Using JAXB to unmarshal/marshal a List<String>

Question

I'm trying to create a very simple REST server. I just have a test method that will return a List of Strings. Here's the code:

@GET
@Path("/test2")
public List test2(){
    List list=new Vector();
    list.add("a");
    list.add("b");
    return list;
}

It gives the following error:

SEVERE: A message body writer for Java type,
class java.util.Vector, and MIME media type,
application/octet-stream, was not found

I was hoping JAXB had a default setting for simple types like String, Integer, etc. I guess not. Here's what I imagined:

<Strings>
  <String>a</String>
  <String>b</String>
</Strings>

What's the easiest way to make this method work?

Answer 1

I have encountered this pattern a few times, I found that the easiest way is to define an inner class with JaxB annotations. (anyways, you'll probably want to define the root tag name)

so your code would look something like this

@GET
@Path("/test2")
public Object test2(){
   MyResourceWrapper wrapper = new MyResourceWrapper();
   wrapper .add("a");
   wrapper .add("b");
   return wrapper ;
}

@XmlRootElement(name="MyResource")
private static class MyResourceWrapper {
       @XmlElement(name="Item")
       List<String> list=new ArrayList<String>();
       MyResourceWrapper (){}

       public void add(String s){ list.add(s);}
 }

if you work with javax.rs (jax-rs) I'd return Response object with the wrapper set as its entity

Answer 2

I used @LiorH's example and expanded it to:

@XmlRootElement(name="List")
public class JaxbList<T>{
    protected List<T> list;

    public JaxbList(){}

    public JaxbList(List<T> list){
     this.list=list;
    }

    @XmlElement(name="Item")
    public List<T> getList(){
     return list;
    }
}

Note, that it uses generics so you can use it with other classes than String. Now, the application code is simply:

    @GET
    @Path("/test2")
    public JaxbList test2(){
     List list=new Vector();
     list.add("a");
     list.add("b");
     return new JaxbList(list);
    }

Why doesn't this simple class exist in the JAXB package? Anyone see anything like it elsewhere?

Answer 3

User1's example worked well for me. But, as a warning, it won't work with anything other than simple String/Integer types, unless you add an @XmlSeeAlso annotation:

@XmlRootElement(name = "List")
@XmlSeeAlso(MovieTicket.class)
public class MovieTicketList {
    protected List<MovieTicket> list;

This works OK, although it prevents me from using a single generic list class across my entire application. It might also explain why this seemingly obvious class doesn't exist in the JAXB package.

Answer 4

@GET
@Path("/test2")
public Response test2(){
   List<String> list=new Vector<String>();
   list.add("a");
   list.add("b");

   final GenericEntity<List<String>> entity = new GenericEntity<List<String>>(list) { };
   return Response.ok().entity(entity).build();
}