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    <p>According to the <a href="http://db.apache.org/jdo/pmf.html" rel="nofollow">JDO Documentation</a> you create one <code>PersistenceManagerFactory</code> per datastore. If you are using JDO to access databases via SQL and you have more than one database, then you will need one <code>PersistenceManagerFactory</code> per database (since you need to specify the JDBC URL, user name and password when you create the <code>PersistenceManagerFactory</code>).</p> <p>For simple use cases, you can just create a <code>PersistenceManager</code> when you need it and close it in a <code>finally</code> clause (see <a href="http://db.apache.org/jdo/pm.html" rel="nofollow">the persistence manager documentation</a>).</p> <p>If you use transactions, and the code for updating entities can be spread across multiple methods or objects, I recommend creating the <code>PersistenceManager</code> on demand and storing it in a <code>ThreadLocal</code> (or request-scoped object if you use Guice or Spring). This will make sure any code that does updates participates in the current transaction. Make sure to close the <code>PersistenceManager</code> at the end of the request.</p> <p>If you only need one persistence manager factory, you can do:</p> <pre><code>public class Datastore { private static PersistenceManagerFactory PMF; private static final ThreadLocal&lt;PersistenceManager&gt; PER_THREAD_PM = new ThreadLocal&lt;PersistenceManager&gt;(); public static void initialize() { if (PMF != null) { throw new IllegalStateException("initialize() already called"); } PMF = JDOHelper.getPersistenceManagerFactory("jdo.properties"); } public static PersistenceManager getPersistenceManager() { PersistenceManager pm = PER_THREAD_PM.get(); if (pm == null) { pm = PMF.getPersistenceManager(); PER_THREAD_PM.set(pm); } return pm; } public static void finishRequest() { PersistenceManager pm = PER_THREAD_PM.get(); if (pm != null) { PER_THREAD_PM.remove(); Transaction tx = pm.currentTransaction(); if (tx.isActive()) { tx.rollback(); } pm.close(); } } } </code></pre> <p>Any code that needs a persistence manager can call <code>Datastore.getPersistenceManager()</code></p> <p>Note: I used all static methods to make it simple for the purposes of answering your question. If I was using a dependency-injection framework like Guice, I would make the methods non-static and bind <code>Datastore</code> as a Singleton.</p> <p>You could call <code>finishRequest</code> in a Servlet Filter:</p> <pre><code>public class PersistenceManagerFilter implements javax.servlet.Filter { public init(FilterConfig filterConfig) { Datastore.initialize(); } public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { try { chain.doFilter(request, response); } finally { Datastore.finishRequest(); } } } </code></pre>
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    1. POHow to use JDO persistence manager?
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    1. COthank you for such a detailed answer.
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    2. COOne issue: do a search/replace and change all occurrences of 'persistAnce' to 'persistEnce' and you'll be much happier. :-)
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    3. COI found this approach doesn't work on google app engine - for whatever reason it works perfectly on localhost but the pmf is closed at unexpected times in production
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