. If the request has no headers, this method
 returns an empty enumeration.

 <p>
 Some servlet containers do not allow servlets to access headers using this method, in which case this method returns
 <code>null</code></div>
<dl>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>an enumeration of all the header names sent with this request; if the request has no headers, an empty
 enumeration; if the servlet container does not allow servlets to use this method, <code>null</code></dd>
</dl>
</li>
</ul>
