java - Measuring time differences using System.currentTimeMillis()

Question

I have a simple java program, and I want to know the time difference between some set of operations. For this question the details are not important, but let us take the following scenario.

long beginTime = System.currentTimeMillis();

//Some operations. Let us asssume some database operations etc. which are time consuming.

//

long endTime = System.currentTimeMillis();

long difference = endTime - beginTime;

When the code is run on a machine, how reliable will the difference be?

Let us say, that the processor starts executing some instructions from my code, then gives context to another process, which executes for some time, and then comes back to execute instructions related to this java process.

So, the time difference should depend on the current state of my machine, i.e. how many processes are running etc? So, in profiling time it takes for some operations to run, is this mechanism not reliable?

Answer 1

The granularity of System.currentTimeMillis() depends on the implementation and on the Operating system and is usually around 10 ms.

Instead use the System.nanoTime() which returns the current value of the most precise available system timer, in nanoseconds. Note that you can only use this to calculate elapsed time, you cannot use its value as an absolute time.

Example:

long startTime = System.nanoTime();
// do something you want to measure
long elapsedTimeNs = System.nanoTime() - startTime;