How can I prevent SLF4J's infamous “multiple bindings” warning at build time?

Question

My program outputs the following annoying message:

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/scratch/events-beware/events-beware/build/install/events-beware/lib/logback-classic-1.1.2.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/scratch/events-beware/events-beware/build/install/events-beware/lib/slf4j-log4j12-1.6.1.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/scratch/events-beware/events-beware/build/install/events-beware/lib/slf4j-simple-1.6.1.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]

The unwanted SLF4J bindings are transitive dependencies of my project, so I configure my dependency manager to exclude the jars containing the unwanted bindings. This works for a while, until a new dependency is added to the project, which pulls in yet another unwanted binding...

How can I use the power of my build system to fail the build if I transitively depend on multiple SLF4J bindings?

Answer 1

At build time you can check each of your dependencies, looking for an SLF4J binding. If you find more than one, you can fail the build.

To do this with Gradle:

task checkSlf4j {
    description 'Ensure only one SFL4j binding is present in the runtime configuration'

    doLast {
        def bindings = []
        configurations.runtime.each {
            zipTree(it).matching { include 'org/slf4j/impl/StaticLoggerBinder.class' }.each { c ->
                bindings << [it.getName(), c]
            }
        }
        if (bindings.size () > 1) {
            throw new GradleException("Multiple SLF4J bindings found: ${bindings*.getAt(0)}")
        }
    }
}

check.dependsOn checkSlf4j

Here's the result:

$ ./gradlew build
:bundleJars UP-TO-DATE
:compileJava UP-TO-DATE
:compileGroovy UP-TO-DATE
:processResources UP-TO-DATE
:classes UP-TO-DATE
:jar UP-TO-DATE
:assemble UP-TO-DATE
:checkSlf4j FAILED

FAILURE: Build failed with an exception.

* Where:
Script '/scratch/events-beware/events-beware/slf4j.gradle' line: 14

* What went wrong:
Execution failed for task ':checkSlf4j'.
> Multiple SLF4J bindings found: [logback-classic-1.1.2.jar, slf4j-log4j12-1.6.1.jar, slf4j-simple-1.6.1.jar]

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

Total time: 2.009 secs