Java Interoperability

Clojure provides very clear and simple syntax for Java interoperatility, using the functions such as . new import

As Clojure is a hosted language on the JVM, its very easy to include any library from other languages that also run on the JVM, i.e. Java, Groovy, Scala, Jython, JRuby, etc.

The Leiningen build tool provides a simple way to include libraries as dependencies, using the :dependencies section of the project.clj file. Maven Central is one of the default repositories libraries are pulled from, the other being Clojars.org.

Clojure includes the java.lang library automatically, so you can call any methods from that library without having to import them or include any dependencies

The syntax

Its very easy to call Java methods and objects from clojure using the following syntax

(.instanceMember instance args*)
(.instanceMember Classname args*)
(.-instanceField instance)
(Classname/staticMethod args*)
Classname/staticField

Convert a string to upper case & show the approximate version of Pi

(.toUpperCase "I was low, but now I'm up")


Math/PI
-> 3.141592653589793

Getting the Java environment

Eariler we used Clojure functions to find information about our environment. We can also used the getProperty() method from the java.lang.System object to ask for the java version and jvm name.

Get version of Java & the JVM, returning those values as a meaningful string. Then get the version of the Clojure project

(str "You are running Java version " (System/getProperty "java.version") "with the JVM" (System/getProperty "java.vm.name")) 

(str "Latest project version: " (System/getProperty "playground.version"))

Use System/getenv to return your system's environment variables as a map

(System/getenv)

You may notice that this is a map data structure that we return, so we can use use destructuring or the maps behaviour itself to pull out information.

A full list of properties can be seen in the getProperty() documentation

There are more examples of Java Interoperability in the next few sections.