定时任务 Timer

原文链接xingzhi.info/?p=117，转载请注明出处

jdk提供了两种定时任务的选择方案，分别是Timer，和ScheduledExecutorService

Timer的用法大体如下：
定义Task，在run方法中写明，这个任务是干什么。

public class XXXXXTask extends TimerTask
{
    @Override
    public void run()
    {
        //该干嘛干嘛
    }
}

然后用Timer来调用他， 下面是在一分钟整点的时候执行。

public class YYYYYScheduler
{
    private Timer timer = new Timer();
   
    public void start()
    {
        Date date = new Date();
        long curTime = date.getTime();
        long period = 60000;// 整1分钟
        long delay = (period - (curTime%period));  // 整1分钟
        timer.scheduleAtFixedRate(new XXXXXTask(), delay, period);
    }
}

实现原理，下面所说是从jdk的源码中抽出来的，版本是1.6。

public void scheduleAtFixedRate(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);
    }
上面是timer.scheduleAtFixedRate源代码
他会最终会调用sched(TimerTask task, long time, long period)这个方法，里面的源码如下：

synchronized(queue) {
            if (!thread.newTasksMayBeScheduled)
                throw new IllegalStateException("Timer already cancelled.");
            synchronized(task.lock) {
                if (task.state != TimerTask.VIRGIN)
                    throw new IllegalStateException(
                        "Task already scheduled or cancelled");
                task.nextExecutionTime = time;
                task.period = period;
                task.state = TimerTask.SCHEDULED;
            }
            queue.add(task);
            if (queue.getMin() == task)
                queue.notify();
        }

对queue进行加锁, 线程加锁的这个queue是task队列

    /**
     * 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 TaskQueue queue = new TaskQueue();

下面的代码，可以看出锁住某个task之后，会对他设定执行的时间，这个时间就是传入的那个开始的延时

 

synchronized(task.lock) {
                if (task.state != TimerTask.VIRGIN)
                    throw new IllegalStateException(
                        "Task already scheduled or cancelled");
                task.nextExecutionTime = time;
                task.period = period;
                task.state = TimerTask.SCHEDULED;
            }

就是这个task.nextExecutionTime = time;。
在Timer的里面有一个run方法，里面有一个mainloop,如下所示：

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

打开这个mainLoop方法就是里面的执行过程，这个Timer运转起来的核心

private void mainLoop() {
while (true) {
    try {
        TimerTask task;
        boolean taskFired;
        synchronized(queue) {
            // Wait for queue to become non-empty
            while (queue.isEmpty() && newTasksMayBeScheduled)
                queue.wait();
            if (queue.isEmpty())
                break; // Queue is empty and will forever remain; die
            // Queue nonempty; look at first evt and do the right thing
            long currentTime, executionTime;
            task = queue.getMin();
            synchronized(task.lock) {
                if (task.state == TimerTask.CANCELLED) {
                    queue.removeMin();
                    continue;  // No action required, poll queue again
                }
                currentTime = System.currentTimeMillis();
                executionTime = task.nextExecutionTime;
                if (taskFired = (executionTime<=currentTime)) {
                    if (task.period == 0) { // Non-repeating, remove
                        queue.removeMin();
                        task.state = TimerTask.EXECUTED;
                    } else { // Repeating task, reschedule
                        queue.rescheduleMin(
                          task.period<0 ? currentTime   - task.period
                                        : executionTime + task.period);
                    }
                }
            }
            if (!taskFired) // Task hasn't yet fired; wait
                queue.wait(executionTime - currentTime);
        }
        if (taskFired)  // Task fired; run it, holding no locks
            task.run();
    } catch(InterruptedException e) {
    }
}
    }

从下面这几句可以看出，Timer的执行是用的绝对时间：

//1. 获取系统当前时间
currentTime = System.currentTimeMillis();
//2.从task里面去除下次执行时间
executionTime = task.nextExecutionTime;
//3.看下次执行时间是不是比当前时间小，接下来再判断是不是重复执行，

if (taskFired = (executionTime<=currentTime)) {
    if (task.period == 0) { // Non-repeating, remove
        queue.removeMin();
        task.state = TimerTask.EXECUTED;
    } else { // Repeating task, reschedule
        queue.rescheduleMin(
          task.period<0 ? currentTime   - task.period
                        : executionTime + task.period);
    }
}

下面这句可以看出，Timer让定时任务等待，用的是wait方法。

 if (!taskFired) // Task hasn't yet fired; wait
                        queue.wait(executionTime - currentTime);

总结：
Timer就是把TimerTask方法任务队列，获取系统当前时间，来判断task是否执行，不到时间的话，就一直wait。
这种方式的话，系统时间调整，Timer就被打乱了。