Compare Lucene memory estimator with terracota's and Alex Shipilev's object-layout

Alex Snaps informed me that there's a sizeof estimator in terracota –

http://svn.terracotta.org/svn/ehcache/trunk/ehcache/ehcache-core/src/main/java/net/sf/ehcache/pool/sizeof/

looks interesting, they have some VM-specific methods. Didn't look too deeply though; if somebody has the time to check out the differences and maybe compare the estimation differences it'd be nice.

There is also another tool by Aleksey Shipilev. It looks very good to me (Aleksey has deep knowledge of JVM internals).
https://github.com/shipilev/java-object-layout/