Adding External Java Libraries to Clojure Projects

10.5.2 Adding External Java Libraries

As experienced Java developers, you are likely familiar with the vast ecosystem of Java libraries available for various tasks, from data processing to web development. One of the strengths of Clojure is its seamless interoperability with Java, allowing you to leverage these libraries within your Clojure projects. In this section, we’ll explore how to add external Java libraries to your Clojure projects using two popular dependency management tools: Leiningen and tools.deps. We’ll also provide examples of calling code from popular Java libraries to illustrate these concepts.

Understanding Dependency Management in Clojure

Before diving into the specifics of adding Java libraries, it’s important to understand how dependency management works in Clojure. Clojure projects typically use either Leiningen or tools.deps to manage dependencies. Both tools allow you to specify the libraries your project depends on, automatically downloading and including them in your project’s classpath.

Leiningen (project.clj)

Leiningen is a build automation tool for Clojure that uses a project.clj file to define project settings, including dependencies. Here’s a basic structure of a project.clj file:

(defproject my-clojure-project "0.1.0-SNAPSHOT"
  :description "A simple Clojure project"
  :dependencies [[org.clojure/clojure "1.10.3"]
                 [org.some/library "1.2.3"]])

• :dependencies: This vector contains the list of dependencies, each specified as a vector with the library’s group ID, artifact ID, and version.

tools.deps (deps.edn)

tools.deps is a more recent addition to the Clojure ecosystem, providing a flexible way to manage dependencies using a deps.edn file. Here’s an example of a deps.edn file:

{:deps {org.clojure/clojure {:mvn/version "1.10.3"}
        org.some/library {:mvn/version "1.2.3"}}}

• :deps: This map contains the dependencies, with each key being the library’s coordinate and the value specifying the version.

Adding Java Libraries to Your Clojure Project

Now that we understand the basics of dependency management, let’s explore how to add external Java libraries to your Clojure project using both Leiningen and tools.deps.

Adding Java Libraries with Leiningen

To add a Java library using Leiningen, you simply need to include it in the :dependencies vector in your project.clj file. For example, let’s add the popular Apache Commons Lang library:

(defproject my-clojure-project "0.1.0-SNAPSHOT"
  :description "A simple Clojure project with Apache Commons Lang"
  :dependencies [[org.clojure/clojure "1.10.3"]
                 [org.apache.commons/commons-lang3 "3.12.0"]])

• Apache Commons Lang: This library provides utility functions for the Java standard library, such as string manipulation and number utilities.

Once you’ve added the dependency, run lein deps to download and include it in your project’s classpath.

Adding Java Libraries with tools.deps

With tools.deps, you add a Java library by specifying it in the :deps map in your deps.edn file. Here’s how you can add Apache Commons Lang:

{:deps {org.clojure/clojure {:mvn/version "1.10.3"}
        org.apache.commons/commons-lang3 {:mvn/version "3.12.0"}}}

After updating your deps.edn file, use the clj command to resolve and download the dependencies:

clj -Stree

This command will show the dependency tree, confirming that the library has been added successfully.

Calling Java Code from Clojure

Once you’ve added a Java library to your project, you can call its code directly from Clojure. Let’s explore how to do this with some examples.

Example: Using Apache Commons Lang

Suppose you want to use the StringUtils class from Apache Commons Lang to check if a string is empty. Here’s how you can do it in Clojure:

(ns my-clojure-project.core
  (:import [org.apache.commons.lang3 StringUtils]))

(defn is-empty? [s]
  (StringUtils/isEmpty s))

;; Usage
(println (is-empty? ""))  ; true
(println (is-empty? "Clojure"))  ; false

• :import: This directive is used to import Java classes into your Clojure namespace.
• StringUtils/isEmpty: Calls the static isEmpty method from the StringUtils class.

Example: Using Google Guava

Google Guava is another popular Java library that provides utilities for collections, caching, and more. Let’s use Guava’s Joiner class to join a list of strings:

(ns my-clojure-project.core
  (:import [com.google.common.base Joiner]))

(defn join-strings [separator strings]
  (-> (Joiner/on separator)
      (.join strings)))

;; Usage
(println (join-strings ", " ["Clojure" "Java" "Scala"]))  ; "Clojure, Java, Scala"

• Joiner/on: Creates a Joiner instance with the specified separator.
• .join: Joins the strings using the Joiner instance.

Comparing with Java Code

To highlight the differences and similarities between Java and Clojure, let’s compare the above examples with their Java counterparts.

Java Code for Apache Commons Lang

import org.apache.commons.lang3.StringUtils;

public class Example {
    public static void main(String[] args) {
        System.out.println(StringUtils.isEmpty(""));  // true
        System.out.println(StringUtils.isEmpty("Clojure"));  // false
    }
}

Java Code for Google Guava

import com.google.common.base.Joiner;

public class Example {
    public static void main(String[] args) {
        Joiner joiner = Joiner.on(", ");
        System.out.println(joiner.join("Clojure", "Java", "Scala"));  // "Clojure, Java, Scala"
    }
}

Try It Yourself

To deepen your understanding, try modifying the examples above:

• Experiment with different libraries: Add another Java library to your project and call its methods from Clojure.
• Change the method calls: Use different methods from the Apache Commons Lang or Google Guava libraries.
• Create a new function: Write a Clojure function that combines methods from multiple Java libraries.

Visualizing Dependency Management

To better understand how dependencies are managed in Clojure, let’s visualize the process using a diagram.

    graph TD;
	    A[Define Dependencies] --> B[Leiningen: project.clj];
	    A --> C[tools.deps: deps.edn];
	    B --> D[Download Dependencies];
	    C --> D;
	    D --> E[Classpath Setup];
	    E --> F[Use Java Libraries in Clojure];

Diagram Description: This flowchart illustrates the process of adding external Java libraries to a Clojure project. It starts with defining dependencies in either project.clj or deps.edn, followed by downloading the dependencies and setting up the classpath, allowing you to use Java libraries in your Clojure code.

Further Reading

For more information on dependency management and Java interoperability in Clojure, consider exploring the following resources:

• Official Clojure Documentation
• Leiningen Documentation
• tools.deps Guide
• Apache Commons Lang
• Google Guava

Exercises

1. Add a New Library: Choose a Java library you frequently use in your Java projects and add it to a Clojure project. Write a Clojure function that utilizes this library.
2. Compare Performance: Implement the same functionality using both Clojure and Java, and compare their performance.
3. Explore tools.deps: Experiment with different options in the deps.edn file, such as specifying a local path or a Git repository for dependencies.

Key Takeaways

• Seamless Interoperability: Clojure’s interoperability with Java allows you to leverage the vast ecosystem of Java libraries.
• Dependency Management: Use Leiningen or tools.deps to manage dependencies in your Clojure projects.
• Calling Java Code: Import and call Java classes and methods directly from Clojure, making it easy to integrate Java libraries.

Now that we’ve explored how to add external Java libraries to your Clojure projects, you’re well-equipped to enhance your applications with powerful Java tools and libraries.

Quiz: Mastering Java Library Integration in Clojure