Re: commons logging class level logging

[Simon Kitching <sk...@apache.org>]

Trenton D. Adams wrote:
> What's the quickest and easiest way of my code knowing what class called 
> a method?  The reason I ask is because we have some wrapper methods in 
> our main class that every other class calls to do logging.  I would like 
> that method to be able to determine what class it was that called, so 
> that I could use the proper logger/category.

I don´t believe it is possible, except by requiring the calling method 
to pass the associated information as a parameter.

Java 1.5 does provide an API to get a stack trace. Earlier versions of 
java provides Throwable.printStackTrace method that can be parsed to 
extract the calling method [WARNING: stack trace format can vary by 
platform and JVM vendor!]. However there is no guarantee that this 
information is actually available at runtime; optimising JVMs can do all 
sorts of things that mean that stacktrace information is incomplete or 
simply not available; inlining method calls is just one case.

You will see that log4j appenders have %C and %M (I think that´s the 
right chars) to insert class and method info into log messages. But I 
know from experience that they don´t always work.

If you don´t want to require the source code to explicitly include the 
caller information as a parameter, then maybe you can use an 
"aspect-oriented" tool (eg aspectJ) to automatically insert the 
necessary parameter.

> 
> Also, for log4j, is Logger.getLogger("classname") an efficient way of 
> getting a logger for each class?  e.g. should I be calling this method 
> every time my *wrapper* log method is called?  How does the commons 
> logging do this properly, surely it has to do something similar since 
> it's a wrapper too?

commons-logging keeps a hashmap of all the Log objects, keyed by the 
category name. Calling getLogger("foo") just does a hashmap lookup to 
find and return the appropriate Log object.

And I´m pretty sure log4j internally does the same thing; it just has a 
hashmap of all the Logger objects that already exist.

Cheers,

Simon

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