Using JAXB to unmarshal/marshal a List

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

@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();
}

Answer 2

From a personal blog post, it is not necessary to create a specific JaxbList < T > object.

Assuming an object with a list of strings:

@XmlRootElement
public class ObjectWithList {

    private List<String> list;

    @XmlElementWrapper(name="MyList")
    @XmlElement
    public List<String> getList() {
        return list;
    }

    public void setList(List<String> list) {
        this.list = list;
    }

}

A JAXB round trip:

public static void simpleExample() throws JAXBException {

    List<String> l = new ArrayList<String>();
    l.add("Somewhere");
    l.add("This and that");
    l.add("Something");

    // Object with list
    ObjectWithList owl = new ObjectWithList();
    owl.setList(l);

    JAXBContext jc = JAXBContext.newInstance(ObjectWithList.class);
    ObjectWithList retr = marshallUnmarshall(owl, jc);

    for (String s : retr.getList()) {
        System.out.println(s);
    } System.out.println(" ");

}

Produces the following:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<objectWithList>
    <MyList>
        <list>Somewhere</list>
        <list>This and that</list>
        <list>Something</list>
    </MyList>
</objectWithList>

Answer 3

Make sure to add @XmlSeeAlso tag with your specific classes used inside JaxbList. It is very important else it throws HttpMessageNotWritableException

Answer 4

In case anyone of you wants to write a list wrapper for lists containing elements of multiple classes and want to give an individual XmlElement name according to the Class type without Writing X Wrapper classes you could use the @XmlMixed annotation. By doing so JAXB names the items of the list according to the value set by the @XmlRootElement. When doing so you have to specify which classes could possibly be in the list using @XmlSeeAlso

Example:

Possible Classes in the list

@XmlRootElement(name="user")
public class User {/*...*/}

@XmlRootElement(name="entry")
public class LogEntry {/*...*/}

Wrapper class

@XmlRootElement(name="records")
@XmlSeeAlso({User.class, LogEntry.class})
public static class JaxbList<T>{

    protected List<T> records;

    public JaxbList(){}

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

    @XmlMixed 
    public List<T> getRecords(){
        return records;
    }
}

Example:

List l = new List();
l.add(new User("userA"));
l.add(new LogEntry(new UserB()));


XStream xStream = new XStream();
String result = xStream.toXML(l);

Result:

<records>
    <user>...</user>
    <entry>...</entry>
</records>

Alternatevily you could specify the XmlElement names directly inside the wrapper class using the @XmlElementRef annotation

@XmlRootElement(name="records")
@XmlSeeAlso({User.class, LogEntry.class})
public static class JaxbList<T>{

    protected List<T> records;

    public JaxbList(){}

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

    @XmlElementRefs({
        @XmlElementRef(name="item", type=Object.class),
        @XmlElementRef(name="user", type=User.class),
        @XmlElementRef(name="entry", type=LogEntry.class)
    })
    public List<T> getRecords(){
        return records;
    }
}

Answer 5

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 6

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?