Good practices to add properties to a custom jira plugin?

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() {"Very Important", JiraPriority.BLOCKER);"Important", JiraPriority.CRITICAL);"Average", JiraPriority.MAJOR);"Not Important", JiraPriority.MINOR);"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?