XStream class cannot be resolved

Question

I'm getting an XStream error with the following setup. I must be crazy. What's wrong?

The request class

@XStreamAlias("RequestTO")
public class RequestTO {

    @XStreamImplicit
    private List<SkuMerchTO> skuNumbers;
...
}

The nested object class

@XStreamAlias("skuMerch")
public class SkuMerchTO {

    @XStreamAlias("skuNumber")
    @XStreamAsAttribute
    private Integer skuNumber;

    @XStreamAlias("dept")
    @XStreamAsAttribute
    private Short department;

    @XStreamAlias("class")
    @XStreamAsAttribute
    private Short cls;

    @XStreamAlias("subClass")
    @XStreamAsAttribute
    private Short subClass;
...
}

XStream code to decode the XML to an object:

XStream stream = new XStream();
stream.processAnnotations(SkuMerchTO.class);
stream.processAnnotations(RequestTO.class);
RequestTO request =  (RequestTO)stream.fromXML(requestXml);

XML input string:

<RequestTO>
     <skuMerch skuNumber="123456" dept="1" class="2" subClass="3"/>
     <skuMerch skuNumber="234567" dept="4" class="5" subClass="6"/>  
</RequestTO>

Error in Stacktrace:
---- Debugging information ----
message : 2 : 2
cause-exception : com.thoughtworks.xstream.mapper.CannotResolveClassException
cause-message : 2 : 2
class : [...]RequestTO
required-type : [...]SkuMerchTO
path : /RequestTO/skuNumberList/skuMerch
line number : 3
.-------------------------------
com.thoughtworks.xstream.converters.ConversionException: 2 : 2 : 2 : 2
---- Debugging information ----
message : 2 : 2
cause-exception : com.thoughtworks.xstream.mapper.CannotResolveClassException
cause-message : 2 : 2
class : [...]RequestTO
required-type :[..]SkuMerchTO
path : /RequestTO/skuNumberList/skuMerch
line number : 3
.-------------------------------

If I create the objects and do toXML I get this:

<RequestTO>
  <skuMerch skuNumber="0" dept="1" class="2" subClass="2"/>
  <skuMerch skuNumber="1" dept="1" class="2" subClass="2"/>
  <skuMerch skuNumber="2" dept="1" class="2" subClass="2"/>
</RequestTO>

EDIT: The hilarious thing is, if do this:

 stream.fromXML(stream.toXML(object));  

It still fails on the from XML part!

Answer 1

The attribute named class has a special meaning to XStream, this question suggests that you can tell XStream to use a different attribute for this purpose via something like

stream.aliasSystemAttribute("__class", "class");

This would cause XStream to use __class as the "special" attribute, and treat class as a normal one. Or, if you know you don't need the XStream magic handling of class for any of your objects you can say

stream.aliasSystemAttribute(null, "class");

to tell it not to use this feature at all.

You may also need to use itemFieldName="skuMerch" on the @XStreamImplicit annotation, as per the XStream annotation tutorial.