Since the first edition of this book was published in November 2013, Clojure has seen a much wider adoption and has witnessed many success stories. The newer versions of Clojure fortify its performance story while staying close to its roots—simplicity and pragmatism. This edition significantly updates the book for Clojure 1.7, and adds a new chapter on the performance measurement.
The Java Virtual Machine plays a huge role in the performance of the Clojure programs. This edition of the book increases the focus on the JVM tools for performance management, and it explores how to use those. This book is updated to use Java 8, though it also highlights the places where features from Java 7 or 8 have been used.
This book is updated mainly to be more of practical use to the readers. I hope that this edition will better equip the readers with performance measurement and profiling tools and with the know-how of analyzing and tuning the performance characteristics of Clojure code.
Chapter 1, Performance by Design, classifies the various use cases with respect to performance, and analyzes how to interpret their performance aspects and needs.
Chapter 2, Clojure Abstractions, is a guided tour of various Clojure data structures, abstractions (persistent data structures, vars, macros, transducers, and so on), and their performance characteristics.
Chapter 3, Leaning on Java, discusses how to enhance performance by using Java interoperability and features from Clojure.
Chapter 4, Host Performance, discusses how the host stack impacts performance. Being a hosted language, Clojure has its performance directly related to the host.
Chapter 5, Concurrency, is an advanced chapter that discusses the concurrency and parallelism features in Clojure and JVM. Concurrency is an increasingly significant way to derive performance.
Chapter 6, Measuring Performance, covers various aspects of performance benchmarks and measuring other factors.
Chapter 7, Performance Optimization, discusses systematic steps that need to be taken in order to identify that the performance bottlenecks obtain good performance.
Chapter 8, Application Performance, discusses building applications for performance. This involves dealing with external subsystems and factors that impact the overall performance.
You should acquire Java Development Kit version 8 or higher for your operating system to work through all the examples. This book discusses the Oracle HotSpot JVM, so you may want to get Oracle JDK or OpenJDK (or Zulu) if possible. You should also get the latest Leiningen version (2.5.2 as of the time of writing) from http://leiningen.org/, and JD-GUI from http://jd.benow.ca/.
This book is for intermediate Clojure programmers who are interested in learning how to write high-performance code. If you are an absolute beginner in Clojure, you should learn the basics of the language first, and then come back to this book. You need not be well-versed in performance engineering or Java. However, some prior knowledge of Java would make it much easier to understand the Java-related chapters.
In this book, you will find a number of text styles that distinguish between different kinds of information. Here are some examples of these styles and an explanation of their meaning.
Code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles are shown as follows: "Note that identical? in Clojure is the same as == in Java."
A block of code is set as follows:
user=> (identical? "foo" "foo") ; literals are automatically interned true user=> (identical? (String. "foo") (String. "foo")) ; created string is not interned false
New terms and important words are shown in bold. Words that you see on the screen, for example, in menus or dialog boxes, appear in the text like this: "Clicking the Next button moves you to the next screen."
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