Java Performance Testing [duplicate]

Answer 1

The code shown in the question is not a good performance measuring code:

1. The compiler might choose to optimize your code by reordering statements. Yes, it can do that. That means your entire test might fail. It can even choose to inline the method under test and reorder the measuring statements into the now-inlined code.

2. The hotspot might choose to reorder your statements, inline code, cache results, delay execution...

3. Even assuming the compiler/hotspot didn't trick you, what you measure is "wall time". What you should be measuring is CPU time (unless you use OS resources and want to include these as well or you measure lock contestation in a multi-threaded environment).

The solution? Use a real profiler. There are plenty around, both free profilers and demos / time-locked trials of commercials strength ones.

Answer 2

That's good when you are comparing one implementation to another or trying to find a slow part in your code (although it can be tedious). It's a really good technique to know and you'll probably use it more than any other, but be familiar with a profiling tool as well.

Answer 3

Well that is just one part of performance testing. Depending on the thing you are testing you may have to look at heap size, thread count, network traffic or a whole host of other things. Otherwise I use that technique for simple things that I just want to see how long they take to run.