Category : Why so many of them

[Modha Kumar <mk...@sentius.com>]

Hi All
I have this question on Categories concept of log4j

In the documentation it says you can have multiple categories for the logger.
And in each class in my project I specify
static Category cat =Category.getInstance(MyClass.class.getName());
so this as i understand will create a category per class. So if I have 50
classes in my project it will create 50 categories. Now why will I use so many
categories.

1. Does it mean that I can control logging at class level.
2. Thinking from a project level how can i say that all classes in a particular
package should log into appender A.
3. Let us say 2 classes are logging into the same file at the same time does
log4j guarentee that they will be entered into the file.
4. In essence I understood the logging at a class level not at a project level.

Can anybody clarify these.
Thanks for suggesting vipan website it was helpful.

Modha/-

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Re: Category : Why so many of them

[Steve Ebersole <st...@austin.rr.com>]

The reason to do this is exactly what you state.  Control over how logging
gets done simply by changing the config.  Using the 1.2 classes/syntax, an
example would be:

Say you have the following two classes:
com.foo.bar.MyMain
com.foo.bar.MyHelper

each which define a logger based on its class name.

Then the following line in the config file:
log4j.logger.com.foo.bar=DEBUG, FILE1

states that all messages to a category starting with "com.foo.bar" (this
also would include "com.foo.bar.util" and com.foo.bar.util.jdo" etc) should
be sent to the defined appender named FILE1

Say later you decide that you want logging from MyHelper to go to a seperate
log file.  Then just do this in the config:
log4j.logger.com.foo.bar=DEBUG, FILE1

log4j.logger.com.foo.bar.MyHelper=WARN, FILE2
# This next line says to not also send it to FILE1
log4j.additivity.com.foo.bar.MyHelper=false

Now everything logs as before, except that output from MyHelper will show up
in the target of the FILE2 appender.

To answer your specific questions:
>>1. Does it mean that I can control logging at class level.
It means you can control logging at any level you want.  The project I work
on, we decided to functionally group the loggers.  For example, we use named
loggers such as "system.managers.models" and "system.managers.cache" etc.
Each class then obtains a Logger based on its functional role:  Logger log =
Logger.getLogger( "system.managers.models" ).  One of the many cool things
about log4j is that is extremely flexible.

>>2. Thinking from a project level how can i say that all classes in a
particular
>>package should log into appender A.
An example was shown above.  Note however that "sub-packages" will also be
included unless you explicitly set up config entries for them also and turn
their additivity flag to false.

>>3. Let us say 2 classes are logging into the same file at the same time
does
>>log4j guarentee that they will be entered into the file.
Now idea what your asking.  If your mean will log statements from both
classes show up in the same file, then the answer is: of course they will.

>>4. In essence I understood the logging at a class level not at a project
level.
Again, not sure I understand the question...



----- Original Message -----
From: "Modha Kumar" <mk...@sentius.com>
To: <lo...@jakarta.apache.org>
Sent: Friday, February 15, 2002 7:59 PM
Subject: Category : Why so many of them


Hi All
I have this question on Categories concept of log4j

In the documentation it says you can have multiple categories for the
logger.
And in each class in my project I specify
static Category cat =Category.getInstance(MyClass.class.getName());
so this as i understand will create a category per class. So if I have 50
classes in my project it will create 50 categories. Now why will I use so
many
categories.

1. Does it mean that I can control logging at class level.
2. Thinking from a project level how can i say that all classes in a
particular
package should log into appender A.
3. Let us say 2 classes are logging into the same file at the same time does
log4j guarentee that they will be entered into the file.
4. In essence I understood the logging at a class level not at a project
level.

Can anybody clarify these.
Thanks for suggesting vipan website it was helpful.

Modha/-

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