/**

      * 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.

      *

      * <p> See the description of the class <code>Date</code> for

      * a discussion of slight discrepancies that may arise between

      * "computer time" and coordinated universal time (UTC).

      *

      * @return  the difference, measured in milliseconds, between

      *          the current time and midnight, January 1, 1970 UTC.

      * @see     java.util.Date

      */

     public static native long currentTimeMillis();

1  以毫秒的方式返回当前时间。请注意，虽然返回值的时间单位是毫秒，但是这个值的粒度取决于底层操作系统并且可能粒度更大。例如，许多操作系统是以几十毫秒为粒度测量时间的。

2  有关于“计算机时间”和协调世界时（UTC）之间的细微差别， 请查阅“Date”类。

3  @return 当前时间和1970年1月1日午夜之间的差值，以毫秒来测量。

4  @see java.util.Date

 /**

      * Returns the current value of the running Java Virtual Machine's

      * high-resolution time source, 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 <i>origin</i> time (perhaps in the future, so values

      * may be negative).  The same origin is used by all invocations of

      * this method in an instance of a Java virtual machine; other

      * virtual machine instances are likely to use a different origin.

      *

      * <p>This method provides nanosecond precision, but not necessarily

      * nanosecond resolution (that is, how frequently the value changes)

      * - no guarantees are made except that the resolution is at least as

      * good as that of {@link #currentTimeMillis()}.

      *

      * <p>Differences in successive calls that span greater than

      * approximately 292 years (2<sup>63</sup> nanoseconds) will not

      * correctly compute elapsed time due to numerical overflow.

      *

      * <p>The values returned by this method become meaningful only when

      * the difference between two such values, obtained within the same

      * instance of a Java virtual machine, is computed.

      *

      * <p> For example, to measure how long some code takes to execute:

      *  <pre> {@code

      * long startTime = System.nanoTime();

      * // ... the code being measured ...

      * long estimatedTime = System.nanoTime() - startTime;}</pre>

      *

      * <p>To compare two nanoTime values

      *  <pre> {@code

      * long t0 = System.nanoTime();

      * ...

      * long t1 = System.nanoTime();}</pre>

      *

      * one should use {@code t1 - t0 < 0}, not {@code t1 < t0},

      * because of the possibility of numerical overflow.

      *

      * @return the current value of the running Java Virtual Machine's

      *         high-resolution time source, in nanoseconds

      * @since 1.5

      */

     public static native long nanoTime();

 1 返回正在运行的Java虚拟机的高分辨率时间源的当前值，以纳秒计。

 2

 3 该方法可能仅仅用于测量已经逝去的时间，并且与任何其它系统或者挂钟时间概念无关。该返回值表示从某个固定但任意的原点时间（可能在未来，所以值可能是负数）开始的纳秒数。在一个java虚拟机实例中，所有该方法的调用都使用相同的原点；其它虚拟机实例很可能使用不同的源头。

 4

 5 该方法提供了纳秒级别的精度，但是不一定是纳秒级分辨率（也就是该值改变的频率）———— 除非这个分辨率至少和currentTimeMillis()一样好，否则将不会做任何保证。

 6

 7 在跨越大于292年（2的63次方纳秒）左右的连续调用中，这个差值将不能正确地计算已经过去的时间，因为数字溢出。

 8

 9 仅仅只有当在同一java虚拟机实例中获取的两个值之间的差值被计算时，返回值才有意义。

10

11 例如，去测量某代码执行花费了多长时间：

12  long startTime = System.nanoTime();

13  //...被测量的代码...

14  long estimatedTime = System.nanoTime() - startTime;

15

16 要比较两个nanoTime的值：

17  long t0 = System.nanoTime();

18  ...

19  long t1 = System.nanoTime()。

20 因为数字溢出的可能性，您应该使用"t1 - t0 < 0"，而不是"t1 < t0"（来判断它们的大小，笔者注）。

21 @return 当前正在运行的java虚拟机的高精度时间资源值，以纳秒为单位。

22 @since 1.5