Clojure Application Packaging: Creating an Uberjar

2.6.4 Packaging Your Application

As experienced Java developers, you’re likely familiar with the concept of packaging applications into JAR files for distribution. In Clojure, we often use Leiningen, a popular build automation tool, to create an uberjar—a standalone executable JAR file that contains all the dependencies your application needs to run. This section will guide you through the process of creating an uberjar for your Clojure application and explain how to run it.

Understanding the Uberjar

An uberjar is a JAR file that includes not only your compiled Clojure code but also all the libraries and dependencies your application requires. This makes it easy to distribute and run your application on any system with a compatible Java Runtime Environment (JRE).

Why Use an Uberjar?

• Portability: An uberjar is self-contained, meaning you can run it on any machine with a JRE without needing to install additional dependencies.
• Simplicity: It simplifies deployment by reducing the complexity of managing dependencies on the target system.
• Consistency: Ensures that the exact versions of libraries used during development are included in the deployment.

Creating an Uberjar with Leiningen

Leiningen simplifies the process of building and packaging Clojure applications. Let’s walk through the steps to create an uberjar.

Step 1: Define Your Project

First, ensure your project is set up correctly. Your project.clj file should define the necessary dependencies and configurations. Here’s an example:

(defproject my-clojure-app "0.1.0-SNAPSHOT"
  :description "A simple Clojure application"
  :url "http://example.com/my-clojure-app"
  :license {:name "Eclipse Public License"
            :url "http://www.eclipse.org/legal/epl-v10.html"}
  :dependencies [[org.clojure/clojure "1.10.3"]
                 [ring/ring-core "1.9.0"]
                 [compojure "1.6.2"]]
  :main ^:skip-aot my-clojure-app.core
  :target-path "target/%s"
  :profiles {:uberjar {:aot :all}})

Key Points:

• :main: Specifies the namespace containing the -main function, which serves as the entry point for your application.
• :profiles: The :uberjar profile is used to specify settings for building the uberjar, such as Ahead-of-Time (AOT) compilation.

Step 2: Implement the Main Function

Ensure your application has a -main function, which is the entry point when the uberjar is executed. Here’s a simple example:

(ns my-clojure-app.core
  (:gen-class))

(defn -main
  "The main entry point for the application."
  [& args]
  (println "Hello, Clojure World!"))

Explanation:

• :gen-class: This directive is necessary for generating a Java class file that can be executed by the JVM.
• -main: This function will be called when the uberjar is executed.

Step 3: Build the Uberjar

To create the uberjar, run the following command in your terminal:

lein uberjar

This command compiles your Clojure code, resolves dependencies, and packages everything into a single JAR file located in the target directory.

Step 4: Run the Packaged Application

Once the uberjar is created, you can run it using the java -jar command:

java -jar target/my-clojure-app-0.1.0-SNAPSHOT-standalone.jar

This command executes the -main function defined in your application, producing the output:

Hello, Clojure World!

Comparing with Java

In Java, creating an executable JAR involves compiling your code and specifying a Main-Class attribute in the MANIFEST.MF file. Clojure’s approach with Leiningen automates much of this process, making it more straightforward, especially when dealing with dependencies.

Java Example:

// Main.java
public class Main {
    public static void main(String[] args) {
        System.out.println("Hello, Java World!");
    }
}

To package this Java application, you would typically use a build tool like Maven or Gradle, which involves more configuration compared to Leiningen’s concise project.clj.

Advanced Packaging Options

Leiningen provides several options to customize the packaging process:

• Excluding Dependencies: You can exclude certain dependencies from the uberjar if they are provided by the runtime environment.
• Customizing the Manifest: Modify the :manifest entry in project.clj to include additional metadata.
• Including Resources: Ensure that any non-code resources (e.g., configuration files) are included in the JAR.

Try It Yourself

To deepen your understanding, try modifying the example application:

1. Add a New Dependency: Include a library like cheshire for JSON processing and update the -main function to parse a JSON string.
2. Change the Entry Point: Create a new namespace with a different -main function and update the :main entry in project.clj.
3. Experiment with Profiles: Define a new profile in project.clj for a different environment (e.g., development vs. production) and observe how it affects the build process.

Visualizing the Packaging Process

Below is a diagram illustrating the flow of creating an uberjar with Leiningen:

    graph TD;
	    A[Define Project] --> B[Implement Main Function];
	    B --> C[Run lein uberjar];
	    C --> D[Generate Uberjar];
	    D --> E[Run with java -jar];

Diagram Explanation: This flowchart shows the steps from defining your project to running the packaged application, highlighting the simplicity and efficiency of using Leiningen for Clojure projects.

Further Reading

For more detailed information on Leiningen and uberjars, consider exploring the following resources:

• Leiningen Official Documentation
• ClojureDocs: Leiningen
• Clojure’s Official Guide on Packaging

Exercises

1. Create a New Clojure Project: Set up a new project with Leiningen, add dependencies, and package it into an uberjar.
2. Modify the Build Process: Experiment with different profiles and observe how they affect the resulting JAR.
3. Integrate with Java: Create a simple Java class that calls your Clojure code packaged in an uberjar.

Key Takeaways

• Uberjars provide a convenient way to package and distribute Clojure applications, ensuring all dependencies are included.
• Leiningen simplifies the build process, making it accessible even for complex projects.
• Understanding how to package applications effectively is crucial for deploying Clojure applications in production environments.

By mastering the process of creating and running uberjars, you can ensure your Clojure applications are ready for deployment in any environment. Now, let’s apply these concepts to build robust, portable applications with Clojure!

Quiz: Mastering Clojure Application Packaging