Why is a field of my entity class null? - jpa

Question

I have these two entities in a one to many relation:
public class Category implements Serializable {
private static final long serialVersionUID = 1L;
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Basic(optional = false)
#Column(name = "id")
private Short id;
#Basic(optional = false)
#Column(name = "name")
private String name;
#OneToMany(cascade = CascadeType.ALL, mappedBy = "categoryId")
private Collection<Product> productCollection;
...
#XmlTransient
public Collection<Product> getProductCollection() {
return productCollection;
}
...
and
public class Product implements Serializable {
...
#JoinColumn(name = "category_id", referencedColumnName = "id")
#ManyToOne(optional = false)
private Category categoryId;
...
generated with NetBeans. The problem is that when the method getProductCollection() is called by the ControllerServlet the Collection of Product is null.
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String userPath = request.getServletPath();
Category selectedCategory;
Collection<Product> categoryProducts;
// if category page is requested
if (userPath.equals("/category")) {
// get categoryId from request
String categoryId = request.getQueryString();
if (categoryId != null) {
// get selected category
selectedCategory = categoryFacade.find(Short.parseShort(categoryId));
// place selected category in request scope
request.setAttribute("selectedCategory", selectedCategory);
// get all products for selected category
categoryProducts = selectedCategory.getProductCollection();
// place category products in request scope
request.setAttribute("categoryProducts", categoryProducts);
}
Notice the null value of productCollection when other fields has been yet initialized
Edit 1: I declared the categoryFacade in the ControllerServlet applying the #EJB annotation
public class ControllerServlet extends HttpServlet {
#EJB
private CategoryFacade categoryFacade;
Edit 2: Here is the persistence.xml document
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1" xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
<persistence-unit name="AffableBeanPU" transaction-type="JTA">
<jta-data-source>jdbc/affablebean</jta-data-source>
<properties/>
</persistence-unit>
</persistence>
Edit 3: I'm using TomEE 7.0.2

Answer 1

Try to initialize the Collection empty like:
#OneToMany(cascade = CascadeType.ALL, mappedBy = "categoryId")
private Collection<Product> productCollection = new HashSet<>();
Then you won't have null values even when there are no results in the lazy loaded relationship. If there are loaded values, they will be added to the collection.