java.util.List is an interface, and JAXB can’t handle interfaces

In my understanding, you will not be able to process a plain List via JAXB, as JAXB has no idea how to transform that into XML.

Instead, you will need to define a JAXB type which holds a List<RelationCanonical> (I’ll call it Type1), and another one to hold a list of those types, in turn (as you’re dealing with a List<List<...>>; I’ll call this type Type2).

The result could then be an XML ouput like this:

<Type2 ...>
    <Type1 ...>
        <RelationCanonical ...> ... </RelationCanonical>
        <RelationCanonical ...> ... </RelationCanonical>
        ...
    </Type1>
    <Type1>
        <RelationCanonical ...> ... </RelationCanonical>
        <RelationCanonical ...> ... </RelationCanonical>
        ...
    </Type1>
    ...
</Type2>

Without the two enclosing JAXB-annotated types, the JAXB processor has no idea what markup to generate, and thus fails.

–Edit:

What I mean should look somewhat like this:

@XmlType
public class Type1{

    private List<RelationCanonical> relations;

    @XmlElement
    public List<RelationCanonical> getRelations(){
        return this.relations;
    }

    public void setRelations(List<RelationCanonical> relations){
        this.relations = relations;
    }
}

and

@XmlRootElement
public class Type2{

    private List<Type1> type1s;

    @XmlElement
    public List<Type1> getType1s(){
        return this.type1s;
    }

    public void setType1s(List<Type1> type1s){
        this.type1s= type1s;
    }
}

You should also check out the JAXB section in the J5EE tutorial and the Unofficial JAXB Guide.