SystemnanoTime

System.nanoTime is a method in the Java standard library (java.lang.System) that provides access to a high-resolution timer. It returns a long value representing nanoseconds elapsed on a monotonic timer with an arbitrary origin. The value is not related to wall-clock time or the epoch, and should not be used as a timestamp for logging or event times.

Because the origin is arbitrary, the number cannot be interpreted as a real-world clock reading. It is intended for measuring elapsed time with high precision, such as benchmarking or duration calculations. System.nanoTime is designed to be monotonic, meaning it should not go backward if the system clock changes, in contrast to System.currentTimeMillis, which reflects wall-clock time and can jump.

Usage is straightforward: capture two readings and subtract to obtain elapsed time in nanoseconds. For example, long start = System.nanoTime(); // code to measure long end = System.nanoTime(); long duration = end - start; The resulting duration is in nanoseconds and can be converted to other units by division, for instance dividing by 1_000_000 to get milliseconds.

Notes and limitations: the actual timer resolution depends on the underlying operating system and hardware, so the reported elapsed time may not be precise to a single nanosecond. The method may return identical values for very short intervals. The return type is long, so the counter can wrap after a very long period (about 292 years); for typical usage this does not affect elapsed-time calculations when subtracting two values.

See also: System.currentTimeMillis, monotonic clocks in other environments.