public static class IntegerListXmlAdapter extends XmlAdapter<Object, List<Integer>> {
        ...
        @Override
        public Object marshal(List<Integer> list) throws Exception {
            return Joiner.on(",").join(list);
        }
}

public class IntegerListXmlAdapterTest {
    @Test
    public void testBasic() throws JsonProcessingException {
        ObjectMapper mapper = (new ObjectMapper()).setAnnotationIntrospector(new JaxbAnnotationIntrospector());
        SomeIntListHolder listHolder = new SomeIntListHolder();
        listHolder.setListOne(asList(1, 2, 3));
        System.out.println(mapper.writeValueAsString(listHolder));
    }

    public static class IntegerListXmlAdapter extends XmlAdapter<Object, List<Integer>> {
        @Override
        public List<Integer> unmarshal(Object value) throws Exception {return null;}

        @Override
        public Object marshal(List<Integer> list) throws Exception {
            return Joiner.on(",").join(list);
        }
    }

    public static class IntegerListToStringXmlAdapter extends XmlAdapter<String, List<Integer>> {
        public List<Integer> unmarshal(String value) throws Exception {return null;}

        public String marshal(List<Integer> list) throws Exception {
            return Joiner.on(",").join(list);
        }
    }

    @XmlRootElement
    @XmlAccessorType(XmlAccessType.NONE)
    public static class SomeIntListHolder {

        @XmlAttribute
        @XmlJavaTypeAdapter(IntegerListXmlAdapter.class)
        private List<Integer> listOne;

        public List<Integer> getListOne() {
            return listOne;
        }

        public void setListOne(List<Integer> listOne) {
            this.listOne = listOne;
        }
    }
}

    @Override
    public void serialize(Object value, JsonGenerator gen, SerializerProvider provider) throws IOException
    {
        Object delegateValue = convertValue(value);
        // should we accept nulls?
        if (delegateValue == null) {
            provider.defaultSerializeNull(gen);
            return;
        }

        //original code:
        //_delegateSerializer.serialize(delegateValue, gen, provider);

        JsonSerializer<Object> delegateSerializer;
        if (_delegateSerializer instanceof UnknownSerializer) {
            delegateSerializer =  provider.findValueSerializer(delegateValue.getClass());
        } else {
            delegateSerializer = _delegateSerializer;
        }

        delegateSerializer.serialize(delegateValue, gen, provider);
    }