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Java Clojure Interop: Integrating Clojure into Your Java Project

03.10.2010
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It's easier to wrap an existing piece of Java code in Clojure than it is to do the inverse, but because Clojure is implemented as a Java class library, it's also relatively simple to embed Clojure in your Java applications, load code, and call functions.  Repl.java and Script.java in the distribution are the normal examples for loading code from a user or from a file.  In this quick tutorial, you'll find out how to use the same underlying machinery to load Clojure code and then manipulate it directly from Java.

First, let's start with this script that defines a simple Clojure function:
; foo.clj
(ns user)

(defn foo [a b]
(str a " " b))
This Java class will load the script and call the of function with arguments in the form of Java objects.  Then it will print the returned object:
// Foo.java

import clojure.lang.RT;
import clojure.lang.Var;

public class Foo {
public static void main(String[] args) throws Exception {
// Load the Clojure script -- as a side effect this initializes the runtime.
RT.loadResourceScript("foo.clj");

// Get a reference to the foo function.
Var foo = RT.var("user", "foo");

// Call it!
Object result = foo.invoke("Hi", "there");
System.out.println(result);
}
}
Finally, you have to compile this and run it to see the printed result.  For compiling Clojure into a .jar, Leiningen is a favorable option since it is made specifically for Clojure.  

There is a good video that explains how to use leiningen.  It has the advantage of letting users write everything in Clojure, rather than writing a bunch of XML code like you would for Ant or Maven.  Leiningen also has "uberjars," which build in Clojure and put all of your Clojure dependencies into one standalone file, meaning less work for you.

If you want to be more Java friendly in your approach, you could add an Ant task to build it along with the Java project.  This will just take a little more work.  You'll need to call 'to-array' on the functions that need to return proper java arrays.  Clojure supports the creation, reading, and modification of Java arrays, but it is recommended that you limit use of arrays to interop with Java libraries that require them as arguments or use them as return values.

Once you have your .jar file, just add it to your java project as a build deployment.  then you can call it directly from Java.  Here is the printed result when you run the .jar:
>javac -cp clojure.jar Foo.java

>java -cp clojure.jar Foo
Hi there

>
If you're doing Java interop from Clojure, things are even more simple.  Clojure programs can use any Java class or interface. The classes in the java.lang package can be used in Clojure just like you would in Java without having to import them. Java classes in other packages are used by either specifying their package when referencing them or using the import function. Invoking Java methods from Clojure code is also pretty simple.  As a result, Clojure doesn't provide functions for many common operations.  Instead, it relies on Java methods.

[http://java.dzone.com/articles/java-clojure-interop-calling]
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