Why can't SLF4J find any providers in my project?

I recently forked a GitHub project and have been trying to bring it up to date. One of my primary goals was to remove the old (vulnerable) Log4J v1.2 and replace it with Log4J 2.2. The original project used an old version of SFL4J with Log4J v1.2.

I don't write much Java and have never used Gradle until I forked this project, so be gentle.

Anyway, no matter what I do or try, SLF4J always reports the following when I run my application:

SLF4J: No SLF4J providers were found.
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#noProviders for further details.

I've read many tutorials that cover the setup, but I can't seem to get this working.

I tried using setup with the following:

slf4j-api-2.0.7.jar
log4j-api-2.20.0.jar
log4j-core-2.20.0.jar
log4j-slf4j2-impl-2.20.0.jar
log4j2.properties

I've also tried ditching log4j2 and using a simpler setup with just the SimpleLogger from SLF4J:

slf4j-api-2.0.7.jar
slf4j-simple-2.0.7.jar

However, SLF4J reports the same issue. I'm currently working out of the following public repo:
https://github.com/jmoscola/PygmyMarmoset/tree/updateLogging

The file default.deps includes the current dependencies. Log4J is currently commented out, but that would be the preferred logger as I would like to maintain a daily rollover for log files.

The file UberjarDaemon.java in subproject PygmyMarmosetLauncher shows the basic Logger/LoggerFactory setup using org.slf4j

Building is quick and easy:

1. ./fetchdeps.pl
2. gradle uberJar

I'd really appreciate it if someone could give me some tips on where I've gone wrong.

Thanks to the comment made by @g00se I was able to get everything working.

• First, I had to put my log4J properties file into my classpath, which was the root of my UberJar. Previously, I was attempting to put my log4J properties file into my /war/WEB-INF/classes folder and that wasn't working out for me. I edited my rootProject build.gradle file to get the properties file where it needed to go.

• I also realized that my log4J properties file had the wrong name. The project previously used log4J 1.2 so it already had an existing 'log4j.properties' file. While reading documentation and tutorials I didn't initially notice that log4J 2.20 uses a new filename, 'log4j2.properties'.

• Lastly, I had some syntax issues as I hadn't properly updated the log4J 1.2 syntax for log4J2.

I now have logging working using the SLF4J API with log4J2. With that said, I STILL see the "No SLF4J providers were found" message in my stdout. But I'm willing to ignore it since logs are being produced. I suspect it has something to do with the existence of multiple subprojects, one of which starts the webapp (and maybe has a different classpath?). But, that's a problem for a different day.

SLF4J 2.x and many other services rely on ServiceLoader to find providers (same for Log4j API 3.x). Services are declared in META-INF/services files and you remove the declarations, in your build.gradle file:

// Copy all of the contents of the Jetty jarfiles into the uberjar staging area.
task copyJettyDeps(type: Copy, dependsOn: [':copyJettyLoggingProperties']) {
	duplicatesStrategy = 'INCLUDE'
	from requiredJars(project(':PygmyMarmosetLauncher')).collect { zipTree(it) }
	into 'build/uberJarStaging'
	exclude 'META-INF/**'
}

You should refine the list of excluded resources:

// Copy all of the contents of the Jetty jarfiles into the uberjar staging area.
task copyJettyDeps(type: Copy, dependsOn: [':copyJettyLoggingProperties']) {
	duplicatesStrategy = 'WARN'
	from requiredJars(project(':PygmyMarmosetLauncher')).collect { zipTree(it) }
	into 'build/uberJarStaging'
	exclude(
			'module-info.class',
			'about*',
			'META-INF/DEPENDENCIES*',
			'META-INF/MANIFEST.MF',
			'META-INF/LICENSE*',
			'META-INF/NOTICE*',
			'META-INF/versions/9/module-info.class'
		)
}

The module-info.class comes from Log4j API and exports only the packages specific to Log4j API, so it should be removed. Same goes for MANIFEST.MF, which is contained in all the JARs you shade. Removing licenses and notices is technically safe and removes a lot of warnings, but if you were to publish the app, you need to consult a lawyer.

Remark: by shading log4j-api you also break location support in Log4j API and related features. To fix it you need to replace:

	manifest {
		attributes 'Main-Class' : 'edu.ycp.cs.pygmymarmoset.main.UberjarMain'
	}

with

	manifest {
		attributes ([
			'Main-Class' : 'edu.ycp.cs.pygmymarmoset.main.UberjarMain',
			'Multi-Release' : 'true'
		])
	}