Where is slf4j coming from and how to make it to use log4j-core 2.17.1

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We have a maven project on scala, where it implicitly using slf4j with log4j of version 1.2.17. Must upgrade it to log4j 2.17.1. In my pom file there is no slf4j dependency and it is somehow still using it. enter image description here

Is there any command to scan the dependency tree to find which dependency is using slf4j? And how can I make it to use log4j-core 2.17.1? enter image description here

<dependencies>
        <dependency>
            <groupId>org.scala-lang</groupId>
            <artifactId>scala-library</artifactId>
            <version>${scala.version}</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.scala-lang</groupId>
            <artifactId>scala-reflect</artifactId>
            <version>${scala.version}</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.apache.spark</groupId>
            <artifactId>spark-core_${scala.binary.version}</artifactId>
            <version>${spark.version}</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.apache.spark</groupId>
            <artifactId>spark-sql_${scala.binary.version}</artifactId>
            <version>${spark.version}</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.apache.spark</groupId>
            <artifactId>spark-hive_${scala.binary.version}</artifactId>
            <version>${spark.version}</version>
            <exclusions>
                <exclusion>
                    <groupId>stax</groupId>
                    <artifactId>stax-api</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        <dependency>
            <groupId>org.apache.spark</groupId>
            <artifactId>spark-mllib_${scala.binary.version}</artifactId>
            <version>${spark.version}</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>ml.dmlc</groupId>
            <artifactId>xgboost4j-spark</artifactId>
            <version>0.81</version>
        </dependency>
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>com.typesafe</groupId>
            <artifactId>config</artifactId>
            <version>1.3.2</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.11</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.mockito</groupId>
            <artifactId>mockito-core</artifactId>
            <version>1.9.5</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.scalatest</groupId>
            <artifactId>scalatest_${scala.binary.version}</artifactId>
            <version>3.0.5</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.scalamock</groupId>
            <artifactId>scalamock-scalatest-support_${scala.binary.version}</artifactId>
            <version>3.6.0</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.mockito</groupId>
            <artifactId>mockito-scala_${scala.binary.version}</artifactId>
            <version>1.0.9</version>
        </dependency>
        <dependency>
            <groupId>com.holdenkarau</groupId>
            <artifactId>spark-testing-base_2.11</artifactId>
            <version>${spark.version}_0.11.0</version>
            <scope>test</scope>
            <exclusions>
                <exclusion>
                    <groupId>asm</groupId>
                    <artifactId>asm</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        <dependency>
            <groupId>com.amazon.deequ</groupId>
            <artifactId>deequ</artifactId>
            <version>1.0.0-rc4</version>
        </dependency>
        <dependency>
            <groupId>software.amazon.awssdk</groupId>
            <artifactId>sqs</artifactId>
            <version>2.9.5</version>
        </dependency>
        <dependency>
            <groupId>org.apache.httpcomponents</groupId>
            <artifactId>httpclient</artifactId>
            <version>4.5.9</version>
        </dependency>
        <dependency>
            <groupId>org.apache.httpcomponents</groupId>
            <artifactId>httpcore</artifactId>
            <version>4.4.9</version>
        </dependency>
    </dependencies>

slf4j maven log4j
1 Answers

I think you are getting the slf4j-api as a transitive dependency from one of your other logging related dependencies. However there is no slf4j bindings in your class path as well which is good. If I answer for your question how you can use slf4j-api, it is purely used as an logging facade. Use slf4j for logging in your application and don't user log4j loggers. In long run it will be beneficial because even if you want to change the logging mechanism you'll not require any code changes. slf4j can be used via @Slf4j or via a logger instance as well. If you use @Slf4j, then you'll have to add lombok to your class path.

Example for logger instance based logging:

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

    Logger logger = LoggerFactory.getLogger(Logger.ROOT_LOGGER_NAME);
    
    public void testLog() {
        logger.info("Test");
    }

Example for @Slf4j annotation based logging:

import lombok.extern.slf4j.Slf4j;

@Slf4j
public class LoggingTest {

    public void testLog() {
        log.info("Test");
    }

}

For further reading please refer slf4j documentations.

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