SLF4J

Simple Logging Facade for Java

Developer(s)	Ceki Gülcü
Stable release	1.7.13[1] / November 10, 2015
Written in	Java
Operating system	Cross-platform
Type	Logging Tool
License	MIT License
Website	www.slf4j.org

Simple Logging Facade for Java (SLF4J) provides a Java logging API by means of a simple facade pattern. The underlying logging backend is determined at runtime by adding the desired binding to the classpath and may be the standard Sun Java logging package java.util.logging,^[2] log4j, logback^[3] or tinylog.^[4][5]

The separation of the client API from the logging backend reduces the coupling between an application and any particular logging framework. This can make it easier to integrate with existing or third-party code or to deliver code into other projects that have already made a choice of logging backend.

SLF4J was created by Ceki Gülcü as a more reliable alternative to Jakarta Commons Logging framework.^[6][7] Research in 2013 on 10,000 GitHub projects found that the most popular Java library is slf4j, with 30.7% of projects using it.^[8]

Similarities and differences with log4j 1.x

• Five of log4j's six logging levels are used (ERROR, WARN, INFO, DEBUG, TRACE). FATAL has been dropped on the basis that inside the logging framework is not the place to decide when an application should terminate and therefore there is no difference between ERROR and FATAL from the logger's point of view. In addition, slf4j markers offer a more general method for tagging log statements. For example, any log statement of level ERROR can be tagged with the "FATAL" marker.
• Logger instances are created via the LoggerFactory, which is very similar in log4j. For example,

   private static final Logger LOG = LoggerFactory.getLogger(Wombat.class);
  

• In Logger, the logging methods are overloaded with forms that accept one, two or more values.^[9] Occurrences of the simple pattern {} in the log message are replaced in turn with the values. This is simple to use yet provides a performance benefit when the values have expensive toString() methods. When logging is disabled at the DEBUG level, the logging framework does not need to evaluate the string representation of the values. In the following example, the values count or userAccountList only need to be evaluated when DEBUG is enabled; otherwise the overhead of the debug call is trivial.

   LOG.debug("There are now " + count + " user accounts: " + userAccountList); // slow
   LOG.debug("There are now {} user accounts: {}", count, userAccountList);    // faster
  

• Similar methods exist in Logger for isDebugEnabled() etc. to allow more complex logging calls to be wrapped so that they are disabled when the corresponding level is disabled, avoiding unnecessary processing.
• Unlike log4j, SLF4J offers logging methods that accept markers. These are special objects that enrich the log messages. At present time, logback is the only framework which makes use of markers.

Similarities and differences with log4j 2.x

Apache log4j 2.x supports all slf4j features.^[10]

See also

• Java logging framework

References

1. "SLF4J News". slfj4.org. 2015-11-22.
2. java.util.logging
3. logback
4. tinylog
5. SLF4J binding for tinylog
6. "Think again before adopting the commons-logging API"
7. "Taxonomy of class loader problems encountered when using Jakarta Commons Logging"
8. "We Analyzed 30,000 GitHub Projects – Here Are The Top 100 Libraries in Java, JS and Ruby".
9. SLF4J api docs: Logger
10. Apache log4j 2.x slf4j Binding

External links

• Official website