How do I time a method's execution in Java?

There is always the old-fashioned way:

long startTime = System.nanoTime();
methodToTime();
long endTime = System.nanoTime();

long duration = (endTime - startTime);  //divide by 1000000 to get milliseconds.

I go with the simple answer. Works for me.

long startTime = System.currentTimeMillis();

doReallyLongThing();

long endTime = System.currentTimeMillis();

System.out.println("That took " + (endTime - startTime) + " milliseconds");

It works quite well. The resolution is obviously only to the millisecond, you can do better with System.nanoTime(). There are some limitations to both (operating system schedule slices, etc.) but this works pretty well.

Average across a couple of runs (the more the better) and you'll get a decent idea.

Come on guys! Nobody mentioned the Guava way to do that (which is arguably awesome):

import com.google.common.base.Stopwatch;

Stopwatch timer = Stopwatch.createStarted();
//method invocation
LOG.info("Method took: " + timer.stop());

The nice thing is that Stopwatch.toString() does a good job of selecting time units for the measurement. I.e. if the value is small, it'll output 38 ns, if it's long, it'll show 5m 3s

Even nicer:

Stopwatch timer = Stopwatch.createUnstarted();
for (...) {
   timer.start();
   methodToTrackTimeFor();
   timer.stop();
   methodNotToTrackTimeFor();
}
LOG.info("Method took: " + timer);

Note: Google Guava requires Java 1.6+