Learn how to create executable JARs in Clojure using Leiningen, enabling seamless deployment and execution of your Clojure applications.
Creating executable JARs is a crucial step in the process of deploying Clojure applications. This section will guide you through the process of packaging your Clojure applications into standalone JAR files using Leiningen, a popular build automation tool for Clojure. We will explore the benefits of using executable JARs, delve into the configuration and execution details, and provide practical examples to ensure you can confidently create and run your own JAR files.
An executable JAR (Java ARchive) file is a package that contains all the necessary components of a Java application, including compiled classes, resources, and metadata. For Clojure applications, an executable JAR allows you to bundle your Clojure code, dependencies, and a manifest file that specifies the entry point of the application. This makes it easy to distribute and run your application on any machine with a compatible Java Runtime Environment (JRE).
Before creating an executable JAR, ensure your Clojure project is properly set up with Leiningen. If you haven’t already, create a new Leiningen project using the following command:
lein new app your-app
This command generates a basic project structure with the necessary files, including project.clj
, which is the configuration file for your Leiningen project.
project.clj
for JAR CreationThe project.clj
file is where you define your project’s configuration, including dependencies, source paths, and build instructions. To create an executable JAR, you need to specify the :main
namespace, which contains the entry point of your application. This is typically the namespace with the -main
function.
Here’s an example project.clj
configuration:
(defproject your-app "0.1.0-SNAPSHOT"
:description "A sample Clojure application"
:url "http://example.com/your-app"
:license {:name "Eclipse Public License"
:url "http://www.eclipse.org/legal/epl-v10.html"}
:dependencies [[org.clojure/clojure "1.10.3"]]
:main ^:skip-aot your-app.core
:target-path "target/%s"
:profiles {:uberjar {:aot :all}})
:main
: Specifies the namespace containing the -main
function. The ^:skip-aot
metadata indicates that the namespace should not be ahead-of-time (AOT) compiled unless building an uberjar.:target-path
: Defines the directory where compiled artifacts will be placed.:profiles
: Contains build profiles. The :uberjar
profile is used to configure settings specific to building an executable JAR, such as AOT compilation.With your project.clj
configured, you can build the executable JAR using the lein uberjar
command. This command compiles your Clojure code, resolves dependencies, and packages everything into a standalone JAR file.
lein uberjar
Upon successful execution, Leiningen creates the JAR file in the target
directory, typically named your-app-standalone.jar
.
To run your application from the JAR file, use the java -jar
command followed by the path to the JAR file:
java -jar target/your-app-standalone.jar
This command launches the Java Virtual Machine (JVM) and executes the -main
function specified in your :main
namespace.
Let’s walk through a practical example of creating an executable JAR for a simple Clojure application. We’ll build a basic “Hello, World!” application and package it into a JAR file.
Create a new Leiningen project:
lein new app hello-world
Navigate to the src/hello_world/core.clj
file and implement the -main
function:
(ns hello-world.core
(:gen-class))
(defn -main
"A simple Hello, World! application."
[& args]
(println "Hello, World!"))
project.clj
Edit the project.clj
file to specify the :main
namespace:
(defproject hello-world "0.1.0-SNAPSHOT"
:description "A simple Hello, World! application"
:url "http://example.com/hello-world"
:license {:name "Eclipse Public License"
:url "http://www.eclipse.org/legal/epl-v10.html"}
:dependencies [[org.clojure/clojure "1.10.3"]]
:main ^:skip-aot hello-world.core
:target-path "target/%s"
:profiles {:uberjar {:aot :all}})
Run the lein uberjar
command to build the executable JAR:
lein uberjar
Run the application using the java -jar
command:
java -jar target/hello-world-standalone.jar
You should see the output:
Hello, World!
Leiningen offers various configuration options to customize the JAR creation process. Here are some advanced options you might consider:
The manifest file in a JAR specifies metadata about the JAR and its contents. You can customize the manifest by adding a :manifest
entry to your project.clj
:
:manifest {"Main-Class" "hello_world.core"
"Implementation-Version" "0.1.0"}
If your application requires additional resources (e.g., configuration files, images), you can include them in the JAR by placing them in the resources
directory of your project. Leiningen automatically includes this directory in the JAR.
To exclude specific files or directories from the JAR, use the :uberjar-exclusions
key in your project.clj
:
:uberjar-exclusions [#"^log4j.properties$"]
If you encounter a ClassNotFoundException
, ensure that all necessary dependencies are included in your project.clj
and that the :main
namespace is correctly specified.
If your application doesn’t start as expected, verify that the :main
namespace contains a valid -main
function and that the namespace is correctly specified in project.clj
.
If your JAR is excessively large, review your dependencies and exclude unnecessary files using :uberjar-exclusions
.
Creating executable JARs is an essential skill for deploying Clojure applications. By leveraging Leiningen’s powerful build automation capabilities, you can efficiently package your applications into standalone JARs, simplifying distribution and execution. With the knowledge and examples provided in this section, you are well-equipped to create and manage executable JARs for your Clojure projects.