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Encouraging lot, aren't we :)
Profilers aren't too hard if you're just trying to get a reasonable idea of where the program's spending most of its time. If you're bothered about high accuracy and minimum disruption, things get difficult.
So if you just want the answers a profiler would give you, go for one someone else has written. If you're looking for the intellectual challenge, why not have a go at writing one?
I've written a couple, for run time environments that the years have rendered irrelevant.
There are two approaches
adding something to each function or other significant point that logs the time and where it is.
having a timer going off regularly and taking a peek where the program currently is.
The JVMPI version seems to be the first kind - the link provided by uzhin shows that it can report on quite a number of things (see section 1.3). What gets executed changes to do this, so the profiling can affect the performance (and if you're profiling what was otherwise a very lightweight but often called function, it can mislead).
If you can get a timer/interrupt telling you where the program counter was at the time of the interrupt, you can use the symbol table/debugging information to work out which function it was in at the time. This provides less information but can be less disruptive. A bit more information can be obtained from walking the call stack to identify callers etc. I've no idea if these is even possible in Java...
Paul.