Configuration.scala

trait Configured {
    val config = ConfigFactory.load().getConfig("spark")
}

object ConfigUtils {
    def asOption[T](t: => T): Option[T] = {
        try {
            Option(t)
        } catch {
            case e: ConfigException.Missing => None
            case e: ConfigException => throw e
            case e: Exception => throw e
        }
    }
}

class SparkConfig extends Configured {
    val sparkHome = config.getString("home")        // forcing types on instantiation
    val localDir = config.getString("local_dir")
    val bufferSize = config.getInt("buffer_size")
    val kryoBufferSizeMb = config.getInt("kryo.buffer_size_mb")
    ...
    val sparkMem = asOption { System.getEnv("SPARK_MEM") } getOrElse config.getString("worker.mem")
}

class SparkContext(..., config: SparkConfig) {
    val home = config.sparkHome                     // type safe
    ...
}

reference.conf

{ "spark": 
    {
        "home": null,
        "local_dir": null,
        "buffer_size": 65536,
        "kryo": {
            "buffer_size_mb": 10,
            "registrator": null
        },
        ...
    }
}