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POEntityManager ThreadLocal pattern with JPA in JSE
 

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    <p>I'm developing a simple "Book Store" project using Struts 1.3 + JPA (with Hibernate as persistence provider). I cannot switch to Spring or any other more sophisticated development environment (e.g., Jboss) and I cannot use any Hibernate-specific technique (e.g., <code>Session</code> class). </p> <p>Given the fact that I'm in a JSE Environment, I need to explicitly manage the whole EntityManager's lifecycle.</p> <p>The <code>Book</code> entity is defined as follows:</p> <pre><code>@Entity public class Book { @Id private String isbn; private String title; private Date publishDate; // Getters and Setters } </code></pre> <p>I defined three <code>Action</code> classes, which are responsible, respectively, of retrieving all book instances, retrieving a single book instance by its ISBN and merging a detached book into the DB.</p> <p>In order to increase separation of concerns between business-logic code and data-access code, I introduced a simple <code>BookDAO</code> object, which is charge of executing CRUD operations. Ideally, all data-access related calls should be delegated to the persistence layer. For example, the <code>ListBookAction</code> is defined as follows:</p> <pre><code>public class ListBookAction extends Action { private BookDAO dao = new BookDAO(); @Override public ActionForward execute(ActionMapping mapping, ActionForm form, HttpServletRequest request, HttpServletResponse response) throws Exception { // Retrieve all the books List&lt;Book&gt; books = dao.findAll(); // Save the result set request.setAttribute("books", books); // Forward to the view return mapping.findForward("booklist"); } } </code></pre> <p>The BookDAO object needs to access an <code>EntityManager</code> instance in order to do any operation. Given that <code>EntityManger</code> is not thread-safe, I introduced an helper class named <code>BookUnitSession</code> which encapsulates EntityManager within a <code>ThreadLocal</code> variable:</p> <pre><code>public class BookUnitSession { private static EntityManagerFactory emf = Persistence.createEntityManagerFactory("BookStoreUnit"); private static final ThreadLocal&lt;EntityManager&gt; tl = new ThreadLocal&lt;EntityManager&gt;(); public static EntityManager getEntityManager() { EntityManager em = tl.get(); if (em == null) { em = emf.createEntityManager(); tl.set(em); } return em; } } </code></pre> <p>Everything seems to work, but I still have some concerns. Namely:</p> <ol> <li>Is this solution the best thing to do? which is the best practice in this case?</li> <li>I still need to explictly close both the EntityManager and the EntityManagerFactory. How can I do that?</li> </ol> <p>Thanks</p>
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