Question

From some library, I’m getting such an input string:

<link>
    <name>button1</name>
    <target>there</target>
</link>
<link>
    <name>button2</name>
    <target>there2</target>
</link>

(note that this is not an XML document as it has no root) and I have this class:

@XmlRootElement(name = "link")
public class TableTagLinkElement {
    private String name;
    private String target;

    // getters and setters
}

How can I unmarshall this easily into a list of TableTagLinkElements, in a generic way such that I could implement such a method:

public <T> List<T> parseCollection(String xmlString, Class<T> rootClass)

i.e., without any previous knowledge of the TableTagLinkElement class or of the name of the <link> tag?

I know about solutions that create a wrapper class with a list, but I think they’re not applicable here, are they?

Answer 1

If you can wrap a <root>...</root> element around the complete XML string, then you could create an XMLStreamReader reading from that string, then loop over the reader, unmarshalling each link as you go. For example (exception handling omitted)

public <T> List<T> parseCollection(String xmlString, Class<T> rootClass) {
  XMLInputFactory inFac = XMLInputFactory.newFactory();
  XMLStreamReader reader = inFac.createXMLStreamReader(
                    new StringReader("<root>" + xmlString + "</root>"));
  reader.nextTag(); // move to the <root> tag
  reader.nextTag(); // move to the first child
  List<T> list = new ArrayList<T>();
  while(reader.getEventType() == XMLStreamConstants.START_ELEMENT) {
    list.add(declaredType.cast(unmarshaller.unmarshal(reader)));

    // unmarshal leaves the reader pointing at the event *after* the
    // closing tag, not the END_ELEMENT event itself, so we can't just
    // do nextTag unconditionally.  We may already be on the next opening
    // tag or the closing </root> but we might need to advance if there
    // is whitespace between tags
    if(reader.getEventType() != XMLStreamConstants.START_ELEMENT &&
       reader.getEventType() != XMLStreamConstants.END_ELEMENT) {
      reader.nextTag();
    }
  }
  reader.close();
  return list;
}