Logback to log4j bridge - log4j

Question

My entire system is logged by slf4j with log4j implementation.
I had a problem when a new module used a logback-classic depdency for logging which cannot be excluded from pom.xml file since it breaks it.
First I tried to look for solution in this 3rd side depdency but couldn't find any solutions so I thought maybe a bridge between them could be something that solves it.
What I mainly looking for is to split my logs between the deafult console logging of the dependency to my own log4j.xml loggers and appenders so I can use the separetly..
Is there any bridge so I could use both logback-classic and log4j under slf4j with example?
Thanks!

Answer 1

You said:
I had a problem when a new module used a logback-classic depdency for logging which cannot be excluded from pom.xml file since it breaks it.
Is this an assumption or did you try it? Also, if you did try it did you remove the logback-core dependency as well? The only way removing these dependencies would break the module is if the module depends on the logging implementation since logback natively implements the slf4j API. If the module does depend on the implementation rather than the API/interface I don't think there's anything you can do without either removing those dependencies (changing the module source code) or writing some stubbed versions of the implementation classes that the module depends on.
When I write code that follows the pattern in the logback manual I'm able to swap the implementation from logback to log4j2 without any issues as long as I don't introduce dependencies on the logback implementation classes.
Here is the example I wrote:
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class Main {
private Logger log = LoggerFactory.getLogger(Main.class);
public static void main(String[] args) {
Main main = new Main();
main.main();
}
public void main(){
log.trace("trace msg");
log.debug("debug msg");
log.info("info msg");
log.warn("warn msg");
log.error("Error msg");
log.info(log.getClass().getName());
}
}
Here are the dependencies in the pom:
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.22</version>
</dependency>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-core</artifactId>
<version>1.2.3</version>
</dependency>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.2.3</version>
</dependency>
Here is the output:
00:48:26.378 [main] DEBUG blah.Main - debug msg
00:48:26.380 [main] INFO blah.Main - info msg
00:48:26.380 [main] WARN blah.Main - warn msg
00:48:26.380 [main] ERROR blah.Main - Error msg
00:48:26.380 [main] INFO blah.Main - ch.qos.logback.classic.Logger
Now I change the pom to replace the logback jars with log4j2:
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.22</version>
</dependency>
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-slf4j-impl</artifactId>
<version>2.7</version>
</dependency>
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-api</artifactId>
<version>2.7</version>
</dependency>
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-core</artifactId>
<version>2.7</version>
</dependency>
Here's the output after making this change:
2017-09-03 00:52:21,630 INFO b.Main [main] info msg
2017-09-03 00:52:21,631 WARN b.Main [main] warn msg
2017-09-03 00:52:21,631 ERROR b.Main [main] Error msg
2017-09-03 00:52:21,632 INFO b.Main [main] org.apache.logging.slf4j.Log4jLogger
So based on this I think you should, if things are implemented the "right way", be able to swap the logback jars with log4j2 and it should "just work".
You also said:
What I mainly looking for is to split my logs between the deafult console logging of the dependency to my own log4j.xml loggers and appenders so I can use the separetly..
Now it's entirely clear to me what you were asking but I think you wanted to have log messages from the module go to console as well as any logs you are using with your log4j2 configuration. If this is the case that's as simple as modifying your log4j2 configuration - add a logger with the appropriate name and assign the appropriate appenders. For example if your module's classes are com.my.package.Class1, com.my.package.Class2, com.my.package.Class3, etc then you could create a logger for com.my.package and give it a console appender along with the appropriate file appenders.
Hope this helps!