extract the class file to create a jar package

build.gradle

//1. Use the zipTree directly in the jar task rather than creating an intermediate zip.
//2. Use eachFile to change the relative path of each file to strip off the first folder.
jar {
  from zipTree('runtime.jar') {
    include 'classes/**/*.class'
    eachFile { it.relativePath = it.relativePath.segments[1..-1] }
  }
}
//Jar tasks have all of the CopySpec methods, which provide the eachFile abilities.