How To attach Logback to smartfox server - log4j

Question

How do I attach logback to smartfox server?
smartfox uses log4j by default. How can I shift logging of my extensions/all of smartfox to logback?
I've tried this but this is failing with this error cause it has already boud to log4j I guess.

Answer 1

SmartFoxServer 2x uses Simple Logging Facade for Java (SLF4J) for logging. The very purpose of SLF4J is to provide a facade over the logging framework so the user can replace one logging framework with another. In your case log4J with Logback. I'll first explain how SLF4J works and how you can change one logging framework with another and briefly discuss some caveats. Then I'll cover some SmartFoxServer specifics.
Simple Logging Facade for Java Architecture
SLF4J provides API which serves as facade (abstraction layer) over the actual logging framework. This allows the user to easily switch between frameworks on deployment time without the need to change the code. But it also means that SLF4J on its own is not enough - the default implementation is no-op. In order to actually log anything you need actual logging framework (such as log4j or logback) and so called "binder" which servers as a bridge or adapter between SLF4J and the logging framework.
Swapping Logging Frameworks
The SLF4J User Manual provides detailed explanation on how you can swap one framework with another during deployment time. In short you just need to delete the old binder and logging framework and add the new binder and the new logging framework. To swap log4 with logback you need to delete slf4j-log4j12 and log4j jars and add logback-classic and logback-core.
Caveats
You should make sure that there is only one binder implementation in the classpath. Having more than one causes waring, not error. SLF4J would just pick one of the implementations. But I would not rule out the possibility for this to cause issues with more complex application (such as SmartFoxServer) that uses multiple class loaders. But more importantly you should make sure the SLF4J API, the binder and the logging framework versions are compatible. For example if you use old version of SLF4J with newer version of logback that may cause ClassNotFoundException. I suspect that this could be reason why you get the error you see.
Swapping lof4j with Logback as logging framework for SmartFoxServer 2x
SmartFoxServer 2x version 2.17.3 uses SLF4J API version 1.7.5. To swap log4j you need to first delete lib/slf4j-log4j12-1.7.5.jar and then add compatible version of logback-classic and logback-core jars. For example logback-classic 1.1.0 and logback-core 1.1.0.
You can delete lib/log4j-1.2.15.jar but I would rather keep it. The binder(lib/slf4j-log4j12-1.7.5.jar) is not meant to be used directly so it should be safe to be deleted. On other hand there are libraries that use log4j directly. I don't know if SmartFoxServer 2x uses any such library but it is safer to keep it just in case. Swapping the binder is enough for SLF4J to use Logback and ignore log4j.
Logger Output
SmartFoxServer 2x parses the logger output to provide some functionality such as the Admin Log Viewer. If you change the log output this may cause this functionality to stop working and maybe even cause other issues (on theory it should not, but you never know). There is configuration file (config/logParser.properties) that would allow you to configure the log parser, but I didn't found any documentation about it. You may try to ask on the SmartFoxServer forum. The developers are actually pretty active there so they may help.
Swapping loggers only for your extension
The instruction I gave swaps the logger for all extensions and SmartFoxServer. If you want you may try to swap them only for your extensions. But I'm not quite sure if and how that would work. Each class extension uses its own class loader but this provides isolation between extensions and SmartFoxServer and extensions, but not between extension and SmartFoxServer. What does this mean is that if you add lib.jar to Extension A classpath it would not be visible to Extension B or to SmartFoxServer code. But if you add lib.jar to the SmartFoxServer classpath it would be visible to both Extension A and Extension B. As SmartFoxServer already contains SLF4J API on its classpath you should not add it to your extension classpath. You can try to add logback-classic and logback-core to you extension classpath. But in this case you'll have two binder implementations in you extension classpath (logback and log4j from the SmartFoxServer classpath). As already discussed, I'm not quite sure how and if this would work.
Conclusion
SLF4J provides an easy way to swap logging frameworks, but there some caveats. And SmartFoxServer adds some caveats on its own. Unless SmartFoxServer team supports swapping of the underlaying logging framework (which judging by some anwers in their forum, they don't), I would be quite careful and do such swap only if there are some benefits and it is not just a matter of personal preference.