LoggerFactory is not a Logback LoggerContext but Logback is on the classpath

1. Overview

In this article, we will see the various possibilities to prevent the error “LoggerFactory is not a Logback LoggerContext but Logback is on the classpath“.

2. Spring Boot default behavior

The Spring Boot auto-configures the logging framework based on the dependencies available in the class path. If the Log back dependency is available in the class path, then Spring boot considers it as the first choice.

The spring-boot-starter-logging is the starter for the LogBack logging. So if you include the spring-boot-starter-logging starter dependency in your project, then Spring boot auto-configures the Log Back as the default logging framework.

The spring-boot-starter-web starter dependency also contains the spring-boot-starter-logging starter. So declaring the web starter in your project transitively pulls the LogBack starter. 

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

If your project has either logging or web starter dependencies in the class path, then the Spring only auto-configures the LogBack library as the first choice.

If your project has Logback and also log4j2 dependencies in the class path, then the Spring Boot configures the Log Back library instead of log4j2 dependency.

2.1. Fix for “LoggerFactory is not a Logback LoggerContext but Logback is on the classpath”

The mentioned error appears when you configure the log 4j library in your Spring Boot project.

Assume you have the web starter dependency and also has the log 4j2 library as below.

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-log4j2</artifactId>
</dependency>

The Spring Boot will still configure the LogBack library because of the web starter. If you want to use log4j, then you must exclude this default logging by using the following ways.

2.2. Exclude default LogBack on the maven

You can prevent the default Log Back configuration in your maven project by excluding the library spring-boot-starter-logging from your class path as below:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter</artifactId>
    <exclusions>
        <exclusion>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-logging</artifactId>
        </exclusion>
    </exclusions>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-log4j2</artifactId>
</dependency>

2.3. Exclude the default LogBack on the Gradle

In Gradle, we have different ways to exclude the default Log Back. You can exclude the spring-boot-starter-logging module from all jar files specified in the dependency.

configurations {
    all*.exclude module : 'spring-boot-starter-logging'
    all*.exclude module : 'logback-classic'
}

Alternatively, you can use the module replacement feature of Gradle. After declaring the log4j 2 dependency, instruct Gradle to replace all occurrences of the default logging (spring-boot-starter-logging) by Log4j 2 starter as below:

dependencies {
    implementation "org.springframework.boot:spring-boot-starter-log4j2"
    modules {
        module("org.springframework.boot:spring-boot-starter-logging") {
            replacedBy("org.springframework.boot:spring-boot-starter-log4j2", "Use Log4j2 instead of Logback")
        }
    }
}

3. Conclusion

To sum up, we have learned the ways to prevent the error LoggerFactory is not a Logback LoggerContext but Logback is on the classpath.