Integrating Clojure with Java and Spring Boot

Introduction

Clojure is a functional programming language that runs on the Java Virtual Machine (JVM). It is known for its simplicity, expressiveness, and facilitation of concurrent software development. Java, on the other hand, is a widely used object-oriented language in corporate development. Spring Boot is a Java-based framework that simplifies the creation of stand-alone, production-ready applications using Spring.

Integrating Clojure with Java and Spring Boot can offer a range of advantages, such as greater code expressiveness, fewer bugs, and more agile development. This article explores how to use Clojure with Java and Spring Boot, using Maven as the build tool.

Advantages of Using Clojure with Java and Spring Boot

1. Interoperability: Clojure can directly call Java code and vice versa, allowing seamless integration.
2. Concurrency: Clojure's immutable nature makes it easier to develop concurrent applications.
3. Expressiveness: Clojure's minimalist and powerful syntax enables writing more concise and expressive code.
4. Flexibility: Using Clojure allows developers to employ functional paradigms within an object-oriented environment, providing more flexibility in the development approach.

Project Setup

1. Project Structure

The project structure can be organized as follows:

my-clojure-springboot-app

│

├── src

│ ? ├── main

│ ? │ ? ├── java

│ ? │ ? │ ? └── com

│ ? │ ? │ ? ? ? └── example

│ ? │ ? │ ? ? ? ? ? └── demo

│ ? │ ? │ ? ? ? ? ? ? ? └── DemoApplication.java

│ ? │ ? └── clojure

│ ? │ ? ? ? └── com

│ ? │ ? ? ? ? ? └── example

│ ? │ ? ? ? ? ? ? ? └── demo

│ ? │ ? ? ? ? ? ? ? ? ? └── core.clj

│ ? └── test

│ ? ? ? ├── java

│ ? ? ? └── clojure

├── pom.xml

└── README.md

2. Maven Configuration

The pom.xml file should include the necessary dependencies for Clojure and Spring Boot:

<project xmlns="https://maven.apache.org/POM/4.0.0"

         xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"

         xsi:schemaLocation="https://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">

    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>

    <artifactId>my-clojure-springboot-app</artifactId>

    <version>0.0.1-SNAPSHOT</version>

    <packaging>jar</packaging>

    <name>my-clojure-springboot-app</name>

    <description>Demo project for Spring Boot with Clojure</description>

    <parent>

        <groupId>org.springframework.boot</groupId>

        <artifactId>spring-boot-starter-parent</artifactId>

        <version>2.7.1</version>

        <relativePath/> <!-- lookup parent from repository -->

    </parent>

    <properties>

        <java.version>17</java.version>

        <clojure.version>1.11.1</clojure.version>

    </properties>

    <dependencies>

        <dependency>

            <groupId>org.springframework.boot</groupId>

            <artifactId>spring-boot-starter</artifactId>

        </dependency>

        <dependency>

            <groupId>org.clojure</groupId>

            <artifactId>clojure</artifactId>

            <version>${clojure.version}</version>

        </dependency>

        <dependency>

            <groupId>org.springframework.boot</groupId>

            <artifactId>spring-boot-starter-test</artifactId>

            <scope>test</scope>

        </dependency>

    </dependencies>

    <build>

        <plugins>

            <plugin>

                <groupId>org.apache.maven.plugins</groupId>

                <artifactId>maven-compiler-plugin</artifactId>

                <configuration>

                    <source>${java.version}</source>

                    <target>${java.version}</target>

                </configuration>

            </plugin>

        </plugins>

    </build>

</project>        

Development

1. Initial Spring Boot Configuration

Create the main application class for Spring Boot in src/main/java/com/example/demo/DemoApplication.java:

package com.example.demo;

import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication

public class DemoApplication {

    public static void main(String[] args) {

        SpringApplication.run(DemoApplication.class, args);

    }

}        

2. Adding Clojure Code

Create a Clojure file in src/main/clojure/com/example/demo/core.clj:

(ns com.example.demo.core

  (:gen-class))

(defn -main []

  (println "Hello from Clojure!"))

(defn add [a b]

  (+ a b))        

3. Calling Clojure from Java

To call Clojure functions from Java, first compile the Clojure code using a tool like leiningen or clojure-maven-plugin.

Example of a Java call to a Clojure function:

package com.example.demo;

import clojure.java.api.Clojure;

import clojure.lang.IFn;

import org.springframework.web.bind.annotation.GetMapping;

import org.springframework.web.bind.annotation.RestController;

@RestController

public class DemoController {

    @GetMapping("/add")

    public String add() {

        IFn require = Clojure.var("clojure.core", "require");

        require.invoke(Clojure.read("com.example.demo.core"));

        IFn add = Clojure.var("com.example.demo.core", "add");

        return "Result: " + add.invoke(10, 20);

    }

}        

Why Companies Like Nubank Use Clojure

Nubank, a leading fintech company in Brazil, uses Clojure extensively in its technology stack. Here are some reasons why companies like Nubank choose Clojure:

1. Simplicity and Productivity: Clojure's concise syntax allows developers to write less code and focus on solving business problems. This leads to increased productivity and faster development cycles.
2. Immutable Data Structures: Clojure's default immutability reduces the complexity associated with concurrent programming, making it easier to write robust, thread-safe code.
3. Interoperability with Java: Since Clojure runs on the JVM, it can leverage the vast ecosystem of Java libraries and tools, enabling seamless integration with existing systems.
4. Functional Programming Paradigm: Clojure's emphasis on functional programming promotes writing pure functions and using higher-order functions, which leads to more predictable and maintainable code.
5. REPL-Driven Development: Clojure's REPL (Read-Eval-Print Loop) allows developers to interactively test and debug code, providing immediate feedback and improving the development experience.

Conclusion

Integrating Clojure with Java and Spring Boot provides a robust and flexible platform for developing modern applications. Using Maven, it is possible to easily configure a project that combines the simplicity and expressiveness of Clojure with the robustness and popularity of the Java/Spring Boot ecosystem. This approach allows developers to leverage the best of both worlds, promoting more agile and less error-prone development.

References

1. Hickey, Rich. "Clojure - Functional Programming for the JVM." Clojure.org. Accessed August 13, 2024. https://clojure.org.
2. Sierra, Kathy, and Bert Bates. "Head First Java." O'Reilly Media, 2003.
3. "Spring Boot Reference Documentation." Spring.io. Accessed August 13, 2024. https://spring.io/projects/spring-boot.
4. Fogus, Michael, and Chris Houser. "The Joy of Clojure." Manning Publications, 2014.
5. "Interoperability between Clojure and Java." Clojure.org. Accessed August 13, 2024. https://clojure.org/reference/java_interop.
6. Anderson, Stuart. "Why Nubank Uses Clojure." Nubank.com. Accessed August 13, 2024. https://nubank.com.br/en/blog/why-nubank-uses-clojure.