Hi Claus,
Claus Priisholm wrote:
> I been using XStream with great results for a number of tasks, but now I
> run into a problem that seems to boil down to the order in which the
> XStream object is configured.
>
> I've made a simplified example:
>
> <doc>
> <intro>
> <link>
> <url>/foo/bar</url>
> </link>
> </intro>
> <link>
> <url>/up/down</url>
> <image>cat.jpg</image>
> </link>
> </doc>
>
> The setup is like:
>
> XStream xstream = new XStream();
> xstream.alias("doc", DocText.class);
> xstream.addImplicitCollection(DocText.class, "links");
> xstream.alias("intro", Paragraph.class);
>
> xstream.alias("link", UrlLink.class); // 1
> xstream.alias("link", ImageLink.class); // 2
The second definition clobbers the first one.
> The gist of the classes:
>
> public class DocText {
> private Paragraph intro;
> private List<ImageLink> links;
> }
>
> public class Paragraph {
> UrlLink link;
> }
>
> public class UrlLink {
> String url;
> }
>
> public class ImageLink {
> private String url;
> private String image;
> }
>
> When the sequence of the lines marked 1 & 2 is as shown it works.
It works only, because the XML actually contains an ImageLink.
> When
> they change place it throws the following exception:
>
> No such field com.codedroids.oc.schema.dao.UrlLink.image
> ---- Debugging information ----
> field : image
> class : com.codedroids.oc.schema.dao.UrlLink
> required-type : com.codedroids.oc.schema.dao.UrlLink
> converter-type :
> com.thoughtworks.xstream.converters.reflection.ReflectionConverter
> path : /doc/link/image
> line number : 9
> class[1] : com.codedroids.oc.schema.dao.DocText
> version : null
> -------------------------------
You'd get the same exception for the first setup if your XML contained an
UrlLink.
> If DocText has a simple field called link rather than a list of them, then
> it works either way (after removing the xstream.addImplicitCollection() of
> course).
>
> The problem is that I am trying to get the xstream configuration generated
> automatically from the related schema definition and thus cannot be sure
> in which order the various alias'es gets invoked.
Your setup *is* ambiguous.
> I haven't tried using annotations since there places where the same class
> is mapped by more than one tag name - this works fine with alias'es but
> with annotation I can only map it to one tag name (or perhaps I'm
> mistaken)
Annotations make no difference here.
See, if you have two boys and name them both "Nick", so would you expect
that they know whom you mean, when you shout after one of them?
Aliases define normally a 1:1 relation. The ReflectionConverter has to
recover from the alias name the required class type to instantiate the
appropriate object. If you use the same alias more then once, than the
latest definition will win. And in case of annotations you cannot even
define the registration sequence.
What you can do is to write a custom converter that can handle all classes
using the same alias. As long as it can detect what it has to create at
deserialization time, anything is fine. In your case it should create an
ImageLink if the "link" element has an additional "image" element as child
and an UrlLink otherwise.
See the converter tutorial, how to write such converters, it's an easy task.
Cheers,
Jörg
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