计算时间差，调用System的nanoTime()还是curre

在日常开发中，经常会有计算方法调用时间的场景，我之前的做法一直是

long start =System.currentTimeMillis();
object.methodinvoke();
long end =System.currentTimeMillis();
long cost=end-start;//????

这里真的有个坑！！！！！！！！！

来看看currentTimeMillis()这个方法的源码注释,这个方法返回的是当前时间的微秒数。恰恰因为返回的是微秒数，而这个值的颗粒度取决于底层的操作系统，所以就可能会很大。例如，很多操作系统的时间颗粒度是10微妙。而且这个时间又可能受NTP影响而产生微调，从而导致时间很不准确。
综上，这种写法是不严谨的。

    /**
     * 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();

正确的操作应该是这个样子的：

long start =System.nanoTime();
object.methodinvoke();
long end =System.nanoTime();
long cost=end-start;// 为什么是准确的呢？

来看看nanoTime()的源码注释，这个方法返回的是JVM运行的纳秒数，它只依赖与当前的jvm，并且不会出现同步的情况，所以是准确的。

 /**
     * 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();

另外,在计算当前日期的时候是要使用currentTimeMillis()的，因为它是相对于 midnight, January 1, 1970 UTC的时间长度，而nanoTime()是相对于jvm的运行时间，这个时间是不确定的。