Beta: Classmexer agent

The beta version of a simple instrumentation tool is available for download from the Javamex site. The Classmexer jar provides various calls for querying the memory usage of Java objects. Via the provided MemoryUtil class.deepMemoryUsageOf() method, it is possible to get an estimate from the JVM of the number of bytes taken up by an object and its "subobjects" (objects referred to by a non-public reference, or by references with other visibility criteria). The memory usage of subobjects is combined recursively (so subobjects of subobjects are considered etc), but without counting the same object more than once.

A variant of the call is also provided which gives the total memory usage of several objects at a time, without counting as duplicates objects referenced by more than one of the objects.

Updates to profiling section

Firstly, some minor corrections and additions to the Java profiling section. The corrections mainly concern a couple of typos that crept into the variable names of the examples. Readers should be reassured that the code, like that of the site in general, is copied and pasted from working, live profiling code. But things such as variable names are occasionally changed or shortened for the purposes of making it clearer on the site, and that seems to be where the errors crept in. I've also taken the opportunity to add a few links to other sections of the site (such as the section on threading, sleep() and yield()) that were added since the profiling tutorial was written.

Readers interested in Java profiling may also be interested in the first page of an upcoming section on Java and memory. This first page looks at how to find out the memory usage of a Java object. The technique involves using the Java Instrumentation framework introduced in version 5 of the language to query the JVM directly for the size of an object. Although slightly fiddly to set up, the technique has the advantage that there's less guesswork involved than if we were to just estimate an object's size (although future pages in the section will nonetheless look at estimation).