Error 415 Unsupported Media Type: POST not reaching REST if JSON, but it does if XML

I am actually new to REST WS but really I don't get this 415 Unsupported Media Type.

I am testing my REST with Poster on Firefox and the GET works fine for me, also the POST (when it's a application/xml) but when I try application/json it doesn't not reach the WS at all, the server rejects it.

This is my URL: http:// localhost:8081/RestDemo/services/customers/add

This is JSON I'm sending: {"name": "test1", "address" :"test2"}

This is XML I'm sending:

<customer>
    <name>test1</name>
    <address>test2</address>
</customer>

and this is my Resource class:

@Produces("application/xml")
@Path("customers")
@Singleton
@XmlRootElement(name = "customers")
public class CustomerResource {

    private TreeMap<Integer, Customer> customerMap = new TreeMap<Integer, Customer>();

    public  CustomerResource() {
        // hardcode a single customer into the database for demonstration
        // purposes
        Customer customer = new Customer();
        customer.setName("Harold Abernathy");
        customer.setAddress("Sheffield, UK");
        addCustomer(customer);
    }

    @GET
    @XmlElement(name = "customer")
    public List<Customer> getCustomers() {
        List<Customer> customers = new ArrayList<Customer>();
        customers.addAll(customerMap.values());
        return customers;
    }

    @GET
    @Path("/{id}")
    @Produces("application/json")
    public String getCustomer(@PathParam("id") int cId) {
        Customer customer = customerMap.get(cId); 
        return  "{\"name\": \" " + customer.getName() + " \", \"address\": \"" + customer.getAddress() + "\"}";

    }

    @POST
    @Path("/add")
    @Produces(MediaType.APPLICATION_JSON)
    @Consumes({MediaType.APPLICATION_XML, MediaType.APPLICATION_JSON})
    public String addCustomer(Customer customer) {
         //insert 
         int id = customerMap.size();
         customer.setId(id);
         customerMap.put(id, customer);
         //get inserted
         Customer result = customerMap.get(id);

         return  "{\"id\": \" " + result.getId() + " \", \"name\": \" " + result.getName() + " \", \"address\": \"" + result.getAddress() + "\"}";
    }

}

EDIT 1:

This is my Customer class:

@XmlRootElement 
public class Customer implements Serializable {

    private int id;
    private String name;
    private String address;

    public Customer() {
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

}

The issue is in the deserialization of the bean Customer. Your programs knows how to do it in XML, with JAXB as Daniel is writing, but most likely doesn't know how to do it in JSON.

Here you have an example with Resteasy/Jackson http://www.mkyong.com/webservices/jax-rs/integrate-jackson-with-resteasy/

The same with Jersey: http://www.mkyong.com/webservices/jax-rs/json-example-with-jersey-jackson/