On 9/11/2019 4:45 AM, Jan Høydahl wrote:
Upgrading to log4j2 in https://issues.apache.org/jira/browse/SOLR-7887 added
dependency on a bunch of log4j libs to solr-core, e.g. log4j-slf4j-impl which
is a binding for slf4j to log using log4j.
This seems wrong since people who consume solr-core through Maven should be
able to freely choose logging framework by throwing in the desired static
binding jars on classpath.
Now you need to <exclude>org.apache.logging.log4j:log4j-slf4j-impl</exclude> in
your solr-core dependency which is a workaround but not in the true spirit of slf4j.
I think it's correct for the Solr server to depend on log4j libraries by
default. The only logging implementation that the admin UI supports is
log4j. If somebody running our binary release wants to use a different
logging implementation on the server side (knowing that they can't
access logging in the admin UI), that can be done by replacing jars in
lib/ext. Which jars to add and remove is specific to the desired
logging implementation. The ref guide probably does need some info there.
If there are any users that integrate solr-core into their own code,
it's currently a bit of a land-mine situation to change logging
implementations. If there's a way we can include log4j jars at build
time, but remove the log4j dependency on the published solr-core
artifact, that might work well. We should do our best to make it so
people can use EmbeddedSolrServer without log4j jars.
If solrj depends on log4j jars, that's a problem. It should only depend
on slf4j-api. I believe this is the way it works now. For the majority
of user code, solrj is what they'll use.
The log4j-slf4j-impl jar contains the org.slf4j.impl package, with 3
classes. It also contains the org.apache.logging.slf4j package, with 7
Unless my investigation was flawed, only one of the 3 classes in
org.sl4j.impl is used in Solr code, and it's in exactly two places:
LogWatcher and StartupLoggingUtils.
The two classes that are used everywhere across Solr are in the
org.slf4j package, named Logger and LoggerFactory. I checked the
WEB-INF/lib directory in the solr-8.2.0 download and could not find any
jars related to logging ... and I expected to find one, preferably
sl4fj-api directly from slf4j.org. Instead, that jar is in lib/ext with
the log4j jars. That location doesn't seem right, as slf4j-api is
ALWAYS required by Solr, regardless of the logging implementation that's
The slf4j-api dependency only provides part of what's used in Solr
server-side code, but it SHOULD fully satisfy compiling dependencies for
solrj. Prior to version 7.4, the org.slf4j.impl package (for
server-side code only) was supplied by the slf4j-log4j12 jar, which came
from the slf4j project. This is the jar that links slf4j and log4j1.
For log4j2, classes in that package come from Apache, not slf4j.
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