[GitHub] [logging-log4j2] ppkarwasz commented on issue #1258: Rolling problem -Dlog4j.configurationFile with two log4j2.xml files

["ppkarwasz (via GitHub)" <gi...@apache.org>]

ppkarwasz commented on issue #1258:
URL: https://github.com/apache/logging-log4j2/issues/1258#issuecomment-1412634661

   @mariusv5,
   
   Your problem is caused by multiple `log4j-api` and `log4j-core` jars being loaded into your Tomcat system (your logs show 3 instances of `ClassLoaderContextSelector`, one for each web application). Each one opens a file descriptor to your log file and each one tries to rotate it. As you can imagine, this does not end well.
   
   Log4j2 is perfectly capable of sharing the same log file between logging context, if a **single** copy of `log4j-core` is deployed in your system. Technically each logging context will then have **its own** copy of `RollingFileAppender`, but the engine that opens files and performs the rotation ([`RollingFileManager`](https://logging.apache.org/log4j/2.x/log4j-core/apidocs/org/apache/logging/log4j/core/appender/rolling/RollingFileManager.html)) will be shared.
   
   Unlike full application servers such as Wildfly, Tomcat has not easy way to share a **single** library between web applications. In order to do that you need to install a set of Log4j2 artifacts in the `$CATALINA_BASE/lib` folder and do **one** of the following:
   
   1. Remove the `log4j-*` artifacts from your web applications (personally I don't like the idea of modifying web applications),
   2. Switch the delegation model of all web application classloaders to "parent first" (this might break some apps if they need different versions of the same library), by adding:
      ```lang-xml
      <Loader delegate="true"/>
      ```
      to their context configuration (e.g. in `$CATALINA_BASE/conf/context.xml`). Cf. [Tomcat documentation](https://tomcat.apache.org/tomcat-9.0-doc/config/loader.html).
   3. Use a custom classloader that looks up `org.apache.logging.log4j.*` classes in the parent classloader **first**, but uses the usual delegation for the other classes. This is basically what Tomcat does with `org.apache.catalina.*` and other internal classes.
   
   You can find such a classloader in my personal repository (cf. [`Log4jWebappClassloader`](https://github.com/copernik-eu/log4j-plugins/blob/main/log4j-tomcat/src/main/java/eu/copernik/log4j/tomcat/Log4jWebappClassLoader.java)). You can find it on Maven Central under the coordinates:
   ```lang-xml
   <dependency>
       <groupId>eu.copernik</groupId>
       <artifactId>log4j-tomcat</artifactId>
       <version>2.18.0</version>
   </dependency>
   ```
   
   To use it, place it in the `$CATALINA_BASE/lib` folder together with `log4j-api` and `log4j-core` and add:
   ```lang-xml
   <Loader loaderClass="eu.copernik.log4j.tomcat.Log4jWebappClassLoader"/>
   ```
   to your `$CATALINA_BASE/conf/context.xml` file.


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