Internals of Java Virtual Machine based on challenges and experience in implementation of one of the most popular code coverage tools for Java - JaCoCo ( http://www.jacoco.org/ ) that is backbone of award-winning Eclipse EclEmma Plugin ( https://www.eclipse.org/eclemma ). In this presentation we will dive into bosom of JVM (bytecode, restrictions, classloaders) answering on following questions: - How by developing JaCoCo we find bugs in OpenJDK? You also can! - How JaCoCo manages to work in OSGi environment? - What makes it fastest? - And which changes were required to support Java 8? Java 9?