I’ve run into this message several times while working on Scala applications:

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.

Following the link in the message is helpful of course. The main takeaways for me from that link are:

This happens when no appropriate SLF4J binding could be found on the class path.

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.

From my understanding, what is going on here is that a library I am bringing in has a compile-time dependency on SLFJ4, and since I am using log4j2 in my project, and not bringing in the SLFJ4 logging module directly, I am hosed. Any logging from my dependencies that use SLF4J will be a No-Op and not displayed. Leading to a bug hunting nightmare.

The way to fix this is to bring in a “bridge” that routes SLF4J log messages to the Log4J implementation. The documentation is on the log4j site under “SLF4J Bridge”.

If existing components use SLF4J and you want to have this logging routed to Log4j 2, then add the following but do not remove any SLF4J dependencies.

Here’s a segment of a build.gradle file which shows this fix in action:

def log4jVersion = "2.9.1"
dependencies {
    compile "org.apache.logging.log4j:log4j-api:$log4jVersion"
    runtime "org.apache.logging.log4j:log4j-core:$log4jVersion"
    // this one is the magic:
    runtime "org.apache.logging.log4j:log4j-slf4j-impl:$log4jVersion"
}

Now the SLF4J logs should appear.

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It’s better than bad, its good