在Java中我们常常使用System.currentTimeMillis()来进行时间计算,它的精确度是毫秒级别的,如果我们想来的更加精确,可以使用

System.nanoTime(),这个是精确到纳米级别的方法。

我们来看一看方法注释:

/**
* Returns the current value of the most precise available system
* timer, in nanoseconds.
*
* <p>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 (2<sup>63</sup>
* nanoseconds) will not accurately compute elapsed time due to
* numerical overflow.
*
* <p> For example, to measure how long some code takes to execute:
* <pre>
* long startTime = System.nanoTime();
* // ... the code being measured ...
* long estimatedTime = System.nanoTime() - startTime;
* </pre>
*
* @return The current value of the system timer, in nanoseconds.
* @since 1.5
*/

此方法只能用于计时,不能用于时间推算。