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https://issues.apache.org/jira/browse/LOG4J2-400?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13889086#comment-13889086
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Matt Sicker commented on LOG4J2-400:
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I believe there are many, many ways to implement this idea. One of the most
direct ways would be creating a custom Felix SCR annotation processor that
parses the {{@Plugin}} annotations to build up the component list.
Another option would be adding the proper SCR annotations to the existing code
so that we can just use the normal annotation processor in Felix's maven plugin.
Another option that conforms more so to the OSGi way of doing things would be
using capabilities on each plugin and such so that the OSGi version can simply
query for services/components/something that provide the proper capabilities.
This idea, while cooler, would take some refactoring effort in order to prevent
a lot of code duplication.
There are other ways to do it, of course, but these seem to be the simplest.
The idea suggested by Roland, while workable, is very fragile to orders of
dependencies and such. Lifecycle hooks like that shouldn't be abused for this;
listeners and the OSGi whiteboard thing are more robust.
> Provide Appender-Bundles
> ------------------------
>
> Key: LOG4J2-400
> URL: https://issues.apache.org/jira/browse/LOG4J2-400
> Project: Log4j 2
> Issue Type: Improvement
> Components: Appenders, Core
> Affects Versions: 2.0-beta9
> Environment: OSGi R4 / R5 (Apache Felix 4.x)
> Reporter: Roland Weiglhofer
> Priority: Critical
> Labels: Appender, Core, Dependency, OSGi, PluginManager,
> lightweight, optional
>
> Instead of deploying all appenders in the core fragment, it would be much
> better if the customer can choose which appender he wants to provide. It's
> easy to hive the appender off in a separate bundle fragment. The host bundle
> is the API bundle. The Plugin Manager (core fragment) finds the deployed
> appenders in the classpath of the host bundle. The PluginManager should parse
> the class path in a separate thread (Startup-Hook) and only once at the start
> of the host bundle, but not for each call (when a consumer bundle aquires a
> logger). Make package-imports optional
> (<Import-Package>*;resolution:=optional</Import-Package>)!!!!
> This reduces the number of dependencies and reduces the startup time of the
> whole system.
> One possible solution for the Plugin Manager is to use the reflections plugin
> during the maven build process. This plugin lists all classes of a project
> within a xml file. This file can be marked as a bundle resource and is stored
> within the appender bundle fragment. The idea is that each appender fragment
> has its own class list. Because the bundle host (log4j2 core) sees all
> resources of its fragments it can load these class lists at runtime. Thus,
> the Plugin Manager gets only those appenders that are installed within
> deployed bundle fragements. The class list is created during the build
> process, the plugin manager must not parse the classpath at runtime. Log4j2
> uses a xml parser by default. An additional new dependency to a xml-parser
> library is not required.
> <plugin>
> <groupId>org.reflections</groupId>
> <artifactId>reflections-maven</artifactId>
> <version>0.9.8</version>
> <executions>
> <execution>
> <goals>
> <goal>reflections</goal>
> </goals>
> <phase>process-classes</phase>
> </execution>
> </executions>
> <configuration>
>
> <destinations>${project.basedir}/META-INF/reflections/${project.artifactId}-reflections.xml</destinations>
> </configuration>
> </plugin>
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