Log4j-slf4j-impl cannot be present with log4j-to-slf4j

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The error message “log4j-slf4j-impl cannot be present with log4j-to-slf4j” typically occurs when you have conflicting dependencies in your project’s classpath.

This error generally occurs when you have both the “log4j-slf4j-impl” and “log4j-to-slf4j” libraries included in your project. These two dependencies serve the same purpose of bridging the gap between Log4j and SLF4J, providing compatibility between the two logging frameworks.

Having both of these libraries in your project’s classpath can lead to conflicts and inconsistent behavior. Therefore, it’s recommended to use only one of these dependencies based on your requirements.

Here are a few scenarios and their corresponding solutions:

  • Scenario 1: Using Log4j as the logging framework:
    If you are using Log4j as the logging framework and want to bridge it with SLF4J, make sure you don’t have the “log4j-to-slf4j” library in your project’s classpath. You only need the “log4j-slf4j-impl” library to redirect Log4j’s logging calls to SLF4J.

    Example dependency declaration:

    <dependency>
        <groupId>org.apache.logging.log4j</groupId>
        <artifactId>log4j-slf4j-impl</artifactId>
        <version>X.X.X</version>
    </dependency>
  • Scenario 2: Using SLF4J as the logging framework:
    If you are using SLF4J as the logging framework and want to bridge it with Log4j, make sure you don’t have the “log4j-slf4j-impl” library in your project’s classpath. You only need the “log4j-to-slf4j” library to redirect SLF4J’s logging calls to Log4j.

    Example dependency declaration:

    <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>log4j-to-slf4j</artifactId>
        <version>X.X.X</version>
    </dependency>
  • Scenario 3: Migrating from Log4j to SLF4J:
    If you are migrating your project from Log4j to SLF4J, you need to remove the Log4j-specific dependencies like “log4j-slf4j-impl” and replace them with the SLF4J equivalent libraries.

It’s important to ensure that your project’s dependency graph is clean and doesn’t have conflicting or redundant dependencies. You can use build tools like Maven or Gradle to manage your project’s dependencies and ensure that there are no conflicting versions of the same library.

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