title: JAXB marshalling a map without adapter
body: <p>I have a Map and I want to marshal it to XML. As long as there are plenty of maps of different types I really don't want to write a custom XmlAdapter for each map.
  I have XmlAdapters for all classes that I use as keys. When I marshal this classes standalone they work perfectly, but when I marshal the maps the keys are just ignored. All I get is the following XML:</p>
  
  <pre><code>&lt;entry&gt;
      &lt;key/&gt;
      &lt;value&gt;
          &lt;id&gt;1&lt;/id&gt;
          &lt;property&gt;something&lt;/property&gt;
      &lt;/value&gt;
  &lt;/entry&gt;
  </code></pre>
  
  <p>What I want instead is:</p>
  
  <pre><code>&lt;entry&gt;
      &lt;key&gt;
          &lt;property&gt;something&lt;/property&gt;
      &lt;/key&gt;
      &lt;value&gt;
          &lt;id&gt;1&lt;/id&gt;
          &lt;property&gt;something&lt;/property&gt;
      &lt;/value&gt;
  &lt;/entry&gt;
  </code></pre>
  
  <p>Is there a way to achieve the required result without coding a custom XmlAdapter for each map?</p>