Q: Can I use SimpleXML to parse XML whose structure is unknown? I am using SimpleXML to parse small XML files used in a communication protocol. This all works fine, but now I am implementing a part of the protocol which includes a kind of free-form XML.
For example, an XML like this:
<telegram>
  <config>
    <foo>yes</foo>
    <bar>no</bar>
  </config>
</telegram>

Where foo and bar might change in the future, or an element baz might be added, without the need to touch the parsing code. I would like to access these elements in Java using a construct like
tree.getConfig().get("bar");   // returns "no"

Can I use SimpleXML to parse that? I looked into the documentation, but couldn't find what I need.

A: 
Can I use SimpleXML to parse that?

Not out of the box - but writing a Converter will do it.

@Root(name = "telegram")
@Convert(Telegram.TelegramConverter.class) // Requires AnnotationStrategy
public class Telegram
{
    private Map<String, String> config;


    public String get(String name)
    {
        return config.get(name);
    }

    public Map<String, String> getConfig()
    {
        return config;
    }

    // ...

    @Override
    public String toString()
    {
        return "Telegram{" + "config=" + config + '}';
    }




    static class TelegramConverter implements Converter<Telegram>
    {
        @Override
        public Telegram read(InputNode node) throws Exception
        {
            Telegram t = new Telegram();

            final InputNode config = node.getNext("config");
            t.config = new HashMap<>();

            // Iterate over config's child nodes and put them into the map
            InputNode cfg = config.getNext();

            while( cfg != null )
            {
                t.config.put(cfg.getName(), cfg.getValue());
                cfg = config.getNext();
            }

            return t;
        }

        @Override
        public void write(OutputNode node, Telegram value) throws Exception
        {
            // Implement if you need serialization too
            throw new UnsupportedOperationException("Not supported yet.");
        }

    }
}

Usage:
final String xml = "<telegram>\n"
        + "  <config>\n"
        + "    <foo>yes</foo>\n"
        + "    <bar>no</bar>\n"
        + "    <baz>maybe</baz>\n" // Some "future element"
        + "  </config>\n"
        + "</telegram>";
/*
 * The AnnotationStrategy is set here since it's
 * necessary for the @Convert annotation
 */
Serializer ser = new Persister(new AnnotationStrategy());
Telegram t = ser.read(Telegram.class, xml);

System.out.println(t);

Result:
Telegram{config={bar=no, foo=yes, baz=maybe}}


A: Because @Convert notation can be put on fields also, I found a bit more concise code than the brilliant answer of "ollo":
    @Root(name = "telegram")
// Requires AnnotationStrategy
public class Telegram {
    @Element
    @Convert(Telegram.ConfigConverter.class)
    private Map<String, String> config;

    public String get(String name) {
        return config.get(name);
    }

    public Map<String, String> getConfig() {
        return config;
    }

    // ...

    @Override
    public String toString() {
        return "Telegram{" + "config=" + config + '}';
    }

    static class ConfigConverter implements Converter<Map<String, String>> {
        @Override
        public Map<String, String> read(final InputNode configNode) throws Exception {
            Map<String, String> map = new HashMap<>();

            // Iterate over config's child nodes and put them into the map
            InputNode cfg = configNode.getNext();

            while (cfg != null) {
                map.put(cfg.getName(), cfg.getValue());
                cfg = configNode.getNext();
            }

            return map;
        }

        @Override
        public void write(OutputNode node, Map<String, String> value) throws Exception {
            // Implement if you need serialization too
            throw new UnsupportedOperationException("Not supported yet.");
        }

    }
}

And the tests:
        final String xml = "<telegram>\n"
            + "  <config>\n"
            + "    <foo>yes</foo>\n"
            + "    <bar>no</bar>\n"
            + "    <baz>maybe</baz>\n" // Some "future element"
            + "  </config>\n"
            + "</telegram>";
    /*
     * The AnnotationStrategy is set here since it's
     * necessary for the @Convert annotation
     */
    Serializer ser = new Persister(new AnnotationStrategy());
    Telegram t = ser.read(Telegram.class, xml);

    assertEquals("yes",t.getConfig().get("foo"));
    assertEquals("no",t.getConfig().get("bar"));
    assertEquals("maybe",t.getConfig().get("baz"));
    assertEquals(3, t.getConfig().size());

