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Log4j vs SLF4J

SLF4J(Simple Logging Fašade for java) is an API designed to give generic access to many logging frameworks, log4j being one of them.

It is basically an abstraction layer. It is not a logging implementation. It means that if you are writing a library and you use SLF4J, you can give that library to someone else to use and they can choose which logging implementation to use with SLF4J, e.g., log4j or the Java logging API. It is used to prevent applications from being dependent on different logging APIs just as they use libraries that are dependent on them.

However, we elaborate the difference between Log4J and SLF4J that deserves only one line answer. i.e., the question itself is wrong. SLF4J and Log4J are different, or they are not similar components. As the name specified, SLF4J is a simple logging fašade for java. It is not a logging component, and even it does not do the actual logging. It is only an abstraction layer to an underlying logging component.

In the case of Log4j, it is a logging component, and it does the logging instructed to do. So we can say that SLF4J and Log4J are logically two different things.

Log4j vs SLF4J

Now, all you have to select, which logging framework you need to use in runtime. For that, you will need to include two jar files:

  • SLF4J binding jar file
  • Desired logging framework jar files

For example, to use log4j in your project, you will need to include given below jar files:

  • slf4j-log4j12-1.7.12.jar
  • log4j-1.2.17.jar

Once you have placed both jar files in your application classpath, SLF4J will automatically detect it and start using log4j for processing the log statements based on the configuration you provided in log4j configuration file.

For example, below code you may write in your project class file:

Why is SLF4J better than Log4J?

It is always difficult to prefer one between the SLF4J and Log4j. If you have a choice, I would suggest you; logging abstraction is always preferable than logging framework. If you use a logging abstraction, SLF4J in particular, we can migrate to any logging framework we require at the time of deployment without opting for single dependency.

Following are the reasons, which are good enough to choose SLF4J over Log4j:

  • It is always better to use abstraction.
  • SLF4J is an open-source library or internal library that makes it independent of any particular logging implementation, which means no need to manage multiple logging configurations for multiple libraries.
  • SLF4J provides placeholder based logging, which improves the readability of code by removing checks like isInforEnabled(), isDebugEnabled(), etc.
  • By using the logging method of SLF4J, we defer the cost of constructing logging messages (string), until you need it, which is both CPU and memory efficient.
  • Since SLF4J uses less number of temporary strings means less work for the garbage collector, which means better throughput and performance for your application.

So essentially, SLF4J does not replace log4j; they both work together. It removes the dependency on log4j from your application and makes it easy to replace it in the future with the more capable library.

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