Jacoco is an open-source code coverage library for Java distributed under the Eclipse Public License. The library is often used in Java projects to measure the coverage of unit or integration tests.

Instrumentation: querying the memory usage of a Java object provides a nice overview of this interface and provides a link to the Classmexer agent, “a simple Java instrumentation agent that ...

Java 5 added the Java Instrumentation mechanism, which allows you to provide “Java agents” that can inspect and modify the byte code of the classes as they are loaded.