in core/src/main/scala/org/apache/livy/sessions/SessionState.scala [32:45]
def apply(s: String): SessionState = s match {
case "not_started" => NotStarted
case "starting" => Starting
case "recovering" => Recovering
case "idle" => Idle
case "running" => Running
case "busy" => Busy
case "shutting_down" => ShuttingDown
case "error" => Error()
case "dead" => Dead()
case "killed" => Killed()
case "success" => Success()
case _ => throw new IllegalArgumentException(s"Illegal session state: $s")
}