Java java.util.Timer is a utility class that can be used to schedule a thread to be executed at certain time in future. Java Timer class can be used to schedule a task to be run one-time or to be run at regular intervals.

Java java.util.Timer是一个实用程序类，可用于安排将来某个时间执行的线程。 Java Timer类可用于安排任务一次运行或定期运行。

Java TimerTask (Java TimerTask)

java.util.TimerTask is an abstract class that implements Runnable interface and we need to extend this class to create our own TimerTask that can be scheduled using java Timer class.

java.util.TimerTask是实现Runnable接口的抽象类 ，我们需要扩展该类以创建可以使用java Timer类进行调度的自己的TimerTask 。

Java计时器示例 (Java Timer Example)

Java Timer class is thread safe and multiple threads can share a single Timer object without need for external synchronization. Timer class uses java.util.TaskQueue to add tasks at given regular interval and at any time there can be only one thread running the TimerTask, for example if you are creating a Timer to run every 10 seconds but single thread execution takes 20 seconds, then Timer object will keep adding tasks to the queue and as soon as one thread is finished, it will notify the queue and another thread will start executing.

Java Timer类是线程安全的，并且多个线程可以共享一个Timer对象，而无需外部同步。 Timer类使用java.util.TaskQueue以给定的固定间隔添加任务，并且在任何时候都只能有一个线程运行TimerTask，例如，如果要创建一个每10秒运行一次的Timer，但是单线程执行需要20秒，然后，Timer对象将继续向队列中添加任务，一旦一个线程完成，它将通知队列，而另一个线程将开始执行。

Java Timer class uses Object wait and notify methods to schedule the tasks.

Java Timer类使用对象等待和通知方法来计划任务。

Here is a simple program for Java Timer and TimerTask example.

这是Java Timer和TimerTask示例的简单程序。

package com.journaldev.threads;import java.util.Date;
import java.util.Timer;
import java.util.TimerTask;public class MyTimerTask extends TimerTask {@Overridepublic void run() {System.out.println("Timer task started at:"+new Date());completeTask();System.out.println("Timer task finished at:"+new Date());}private void completeTask() {try {//assuming it takes 20 secs to complete the taskThread.sleep(20000);} catch (InterruptedException e) {e.printStackTrace();}}public static void main(String args[]){TimerTask timerTask = new MyTimerTask();//running timer task as daemon threadTimer timer = new Timer(true);timer.scheduleAtFixedRate(timerTask, 0, 10*1000);System.out.println("TimerTask started");//cancel after sometimetry {Thread.sleep(120000);} catch (InterruptedException e) {e.printStackTrace();}timer.cancel();System.out.println("TimerTask cancelled");try {Thread.sleep(30000);} catch (InterruptedException e) {e.printStackTrace();}}}

Notice that one thread execution will take 20 seconds but Java Timer object is scheduled to run the task every 10 seconds. Here is the output of the program:

请注意，一个线程执行将花费20秒，但是Java Timer对象被安排为每10秒运行一次任务。 这是程序的输出：

TimerTask started
Timer task started at:Wed Dec 26 19:16:39 PST 2012
Timer task finished at:Wed Dec 26 19:16:59 PST 2012
Timer task started at:Wed Dec 26 19:16:59 PST 2012
Timer task finished at:Wed Dec 26 19:17:19 PST 2012
Timer task started at:Wed Dec 26 19:17:19 PST 2012
Timer task finished at:Wed Dec 26 19:17:39 PST 2012
Timer task started at:Wed Dec 26 19:17:39 PST 2012
Timer task finished at:Wed Dec 26 19:17:59 PST 2012
Timer task started at:Wed Dec 26 19:17:59 PST 2012
Timer task finished at:Wed Dec 26 19:18:19 PST 2012
Timer task started at:Wed Dec 26 19:18:19 PST 2012
TimerTask cancelled
Timer task finished at:Wed Dec 26 19:18:39 PST 2012

The output confirms that if a task is already executing, Timer will wait for it to finish and once finished, it will start again the next task from the queue.

输出确认如果一个任务已经在执行，Timer将等待它完成，一旦完成，它将再次从队列中开始下一个任务。

Java Timer object can be created to run the associated tasks as a daemon thread. Timer cancel() method is used to terminate the timer and discard any scheduled tasks, however it doesn’t interfere with the currently executing task and let it finish. If the timer is run as daemon thread, whether we cancel it or not, it will terminate as soon as all the user threads are finished executing.

可以创建Java Timer对象以将其作为守护程序线程运行。 Timer cancel（）方法用于终止计时器并丢弃所有计划的任务，但是它不会干扰当前正在执行的任务并使其完成。 如果计时器作为守护程序线程运行，则无论我们是否取消它，它都会在所有用户线程完成执行后立即终止。

Timer class contains several schedule() methods to schedule a task to run once at given date or after some delay. There are several scheduleAtFixedRate() methods to run a task periodically with certain interval.

计时器类包含几个schedule （）方法，用于调度任务在给定日期或延迟一段时间后运行一次。 有几种scheduleAtFixedRate （）方法可按一定间隔定期运行任务。

While scheduling tasks using Timer, you should make sure that time interval is more than normal thread execution, otherwise tasks queue size will keep growing and eventually task will be executing always. That’s all for a quick roundup on Java Timer and Java TimerTask.

使用Timer计划任务时，应确保时间间隔大于正常线程执行的时间，否则任务队列大小将不断增长，最终任务将始终执行。 这就是对Java Timer和Java TimerTask的快速总结。

翻译自: https://www.journaldev.com/1050/java-timer-timertask-example