logging-log4j2

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Problem Description

Our web application, which runs in Tomcat 11 and Java 21, uses SLF4J + Logback for logging. However, it allows dynamically importing extensions along with JARs at runtime using the server classloader. One of these extensions depends on log4j-api-2.17.1.jar, which is correctly present in the classpath.

After deploying the extension, calling a service that uses Log4j results in the following error:

java.lang.NoClassDefFoundError: org/apache/logging/log4j/spi/ExtendedLogger

The error mentioned above is being received by executing this line Logger logger = LogManager.getLogger(); in extension service.

However, other Log4j classes, such as LogManager and Logger, are loading without issues.

Environment Details

• Application Type : WAR-based Java web application

• Application Server : Apache Tomcat 11

• Java Version : 21

• Logging Framework in Main App : SLF4J with Logback

• Extension Support : Allows dynamic loading of JARs at runtime using the server classloader

• Problematic Dependency : log4j-api-2.17.1.jar (used by an extension)

Debugging Steps & Findings

1. Verified that log4j-api-2.17.1.jar is present in the classpath and contains org/apache/logging/log4j/spi/ExtendedLogger.class.
2. Checked for multiple versions of log4j-api dependency in our application. Result : Only one instance of log4j-api-2.17.1.jar is found.
3. Printed the class loaders for Log4j classes:

   System.out.println(org.apache.logging.log4j.LogManager.class);
   System.out.println(org.apache.logging.log4j.Logger.class);
   System.out.println(org.apache.logging.log4j.spi.ExtendedLogger.class);  
   

Result: LogManager and Logger printed correctly. ExtendedLogger.class throws NoClassDefFoundError.

Additional Notes

• Same application worked in Tomcat 9 without requiring a restart.

• Tomcat 11 requires a restart for ExtendedLogger to be found.

• After restarting Tomcat 11, everything works correctly.

Request for Help

We need guidance on :

• Why is ExtendedLogger missing, even though other Log4j classes load fine?

• Does Tomcat 11’s classloader affect Log4j differently compared to Tomcat 9?

• Are there additional configurations we should try in Tomcat 11?

Would appreciate any insights from the community!