RFR 8068730: Increase the precision of the implementation of java.time.Clock.systemUTC()
david.holmes at oracle.com
Mon Jan 12 02:20:58 UTC 2015
Looking at the hotspot part ...
On 10/01/2015 2:56 AM, Daniel Fuchs wrote:
> Please find below a webrev for:
> 8068730: Increase the precision of the implementation
> of java.time.Clock.systemUTC()
> 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.
> 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)
Everything seems consistent with javaTimeMillis code. Only nit on
Windows is that windows_time_stamp isn't a very descriptive name (no
idea what it returns) - perhaps windows_time_as_micros and adjust the
> - 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)
IIUC JVM_GetNanoTimeAdjustment(jlong offset_secs) returns the difference
in nanoseconds between current UTC time (from Epoch) and the UTC time
since the Epoch that offset_secs represents - is that right? I don't
find the name particularly intuitive and I guess you renamed it
somewhere along the way as the mapfiles are no longer in alphabetical
order (but would be if this was JVM_GetTimeXXXX). So please fix mapfiles.
> java (jdk) side:
> - adds a native sun.misc.VM.getNanoTimeAdjustment method
> (which is bound to JVM_GetNanoTimeAdjustment)
> (see VM.java and VM.c)
Just curious: why the indirection through sun.misc.VM?
> - modifies java.time.Clock.SystemClock to call the new
> sun.misc.VM.getNanoTimeAdjustment instead of
> - fixes java.util.Formatter - which wasn't expecting
> greater than millisecond precision.
> - 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
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