title: Jaxb. Customize field naming behavior(camelCase to underscore_case)
body: <p>I want to serialize my fields with underscored names. For example: <code>userName</code> -> <code>user_name</code>. </p>
  
  <p>I know that it can be done with the <code>@XmlElement(name = "user_name")</code> annotation, but it's not very handy for my case. </p>
  
  <p>Is there any way to set up a default naming policy for JAXB?</p>