[LOG4J2-3640] @Plugin not compatible between 2.x and 3.x

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Not A Bug
• Affects Version/s: 3.0.0
• Component/s: Plugins
• Assignee: Unassigned
• Reporter: Rob Spoor
• Created: 03/Dec/22 13:58
• Updated: 30/Dec/22 19:26
• Resolved: 09/Dec/22 18:12

Description

I had an issue with resolving Log4j2 2.19.0 as a module. While I resolved the issue by adding an explicit --add-opens clause, I still found something that appears to be an issue.

According to this comment from rgoers,

Plugins should NOT need to be recompiled

Because of an earlier comment on my question I had tried to use version 3.0.0-SNAPSHOT. That produced some compiler errors. While some were easy enough to solve to be compatible with versions 2.19.0 and 3.0.0-SNAPSHOT, the moved @Plugin annotation meant I couldn't compile my project for both versions 2.19.0 and 3.0.0-SNAPSHOT. That made me believe that plugins written for 2.19.0 are not compatible with 3.0.0.

I have created a little POC: https://github.com/robtimus/log4j-compat-test. This consists of 3 Maven modules:

• A module with only a simple plugin
• A module to test this plugin in Log4j2 2.19.0
• A module to test this plugin in Log4j2 3.0.0-SNAPSHOT

The entire project can be tested using mvn test. Right now this fails because, as expected, the moving of the @Plugin annotation caused the plugin to not be resolved.
I've also included a "legacy" plugin class in the 3.0.0-SNAPSHOT test module that shows that the deprecated annotations org.apache.logging.log4j.core.config.plugins.PluginAttribute and org.apache.logging.log4j.core.config.plugins.PluginFactory still work; it's the (forced) replacement of org.apache.logging.log4j.core.config.plugins.Plugin with both org.apache.logging.log4j.plugins.Plugin and org.apache.logging.log4j.plugins.Configurable that causes the plugin written for version 2.19.0 to not work.

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I think that the following should be done, to prevent forcing people rewrite all of their existing plugins:

• Re-adding the org.apache.logging.log4j.core.config.plugins.Plugin annotation, but make it deprecated.
• The plugin registration code should treat this plugin as a combination of org.apache.logging.log4j.plugins.Plugin and org.apache.logging.log4j.plugins.Configurable. There may be another one, for the category attribute of the old annotation, but I couldn't find out which one yet.