September 21, 2015

Gradle Goodness: Pass Java System Properties To Java Tasks

Gradle is of course a great build tool for Java related projects. If we have tasks in our projects that need to execute a Java application we can use the JavaExec task. When we need to pass Java system properties to the Java application we can set the systemProperties property of the JavaExec task. We can assign a value to the systemProperties property or use the method systemProperties that will add the properties to the existing properties already assigned. Now if we want to define the system properties from the command-line when we run Gradle we must pass along the properties to the task. Therefore we must reconfigure a JavaExec task and assign System.properties to the systemProperties property.

In the following build script we reconfigure all JavaExec tasks in the project. We use the systemProperties method and use the value System.properties. This means any system properties from the command-line are passed on to the JavaExec task.

apply plugin: 'groovy'
apply plugin: 'application'

mainClassName = 'com.mrhaki.sample.Application'


dependencies {
    compile 'org.codehaus.groovy:groovy-all:2.4.4'

// The run task added by the application plugin
// is also of type JavaExec.
tasks.withType(JavaExec) {
    // Assign all Java system properties from 
    // the command line to the JavaExec task.
    systemProperties System.properties

We write a simple Groovy application that uses a Java system property app.greeting to print a message to the console:

// File: src/main/groovy/com/mrhaki/sample/Application.groovy
package com.mrhaki.sample

println "Hello ${System.properties['app.greeting']}"

Now when we execute the run task (of type JavaExec) and define the Java system property app.greeting in our command it is used by the application:

$ gradle -Dapp.greeting=Gradle! -q run
Hello Gradle!

Written with Gradle 2.7.