RFR 8068730: Increase the precision of the implementation of java.time.Clock.systemUTC()

[Daniel Fuchs]

Hi,

Please find below a webrev for:

8068730: Increase the precision of the implementation
          of java.time.Clock.systemUTC()
https://bugs.openjdk.java.net/browse/JDK-8068730

The java.time.Clock.system() method (and variants thereof) are
specified to "obtain a clock that returns the current instant
using best available system clock". However the current
implementation of the clock returned is based on
System.currentTimeMillis() whereas the underlying native clock
used by System.currentTimeMillis() has often a greater precision
than milliseconds (for instance, on Linux, System.currentTimeMillis()
is based on gettimeofday, which offers microseconds precision).

This patch enhances the implementation of the
java.time.Clock.SystemClock, so that it offer at least the
same precision than the underlying clock available on the system.

There is no change in the public API.

http://cr.openjdk.java.net/~dfuchs/webrev_8068730/webrev.00/

Some more details on the patch:

native (hotspot) side:

  - adds a new method to the os class:
    os::javaTimeSystemUTC(jlong &seconds, jlong &nanos)
    which allows to get the time in the form of a number
    of seconds and number of nanoseconds
    (see os.hpp and various os_<os>.cpp files)

  - adds a JVM_GetNanoTimeAdjustment method, which takes
    an offset (in seconds) as parameter and returns a
    nano time adjustment compared to the offset.
    Calls os::javaTimeSystemUTC to compute the adjustment
    (see jvm.cpp)

java (jdk) side:

  - adds a native sun.misc.VM.getNanoTimeAdjustment method
    (which is bound to JVM_GetNanoTimeAdjustment)
    (see VM.java and VM.c)

  - modifies java.time.Clock.SystemClock to call the new
    sun.misc.VM.getNanoTimeAdjustment instead of
    System.currentTimeMillis.

  - fixes java.util.Formatter - which wasn't expecting
    greater than millisecond precision.

testing:

  - A new test is added to test sun.misc.VM.getNanoTimeAdjustment
    In particular it checks the edge cases.
  - New tests are added to TestClock_System.java to check for
    the increased precision.
    There is also a test for the edge cases there.
  - Some java.time tests where tripped by the increased precision,
    and had to be modified to take that into account

Note: comparing the new os::javaTimeSystemUTC with os::javaTimeMillis
       can help in reviewing the changes.

best regards,

-- daniel