定时器Timer源码解析

一 Timer

1、Timer

Timer较之Quartz结构相对简单，其原理更容易动，并且两个会有相似之处，可以在了解Timer之后在看Quartz可能会相对容易通透一点，在Quartz之前先了解一下Timer定时器，以下是JDK Api中的介绍：

• 线程调度任务以供将来在后台线程中执行的功能。 任务可以安排一次执行，或定期重复执行。
• 对应于每个Timer对象是单个后台线程，用于依次执行所有定时器的所有任务。 计时器任务应该快速完成。 如果一个定时器任务需要花费很多时间来完成，它会“计时”计时器的任务执行线程。 这可能会延迟随后的任务的执行，这些任务在（和）如果违规任务最后完成时，可能会“束起来”并快速执行。
• 在最后一次对Timer对象的引用后*，所有未完成的任务已完成执行，定时器的任务执行线程正常终止（并被收集到垃圾回收）。但是，这可能需要任意长时间的发生。默认情况下，任务执行线程不作为守护程序线程*运行，因此它能够使应用程序终止。如果主叫方想要快速终止定时器的任务执行线程，则调用者应该调用定时器的cancel方法
• 这个类是线程安全的：多个线程可以共享一个单独的Timer对象，而不需要外部同步。
• 如果定时器的任务执行线程意外终止，例如，因为它调用了stop方法，那么在计时器上安排任务的任何进一步的尝试将会产生一个IllegalStateException ，就像定时器的cancel方法被调用一样。

2、源码解析

2.1 Timer

​ 初始化Timer时，会将其中的TaskQueue以及TimerThread也进行相应的初始化并且，会启动线程，使thread在一个wait状态。

   /**
     * 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);
	/**
     * Creates a new timer whose associated thread has the specified name.
     * The associated thread does <i>not</i>
     * {@linkplain Thread#setDaemon run as a daemon}.
     *
     * @param name the name of the associated thread
     * @throws NullPointerException if {@code name} is null
     * @since 1.5
     */
	//初始化时会将thead线程启动，使其在一个wait状态。
    public Timer(String name) {
        thread.setName(name);
        thread.start();
    }
	/**
     * Schedules the specified task for execution after the specified delay.
     *
     * @param task  task to be scheduled.
     * @param delay delay in milliseconds before task is to be executed.
     * @throws IllegalArgumentException if <tt>delay</tt> is negative, or
     *         <tt>delay + System.currentTimeMillis()</tt> is negative.
     * @throws IllegalStateException if task was already scheduled or
     *         cancelled, timer was cancelled, or timer thread terminated.
     * @throws NullPointerException if {@code task} is null
     */
    public void schedule(TimerTask task, long delay) {
        if (delay < 0)
            throw new IllegalArgumentException("Negative delay.");
        sched(task, System.currentTimeMillis()+delay, 0);
    }

​

​ 其中的核心schedule方法提供多个重载方法，提供给使用者，最终会调用sched方法。参数含义如下：task（具体执行动作），time（下一次执行时间），perid（每次执行时间间隔）。

 	/**
     * Schedule the specified timer task for execution at the specified
     * time with the specified period, in milliseconds.  If period is
     * positive, the task is scheduled for repeated execution; if period is
     * zero, the task is scheduled for one-time execution. Time is specified
     * in Date.getTime() format.  This method checks timer state, task state,
     * and initial execution time, but not period.
     
     * 计划指定的计时器任务，以便在指定的时间和指定的时间段执行，以毫秒为单位。
     * 如果周期为正，则计划重复执行任务；如果周期为零，则计划一次性执行任务。
     * 时间以日期指定。getTime（）格式。此方法检查计时器状态、任务状态、和初始执行时间，但不检查周期。
     *
     * @param task 具体执行动作。
     * @param time 下次执行时间，时间戳。
     * @param period 每次执行时间间隔，时间戳。
     */
    private void sched(TimerTask task, long time, long period) {
        if (time < 0)
            throw new IllegalArgumentException("Illegal execution time.");

