ScheduledExecutorService 定时任务执行时间有偏差

[u010217111]

public class Task {

/**
* 每天晚上8点执行一次
* 每天定时安排任务进行执行
*/
public static void executeEightAtNightPerDay() {
ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);

long oneHour = 2 * 60 * 1000;

Calendar c = Calendar.getInstance();
//加一个小时，设置对应的分和秒为0则得到下一个整点
c.add(Calendar.HOUR, 1);
c.set(Calendar.MINUTE, 0);
c.set(Calendar.SECOND, 0);

System.out.println(StringUtil.dateToString(c.getTime()));
long initDelay = c.getTime().getTime() - System.currentTimeMillis();
initDelay = initDelay > 0 ? initDelay : oneHour + initDelay;

executor.scheduleAtFixedRate(
new TestTask(),
initDelay,
oneHour,
TimeUnit.MILLISECONDS);
}

/**
* 获取指定时间对应的毫秒数
* @param time "HH:mm:ss"
* @return
*/
private static long getTimeMillis(String time) {
try {
DateFormat dateFormat = new SimpleDateFormat("yy-MM-dd HH:mm:ss");
DateFormat dayFormat = new SimpleDateFormat("yy-MM-dd");
Date curDate = dateFormat.parse(dayFormat.format(new Date()) + " " + time);
return curDate.getTime();
} catch (ParseException e) {
e.printStackTrace();
}
return 0;
}
}





public class TestTask implements Runnable{

public void run() {
try {
Thread.sleep(50);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("This is a echo server. The current time is " + dateToString(new Date()) + ".");
}

public static String dateToString(Date date) {
String str = "";
if (date != null) {
SimpleDateFormat dateFormat = new SimpleDateFormat(
"yyyy-MM-dd HH:mm:ss");
str = dateFormat.format(date);
}

return str;
}
}



public class T {

public static void main(String[] args) {
Task task = new Task();
task.executeEightAtNightPerDay();
}
}



第一次任务执行时间是下一个整点开始执行，以后每隔两小时执行一次。任务运行一段时间后发现TestTask中的运行时间不是按照整点来运行的？求解