Java Performance, 2nd Edition

Chapter 4. Working with the JIT Compiler

The just-in-time (JIT) compiler is the heart of the Java Virtual Machine; nothing controls the performance of your application more than the JIT compiler.

This chapter covers the compiler in depth. It starts with information on how the compiler works and discusses the advantages and disadvantages of using a JIT compiler. Until JDK 8 came along, you had to choose between two Java compilers. Today, those two compilers still exist but work in concert with each other, though in rare cases choosing one is necessary. Finally, we’ll look at some intermediate and advanced tunings of the compiler. If an application is running slowly without any obvious reason, those sections can help you determine whether the compiler is at fault.

Just-in-Time Compilers: An Overview

We’ll start with some introductory material; feel free to skip ahead if you understand the basics of just-in-time compilation.

Computers—and more specifically CPUs—can execute only a relatively few, specific instructions, which are called machine code. All programs that the CPU executes must therefore be translated into these instructions.

Languages like C++ and Fortran are called compiled languages because their programs are delivered as binary (compiled) code: the program is written, and then a static compiler produces a binary. The assembly code in that binary is targeted to a particular CPU. Complementary CPUs can execute the same binary: for example, AMD and Intel CPUs share a ...