[slf4j-user] Best practice on declaring Loggers

[Ceki Gülcü]

At 12:30 PM 6/13/2006, Konstantinos Karadamoglou wrote:
>Hello all,
>
>Which is the best practice of declaring Loggers? For instance, should they 
>be static, private, final? and why?

I recommend that loggers members be declared as instance variables instead 
of static.

Static logger members cost a single variable reference for all instances of 
the class whereas an instance logger member will cost a variable reference 
for every instance of the class. For simple classes instantiated thousands 
of times there might be a noticeable difference.

However, more recent logging systems, e.g log4j or logback, support a 
distinct logger context for each application running in the application 
server. Thus, even if a single copy of log4j.jar or logback-classic.jar is 
deployed in the server, the logging system will be able to differentiate 
between applications and offer a distinct logging environment for each 
application.

More specifically, each time a logger is retrieved by invoking 
LoggerFactory.getLogger() method, the underlying logging system will return 
an instance appropriate for the current application. Please note that 
within the same application retrieving a logger by a given name will always 
return the same logger.

If the logger is static, then it will only be retrieved once when the 
hosting class is loaded into memory. If the hosting class is used in only 
in one application, there is not much to be concerned about. However, if 
the hosting class is shared between several applications, then all 
instances of the shared class will log into the context of the application 
which happened to fist load the shared class into memory.

In summary, except for classes with few members and instantiated very 
frequently, logger members should be instance variables.

I hope this helps,

-- 
Ceki Gülcü
http://ceki.blogspot.com/