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https://issues.apache.org/jira/browse/AMBARI-19364?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Jonathan Hurley updated AMBARI-19364:
-------------------------------------
Status: Patch Available (was: Open)
> Investigate Changing the Default Container Policy in JPA From Vector to
> ArrayList
> ---------------------------------------------------------------------------------
>
> Key: AMBARI-19364
> URL: https://issues.apache.org/jira/browse/AMBARI-19364
> Project: Ambari
> Issue Type: Task
> Components: ambari-server
> Affects Versions: 2.5.0
> Reporter: Jonathan Hurley
> Assignee: Jonathan Hurley
> Fix For: 2.5.0
>
> Attachments: AMBARI-19364.patch
>
>
> Recently I noticed that collections coming back from JPA were using a
> {{Vector}} as their concrete collection. This seems very wrong as {{Vector}}
> is a completely synchronized collection.
> Tracing through EclipseLink, it seems like this really only affects
> [ReadAllQuery|http://grepcode.com/file/repo1.maven.org/maven2/org.eclipse.persistence/eclipselink/2.5.2/org/eclipse/persistence/queries/ReadAllQuery.java/#68].
> Essentially, EclipseLink uses a
> [ContainerPolicy|http://grepcode.com/file/repo1.maven.org/maven2/org.eclipse.persistence/eclipselink/2.5.2/org/eclipse/persistence/internal/queries/ContainerPolicy.java#ContainerPolicy]
> to determine how the result set should be collected. There are policies for
> {{Vector}}, {{ArrayList}}, {{HashSet}}, etc.
> The interesting part here is that this really only affects the
> {{ReadAllQuery}} which is used by JPA's {{Query#getResultList()}} ... anytime
> it needs to create a collection of entities, it's going to use a {{Vector}}.
> There's actually very little documentation on this. In fact, the only way to
> change this is either set a global value for the default container policy, or
> set a per-query policy. An example of setting the global policy would be:
> {code}
> public class EclipseLinkSessionCustomizer implements SessionCustomizer {
> /**
> * {@inheritDoc}
> * <p/>
> * This class exists for quick customization purposes.
> */
> @Override
> public void customize(Session session) throws Exception {
> // ensure db behavior is same as shared cache
> DatabaseLogin databaseLogin = (DatabaseLogin)
> session.getDatasourceLogin();
>
> databaseLogin.setTransactionIsolation(DatabaseLogin.TRANSACTION_READ_COMMITTED);
> // for some reason, read-all queries use a Vector as their container for
> // result items - this seems like an unnecessary performance hit since
> // Vectors are synchronized and there's no apparent reason to provide a
> // thread-safe collection on a read all query
>
> ContainerPolicy.setDefaultContainerClass(CoreClassConstants.ArrayList_class);
> }
> }
> {code}
> This indeed causes collections returned by queries to be backed by
> ArrayLists.
> There is some discussion on this topic already:
> https://bugs.eclipse.org/bugs/show_bug.cgi?id=255634
> It seems like this was kept as a {{Vector}} purely for backward compatibility
> reasons.
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