executeTime+delay$, $initialDelay+2executeTime+2delay$, ...
    
    由此可见，ScheduleAtFixedRate 是基于固定时间间隔进行任务调度，ScheduleWithFixedDelay 取决于每次任务执行的时间长短，是基于不固定时间间隔的任务调度。

转自上面链接的博客

举个栗子：

ScheduledExecutorService scheduledExecutorService= Executors.newScheduledThreadPool(10);
            scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
                @Override
                public void run() {
                    i=i/0;
                    System.out.println(i);
                }
            },1,1, TimeUnit.SECONDS);
            scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
                @Override
                public void run() {
                    i=i+1;
                    System.out.println(i);
                }
            },1,1, TimeUnit.SECONDS);

出错了也不会影响后面的执行

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