One of the JavaProfilers, like SitrakaJprobe or AlphaworksJinsight. 

''OptimizeIt is slick and light by Jprobe standards, but lacks a number of nice features.  Furthermore, I found the Solaris version to be buggy to the point of uselessness.'' -- RobertChurch

OptimizeIt has done Wonders for my programs and saved my bacon a few times. I never realised just how little I understood my programs before I ran them with it :-) -- LukeGorrie