What are some interesting uses for Java Agents?

Question

Starting with Java 5 there's an option to add Java Agents to the class loader.

Have you written any Agents? have you used any Agents? What are interesting uses of Agents?

Answer 1

I have used various java profilers that use this feature. A good open source java profiler that uses this is Profiler4j (http://profiler4j.sourceforge.net).

This is a great tool for finding performance bottlnecks in your java code.