Why does the new Java 8 Date Time API not have nan

问题:

This question already has an answer here:

• Java 8 Instant.now() with nanosecond resolution? 4 answers

One of the features of the new Date Time API in Java 8 is supposed to be nanosecond precision. However when I print the current Date Time to the console like so

DateTimeFormatter formatter = DateTimeFormatter
    .ofPattern("yyyy-MM-dd'T'HH:mm:ss,nnnnnnnnnZ");
System.out.println(OffsetDateTime.now().format(formatter)); 

I only see millisecond precision: 2015-11-02T12:33:26,746000000+0100

The operating system does seem to support nanosecond precision. When I print the current date time via the Terminal

date -Ins

I see 2015-11-02T12:33:26,746134417+0100

How do I get nanosecond precision in Java? I'm running Oracle Java 1.8.0_66 on Ubuntu 14.04 64-bit

回答1:

The java.time API in general does have nanosecond precision. For example:

DateTimeFormatter formatter = DateTimeFormatter
    .ofPattern("yyyy-MM-dd'T'HH:mm:ss,nnnnnnnnnZ");
OffsetDateTime odt = OffsetDateTime.of(2015, 11, 2, 12, 38, 0, 123456789, ZoneOffset.UTC);
System.out.println(odt.format(formatter));

Output:

2015-11-02T12:38:00,123456789+0000

However, it's the clock value returned by OffsetDateTime.now() which is returning a value which only has milliseconds.

From Clock implementation in Java 8:

The clock implementation provided here is based on System.currentTimeMillis(). That method provides little to no guarantee about the accuracy of the clock. Applications requiring a more accurate clock must implement this abstract class themselves using a different external clock, such as an NTP server.

So there's nothing inherently imprecise here - just the default implementation of Clock using System.currentTimeMillis(). You could potentially create your own more precise subclass. However, you should note that adding more precision without adding more accuracy probably isn't terribly useful. (There are times when it might be, admittedly...)

回答2:

To make an important addition to the answer of Jon Skeet, Java 9 is supposed to deliver a clock in improved precision - see the bug log. Background: On many operating systems (especially Linux), there are better clocks available.

The Java SE 8 specification for java.time.Clock states that "The system factory methods provide clocks based on the best available
system clock. This may use System.currentTimeMillis(), or a higher resolution clock if one is available.". In JDK 8 the implementation
of the clock returned was based on System.currentTimeMillis(), and thus has only a millisecond resolution. In JDK 9, the implementation
is based on the underlying native clock that System.currentTimeMillis() is using, providing the maximum resolution available from that clock. On most systems this can be microseconds, or sometimes even tenth of microseconds.

An application making the assumption that the clock returned by these system factory methods will always have milliseconds precision and actively depends on it, may therefore need to be updated in order to take into account the possibility of a greater resolution, as was
stated in the API documentation.

It should also be noted the (exotic) fact that second precision will not exist near leap seconds - not even in Java 9.