Maven Class path error multiple SLF4J bindings

Question

I have been getting this error while trying to do a MAVEN INSTALL. I tried exclusions, but not sure the where to include in pom file. Let me how and what exclusion tags should i include in my pom file. I am also attaching my pom file snippet where to include the exclusions`SLF4J: Class path contains multiple SLF4J bindings.

SLF4J: Found binding in [jar:file:/C:/Users/147188/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar!/org/slf4j/impl/StaticLoggerBinder.class]

SLF4J: Found binding in [jar:file:/C:/Users/147188/.m2/repository/org/apache/logging/log4j/log4j-slf4j-impl/2.10.0/log4j-slf4j-impl-2.10.0.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 [ch.qos.logback.classic.util.ContextSelectorStaticBinder]

POM file:

<!-- Start of required part to make log4j work -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
    <exclusions>
    <exclusion>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-log4j12</artifactId>
    </exclusion>
</exclusions> 
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter</artifactId>

    <exclusions>
    <exclusion>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-log4j12</artifactId>
    </exclusion>
        <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>
    <exclusions>
    <exclusion>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-log4j12</artifactId>
    </exclusion>
</exclusions> 
</dependency>
        <!-- End of required part to make log4j work -->

Answer 1

1 Run

mvn dependency:tree

to see which package import org.slf4j

2 keep one, and exclude other

   <exclusion>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-log4j12</artifactId>
    </exclusion>

Answer 2

Correct way to exclude default logging, and configure log4j for logging. Add this dependency in Spring Boot project, if it is not already there

<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>

Refer Spring Logging - How To.

Answer 3

That message indicates that you are bringing in both logback-classic and log4j-slf4j-impl, which both want to be the logging framework for SLF4J to bind to. If you're not sure which dependency is bringing in which other dependencies, I find it very useful to run "mvn dependency:tree" to see the tree of dependencies being used. That should give you enough information to figure out which logging framework binding you need to exclude.

As the documentation that the warning points to you says,

Embedded components such as libraries or frameworks should not declare a dependency on any SLF4J binding but only depend on slf4j-api. When a library declares a compile-time dependency on a SLF4J binding, it imposes that binding on the end-user, thus negating SLF4J's purpose. When you come across an embedded component declaring a compile-time dependency on any SLF4J binding, please take the time to contact the authors of said component/library and kindly ask them to mend their ways.

You pretty much want to exclude all actual logging frameworks from all dependencies, so that the only logging framework being used is the one you've explicitly added. I even often find it useful to set up some maven-enforcer-plugin bannedDependencies rules to ensure that I don't accidentally bring in another logging framework when updating my dependencies. It can also be helpful to use dependencyManagement sections in your POM to ensure that all your dependencies use the same version of slf4j-api.

Answer 4

I ran into same log4j-slf4j multiple binding issue. There are multiple reasons which caused this issue (Struggled a lot for this issue :) ). Please find below comments.

1. I was using Spring-Boot version 1.4.7, which internally configured with slf4j-1.7.25 version. Below is the url link, where you can find Spring-Boot version and respective module versions.

https://repo1.maven.org/maven2/org/springframework/boot/spring-boot-dependencies/

It seems when you use "spring-boot-starter" dependency, SB(Spring-Boot) will automatically download all these dependent modules. In my project, i was using slf4j-1.7.21, which created multiple slf4j version issue. In short, SB was not able to identify which slf4j version to use at runtime. So, first I changed my slf4j version to 1.7.25(which is compatible with Spring-Boot version 1.4.7).

2. Next you need to exclude right module (in my case exclusion of log4j-over-slf4j, logback-classic worked). You need to add below exclusions in all of spring-boot-starter-** module. For Eg : Here I am adding exclusion for spring-boot-starter-data-redis.

Hope this will resolve issue.