Can't see my own application methods in Java VisualVM

Question

I'm trying to profile my java app, just to find out the methods in which most time is being spent. Given the poor reactions here to TPTP, I thought I'd give Java VisualVM a go.

It all seemed rather simple to use - except that I can't seem to get anything consistent or useful out of it.

I can't seem to see anything relating to MY OWN code - all I get is a whole bunch of calls to things like java.* methods.

I've tried restricting instrumentation to only my own packages, which seems to cut down the number of methods instrumented, but still I don't ever seem to see my own.

Each time I run, I get varying numbers of methods instrumented, ranging from 10's to 1000's. I've tried putting in a sleep at the start of my app, to make sure I get VisualVM up and running before my app starts to do anything interesting, to make sure it's profiling when the interesting stuff is running.

Is there something I have to do to ensure my classes get instrumented ? Are there timing issues ? ..like, have to wait for classes to be loaded etc ? I've also tried running the guts of the code twice, to make sure all the code does get exercised...

I'm just running an app, with a main, from Eclipse. I've tried using the Eclipse integration so that VisualVM starts up when I start the app - results are the same. I've also tried exporting the app as a runnable app, and running it standalone from the command line, rather than through Eclipse - same result.

My app is not a long running web app etc - just a main that calls some other of my own classes to do some processing, then quits.

I'd be grateful for any advice about what I might be doing wrong ! :)

Thanks !

Answer 1

I assume this isn't just an academic question - you would like to see if you could make the app run faster. I assume you also wouldn't mind a little "out of the box" thinking. There are many popular ideas about performance that are actually pretty fuzzy.

For example, you say you're looking for "methods in which most time is being spent". If by that you mean "self time" (program counter actually in the method) there is probably very little, unless you've got some intense loops. Methods generally spend time by calling other methods, sometimes doing I/O.

Another fuzzy idea is that measuring method time or counting the number of calls can tell you very much about where bottlenecks are. Bottlenecks are specific lines of code, not methods, so even if you know approximately where to look, you're still playing detective.

So those are a few of the fuzzy ideas. Here is a bunch more. Let me suggest how one should think about it, and how that leads to results.

When you eventually fix something, it will reduce execution time by some percent, like (pick a number) 30%, right? (Otherwise you didn't fix anything.) OK, during that 30% it was doing something, something that it didn't need to do because later you got rid of it. So, you don't need to measure. You do need to find out what it is doing in that time, so you know what to get rid of.

A very simple way is to "pause" it 10 (or some number of) times at random. Understand what it is doing and why, by looking at the call stack and possibly some of the data. On about 3 of those times you will see it doing something you could get rid of.

You will know approximately how much it will save by seeing what percent of samples is showing it. Approximate is good enough. You can easily see exactly how much time is saved by stopwatching it before and after.

Then, don't stop. You've made the app faster. Do it again, and make it faster yet. Sooner or later you get to a point where you can't make it any faster, but it's probably in more than one step.