Re: System.currentTimeMillis() compile to native, accurate?

["Craig A. Vanderborgh" <craigv at voxware dot com>]

Rui Wang wrote:

> Hi, 
>
> I wonder that how gcj maps Java's "System.currentTimeMillis()" to native
> machine code. 
>
> In JVM, the time returned from "System.currentTimeMillis()" is not
> accurate due to various of issues, such as: garbage collection, 
> thread management model, configurations, etc. 
>
> In native environment, what will happen? Is it accurate? 
>
> Thank you 
> Rui 
>
>
>  
The source code for System.currentTimeMillis(), from natSystem.cc, looks 
like this:

jlong
java::lang::System::currentTimeMillis (void)
{
 return _Jv_platform_gettimeofday ();
}

In turn, _Jv_platform_gettimeofday() looks like this:

// gettimeofday implementation.
jlong
_Jv_platform_gettimeofday ()
{
#if defined (HAVE_GETTIMEOFDAY)
 timeval tv;
 gettimeofday (&tv, NULL);
 return (tv.tv_sec * 1000LL) + (tv.tv_usec / 1000LL);
#elif defined (HAVE_TIME)
 return time (NULL) * 1000LL;
#elif defined (HAVE_FTIME)
 struct timeb t;
 ftime (&t);
 return (t.time * 1000LL) + t.millitm;
#elif defined (ECOS)
 // FIXME.
 return _clock();
#else
 // In the absence of any function, time remains forever fixed.
 return 23000;
#endif
}

So in answer to your question: "is it accurate"?  It depends.  It 
depends on what else is going on in your program.  Under conditions 
where currentTimeMillis() is called and your next lines of code receive 
the result without being interrupted by another thread or by garbage 
collection, currentTimeMillis() is sure to very closely reflect what 
gettimeofday() would produce in a C program.

By the way, if I'm not mistaken "real Java" also uses a native method 
implentation in JNI to provide about the same implementation for 
currentTimeMillis().  In this regard, GCJ and JVM are more similar than 
different.

hth,
craig vanderborgh
voxware incorporated



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