Groovy Scripting for Next-Level Jenkins Pipelines

Why Groovy?

Jenkins Pipelines are built on a DSL (Domain Specific Language) based on Groovy. While “Declarative Pipelines” cover simple cases, “Scripted Pipelines” and “Shared Libraries” require a solid grasp of Groovy logic.

Core Concepts

1. Closures

The bread and butter of Groovy. A closure is an anonymous block of code that can take parameters and can be passed around.

def myLogger = { msg -> println "[LOG]: ${msg}" }
myLogger("Hello Jenkins")

2. String Interpolation (GStrings)

Use double quotes to embed expressions.

def version = "1.0.1"
println "Building version: ${version}"

3. Power of Dynamic Typing

Groovy allows def for variables, making scripts much more concise than Java.

Practice Exercise: A Dynamic Jenkinsfile

We will create a Mock Jenkinsfile that dynamically chooses a build environment based on a JSON config.

Step 1: The Scripted Logic

node {
    stage('Initialize') {
        // Groovy's JsonSlurper
        def config = new groovy.json.JsonSlurper().parseText('{"env": "prod", "retries": 3}')

        env.DEPLOY_TARGET = config.env
        env.RETRY_COUNT = config.retries
    }

    stage('Deploy') {
        retry(env.RETRY_COUNT.toInteger()) {
            echo "Deploying to ${env.DEPLOY_TARGET}..."
            // Deployment script here
        }
    }
}

Step 2: Shared Library (Standardizing)

If multiple projects use the same build logic, move it to a Shared Library in vars/standardBuild.groovy:

def call(Map config) {
    pipeline {
        agent any
        stages {
            stage('Build') {
                steps {
                    sh "mvn clean install -Denv=${config.env}"
                }
            }
        }
    }
}

Why This Works

• JVM Compatibility: Since Groovy runs on the JVM, you can use any Java library (like java.net.HttpURLConnection) directly inside your Jenkinsfile.
• Declarative vs. Scripted: Declarative is preferred for readability, but having Groovy allows you to drop into script { ... } blocks for complex looping or conditional logic that the strict DSL can’t handle.

Security Tip: Sandbox and Script Approval

Jenkins runs Groovy scripts in a “Sandbox.” If you use restricted Java methods (like System.exit()), an admin must manually approve them in the “In-process Script Approval” page.

Summary

Groovy is the secret sauce that makes Jenkins the most powerful automation server. By mastering its basic syntax, you move from “Pipeline user” to “Master of automation.”