XStream annotation not working in scala - scala

Question

I try to use XStream in scala, but it looks like the annotation in XSteam is not working in scala. In the following code, only #XStreamAlias work, I also attach one sample output, could anyone help this ?
#XStreamAlias("posts")
case class Posts(
#XStreamAsAttribute tag: String,
#XStreamAsAttribute total: String,
#XStreamAsAttribute user: String,
#XStreamImplicit(itemFieldName="post") posts: JList[Post])
#XStreamAlias("post")
case class Post(
#XStreamAsAttribute description: String,
#XStreamAsAttribute extended: String,
#XStreamAsAttribute hash: String,
#XStreamAsAttribute href: String,
#XStreamAsAttribute shared: String,
#XStreamAsAttribute tag: String,
#XStreamAsAttribute time: String)
<posts>
<tag>a</tag>
<total>b</total>
<user>c</user>
<posts>
<post>
<description></description>
<extended></extended>
<hash></hash>
<href></href>
<shared></shared>
<tag></tag>
<time></time>
</post>
</posts>
</posts>

Answer 1

This is an old question, but I recently had a similar problem and I'd like to add my solution to it for future reference to others.
I wanted to use an #XStreamAlias for a case class and its fields and to deserialize an XML from a string according to the case class annotations:
import com.thoughtworks.xstream.XStream
import com.thoughtworks.xstream.annotations.XStreamAlias
import com.thoughtworks.xstream.io.xml.DomDriver
#XStreamAlias("TOP")
case class example(
#XStreamAlias("SUB")
val param : String) {
}
val xs = new XStream(new DomDriver())
xs.setClassLoader(getClass.getClassLoader)
xs.processAnnotations(example.getClass)
//OK
xs.fromXML(<TOP><param>x</param></TOP>.toString())
//Error - No such field sub
xs.fromXML(<TOP><SUB>x</SUB></TOP>.toString())
The problem was that the annotation didn't work for the class field.
After searching a bit I found out that scala generates several accessors to a class field (cf. scala.annotation.meta) but by default an annotation is only applied to the constructor parameter and not to all the accessors.
To get the annotation to apply to the field as well (the spec/library is ambiguous about these concepts so I will assume it only applies to the field and not to the accessors) you can use #(XStreamAlias #field)("SUB"). This solved my problem:
#XStreamAlias("TOP")
case class example2 (
#(XStreamAlias #scala.annotation.meta.field)("SUB")
val param : String) {
}
val xs2 = new XStream(new DomDriver())
xs2.setClassLoader(getClass.getClassLoader)
xs2.processAnnotations(example.getClass)
//OK
val obj = xs2.fromXML(<TOP><SUB>x</SUB></TOP>.toString()).asInstanceOf[example2]
print(obj)
//> example2(x)

Answer 2

It looks like java annotations do not play well with Scala. :-) You'd have to rely on setting this with plain old method syntax. See below for a code snippet that I knocked up looking at their API documentation.
import com.thoughtworks.xstream.annotations._
import com.thoughtworks.xstream.XStream
import java.util.{ ArrayList => JList }
class Posts(
val tag: String,
val total: String,
val user: String,
val post: JList[Post])
class Post(
val description: String,
val extended: String,
val hash: String,
val href: String,
val shared: String,
val tag: String,
val time: String)
object Main extends App {
val xst = new XStream();
val pp = new JList[Post]
val rstv = new Post("a", "b", "c", "d", "e", "f", "g")
pp.add(rstv)
val postClazz = classOf[Post]
val postsClazz = classOf[Posts]
val pstv = new Posts("p", "q", "r", pp)
xst.useAttributeFor(postsClazz, "tag")
xst.useAttributeFor(postsClazz, "total")
xst.useAttributeFor(postsClazz, "user")
xst.useAttributeFor(postClazz, "description")
xst.useAttributeFor(postClazz, "extended")
xst.useAttributeFor(postClazz, "hash")
xst.useAttributeFor(postClazz, "href")
xst.useAttributeFor(postClazz, "shared")
xst.useAttributeFor(postClazz, "tag")
xst.useAttributeFor(postClazz, "time")
val foo = xst.toXML(pstv)
println(foo)
}
Note that all classes must have fields set up to be looked up by XStream. With this code, I got the below output:
<Posts tag="p" total="q" user="r">
<post>
<Post description="a" extended="b" hash="c" href="d" shared="e" tag="f" time
="g"/>
</post>
</Posts>
Hope this helps.
EDIT: To expand on what I said above, annotations such as #XStreamAlias et al, were completely stripped from compiled bytecode. This can be seen by running javap or scalap on those compiled classes. This led me to conclude that java annotations were not treated on par with scala annotations(Though, ideally it should be - Feel free to chime in if I made any mistake). Would be glad to learn something. :-)