Common problems and solutions for log4j implementation in applications running on application servers like Weblogic, Websphere

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Agenda
 Current Log Implementation Problems!!!
 Log4j Overview
 Log4j And Web Applications
 Log4j And Class Loaders In Application Servers
 Log4j And The “Logging Separation” Problem
 Log4j Implementation In Web Applications
 Log4j And The “Log Rotation” Problem
 Dated Logger Implementation In Web Applications
 For You….

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Current Log Implementation
Problems!!!

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Log4j Overview

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Log4j normally needs to be configured only once. Some new users try to
configure log4j in each and every class. This is very inefficient and just plain
wrong.
A logger is said to be an ancestor of another logger if its name followed by a
dot is a prefix of the descendant logger name.
The root logger resides at the top of the logger hierarchy.
e.g. x.y is parent of x.y.z

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Log4j And Web Applications

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In practice placing log4j-VERSION.jar in the WEB-INF/lib directory of your
web-application will cause log4j classes to be loaded/unloaded whenever your
web application is loaded/unloaded. Moreover, each copy of the log4j classes
will be treated as a separate unrelated copy by the JVM.
Under Tomcat 3.x and 4.x, you should place the log4j.xml or
log4j.properties under the WEB-INF/classes directory of your web-
applications. Log4j will find the properties file and initialize itself.
WEB-INF
Lib
• log4j-version.jar
Classes
• log4j.xml/log4j.properties

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Log4j And Class Loaders In
Application Servers

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The Java class loader delegation model gives precedence to parent class
loaders. This means that if log4j.jar is available on the CLASSPATH, or
under JAVA_HOME/jar/lib/ext or to any class loader which is a parent of the
webapplication's class loader, then that copy of log4j will be loaded into
memory and shared by all web-applications.

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Log4j And The “Logging Separation”
Problem

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In a separated logging environment, each web-application can configure log4j
in different ways such that the configuration of one web-application cannot
interfere with the logging configuration of another web application.
Since time immemorial users have struggled to control the logging
configuration of multiple web-applications deployed on the same Servlet
Container (e.g. Tomcat).

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Log4j Implementation In Web
Applications

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Assuming each web-application is loaded by a distinct class loader, then
placing a copy of log4j.jar under WEB-INF/lib/ directory of each web-
application will automatically result in distinct log4j-logging universes. Simply
put, each webapplication will load its own distinct copy of log4j classes into
memory. All such copies are invisible and inaccessible to each other.

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Log4j allows different applications live in their own parallel universe by using
a different LoggerRepository for each application. Given that each hierarchy
(i.e. logger repository) manages its own separate logger tree, logging
separation is a direct consequence of this approach.
The Java Servlet API mandates a unique ServletContext for each web
application. Thus, a web-application can set an attribute for a Servlet context
which can be shared by all Servlet and jsp pages of a web-application but
remain invisible to other web-applications.
In particular, an initialization Servlet can create, set and configure an
independent logger hierarchy in the Servlet context. Subsequently, other
Servlet can obtain the hierarchy stored in the Servlet context in order to retrieve
logger instances.
These logger instances will be attached to the particular hierarchy specific to
the web-application.

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This solution relies on the Servlet Container to keep track of the
execution context and provide a different logging environment for each
context.
Each web application has its own class loader and Tomcat sets the Thread
Context Classloader, or TCL, to be the class loader of the currently
executing web application.
Servlet Container provides a separate hierarchy instance for each
web-application. Each logger object that log4j creates is attached to a
hierarchy. The Hierarchy class implements the LoggerRepository interface by
arranging logger objects in a tree according to their name.

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Log4j And The “Log Rotation”
Problem

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The log file rotation on Application servers fails with following error
log4j: ERROR Failed to rename [<file
name>.log] to [<file name>.log.2010-03-
23].

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Log4j And “Multiple Line Printing”
Problem

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Logger Added Additivity
Output Targets Comment
Name Appenders Flag
The root logger does
root A1 not applicable A1
not have a parent.
Appenders in root
x A-x1, A-x2 true A1, A-x1, A-x2 are added to
appenders in "x".
Appenders of "x" and
x.y none true A1, A-x1, A-x2
root.
A1, A-x1, A-x2, A- Appenders in "x.y.z",
x.y.z A-xyz1 true
xyz1 "x" and root.
No appender
accumulation as the
security A-sec false A-sec
additivity flag is set
to false.
Only appenders of
"security" as the
security.ACL none true A-sec additivity flag in
"security" is set to
false.

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Dated Logger Implementation In
Web Applications

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DatedFileAppender is an "appender" object designed for use with the
Apache Log4j logging system.
It provides an Apache Tomcat style FileLogger implementation that differs
from the file loggers provided with Log4j.
While the DatedFileAppender was written with Apache Tomcat in mind, it does
not depend on Tomcat and may be used in any other environment.
How to use it?
•Copy the datedFileAppender-1.0.2.jar to the same directory as your other
application jar files and add the jar file name to your CLASSPATH.
•Modify your log4j.properties file to use the DatedFileAppender class (see
below for configuration options).

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For You….

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Still Have Any
Question? 

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Thank You !!!