How can i benchmark method execution time in java?

Other than using a profiler, a simple way of getting what you want is the following:

public class SomeClass{
   public void somePublicMethod()
   {
       long startTime = System.currentTimeMillis();
       someMethodWhichYouWantToProfile();
       long endTime = System.currentTimeMillis();
       System.out.println("Total execution time: " + (endTime-startTime) + "ms"); 
   }
 }

If the bottleneck is big enough to be observed with a profiler, use a profiler.

If you need more accuracy, the best way of measuring an small piece of code is the use of a Java microbenchmark framework like OpenJDK's JMH or Google's Caliper. I believe they are as simple to use as JUnit and not only you will get more accurate results, but you will gain the expertise from the community to do it right.

Follows a JMH microbenchmark to measure the execution time of Math.log():

private double x = Math.PI;

@Benchmark
public void myBenchmark() {
    return Math.log(x)
}

Using the currentMillis() and nanoTime() for measuring has many limitations:

1. They have latency (they also take time to execute) which bias your measurements.
2. They have VERY limited precision, this means you can mesure things from 26ns to 26ns in linux and 300 or so in Windows has described here
3. The warmup phase is not taken into consideration, making your measurements fluctuate A LOT.

The currentMillis() and nanoTime() methods in Java can be useful but must be used with EXTREME CAUTION or you can get wrong measurements errors like this where the order of the measured snippets influence the measurements or like this where the author wrongly conclude that several millions of operations where performed in less than a ms, when in fact the JMV realised no operations where made and hoisted the code, running no code at all.

Here is a wonderful video explaining how to microbenchmark the right way: https://shipilev.net/#benchmarking