How to get all field in jira system?

I want to get all field in Jira System

  • default field from Jira System ( issueKey, summary,…)
  • And custom field

I try with FieldManager but I can find method getAll field.
https://docs.atlassian.com/software/jira/docs/api/7.0.3/com/atlassian/jira/issue/fields/FieldManager.html

Many thanks,