How to use Java timer Timer

timer function repeatedly triggers the timer event of the specified window within the specified time interval. Syntax Timer (interval {, windowname }) Parameter interval: Specify the time interval between triggering the Timer event twice. The valid value is between 0 and 65. This article mainly introduces the use of Java timer Timer in detail, which has certain reference value. Interested friends can refer to it.

The scheduled task function in Java mainly uses the Timer object, which uses multi-threading for processing internally, so it is still very closely related to multi-threading technology. In the JDK, the Timer class is mainly responsible for the function of scheduling tasks, that is, starting a task at a specified time, but the class that encapsulates the task is the TimerTask class.

By inheriting the TimerTask class and implementing the run() method to customize the task to be executed:

public class Mytask extends TimerTask {
  @Override
  public void run()
  {
    DateFormat dateFormat = TimeUtil.df.get();
    System.out.println("我的任务运行了" + dateFormat.format(new Date()));
  }
}

By executing Timer.schedule(TimerTask task,Date time) Run the task at execution time:

public class Run {
  private static Timer timer=new Timer();

  public static void main(String[] args) throws ParseException
  {
    timer.schedule(new Mytask(), TimeUtil.df.get().parse("2017-09-14 09:19:30"));
  }
}

Remarks: Time conversion tool class to ensure thread safety:

public class TimeUtil
{
  public static final ThreadLocal<DateFormat> df = new ThreadLocal<DateFormat>() {
    @Override
    protected DateFormat initialValue() {
      return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    }
  };
}

2. Notes on Timer class

1. Creating a Timer object means starting a new thread, but This newly started thread is not a daemon thread. It has been running in the background. You can set the newly started Timer thread as a daemon thread as follows.

private static Timer timer=new Timer(true);

2. Advance: When the planned time is earlier than the current time, the task will be run immediately.

3. Delay: TimerTask is run sequentially one by one in a queue, so the execution time may not be consistent with your expected time, because the previous task may take longer, and the later tasks may The running time will be delayed. The specific start time of the delayed task is based on the "end time" of the previous task

4. Periodic operation: Timer.schedule(TimerTask task,Date firstTime,long period ) Execute the task every period milliseconds starting from firstTime:

5. schedule(TimerTask task, long delay) The current time is the reference time, and the delay is based on this time Execute a TimerTask task after the specified number of milliseconds.

6. Schedule (TimerTask task, long delay, long period) The current time is the reference time. Based on this, the specified number of milliseconds is delayed, and then a certain task is executed an unlimited number of times at a certain interval.

7. What is the difference between Timer’s cancel() and TimerTask’s cancel()?

As mentioned earlier, tasks are executed one by one in a queue. TimerTask.cancel() refers to canceling the current task from the task queue. The value of Timer.cancel() is to cancel all tasks in the current task queue. It is worth noting that Timer's cancel() sometimes does not necessarily stop executing the scheduled task, but executes it normally. This is because the cancel() method in the Timer class sometimes does not compete for the queue lock, so the tasks in the TimerTask class continue to execute normally.

3. The difference between scheduleAtFixedRate(TimerTask task,Date firstTime,long period) and schedule(TimerTask task,Date firstTime,long period)

Same points:

1. The method schedule and the method scheduleAtFixedRate will be executed in order, so there is no need to consider non-thread safety.

2. Method schedule and method scheduleAtFixedRate If the execution time of the task is not delayed, then the execution time of the next task is calculated with reference to the "start" time of the previous task.

3. Method schedule and method scheduleAtFixedRate If the execution time of the task is delayed, then the execution time of the next task is calculated with reference to the time when the previous task "ended".

Difference:

There is basically no difference in usage between method schedule and method scheduleAtFixedRate , that is, scheduleAtFixedRate has catch-up execution. What does it mean? That is, if a task is interrupted during periodic running, scheduleAtFixedRate will try to make up for the previously dropped tasks. Ignore the schedule and just run the next task. You can refer to this blog, which is very vivid.

The above content is how to use Java timer Timer. I hope it can help everyone.

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