Good practices to add properties to a custom jira plugin?

1

In a jira plugin which I had built there is a mapping class like this:

public class MyMapper implements PriorityMapper {

    private Map<String, JiraPriority> map = new HashMap<>();

    public MyMapper() {
        this.map.put("Very Important", JiraPriority.BLOCKER);
        this.map.put("Important", JiraPriority.CRITICAL);
        this.map.put("Average", JiraPriority.MAJOR);
        this.map.put("Not Important", JiraPriority.MINOR);
        this.map.put("To ignore", JiraPriority.TRIVIAL);
    }

    @Override public JiraPriority toJiraPriority ( IncomingMsg incomingMsg ) {
        return map.get(incomingMsg.getPriority());
    }
}

How can I move this configuration to external file, for instance .xml? What are the best practices?