Gradle: Starting Simple

Gradle is an “open-source build automation tool focused on flexibility and performance”, and is my personal goto build tool for Java software projects.

We’ll be using Gradle’s Kotlin DSL for our examples. First, the project structure. If you have experience using Gradle there shouldn’t be any surprises here. If you haven’t, this is the standard Gradle boilerplate.

gradle-template
|---src
|   |---main
|   |   |---java
|   |   |   |---io.github.tduncan
|   |   |   |   |---Main.java
│---build.gradle.kts
|---settings.gradle.kts  

Our settings.gradle.kts file is very basic, defining only the name of our root project.

rootProject.name = "gradle-template"

The settings.gradle.kts is how we teach Gradle about the specific projects that need to be evaluated and built. Our example is very simple; we only need a top-level project, but more complicated projects likely have a number of sub projects that also need to be built by Gradle.

Finally, build.gradle.kts script itself. This is where we will define how Gradle should evaluate our project and, by extension, what Gradle tasks are available to execute.

plugins {
    java
    application
}

application {
    mainClassName = "io.github.tduncan.Main"
}

At the top of the build script we immediately inform Gradle what type of application we are building by the plugins being applied to the project. Among other things, using the java plugin will configure Gradle to expect Java source code in the src/main/java directory.

Similarily, the application configures Gradle to expect a fully qualified class name to be provided as the mainClassName. This class needs to contain a valid Java main method, and be located within the src/main/java directory. As long as we do this, we can take advantage the run task included by default by the application plugin to quickly run our program from the command line.

Main.java is, as the name implies, a very simple “Hello World!” main method.

package io.github.tduncan;

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

Now, open up your favorite terminal and execute the run command.

$ gradle run                                 
                                             
> Task :run                                  
Hello World!                                 
                                             
BUILD SUCCESSFUL in 838ms                    
2 actionable tasks: 1 executed, 1 up-to-date