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

------------>
This TimerTask's time is 2017-12-10 22:00:35
This 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 22:04:33
Timer最近会执行的Task的时间：2017-12-10 22:04:35
------------>
This TimerTask's time is 2017-12-10 22:04:35
This 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;
	}
	@Override
	public 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 22:07:28
This TimerTask's time is 2017-12-10 22:07:28
This TimerTask's name is near time
<------------

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