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v1.3alpha8 :: examples / tiny-webapp /

INSTALL.txt


The tiny web-applications Hello and Tata demonstrate how multiple
web-apps can live in separate logging contexts.

Here are the installation steps to run the examples.

Steps performed on the Java Web Server (Servlet Container)
=========================================================

- Place log4j-VERSION.jar, where VERSION is 1.3 or above, in your web
  server's (YES, server's) shared class directory.

  For example, for Tomcat versions 4 or 5, that would be ./common/lib/ 
  under the Tomcat installation folder, for Resin version 2.1.x that
  would be the ./lib/ directory under the Resin installation folder. 
  Other Servlet containers such as Jetty also have folders for 
  shared jar files.

- On the java command line launching your java web *server*, make sure
  to add te log4j.repositorySelector system property. For the
  JNDIContextSelector the exact system property to add is:

     -Dlog4j.repositorySelector=org.apache.log4j.selector.ContextJNDISelector
  
  or in its equivalent but shorter form
  
     -Dlog4j.repositorySelector=JNDI 
     
  For Tomcat, you can add this line directly in ./bin/catalina.sh or
  ./bin/catalina.bat.

  After you start Tomcat, the $TOMCAT_HOME/logs/catalina.out file
  should contain a line with the following text.

  *** Will use ContextJNDISelector **

  If the above line exists, then you can be certain that setting the
  log4j.repositorySelector system property had the desired affect.

- You can now run the supplied web-applications hello.war and
  tata.war.

  Alternatively, you can instruct Tomcat to run Hello and Tata
  directly form LOG4J_HOME. Here is an excerpt from our 'server.xml'
  file, that is Tomcat's configration file.

  <Host name="localhost">

    <Context docBase="LOG4J_HOME/examples/tiny-webapp/Hello/WebRoot"
             path="/hello"/>

    <Context docBase="LOG4J_HOME/examples/tiny-webapp/Tata/WebRoot"
             path="/tata"/>
  </Host>    


- Optionally, you can add a configuration file such as log4j.xml or
  log4j.properties in the class directory of your *web-server*. For
  Tomcat versions 4 or 5, that would be ./common/classes/ directory.

  Thus, when log4j is first loaded into memory, it will configure the
  default logging repository using the configuration file found in
  ./common/classes/ directory.

- Instructing Tomcat to use log4j

  If you would like Tomcat to use log4j and its default repository,
  you also need to tell Tomcat to use log4j by placing
  commons-logging.jar in ./common/lib directory.
  
  This way the default (log4j) logger repository will be used by
  Tomcat for its logging and the default logger repository will be
  controlled by the configuration file log4j.xml or log4j.properties
  found in ./common/classes/. (Tested on Tomcat 5.0.19 and 5.0.27),

Steps performed per web-application
==================================

- You will need log4j-VERSION.jar to compile the
  web-applications. However, log4j-VERSION.jar file need not and
  should not be included within the web-application's WAR file.
 
- In each web-application's web.ml file add a JNDI environment entry
  for the log4j logging context name. For the "Hello" web-application
  this takes the following form:

  <env-entry>
   <description>JNDI logging context for this app</description>
   <env-entry-name>log4j/context-name</env-entry-name>
   <env-entry-value>hello</env-entry-value>
   <env-entry-type>java.lang.String</env-entry-type>
  </env-entry>
 
  See also the file examples/tiny-webapp/Hello/src/WEB-INF/web.xml

- Specify the URL for this context's configuration resource using 
  the "log4j/configuration-resource" environment entry.
  The repository selector (ContextJNDISelector) will use the 
  specified resource to configure the log4j repository.

  <env-entry>
   <description>URL for configuring log4j context</description>
   <env-entry-name>log4j/configuration-resource</env-entry-name>
   <env-entry-value>urlOfConfigrationResource</env-entry-value>
   <env-entry-type>java.lang.String</env-entry-type>
  </env-entry>
  
  Note if the  "log4j/configuration-resource" environment entry 
  is missing, then the logger repository for your application's logging
  context will NOT be configured.
  
  For more information on the available options see the javadoc for
  org.apache.log4j.selector.ContextJNDISelector.

- When the web-application is recycled or shutdown, it is very often useful
  to recycle the associated logging repository. This can be done by
  installing a ServletContextListener which will detach the repository
  from the repository selector and shut it down.

  The ContextDetachingSCL, where SCL stands ServletContextListener, which 
  ships with log4j does exactly that. To install it, add the following 
  lines to your web-application's web.xml file.

   <listener>
     <listener-class>org.apache.log4j.selector.servlet.ContextDetachingSCL</listener-class>
   </listener>

  See also the file examples/tiny-webapp/Hello/src/WEB-INF/web.xml

- You are also encouraged to name your web-application in the web
  descriptor file. As in

  <display-name>Hello sample web-application</display-name>


CAVEAT
======

We recommend that you name log4j configuration resources uniquely. In
particualar, avoid naming the log4j configuration resource as
'log4j.xml' or as 'log4j.properties' for a non-default logger
repository. For example, the following env-entry is error-prone:

<env-entry>
   <description>URL for configuring log4j context</description>
   <env-entry-name>log4j/configuration-resource</env-entry-name>
   <env-entry-value>log4j.xml</env-entry-value>
   <env-entry-type>java.lang.String</env-entry-type>
</env-entry>

While trying to configure the web-application log4j would search for
the resource 'log4j.xml' using the thread context classloader.  Thus,
it would first attempt to locate 'log4j.xml' file using the
classloader specific to the web-application.  However, if the file
'log4j.xml' did not exist there (if you forgot to put a custom one in
WEB-INF/classes), and if the file 'log4j.xml' existed higher up in the
classloader tree, we could end up in a situation where the logger
repository for your web-application would be configured using the same
file as that used to configure the default repository. Such
involuntary sharing of the same configuration by multiple repositories
will result in corrupt log output.
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