Matt Sicker created LOG4J2-2853:
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Summary: Add plugin/bean qualifiers to unify instance selection
for dependency injection
Key: LOG4J2-2853
URL: https://issues.apache.org/jira/browse/LOG4J2-2853
Project: Log4j 2
Issue Type: New Feature
Components: Core
Reporter: Matt Sicker
Assignee: Matt Sicker
In the 2.x plugin API, we have numerous ways of specifying metadata to a
variable or performing manual configuration tree traversal in order to resolve
the value of that variable from the user configuration. These include most of
the {{@Plugin}} annotations as well as some system properties.
In the 3.0 API, we should unify this into a "qualifier" concept. A plugin/bean
can contain one or more qualifiers which are specified by annotations and
matched at runtime to the injection target qualifiers. Qualifiers are inspired
by the [corresponding concept in
javax.inject|https://docs.oracle.com/javaee/6/api/javax/inject/Inject.html],
though they shouldn't be limited to only one qualifier on an injection target
or plugin/bean. Like in CDI, multiple qualifiers should be allowed to be
specified.
Within a scope (LOG4J2-2852), plugins/beans will have at most one instance per
unique set of qualifiers. Qualifiers will typically be placed on plugin classes
or on injection targets, while configuration trees will emit specially
qualified plugins/beans for injection into Core plugins which take injected
parameters. Existing plugin annotations should be updated to work as
corresponding qualifiers without requiring code updates on the plugin side
(even though this will allow us to simplify and clean up various parts of our
own code base over time).
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