This book is a collection of notes and tutorial codes I wrote while I was trying different implementations of the JVM (Java Virtual Machine). Topics include: CDS, Class Data Sharing, Garbage collection, GC, HotSpot, JDK 1.3.0, JDK 1.4.0, JDK 1.4.2, JDK 1.5.0, J2SDK, JRockit, Memory, Out of memory, Performance, Stack overflow, String buffer, etc.