gmiscione opened a new issue, #3819:
URL: https://github.com/apache/logging-log4j2/issues/3819
## Description
We have a web application packaged as a .war file deployed in Tomcat 11. The
web application uses logback as the main logger and slf4j as a facade. Then we
use log4j-to-slf4j as a bridge to redirect logs from dependencies using log4j2
to the logback logger. When we undeploy the web application or when we stop
Tomcat, we see these line in the catalina.out file:
```
11-Jul-2025 15:29:48.796 SEVERE [Catalina-utility-5]
org.apache.catalina.loader.WebappClassLoaderBase.checkThreadLocalMapForLeaks
The web application [smartkyc] created a ThreadLocal with key of type
[java.lang.ThreadLocal.SuppliedThreadLocal] (value
[java.lang.ThreadLocal$SuppliedThreadLocal@22437c61]) and a value of type
[org.apache.logging.slf4j.SLF4JLogBuilder] (value
[org.apache.logging.slf4j.SLF4JLogBuilder@2be61ed4]) but failed to remove it
when the web application was stopped. Threads are going to be renewed over time
to try and avoid a probable memory leak.
```
We investigated the issue and we found that in method
`org.apache.logging.slf4j.SLF4JLogger#getLogBuilder` there is this code:
```java
protected LogBuilder getLogBuilder(final Level level) {
final SLF4JLogBuilder builder = logBuilder.get();
return Constants.ENABLE_THREADLOCALS && !builder.isInUse()
? builder.reset(this, level)
: new SLF4JLogBuilder(this, level);
}
```
`Constants.ENABLE_THREADLOCALS` is `false` since the code correctly detects
that it is running in a web application, but still the line `final
SLF4JLogBuilder builder = logBuilder.get();` stores the initial value into the
ThreadLocal. That value is then ignored and is never cleared.
The value of `Constants.ENABLE_THREADLOCALS` should be checked before
`logBuilder.get()` is even called.
## Configuration
**Version:** 2.25.0
**Operating system:** Any + Tomcat 11.0.6
**JDK:** 21.0.7
## Logs
From catalina.out file:
```
11-Jul-2025 15:29:48.796 SEVERE [Catalina-utility-5]
org.apache.catalina.loader.WebappClassLoaderBase.checkThreadLocalMapForLeaks
The web application [smartkyc] created a ThreadLocal with key of type
[java.lang.ThreadLocal.SuppliedThreadLocal] (value
[java.lang.ThreadLocal$SuppliedThreadLocal@22437c61]) and a value of type
[org.apache.logging.slf4j.SLF4JLogBuilder] (value
[org.apache.logging.slf4j.SLF4JLogBuilder@2be61ed4]) but failed to remove it
when the web application was stopped. Threads are going to be renewed over time
to try and avoid a probable memory leak.
```
## Reproduction
1. Create a web application using log4j-to-slf4j, slf4j and logback;
2. Deploy the web application in tomcat 11.0.6 and execute code that uses a
log4j2 logger to write something;
3. Undeploy the web application and check the catalina.out file.
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