所以这里提醒做非常精确的时间统计的朋友，谨慎使用System.currentTimeMillis() 。
在Java中可以通过System.currentTimeMillis()或者System.nanoTime() (JDK>=5.0) 方法获得当前的时间的精确值。但是通过阅读Javadoc，我们发现这两个方法并不一定保证得到你所期望的精度。先来看 System.currentTimeMillis()：
Returns the current time in milliseconds. Note that while the unit of time of the return value is a millisecond, the granularity of the value depends on the underlying operating system and may be larger. For example, many operating systems measure time in units of tens of milliseconds.
JDK5.0加入的System.nanoTime()方法。Javadoc对该方法的描述如下：
Returns the current value of the most precise available system timer, in nanoseconds.
This method can only be used to measure elapsed time and is not related to any other notion of system or wall-clock time. The value returned represents nanoseconds since some fixed but arbitrary time (perhaps in the future, so values may be negative). This method provides nanosecond precision, but not necessarily nanosecond accuracy. No guarantees are made about how frequently values change. Differences in successive calls that span greater than approximately 292 years (263 nanoseconds) will not accurately compute elapsed time due to numerical overflow.
它返回系统能够提供的最为精确的计时，以纳秒(10亿分之一秒)为单位，但并不保证纳秒级精度。
如果你的Java程序需要高精度的计时，如1毫秒或者更小，使用System.nanoTime()方法，它完全可以满足你的需求。