Best Practices

• Libraries shouldn't depend to a logging implementation (like log4j, logback or emaze-logging)
• Libraries shouldn't depend to commons-logging since it is deprecated and introduces a lot of complexity in the classloader
• Libraries shouldn't use java util logging sinse it has serious performance problems
• Libraries should use simply the SLF4J API
• Only the applications are logging-aware and configure the proper backend

Otherwise

• You limit your applications to the logging backend decided by the libraries
• Conflicts are inevitable

Caused by: java.lang.IllegalStateException: Detected both log4j-over-slf4j.jar AND slf4j-log4j12.jar on the class path, preempting StackOverflowError. See also http://www.slf4j.org/codes.html#log4jDelegationLoop for more details.

Excluding jars from target

You can exclude dependencies at the top level configuring the maven-war-plugin properly (see Including and Excluding Files From the WAR).

In this example it excludes the slf4j-log4j12 binding imported transitively by some dependency (since the application would use logback), but it keeps the log4j-over-slf4j ensuring that all logging performed used log4j directly is forwarded to SLF4J.

<plugin>
    <artifactId>maven-war-plugin</artifactId>
            ...
            <configuration>
                <packagingExcludes>
                    WEB-INF/lib/slf4j-log4j12-*.jar,
                    %regex[WEB-INF/lib/log4j-(?!over-slf4j).*.jar]
                </packagingExcludes>
            </configuration>
            ...
</plugin>

Using Spring Boot the wrong dependencies should be excluded from the bundle too:

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
            ...
            <configuration>
                <excludes>
                    <exclude>
                        <groupId>log4j</groupId>
                        <artifactId>log4j</artifactId>
                    </exclude>
                    <exclude>
                        <groupId>org.slf4j</groupId>
                        <artifactId>slf4j-log4j12</artifactId>
                    </exclude>
                </excludes>
            </configuration>
            ...
</plugin>