[jira] [Created] (LOG4J2-3496) Support injection via container types

["Volkan Yazici (Jira)" <ji...@apache.org>]

Volkan Yazici created LOG4J2-3496:
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             Summary: Support injection via container types
                 Key: LOG4J2-3496
                 URL: https://issues.apache.org/jira/browse/LOG4J2-3496
             Project: Log4j 2
          Issue Type: New Feature
          Components: Plugins
            Reporter: Volkan Yazici
             Fix For: 3.0.0


Plugin system should support the injection via container types. Since this is not currently the case, whenever a, for instance, collection of plugins need to be injected (e.g., Pattern and JSON Template Layouts), ad-hoc access to {{PluginManager}} or {{PluginUtils}} is needed. This approach has certain drawbacks:

* Duplication of ad-hoc code
* Not aligned with the way plugins are loaded, that is, via {{@Inject}}
* Impossible to test

h3. Container types

Ideally, the plugin system should support injection via the following container types:

* {{Optional<P>}}
* {{Collection<P>}}
* {{Iterable<P>}}
* {{Set<P>}}
* {{Stream<P>}}
* {{List<P>}}
* {{Map<N, P>}}

Above {{P}} denotes the type the plugin is assignable from and {{N}} denotes the plugin name.

h3. Ordering problem

A {{@PluginOrder}} annotation needs to be introduced to address the following corner cases:

* In the case of {{Optional<P>}}, if there are multiple matches, the first one needs to be picked.
* In the case of collection and map types, elements need to be sorted.

Sorting can be done in the following way:

* If one or more of the matches are annotated with {{@PluginOrder}}, they will have the precedence and get sorted according to {{@PluginOrder}} values.
* The rest will be sorted according to their discovery order.

The order in collection types can be preserved using the following implementations:

* {{Set}} -> {{LinkedHashSet}}
* {{Map}} -> {{LinkedHashMap}}
* {{Collection}}, {{Iterable}}, {{Stream}} -> {{List}}



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