关于TimerTask里面的scheduledExecutionTime()方法，一些见解

笔者为了测试TimerTask里面的scheduledExecutionTime()这个方法，编写了如下测试代码

public class MyTimerTest02 {public static void main(String[] args) {Timer timer=new Timer();//Calendar calendar=Calendar.getInstance();Date date=new Date();SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-hh HH:mm:ss");System.out.println(sdf.format(date));//calendar.add(Calendar.SECOND, 2);//myTimerTask02.setName("test TimerTask Cancel");////timer.schedule(myTimerTask02, 2000, 1000);TimerTask03 myTimerTask03=new TimerTask03("near time");timer.schedule(myTimerTask03,2000,1000);System.out.println("Timer最近会执行的Task的时间："+sdf.format(myTimerTask03.scheduledExecutionTime()));}}

public class TimerTask03 extends TimerTask{private String name;public TimerTask03(String name){this.name=name;}@Overridepublic void run() {// TODO Auto-generated method stub//Calendar calendar=Calendar.getInstance();System.out.println("------------>");SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-hh HH:mm:ss");//System.out.println("Timer最近会执行的Task的时间："+sdf.format(scheduledExecutionTime()));System.out.println("This TimerTask's time is "+sdf.format(new Date()));System.out.println("This TimerTask's name is "+name);System.out.println("<------------");System.out.println();}public String getName() {return name;}public void setName(String name) {this.name = name;}}

执行结果截取

2017-12-10 22:00:32Timer最近会执行的Task的时间：2017-12-10 22:00:33------------>This TimerTask's time is 2017-12-10 22:00:34This TimerTask's name is near time<------------------------>This TimerTask's time is 2017-12-10 22:00:35This TimerTask's name is near time<------------

由此看来当你在测试类下面直接调用scheduledExecutionTime()预计时间会出现偏差

但是如果你把

timer.schedule(myTimerTask03,2000,1000);

改成如下

timer.schedule(myTimerTask03,2000);

结果就是

2017-12-10 22:04:33Timer最近会执行的Task的时间：2017-12-10 22:04:35------------>This TimerTask's time is 2017-12-10 22:04:35This TimerTask's name is near time<------------

小结：如果把scheduledExecutionTime()放在测试类使用测试出来的预计执行时间结果分两种情况

1、timer使用两个参数的schedule是准确的

2、timer使用三个参数的schedule是会比实际执行情况少一分钟

如果我把测试类改成这样

public class MyTimerTest02 {public static void main(String[] args) {Timer timer=new Timer();//Calendar calendar=Calendar.getInstance();Date date=new Date();SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-hh HH:mm:ss");System.out.println(sdf.format(date));//calendar.add(Calendar.SECOND, 2);//myTimerTask02.setName("test TimerTask Cancel");////timer.schedule(myTimerTask02, 2000, 1000);TimerTask03 myTimerTask03=new TimerTask03("near time");timer.schedule(myTimerTask03,2000,1000);//System.out.println("Timer最近会执行的Task的时间："+sdf.format(myTimerTask03.scheduledExecutionTime()));}}

把TimerTask03改成

public class TimerTask03 extends TimerTask{private String name;public TimerTask03(String name){this.name=name;}@Overridepublic void run() {// TODO Auto-generated method stub//Calendar calendar=Calendar.getInstance();System.out.println("------------>");SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-hh HH:mm:ss");System.out.println("Timer最近会执行的Task的时间："+sdf.format(scheduledExecutionTime()));System.out.println("This TimerTask's time is "+sdf.format(new Date()));System.out.println("This TimerTask's name is "+name);System.out.println("<------------");System.out.println();}public String getName() {return name;}public void setName(String name) {this.name = name;}}

结果是

2017-12-10 22:07:25------------>Timer最近会执行的Task的时间：2017-12-10 22:07:28This TimerTask's time is 2017-12-10 22:07:28This TimerTask's name is near time<------------------------>Timer最近会执行的Task的时间：2017-12-10 22:07:29This TimerTask's time is 2017-12-10 22:07:29This TimerTask's name is near time<------------

小结：在Task类里面调用scheduledExecutionTime()

因为是预计执行时间，取的是最近注意是最近，在Task里面调用之后，时间就是当前调用Task的时间。

总结：这个scheduledExecutionTime()还是有略微的问题的，我就不分析底层了。