Where do I configure log4j in a JUnit test class?

I generally just put a log4j.xml file into src/test/resources and let log4j find it by itself: no code required, the default log4j initialisation will pick it up. (I typically want to set my own loggers to 'DEBUG' anyway)

The LogManager class determines which log4j config to use in a static block which runs when the class is loaded. There are three options intended for end-users:

1. If you specify log4j.defaultInitOverride to false, it will not configure log4j at all.

2. Specify the path to the configuration file manually yourself and override the classpath search. You can specify the location of the configuration file directly by using the following argument to java:

   -Dlog4j.configuration=<path to properties file>

   in your test runner configuration.

3. Allow log4j to scan the classpath for a log4j config file during your test. (the default)

See also the online documentation.

You may want to look into to Simple Logging Facade for Java (SLF4J). It is a facade that wraps around Log4j that doesn't require an initial setup call like Log4j. It is also fairly easy to switch out Log4j for Slf4j as the API differences are minimal.

I use system properties in log4j.xml:

...
<param name="File" value="${catalina.home}/logs/root.log"/>
...

and start tests with:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-surefire-plugin</artifactId>
    <version>2.16</version>
    <configuration>
        <systemProperties>
            <property>
                <name>catalina.home</name>
                <value>${project.build.directory}</value>
            </property>
        </systemProperties>
    </configuration>
</plugin>