String nested = "one/two"

// Creates a single folder named "one/two" with / embedded in the name
createProject(Folder.class, nested)

// Looks for a subfolder named "two" inside a folder named "one"
getItemByFullName(nested)

import com.cloudbees.hudson.plugins.folder.Folder

String folder = "nested/folders"

create_folder_if_missing(folder)
create_folder_if_missing(folder)

void create_folder_if_missing(String folder) {
  def item = Jenkins.instance.getItemByFullName(folder)

  if (item == null) {
    println(folder + " not found. Creating.")
    Jenkins.instance.createProject(Folder.class, folder)
    println("Created " + folder)
  }
  else {
    println(folder + " found. Skipping.")
  }
}

nested/folders not found. Creating.
Created nested/folders
nested/folders not found. Creating.
java.lang.IllegalArgumentException: nested/folders already exists
	at hudson.model.Items.verifyItemDoesNotAlreadyExist(Items.java:641)