java - Resolve multiple SLF4J bindings in maven project

Question

This is a question sounds like bunch of similar questions on SE sites, so I should be quite verbose to make my question clear. So, here is project's minimal pom.xml:

<dependencies>
     <dependency>
        <groupId>ch.qos.logback</groupId>
        <artifactId>logback-classic</artifactId>
        <version>1.0.6</version>
    </dependency>

   <dependency>
        <groupId>org.codehaus.gmaven.runtime</groupId>
        <artifactId>gmaven-runtime-1.7</artifactId>
        <version>1.3</version>
   </dependency>

</dependencies>
<build>
    <plugins>
        <plugin>
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>exec-maven-plugin</artifactId>
            <version>1.2.1</version>
            <configuration>
                <mainClass>org.shabunc.App</mainClass>
            </configuration>
        </plugin>
    </plugins>
</build>

Here is the dependency tree produced by maven.

mvn dependency:tree -Dverbose -Dincludes=org.slf4j:

[INFO] [dependency:tree {execution: default-cli}]
[INFO] org.shabunc:logdebug:jar:1.0-SNAPSHOT
[INFO] - ch.qos.logback:logback-classic:jar:1.0.6:compile
[INFO]    - org.slf4j:slf4j-api:jar:1.6.5:compile

Now, let's remove exclusion and check dependencies again. We'll get:

 [INFO] org.shabunc:logdebug:jar:1.0-SNAPSHOT
[INFO] +- ch.qos.logback:logback-classic:jar:1.0.6:compile
[INFO] |  - org.slf4j:slf4j-api:jar:1.6.5:compile
[INFO] - org.codehaus.gmaven.runtime:gmaven-runtime-1.7:jar:1.3:compile
[INFO]    +- (org.slf4j:slf4j-api:jar:1.5.10:compile - omitted for conflict with 1.6.5)
[INFO]    +- org.codehaus.gmaven.feature:gmaven-feature-support:jar:1.3:compile
[INFO]    |  - (org.slf4j:slf4j-api:jar:1.5.10:compile - omitted for conflict with 1.6.5)
[INFO]    - org.codehaus.gmaven.runtime:gmaven-runtime-support:jar:1.3:compile
[INFO]       +- (org.slf4j:slf4j-api:jar:1.5.10:compile - omitted for conflict with 1.6.5)
[INFO]       - org.sonatype.gshell:gshell-io:jar:2.0:compile
[INFO]          - org.sonatype.gossip:gossip:jar:1.0:compile
[INFO]             - (org.slf4j:slf4j-api:jar:1.5.8:compile - omitted for conflict with 1.6.5)

So, as we can see, everything works as expected, and conflicting dependency is actually get excluded. But the thing is that even with dependency excluded I still get following message while compiling and calling mvn exec:java:

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/home/shabunc/.m2/repository/ch/qos/logback/logback-classic/1.0.6/logback-classic-1.0.6.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/home/shabunc/.m2/repository/org/sonatype/gossip/gossip/1.0/gossip-1.0.jar!/org/slf4j/impl/StaticLoggerBinder.class]

The question is: Why I still see this warning and what exactly should be done to make only one version of slf4j reachable during execution?

Answer 1

Your problem isn't getting two copies of the SLF4J API, it's getting two different SLF4J implementations. You need to exclude Gossip, not the API. That means something like:

<dependency>
    <groupId>org.codehaus.gmaven.runtime</groupId>
    <artifactId>gmaven-runtime-1.7</artifactId>
    <version>1.3</version>
    <exclusions>
      <exclusion>
        <groupId>org.sonatype.gossip</groupId>
        <artifactId>gossip</artifactId>
      </exclusion>
    </exclusions>
</dependency>

The Gossip dependency is declared by gshell-io; hopefully, it doesn't actually need Gossip, it just needs an SLF4J SLF4J, which you are supplying in the shape of Logback.