Marshal/Un marshal List objects in Jersey JAX-RS using JAXB

Question

Good Morning. Today morning when I am going through Jersey Entity providers MessageBodyReaders and MessageBodyWriters I came across the following problem.

I want to write a resource method and client that returns a list of custom objects and media type is application/xml. So I would like to use JAXB (I am new to JAXB). I was able to achieve this by writing my own extended MessageBodyReader and MessageBodyWriter. But I am afraid of the way I am following. Just look at the way I implemented:

Resource method:

@Path("productlist/xml")
@GET
public RetObjects getProductsXml(){
    List<Product> pList = new ArrayList<Product>();
    pList.add(new Product("1","Dell latitude E6000",2900,500));
    pList.add(new Product("2","Xperia Z2",549,400));
    RetObjects obj = new RetObjects();
    obj.setObject(pList);
    return obj;
}

My custom objects:

@Entity
@Table (name="PRODUCT")
@XmlRootElement(name="product")
public class Product {

    @Id
    @Column(name = "CODE")
    private String code;
    ...
    // rest of the fields, constructors, getters and setters
 }

Object that wraps my list of custom object:

 @XmlRootElement(name = "products")
 @XmlAccessorType (XmlAccessType.FIELD)
 public class RetObjects {

     @XmlElement(name = "product")
     private List<Product> object = null;

     public List<Product> getObject() {
         return object;
     }

     public void setObject(List<Product> object) {
         this.object = object;
     }
 }

MessageBodyReader/Writer are straight forward just using Jaxb unmarshaller and marshaller over the RetObjects object.

With this implementation it is working fine as expected and i am able to fetch the RetObjects wrapping the list of Products perfectly fine at client.

Here my question is, instead of wrapping my List of Products into a intermediate object, RetObjects in my case, couldn't I marshal and unmarshal List of Products object directly. If I want to write another service that returns List of Orders, I need to wrap this with one more intermediate object. What is the right approach to achieve this? How could I do this without intermediate objects?

Solution

First

You don't need your own MessageBodyWriter/Reader. Jersey/JAX-RS alread has standard support for this. I would stick with the default, unless you have a really, really good reason for needed to whip up your own.

Second

We don't need the wrapper, you can simple return a GenericEntity. This will automatically wrap the elements in a "plural wrapper" element, i.e. <product> -> <products>.

List<Product> list = new ArrayList<>();
GenericEntity<List<String>> entity = new GenericEntity<List<Product>>(list) {};
Response response = Response.ok(entity).build();

For accepting a body in resource method, simply accepting List<Product> as an argument is enough. It will accept <products><product/><product/></products>

---

UPDATE

To retrieve the List<Product> on the client side, we should make use of GenericType. Se this post.

Jersey 1

WebResource resource = client.resource("...");
List<Product> products = resource.get(new GenericType<List<Product>>(){});

Jersey 2/JAX-RS 2

Response response = ...
List<Product> products = response.readEntity(new GenericType<List<Product>>(){});

Comments

[HJK]

WRT second point, you are true @peeskillet. When i am doing this, as you said I am getting <products><product></product></products>. In this case how could I consume at client side in Java POJO objects?

[HJK]

I am not able to do response.readEntity(new RetObject2<List<Product>>(){}); I am getting compilation error saying readEntity(Class<T>) is not applicable for readEntity(RetObject2<List<Product>>(){}). Here RetObject2 is a new generic class like RetObject2<T> with one field of type T. Could you elaborate a bit.

[Paul Samsotha]

Why Are you using RepObject2? It should be GenericType

[Paul Samsotha]

The whole point of your question is to get rid of the wrapper object isn't it?

[Paul Samsotha]

There is an overloaded readEntity which takes a GenericType argument.

[Paul Samsotha]

See javax.ws.rs.core.GenericType. It's an actual class. You are not supposed to provide your own "Generic type" :-) Sorry if I did not make that clear initially.

[HJK]

Thanks Peeskillet. Its working fine. I thought genericType is the abstract name you gave.

[HJK]

Let us continue this discussion in chat.