Clojure is a language implemented in Java and also in Clojure itself. The language implementation (compiler and runtime) is distributed as a jar file (see below) and runs on the Java virtual machine (“JVM”). Your Clojure programs will be compiled by the Clojure implementation into JVM bytecode and then run on the JVM (see below for explanations of Java-related terms).

The compiling and running just described is all neatly handled for you by a standard tool, described later.

Clojure comes with a “REPL” — a “read/eval/print loop”. This is an interactive prompt (like the Python prompt) where you can type in and run some Clojure code and see what it does and what it returns (evaluates to).

Clojure source code filenames end in “.clj”.

For more about what Clojure is, see http://clojure.org/.

1 Some Info About Java

Since Clojure runs on the JVM, makes use of the Java platform, and can interoperate so easily with Java, it helps to know a little about Java.

Note: The rest of this section is just optional extra background info.

Here are explanations of some terms and also a few other morsels of useful background information:

• Java source code gets compiled to “bytecode” by a compiler program usually named “javac” (which you won’t need to use directly while working with Clojure).

• The JVM (started by executing the java command) runs bytecode. It doesn’t matter where the bytecode came from; it could’ve been compiled from Java source, or Clojure source, or some other language.

• Bytecode is stored in “.class” files. You generally won’t see these when working with Clojure. java runs the bytecode in .class files.

• By default, java doesn’t know where to find .class files which aren’t part of the Java standard class library. It maintains a list of places to look called the “classpath”. To tell java to add a location to its classpath, you would use the -cp path/to/classfile/dir option on the command line (or less-commonly, you’d set an environment variable). Happily, the standard Clojure project management tool (discussed later) takes care of dealing with the java command and the classpath so you don’t have to.

  (Note: this brief guide doesn’t cover how to use Java code in your Clojure projects, except to provide a link to additional info.)

• For distribution purposes, one or more .class files for a given library or application are typically packed together into a “.jar” archive file. Jar files are not unlike “tar” or “zip” files, and can contain other files besides .class files. (The Clojure implementation is distributed as clojure.jar.)

• Just like you can tell java additional directories where it can find .class files (using the -cp path/to/classfile/dir option), you can also tell it to find them packed inside a jar file: -cp path/to/jarfile.jar.

• If a jar file contains a .class file with a “main” function, and if that jar file also contains a special bit of metadata indicating which .class file that is, then the jar can be run by the JVM as a standalone program like so: java -jar path/to/my-cool-app.jar

  So:

  • -cp path/to/jarfile.jar is for telling java to find classfiles in jarfile.jar, and
  • -jar path/to/jarfile.jar is for telling java to run jarfile.jar.

• Usage of the jar command is somewhat similar to the familiar tar command.

• Some desktop operating systems may let you just double-click on jar files to run them.

• Java comes with a rather large standard library (known as the Java Class Library), all of which is easily accessible from Clojure.

• There are many other 3rd-party Java libraries available as well, and these are also easily accessible from Clojure.

• Java is readily available for most platforms. On Debian-based GNU/Linux distributions it may already be installed (“openjdk-8-jre”). On MS Windows you need to download an installer from http://www.java.com/. On Mac OS X, either it’s already installed (try java -version to see) or else you need to download an installer from Apple.

• When installing Java, you can either install just the runtime environment (openjdk-8-jre), or else the whole development kit (openjdk-8-jdk). Running your Clojure programs only requires the runtime/jre.

Regarding the Java language itself, it’s is your garden-variety, statically-typed, imperative, semi-colon curly-braces object-oriented language. You’ve got classes, and can instantiate objects from those classes. Functions (Java calls them “methods”) can be defined only inside classes. There are functions you can call via a class (where the class just acts as a namespace for the function), and there are functions you can call on behalf of an object (which often modifies its state).

There are many Java tutorials out there.

Personally, for learning Java, I like the “Core Java” book by Horstmann & Cornell.