Log4j-Loggers are cached in static fields.

See org.apache.log4j.Log4jLoggerFactory:

public static synchronized Logger getLogger(String name) {
if (log4jLoggers.containsKey(name))

{ return (org.apache.log4j.Logger) log4jLoggers.get(name); }

else

{ Logger log4jLogger = new Logger(name); log4jLoggers.put(name, log4jLogger); return log4jLogger; }

}

A Log4j-Logger caches a reference to a Slf4j-Logger.

See org.apache.log4j.Category:

Category(String name) {
this.name = name;
slf4jLogger = LoggerFactory.getLogger(name);
if (slf4jLogger instanceof LocationAwareLogger)

{ locationAwareLogger = (LocationAwareLogger) slf4jLogger; }

}

The problem is that the result of LoggerFactory.getLogger(name) is not cacheable because it could depend on Logback-ContextSelectors. One example: the JNDI-ContextSelector switches between different JNDI-Contexts. If the log4j-over-slf4j bridge jar is installed in a shared classloader the first request to getLogger() would tie the logger to the respective JNDI-Context.

And it is not a solution to move the bridge jar to each web application because the context selector could switch e. g. between users.