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JVM performance optimization, Part 1: A JVM technology primer

Java performance for absolute beginners

By Eva Andreasson, JavaWorld.com, 08/21/12

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What is a Java virtual machine?

Simply speaking, a JVM is the software module that executes Java application bytecode and translates the bytecode into hardware- and operating system-specific instructions. By doing so, the JVM enables Java programs to be executed in different environments from where they were first written, without requiring any changes to the original application code. Java's portability is key to its popularity as an enterprise application language: developers don't have to rewrite application code for every platform because the JVM handles the translation and platform-optimization.

A JVM basically is a virtual execution environment acting as a machine for bytecode instructions, while assigning execution tasks and performing memory operations through interaction with underlying layers.

A JVM also takes care of dynamic resource management for running Java applications. This means it handles allocating and de-allocating memory, maintaining a consistent thread model on each platform, and organizing the executable instructions in a way that is suited for the CPU architecture where the application is executed. The JVM frees the programmer from keeping track of references between objects and knowing how long they should be kept in the system. It also frees us from having to decide exactly when to issue explicit instructions to free up memory -- an acknowledged pain point of non-dynamic programming languages like C.

You could think about the JVM as a specialized operating system for Java; its job is to manage the runtime environment for Java applications. A JVM basically is a virtual execution environment acting as a machine for bytecode instructions, while assigning execution tasks and performing memory operations through interaction with underlying layers.

JVM components overview

There's a lot more to write about JVM internals and performance optimization. As foundation for upcoming articles in this series, I'll conclude with an overview of JVM components. This brief tour will be especially helpful for developers new to the JVM, and should prime your appetite for more in-depth discussions later in the series.

From one language to another -- about Java compilers

A compiler takes one language as an input and produces an executable language as an output. A Java compiler has two main tasks:

  1. Enable the Java language to be more portable, not tied into any specific platform when first written
  2. Ensure that the outcome is efficient execution code for the intended target execution platform

Compilers are either static or dynamic. An example of a static compiler is javac. It takes Java code as input and translates it into bytecode -- a language that is executable by the Java virtual machine. Static compilers interpret the input code once and the output executable is in the form that will be used when the program executes. Because the input is static you will always see the same outcome. Only when you make changes to your original source and recompile will you see a different result.

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Resources
  • "To Collect or Not To Collect." (Eva Andreasson, Frank Hoffmann, Olof Lindholm; JVM-02: Proceedings of the Java Virtual Machine Research and Technology Symposium, 2002): Presents the authors' research into an adaptive decision process that determines which garbage collector technique should be invoked and how it should be applied.
  • "Reinforcement Learning for a dynamic JVM" (Eva Andreasson, KTH Royal Institute of Technology, 2002): Master thesis report on how to use reinforcement learning to better optimize the decision of when to start concurrent garbage collection for a dynamic workload.
  • "Deterministic Garbage Collection: Unleash the Power of Java with Oracle JRockit Real Time" (An Oracle White Paper, August 2008): Learn more about the Deterministic Garbage Collection algorithm in JRockit Real Time.
  • Why is Java faster when using a JIT vs. compiling to machine code? (Stackoverflow, December 2009): A thread discussion for learning more about Just-in-Time compiler technology.
  • Zing: A fully Java compliant highly scalable software platform that includes an application-aware resource controller and zero overhead, always-on production visibility and diagnostic tools. Zing incorporates industry-leading, proven technology to allow TBs memory heap sizes per instance with sustained throughput under dynamic load and extreme memory allocation rates common for Java applications.
  • "G1: Java's Garbage First Garbage Collector" (Eric Bruno, Dr. Dobb's, August 2009): A good overview of GC and introduction to the G1 garbage collector.
  • Oracle JRockit: The Definitive Guide (Marcus Hirt, Marcus Lagergren; Packt Publishing, 2010): A complete guide to the JRockit JVM.