Simple gradle sign jar task using ant task.

signJar_build.gradle

task signJar << {
    description = "Signs JAR"

    def config = loadProperties(project.ext.keystorePropsFilename)
    ant.signjar(jar: project.ext.jarToSign,
                alias: config.key.alias,
                keystore: config.key.store,
                storepass: config.key.'store.password',
                keypass: config.key.'alias.password')
}

def loadProperties(propsFilename) {
    def props = new Properties()
    new File(propsFilename).withInputStream {
        stream -> props.load(stream)
    }
    return new ConfigSlurper().parse(props)
}