How to use variables in a multi-project gradle build system?

Question

I have a multi-project structure managed with gradle. All the java sub-projects have this jar block:

def main_class = "foo.Main"
jar {
    manifest {
        attributes 'Main-Class': main_class
    }
}

So I decided that I'm going to move the jar block to the root project and have only the def main_class in the sub-projects, but no matter how I defined the main_class variable, it didn't work. I tried project.ext and properties {} as well.

I had a task, called dist which worked with project.ext :

root project:

subprojects {
    task dist(dependsOn: jar) << {
        copy {
            from('build/libs') {
                include (build_jar)
            }
            into('.')
            rename(build_jar, app_jar)
        }
    }
}

a sub-project:

project.ext {
    build_jar = "..."
    app_jar = "..."
}

How can I define a variable that works with jar like with the dist task?

Answer 1

You can just do:

ext {
  main_class = 'com.Main'
}

subprojects {
  apply plugin: 'java'
  jar {
    manifest {
        attributes 'Main-Class': main_class
    }
  }
}

in PROJECT_ROOT/build.gradle file.

Answer 2

To override extra properties in subproject's build.gradle you must put your desired task (in this case the jar task ) inside the afterEvaluate block.

root project build.gradle:

subprojects {
    apply plugin: 'java'
    apply plugin: 'application'

    ext {
        // the property that should be overridden in suproject's build.gradle
        main_class = ''
    }

    afterEvaluate {
        jar {
            manifest {
                attributes 'Main-Class': main_class
            }
        }
    }
}

sub project build.gradle:

mainClassName = 'project.specific.Main'
ext.set("main_class", mainClassName)
// you could also use this notation, but I prefer the first one
// since it is more clear on what it is doing
//main_class = mainClassName