Optimizing Java

Chapter 10. Understanding JIT Compilation

In this chapter we will delve deeper into the inner workings of the JVM’s JIT compiler. The majority of the material is directly applicable to HotSpot, and is not guaranteed to be the same for other JVM implementations.

Having said that, the general science of JIT compilation is quite well studied, and JIT implementations occur in many modern programming environments, not just the JVM. As a result, many of the same JIT techniques are applicable to other JIT compilers.

Due to the abstract and technically complex nature of the subject matter, we will rely upon tooling to help understand and visualize the inner workings of the JVM. The primary tool we will be using is JITWatch, and we introduce it at the start of the chapter. Following this, we will be able to explain specific JIT optimizations and features and show how the technique and its effects can be observed from JITWatch.

Introducing JITWatch

JITWatch is an open source JavaFX tool designed and built by one of this book’s authors (Chris Newland) as a personal project. The tool is now hosted under the AdoptOpenJDK initiative, as part of a program run by the London Java Community to improve the amount of community participation in the Java ecosystem.

JITWatch allows a development or devops team to better understand what HotSpot actually did to the bytecode during the execution of the application. It is possible to tune compilation switches to improve the performance of our applications, ...