SLF4J: Failed to load class “org.slf4j.impl.StaticLoggerBinder” is an error that people face very often. Here we show what is the reason and how to fix it!
Failing Java project
SLF4J-API is only an API
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.21</version>
</dependency>
It means that the following JAR contains only interfaces (e.g. Logger) and helper classes, but it is not a logging framework like log4j. It’s just a facade!
Java code that calls SLF4J-API
package com.farenda.loggers;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class LoggingWithSlf4j {
private static final Logger log
= LoggerFactory.getLogger(LoggingWithSlf4j.class);
public static void main(String[] args) {
log.info("Hello Slf4j!");
}
}
The error message when no logging framework present
The above code produces the following output:
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder". SLF4J: Defaulting to no-operation (NOP) logger implementation SLF4J: See https://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
SLF4J haven’t detected any logging framework so it switched to the only API implementation it has – NOP – that does nothing. :-)
Binding SLF4J to a logging framework
To fix that we need to add a logging framework to our project configuration:
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-simple</artifactId>
<version>1.7.21</version>
<scope>runtime</scope>
</dependency>
Now, SLF4J will detect at runtime that there is a logging framework on a classpath and will use it for logging. Notice that we’ve declared slf4j-simple in runtime scope, because it is not needed for compilation – our code is using the facade.
Running the same code prints to logs as expected:
[main] INFO com.farenda.loggers.LoggingWithSlf4j - Hello Slf4j!
Logging frameworks bindings
- slf4j-log4j12: binding for log4j version 1.2
- slf4j-jdk14: binding for java.util.logging
- slf4j-nop: binding for NOP (No OPeration)
- slf4j-simple: binding that prints to System.err
- slf4j-jcl: binding for Jakarta Commons Logging
- logback-classic: native implementation.
