What is the JVM? Introducing the Java virtual machine

To implement the Java virtual machine correctly, you need only be able to read the class file format and correctly perform the operations specified therein.

J.S. Bach once described creating music similarly:

All you have to do is touch the right key at the right time.

So, all the JVM has to do is make class files behave correctly. Sounds simple, and might even look simple from the outside, but it is a massive undertaking, especially given the power and flexibility of the Java language.

JVM implementations

Implementing the JVM specification results in an actual software program, which is a JVM implementation. In fact, there are many JVM implementations, both open source and proprietary. OpenJDK’s HotSpot is the JVM reference implementation. It remains one of the most thoroughly tried-and-tested codebases in the world.