An example of a declarative Jenkins Pipeline

declarative.groovy

pipeline {
    stages {
        stage('Preparation') {          
            env.mvnHome = tool 'M3'
        }
        stage('Build') {
            sh "'${mvnHome}/bin/mvn' -Dmaven.test.failure.ignore clean package"
        }
    }
    post {
        always {
            junit '**/target/surefire-reports/TEST-*.xml'
            archive 'target/*.jar'
        }
        success {
            slackSend color: 'good', message: "$JOB_NAME - Successfully built #$BUILD_NUMBER (<$BUILD_URL|Open>)"
        }
        failure {
            slackSend color: 'danger', message: "$JOB_NAME - Failed #$BUILD_NUMBER (<$BUILD_URL|Open>)"
        }
    }
}