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From "Jonathan Hurley (JIRA)" <>
Subject [jira] [Created] (AMBARI-19364) Investigate Changing the Default Container Policy in JPA From Vector to ArrayList
Date Wed, 04 Jan 2017 16:39:58 GMT
Jonathan Hurley created AMBARI-19364:

             Summary: Investigate Changing the Default Container Policy in JPA From Vector
to ArrayList
                 Key: AMBARI-19364
             Project: Ambari
          Issue Type: Task
          Components: ambari-server
    Affects Versions: 2.5.0
            Reporter: Jonathan Hurley
             Fix For: 2.5.0

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|].
Essentially, EclipseLink uses  a [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:

public class EclipseLinkSessionCustomizer implements SessionCustomizer {

   * {@inheritDoc}
   * <p/>
   * This class exists for quick customization purposes.
  public void customize(Session session) throws Exception {
    // ensure db behavior is same as shared cache
    DatabaseLogin databaseLogin = (DatabaseLogin) session.getDatasourceLogin();

    // 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

This indeed causes collections returned by queries to be backed by ArrayLists. 

There is some discussion on this topic already:

It seems like this was kept as a {{Vector}} purely for backward compatibility reasons. 

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