org.apache.logging.log4j loggers not supported like test shows

I’m working with apache.logging.log4j on existing classes, and followed the example test as below.

//Basic example class
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

public class FooService {

    private static final Logger LOGGER = LogManager.getLogger(FooService.class);

    public void sayHello() {
        LOGGER.info("Keyboard not responding. Press any key to continue...");
        LOGGER.warn("Congratulations, you are pregnant!");
    }

}

//Unit test:
import nl.altindag.log.LogCaptor;

public class FooServiceShould {
    @Test
    public void logInfoAndWarnMessages() {
        String expectedInfoMessage = "Keyboard not responding. Press any key to continue...";
        String expectedWarnMessage = "Congratulations, you are pregnant!";

        LogCaptor logCaptor = LogCaptor.forClass(FooService.class);
        //...
    }
}

However, i get the error shown below log4j-slf4j-impl cannot be present with log4j-to-slf4j using the same libraries as the demo.

import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.LogManager; //all existing classes are already setup with these libs

public class Address {
    
    private static final Logger logger = LogManager.getLogger(Address.class);
    
    public Address(int a, int b, int c, int d) {
    	//other code
        logger.error("Address is malformed.");
    }
}

import nl.altindag.log.LogCaptor;

public class AddressTest {
    @Test
    public void testAddressValidation() {
        LogCaptor logCaptor = LogCaptor.forClass(Address.class);
        
        new Address(999,0,0,0);
        
        assertTrue(logCaptor.getErrorLogs().contains("Address is malformed."));
    }
}

org.apache.logging.log4j.LoggingException: log4j-slf4j-impl cannot be present with log4j-to-slf4j
	at org.apache.logging.slf4j.Log4jLoggerFactory.validateContext(Log4jLoggerFactory.java:49)
	at org.apache.logging.slf4j.Log4jLoggerFactory.newLogger(Log4jLoggerFactory.java:39)
	at org.apache.logging.slf4j.Log4jLoggerFactory.newLogger(Log4jLoggerFactory.java:30)
	at org.apache.logging.log4j.spi.AbstractLoggerAdapter.getLogger(AbstractLoggerAdapter.java:54)
	at org.apache.logging.slf4j.Log4jLoggerFactory.getLogger(Log4jLoggerFactory.java:30)
	at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:363)
	at nl.altindag.log.LogCaptor.<init>(LogCaptor.java:49)
	at nl.altindag.log.LogCaptor.forClass(LogCaptor.java:69)
	at ....AddressTest.testAddressValidation(AddressTest.java:20)

and the console output

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:~/.m2/repository/org/apache/logging/log4j/log4j-slf4j-impl/2.13.3/log4j-slf4j-impl-2.13.3.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:~/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
SLF4J: Actual binding is of type [org.apache.logging.slf4j.Log4jLoggerFactory]

I’m not sure if i’m applying the pom exclusions properly, but it seems like it might be easier to make a version that is compatible with org.apache.logging.log4j instead

[dereknheiley]

Aha #2, removing the exclusion we tried adding for one of the above problems finally fixed it. So in summary, import the library before other log4j dependencies.

[Hakky54]

Amazing, you found the solution. Interesting to see that the order of declaring the dependency in this specific use-case worked for you! I will note this solution down. Good luck asserting your log messages!

By the way I published here Java tutorials - LogCaptor isolated examples with different logging frameworks include this example and the adjustments to the pom which are included to get it working.

Update 14-07-2021 I noticed that you are using spring-boot-starter-log4j2. This library has some transitive dependencies which indeed requires adjustments to your pom. I have added an example for this use case, see the example project here: https://github.com/Hakky54/java-tutorials/tree/main/log-captor-examples/log-captor-with-spring-boot-starter-log4j2

So you basically need to add the following configuration to your pom:

<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>

This will exclude org.apache.logging.log4j:log4j-slf4j-impl from the classpath only during the test phase I mentioned this solution also at the beginning of our conversation, see here: https://github.com/Hakky54/log-captor/issues/6#issuecomment-770237736