How to get a meaningful result from subtracting 2 nanoTime objects?

Question

I created a filter that monitors the length of a request.

long start = System.nanoTime();

...

long end = System.nanoTime();

How can I get the number of milliseconds from this now?

Answer 1

Just subtract them and divide result by 10^6.

1 nanosecond is 10^-9 seconds and, correspondingly, 10^-6 milliseconds.
http://en.wikipedia.org/wiki/Nano-

Answer 2

(end - start) / 1000000

1 microsecond = 1000 nanoseconds

1 millisecond = 1000 microseconds

Note, that the result will be rounded down, but you usually don't get true nanosecond accuracy anyway (accuracy depends on the OS). From the Javadoc on nanoTime():

This method provides nanosecond precision, but not necessarily nanosecond accuracy.

Answer 3

You could just use System.currentTimeMillis().

Caveat:

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.

Answer 4

To get a meaningful result:

void procedure ( ... )
{
     ...
}

double measureProcedure ( double epsilon , ... )
{
    double mean ;
    double stderr = 2 * epsilon ;
    while ( stderr > epsilon )
    {
         long start = System.nanoTime();
         procedure ( ... ) ;
         long end = System.nanoTime();
         // recalculate mean , stderr 
    }
    return ( mean / 1000000 ) ;
}