        // Constrain value of period sufficiently to prevent numeric
        // overflow while still being effectively infinitely large.
        // 限制时间间隔的最大值
        if (Math.abs(period) > (Long.MAX_VALUE >> 1))
            period >>= 1;
		//同一个Timer并发调用sched时加重锁
        synchronized(queue) {
            if (!thread.newTasksMayBeScheduled)
                throw new IllegalStateException("Timer already cancelled.");
			//不同Timer调用同一个task时，task加锁应用，task.lock为TimerTask类中的加锁标识。
            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;
            }
			//将task加入执行计划queue中。
            queue.add(task);
            if (queue.getMin() == task)
                //getMin()获取执行计划中下一次执行，如果当前task为下次执行，则通知queue，不需要在wait
                queue.notify();
        }
    }

2.2 TimerThread（thread）

​ TimerThread主要用于管理task重复的启用以及非重复task任务的删除，在上述Timer中，初始化时会启动以下线程，使thread到达运行状态，执行mainLoop方法,进而在该方法中在到等待状态。

	/**
     * This flag is set to false by the reaper to inform us that there
     * are no more live references to our Timer object.  Once this flag
     * is true and there are no more tasks in our queue, there is no
     * work left for us to do, so we terminate gracefully.  Note that
     * this field is protected by queue's monitor!
     */
	//是否继续执行，true标识继续，false标识结束，最终结束定时器，还需要清空执行计划queue。
    boolean newTasksMayBeScheduled = true;
	/**
     * Our Timer's queue.  We store this reference in preference to
     * a reference to the Timer so the reference graph remains acyclic.
     * Otherwise, the Timer would never be garbage-collected and this
     * thread would never go away.
     */
    private TaskQueue queue;

    TimerThread(TaskQueue queue) {
        this.queue = queue;
    }

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

    /**
     * The main timer loop.  (See class comment.)
     */
    private void mainLoop() {
        while (true) {
            //？？？？？？无线执行for循环完成自动服务。		
            try {
                TimerTask task;
                boolean taskFired;
                //将一个执行计划queue，初始化多个TimerThread，并启动线程时，需要将queue加锁
                synchronized(queue) {
                    // Wait for queue to become non-empty
                    while (queue.isEmpty() && newTasksMayBeScheduled)
                        //执行计划中五可执行任务，并且为可执行状态时，则执行计划等待任务加入
                        queue.wait();
                    if (queue.isEmpty())
                        //如果如果queue为空，则退出不在进行，clear时
                        break; // Queue is empty and will forever remain; die

                    // Queue nonempty; look at first evt and do the right thing
                    //executionTime 下一次执行时间
                    long currentTime, executionTime;
                    //获取下一次执行计划
                    task = queue.getMin();
                    //场景同一个task列入多个执行计划时，
                    synchronized(task.lock) {
                        if (task.state == TimerTask.CANCELLED) {
                            //task状态为取消状态时，queue执行计划需要将task删除，并将queue按下一次执行时间进行排序。
                            queue.removeMin();
                            continue;  // No action required, poll queue again
                        }
                        currentTime = System.currentTimeMillis();
                        executionTime = task.nextExecutionTime;
                        if (taskFired = (executionTime<=currentTime)) {
                            //taskFired（true） 下次执行时间小于等于当前时间时
                            if (task.period == 0) { // Non-repeating, remove
                                //不存在时间间隔时，并且次执行时间小于等于当前时间时，标识非重复任务，则将之从执行计划中删除
                                queue.removeMin();
                                //并更新该task的执行状态
                                task.state = TimerTask.EXECUTED;
                            } else { // Repeating task, reschedule
                                //改task任务为重复执行任务，则更新下次执行时间，并且将queue执行计划按下次执行时间进行排序。
                                queue.rescheduleMin(
                                  task.period<0 ? currentTime   - task.period
                                                : executionTime + task.period);
                            }
                        }
                    }
                    if (!taskFired) // Task hasn't yet fired; wait
                        //taskFired（false） 下次执行时间大于当前时间时，则等待
                        queue.wait(executionTime - currentTime);
                }
                if (taskFired)  // Task fired; run it, holding no locks
                    //满足条件则执行task
                    task.run();
            } catch(InterruptedException e) {
            }
        }
    }

