CollectionSpy is a Java profiler that focuses on tracking and analyzing your program's usage of Collection Framework containers. It detects hashing container (e.g. HashMap) corruption due to mutating keys. It tracks expensive internal rehashing of containers whose capacity needs expanding. It detects multithreaded access to any unsynchronized container (e.g. HashMap, ArrayList). It visualizes hashing container bucket list lengths, allowing you to diagnose worst-case access performance.