Question

I am writing a very basic library application in Java EE, just to understand how it works. This application lets the user add a book, which is associated to a shelf. The association is bidirectional, many-to-one, so I would expect to be able to get the shelf which the book belongs to book.getShelf() and the books that a shelf contains shelf.getBooks().

Unfortunately, if I add a new Book to a Shelf, this Book is not returned by shelf.getBooks() until I redeploy my app. I would need your help to understand what I’m doing wrong.

Here is part of the code of the entities:

@Entity
public class Book implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    protected String title;
    protected String author;
    @ManyToOne(fetch=FetchType.EAGER)
    protected Shelf shelf;

    //getters and setters follow
}

@Entity
public class Shelf implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @OneToMany(mappedBy = "shelf")
    private List<Book> books;

    protected String genre;

    //getters and setters follow
}

The persistence of Book and Shelf is managed by the following stateless session bean, BookManager. It also contains the method to retrieve the list of books in a shelf (getBooksInShelf).

@Stateless
@LocalBean
public class BookManager{
    @EJB
    private ShelfFacade shelfFacade;
    @EJB
    private BookFacade bookFacade;

    public List<Shelf> getShelves() {
        return shelfFacade.findAll();
    }

    public List<Book> getBooksInShelf(Shelf shelf) {
        return shelf.getBooks();
    }

    public void addBook(String title, String author, String shelf) {
        Book b = new Book();
        b.setName(title);
        b.setAuthor(author);
        b.setShelf(getShelfFromGenre(shelf));
        bookFacade.create(b);
    }

    //if there is a shelf of the genre "shelf", return it
    //otherwise, create a new shelf and persist it
    private Shelf getShelfFromGenre(String shelf) {
        List<Shelf> shelves = shelfFacade.findAll();
        for (Shelf s: shelves){
            if (s.getGenre().equals(shelf)) return s;
        }
        Shelf s = new Shelf();
        s.setGenre(shelf);
        shelfFacade.create(s);
        return s;
    }

    public int numberOfBooks(){
        return bookFacade.count();
    }

}

In the JSP: (I’m writing only the portion of code for book presentation)

<jsp:useBean id="bookManager" class="sessionBean.BookManager" scope="request"/>
// ...
<%  List<Book> books;
    for(Shelf s: shelves){
        books = bookManager.getBooksInShelf(s);
%>
        <h2><%= s.getGenre() %></h2>
        <ul>
<%      if (books.size()==0){
%>          <p>The shelf is empty.</p>
<%      }
        for (Book b: books){
%>          <li> <em><%= b.getAuthor()%></em>, <%= b.getName() %> </li>
<%      }
%>      </ul>
<%  } 
%>

Answer 1

You must maintain bi-directional relationships. When you create a new Book and set its Shelf, you must add the Book to the Shelf’s books.