2.3 TaskQueue(queue)

    /**
     * Priority queue represented as a balanced binary heap: the two children
     * of queue[n] are queue[2*n] and queue[2*n+1].  The priority queue is
     * ordered on the nextExecutionTime field: The TimerTask with the lowest
     * nextExecutionTime is in queue[1] (assuming the queue is nonempty).  For
     * each node n in the heap, and each descendant of n, d,
     * n.nextExecutionTime <= d.nextExecutionTime.
     */
    //默认数组长度128
	private TimerTask[] queue = new TimerTask[128];
	/**
     * Adds a new task to the priority queue.
     */
	//判断长度，并采用数组扩容
    void add(TimerTask task) {
        // Grow backing store if necessary
        if (size + 1 == queue.length)
            queue = Arrays.copyOf(queue, 2*queue.length);

        queue[++size] = task;
        fixUp(size);
    }

    /**
     * Return the "head task" of the priority queue.  (The head task is an
     * task with the lowest nextExecutionTime.)
     */
    TimerTask getMin() {
        return queue[1];
    }
	/**
     * Remove the head task from the priority queue.
     */
    void removeMin() {
        queue[1] = queue[size];
        queue[size--] = null;  // Drop extra reference to prevent memory leak
        fixDown(1);
    }
	/**
     * Sets the nextExecutionTime associated with the head task to the
     * specified value, and adjusts priority queue accordingly.
     */
    void rescheduleMin(long newTime) {
        queue[1].nextExecutionTime = newTime;
        fixDown(1);
    }
	/**
     * Establishes the heap invariant (described above) assuming the heap
     * satisfies the invariant except possibly for the leaf-node indexed by k
     * (which may have a nextExecutionTime less than its parent's).
     *
     * This method functions by "promoting" queue[k] up the hierarchy
     * (by swapping it with its parent) repeatedly until queue[k]'s
     * nextExecutionTime is greater than or equal to that of its parent.
     */
	//将将执行计划queue[k]与queue[1]进行比较，执行计划靠前的则放入queue[1]中
    private void fixUp(int k) {
        while (k > 1) {
            int j = k >> 1;
            if (queue[j].nextExecutionTime <= queue[k].nextExecutionTime)
                break;
            TimerTask tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
            k = j;
        }
    }

    /**
     * Establishes the heap invariant (described above) in the subtree
     * rooted at k, which is assumed to satisfy the heap invariant except
     * possibly for node k itself (which may have a nextExecutionTime greater
     * than its children's).
     *
     * This method functions by "demoting" queue[k] down the hierarchy
     * (by swapping it with its smaller child) repeatedly until queue[k]'s
     * nextExecutionTime is less than or equal to those of its children.
     */
    private void fixDown(int k) {
        int j;
        while ((j = k << 1) <= size && j > 0) {
            if (j < size && queue[j].nextExecutionTime > queue[j+1].nextExecutionTime)
                j++; // j indexes smallest kid
            if (queue[k].nextExecutionTime <= queue[j].nextExecutionTime)
                break;
            TimerTask tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
            k = j;
        }
    }

简单示例

public class SimpleTimer {
  public static void main(String[] args) {
    Timer simpleTimer = new Timer("firstTime");

    SimpleTask task = new SimpleTask();
    //Object lock = new Object();
    //1000毫秒后开始执行，每次间隔2000毫秒
    simpleTimer.schedule(task,1000,2000);
    simpleTimer.schedule(task,1000,2000);

  }


  public static class SimpleTask extends TimerTask{
    Integer index = 0;

    @Override
    public void run() {
      index = index + 1;
      System.out.println("************"+index);
      if(index == 10 ){
        cancel();
      }
    }

  }
}

注：由于Timer采用单线程执行，未采用线程池，在遇到耗时较长的Job工作时，时间校准会出现误差