10.6.2 Invoking Clojure Functions from Java

Invoking Clojure Functions from Java

In this section, we’ll explore the seamless integration of Clojure functions into Java applications. By the end of this tutorial, you’ll understand how to invoke Clojure functions from Java using both precompiled classes and dynamic invocation through the Clojure Java API. Let’s dive in!

Compiled Classes Approach

To call Clojure code through compiled classes, you typically compile your Clojure code to class files and integrate them with your Java application. Here’s a simple example:

Step-by-Step

1. Compile Clojure Code: Use tools like Leiningen or the Clojure CLI to compile your Clojure files into .class files stored in a Java-compatible structure.

   ;; Example CLI command
   clj -M -m clojure.main -e "(compile 'my.namespace)"
   

2. Java Access: Use the compiled class files in your Java classpath.

3. Java Code Example: Access and invoke a function compiled in the my.namespace namespace.

   // Java representation for invoking Clojure's 'add' function in my.namespace
   import my.namespace.MyNamespaceFn;
   
   public class Invoker {
       public static void main(String[] args) {
           // Access the Clojure compiled function
           MyNamespaceFn addFunction = new MyNamespaceFn();
           // Invoke function
           int sum = addFunction.invoke(5, 10);
           System.out.println("Sum: " + sum);
       }
   }
   

Dynamic Invocation via Clojure Java API

Invoke Clojure code dynamically using the Clojure Java API for more flexibility:

Step-by-Step

1. Add Clojure Dependency: Include the Clojure library in your project dependencies.

2. Java Code Example: Demonstrating dynamic invocation.

   import clojure.java.api.Clojure;
   import clojure.lang.IFn;
   
   public class DynamicInvoker {
       public static void main(String[] args) {
           // Load the Clojure.rt namespace and require your needed namespace
           IFn require = Clojure.var("clojure.core", "require");
   
           // Require the Clojure namespace
           require.invoke(Clojure.read("my.namespace"));
   
           // Reference a function in the namespace
           IFn addFunction = Clojure.var("my.namespace", "add");
   
           // Call the function dynamically
           Object result = addFunction.invoke(5, 15);
           System.out.println("Result of dynamic invocation: " + result);
       }
   }
   

Keypoints

• Flexibility: Dynamic invocation allows for flexible use of Clojure functions without precompiling, ideal for quick prototyping.
• Performance: Compiled classes could offer better performance for functions frequently used due to JIT optimizations.
• Development Tools: Frameworks like Leiningen streamline the compilation process, bundling the conversion of Clojure namespaces into Java class files.

Quiz

By the end of this, you should have a deeper understanding and practical skills to integrate Clojure code with Java using the contextually appropriate method. Enjoy experimenting and optimizing your Java-Clojure interlanguage operations!