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https://issues.apache.org/jira/browse/LOG4J2-745?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14075815#comment-14075815
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Scott Harrington commented on LOG4J2-745:
-----------------------------------------
Matt: 'Map.putIfAbsent' is indeed not available in Java 7. Here's a quick fix
to PluginRegistry.getCategory:
{code:java}
Map<String, T> result = categories.get(key);
if (result == null) {
categories.put(key, result = new LinkedHashMap<String, T>());
}
return result;
{code}
I had made the same change you did -- replace the ConcurrentMaps with
LinkedHashMap -- but I'm concerned what works for my simple usage (where
Log4j2Plugins.dat is read once at startup) will fail in infrequent but nasty
ways* for folks who might dynamically add new plugins at runtime, if that's
even possible. As I described above, the ConcurrentHashMaps were not correctly
exposing a completely-parsed registry, but at least they would not have the
potential to loop infinitely. There will need to be a synchronized block
somewhere.
*HashMap.get can loop infinitely if another concurrent thread has done a put
that causes a resize:
http://confusion.tweakblogs.net/blog/2019/nasty-java-hashmap-race-condition.html
> Plugins can cause ConverterKeys collisions with unpredictable results
> ---------------------------------------------------------------------
>
> Key: LOG4J2-745
> URL: https://issues.apache.org/jira/browse/LOG4J2-745
> Project: Log4j 2
> Issue Type: Improvement
> Reporter: Scott Harrington
> Assignee: Matt Sicker
> Priority: Minor
> Attachments: LinkedHashMap_Plugin_Manager.patch
>
>
> If I create a Converter plugin with ConverterKeys of "d" or "m" then there
> will be a collision with the built-in DatePatternConverter or
> MessagePatternConverter.
> It is unpredictable which plugin gets used.
> I see two resolutions:
> (1) detect collisions in PatternParser and emit a warning so we know which
> implementation will be used
> (2) use whichever Log4j2Plugins.dat appeared first in the CLASSPATH
> Predictable iteration order is usually accomplished by replacing HashMaps
> with LinkedHashMaps. Could easily do this for thie PluginManager.plugins
> field. But PluginRegistry uses a ConcurrentHashMap.
> Is there a good reason to use ConcurrentHashMaps in PluginRegistry? It
> doesn't really give you any concurrency -- a caller to
> PluginManager.getPlugins could see a partially-loaded map if collectPlugins
> was still running. Why not synchronize collectPlugins and/or loadPlugins, and
> force any concurrent caller to getPlugins to wait until the loading was
> complete.
> I would give it a stab but I see other more important changes are probably
> underway for LOG4J2-741 and LOG4J2-673 and this can probably wait.
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