Optimizing Java

Chapter 2. Overview of the JVM

There is no doubt that Java is one of the largest technology platforms on the planet, boasting roughly 9–10 million developers (according to Oracle). By design, many developers do not need to know about the low-level intricacies of the platform they work with. This leads to a situation where developers only meet these aspects when a customer complains about performance for the first time.

For developers interested in performance, however, it is important to understand the basics of the JVM technology stack. Understanding JVM technology enables developers to write better software and provides the theoretical background required for investigating performance-related issues.

This chapter introduces how the JVM executes Java in order to provide a basis for deeper exploration of these topics later in the book. In particular, Chapter 9 has an in-depth treatment of bytecode. One strategy for the reader could be to read this chapter now, and then reread it in conjunction with Chapter 9, once some of the other topics have been understood.

Interpreting and Classloading

According to the specification that defines the Java Virtual Machine (usually called the VM Spec), the JVM is a stack-based interpreted machine. This means that rather than having registers (like a physical hardware CPU), it uses an execution stack of partial results and performs calculations by operating on the top value (or values) of that stack.

The basic behavior of the JVM interpreter ...