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    <p>During the last few days I designed a possible solution. What I was trying to construct with the <code>BookUnitSession</code> class was actually the <code>EntityManagerHelper</code> class:</p> <pre><code>public class EntityManagerHelper { private static final EntityManagerFactory emf; private static final ThreadLocal&lt;EntityManager&gt; threadLocal; static { emf = Persistence.createEntityManagerFactory("BookStoreUnit"); threadLocal = new ThreadLocal&lt;EntityManager&gt;(); } public static EntityManager getEntityManager() { EntityManager em = threadLocal.get(); if (em == null) { em = emf.createEntityManager(); threadLocal.set(em); } return em; } public static void closeEntityManager() { EntityManager em = threadLocal.get(); if (em != null) { em.close(); threadLocal.set(null); } } public static void closeEntityManagerFactory() { emf.close(); } public static void beginTransaction() { getEntityManager().getTransaction().begin(); } public static void rollback() { getEntityManager().getTransaction().rollback(); } public static void commit() { getEntityManager().getTransaction().commit(); } } </code></pre> <p>Such a class ensures that each thread (i.e., each request) will get its own <code>EntityManager</code> instance. Consequently, each DAO object can obtain the correct <code>EntityManager</code> instance by calling <code>EntityManagerHelper.getEntityManager()</code></p> <p>According to the session-per-request pattern each request must open and close its own <code>EntityManager</code> instance, which will be in charge of encapsulating the required unit of work within a transaction. This can be done by means of an intercepting filter implemented as a <code>ServletFilter</code>:</p> <pre><code>public class EntityManagerInterceptor implements Filter { @Override public void destroy() {} @Override public void init(FilterConfig fc) throws ServletException {} @Override public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException { try { EntityManagerHelper.beginTransaction(); chain.doFilter(req, res); EntityManagerHelper.commit(); } catch (RuntimeException e) { if ( EntityManagerHelper.getEntityManager() != null &amp;&amp; EntityManagerHelper.getEntityManager().isOpen()) EntityManagerHelper.rollback(); throw e; } finally { EntityManagerHelper.closeEntityManager(); } } } </code></pre> <p>This approach also allows the View (e.g., a JSP page) to fetch entity's fields even if they have been lazy initialized (Open Session in View pattern). In a JSE environment the <code>EntityManagerFactory</code> needs to be explicitly closed when the servlet container is shutdown. This can be done by using a <code>ServletContextListener</code> object:</p> <pre><code>public class EntityManagerFactoryListener implements ServletContextListener { @Override public void contextDestroyed(ServletContextEvent e) { EntityManagerHelper.closeEntityManagerFactory(); } @Override public void contextInitialized(ServletContextEvent e) {} } </code></pre> <p>The <code>web.xml</code> deployment descriptor:</p> <pre><code>&lt;listener&gt; &lt;description&gt;EntityManagerFactory Listener&lt;/description&gt; &lt;listener-class&gt;package.EntityManagerFactoryListener&lt;/listener-class&gt; &lt;/listener&gt; &lt;filter&gt; &lt;filter-name&gt;interceptor&lt;/filter-name&gt; &lt;filter-class&gt;package.EntityManagerInterceptor&lt;/filter-class&gt; &lt;/filter&gt; &lt;filter-mapping&gt; &lt;filter-name&gt;interceptor&lt;/filter-name&gt; &lt;url-pattern&gt;*.do&lt;/url-pattern&gt; &lt;/filter-mapping&gt; </code></pre>
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    1. CONice work, but you probably mean JEE instead of JSE.
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    2. COThank you. I'm currently using Tomcat 7 and Struts 1.3 without any application server. Therefore, I'd say I'm not completely JEE compliant. From the view point of persistency, it is like being in a JSE environment. That's why I stated JSE in the title.
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