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POHow to design for JPA in a non-Java EE web server (e.g. Tomcat)
 

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  1. POHow to design for JPA in a non-Java EE web server (e.g. Tomcat)
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    <p>Considering there is no @PersistenceContext available to inject the EntityManager, plus you need to manually manage Transactions, what is the best way to design such an application?</p> <p>For the EntityManagerFactory/EntityManager, as far as I can see, you must have each DAO accepting an EntityManager in the costructor e.g.</p> <pre><code>public class DAOImpl implements DAO { private EntityManager em; DAOImpl(EntityManager em){ this.em = em; } //all CRUD operations follow } </code></pre> <p>The first question that rises is when do you call <em>EntityManager#close()</em>?</p> <p><strong>Point A:</strong> The way I see it, you are better off doing this in a <a href="http://java.sun.com/javaee/5/docs/api/javax/servlet/Filter.html" rel="nofollow noreferrer">Filter</a> at the end of the request cycle, which implies that you associate the EntityManager with the current thread (using ThreadLocal?)</p> <p>Second question is, how and when do you inject the EntityManager?</p> <p>Considering there is a <a href="http://java.sun.com/javaee/5/docs/api/javax/servlet/ServletContextListener.html" rel="nofollow noreferrer">ServletContextListener</a> where we create and close the EntityManagerFactory, we could have a static method as follows</p> <pre><code>public static EntityManager createEntityManager(){ return entityManagerFactory.createEntityManager(PERSISTENT_NAME); } </code></pre> <p>but since we want to encapsulate creating the DAO, we could use a factory e.g.</p> <pre><code>public class DAOFactory { public static DAO dao(){ //return a new DAO } } </code></pre> <p>As per <strong>Point A</strong> we should use a ThreadLocal to create the DAO using the EntityManager for the current Thread.</p> <p>For managing Transactions.</p> <p>The best way I can think of (which mimics the JPA spec) is to create your own Transaction annotation and use reflection to inject the begin/commit/rollback operations.</p> <p>You should then return a <a href="http://java.sun.com/javase/6/docs/api/java/lang/reflect/Proxy.html" rel="nofollow noreferrer">Proxy</a> from the DAOFactory which handles the transactions</p>
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