引起：org.apache.logging.log4j.LoggingException: log4j-slf4j-impl cannot be present with log4j-to-slf4j Issue

Question

I am writing Junits to test my logs. I am using LogCaptor. I have attached my actual method and my testcases along with pom.xml.

This is my method where i have logs defined.

@Override
public ApplicationResponse createApplication (final String serviceId, final String appId) throws KMSException     {
    log.debug("Entering createApplication");
    try {
        final Application application =
            applicationOperations.createApplication(appId, serviceId);
        return applicationResponse(application);
    }
    catch (KMSCommonException exception) {
        throw new KMSInternalServerErrorException("Not able to create application", exception);
    }
}

This is my test method to test Loggers:

@Test
public void testCreateApplicationLogger () throws KMSException {
    String expectedInfoMessage = "Entering createApplication";

    LogCaptor logCaptor = LogCaptor.forClass(Test.class);
    adminAPIApplicationOpService.createApplication(TEST_SERVICE, TEST_APP + 1);
    assertTrue( logCaptor.getInfoLogs().contains(expectedInfoMessage));

}

Answer 1

It is a conflict. Run the command

mvn dependency:tree -Dverbose -Dincludes=log4j-to-slf4j

Check from where you import that dependency and exclude it.

Reason is log4j is an API. You must include in your project an implementation for that API to work. Both log4j-slf4j-impl and log4j-to-slf4j are implementations for log4j API. When you include multiple implementations conflicts arise.

Answer 2

I am the library maintainer and this behaviour is normal when there are multiple slf4j implementations present on the classpath. This known issue is also mentioned at the github page, see here: Using Log Captor alongside with other logging libraries

The library ships with couple out of the box options, a utility to redirect Java Util Logging to SLF4J and it is also able to redirect Log4j (Apache logging) to SLF4J. Redirecting the Log4j logging is enabled by providing an slf4j implementation log4j-to-slf4j see here for the details: https://github.com/Hakky54/log-captor/blob/3732e25c041af78153bfa7c0c83fb2874fd547b0/pom.xml#L84 The main reason why I have added this is to make it easier for end-users who are using Apache Log4j Core for their logging. It was a design decision... I didn't want the end-user to add manually log4j-to-slf4j when they wanted to use LogCaptor . The downside of this option is that you will get an error if you have log4j-slf4j-impl because SLF4J won't allow you to have multiple implementations.

The solution is pretty simple. Just exclude log4j-slf4j-impl during the test phase from your classpath. Below is an example with maven and surefire/failsafe:

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-surefire-plugin</artifactId>
            <configuration>
                <classpathDependencyExcludes>
                    <classpathDependencyExclude>org.apache.logging.log4j:log4j-slf4j-impl</classpathDependencyExclude>
                </classpathDependencyExcludes>
            </configuration>
        </plugin>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-failsafe-plugin</artifactId>
            <configuration>
                <classpathDependencyExcludes>
                    <classpathDependencyExclude>org.apache.logging.log4j:log4j-slf4j-impl</classpathDependencyExclude>
                </classpathDependencyExcludes>
            </configuration>
        </plugin>
    </plugins>
</build>

I have also created an example project for this specific use case as a cheat sheet for other developers, see here: https://github.com/Hakky54/java-tutorials/tree/main/log-captor-examples/log-captor-with-slf4j-log4j