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From "Nick Williams (JIRA)" <j...@apache.org>
Subject [jira] [Created] (LOG4J2-270) Improve logging initialization in Servlet containers; reduce amount of extra configuration needed in these contexts
Date Wed, 29 May 2013 14:45:20 GMT
Nick Williams created LOG4J2-270:
------------------------------------

             Summary: Improve logging initialization in Servlet containers; reduce amount
of extra configuration needed in these contexts
                 Key: LOG4J2-270
                 URL: https://issues.apache.org/jira/browse/LOG4J2-270
             Project: Log4j 2
          Issue Type: Improvement
          Components: Core
    Affects Versions: 2.0-beta6, 2.0-beta7
            Reporter: Nick Williams
            Assignee: Nick Williams


[Note: This has been pulled from discussions surrounding LOG4J2-223. Specifically, the description
below comes from [this comment|https://issues.apache.org/jira/browse/LOG4J2-223?focusedCommentId=13668682&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13668682].]

Okay, based on all of this, here are my suggestions on how I would improve this (sorry for
my scarcity lately ... trying to catch up on chapters for the book). These suggestions are
based on the following goals:

# Design web application support so that users of simple Log4j configurations (not using JNDI
lookups for logging configuration) don't have to do any additional configuration to get Log4j
to "just work" in a Servlet container. This goal is made with the understanding that this
is only possible for users of Servlet 3.0, 3.1, or higher. Users of Servlet 2.5 will have
to manually configure listeners and filters. It's the only way.
# Eliminate the need for web applications to have an additional JAR just to support proper
configuration.
# Improve the documentation considerably so that it is abundantly clear how Log4j works in
web applications.

With these goals in mind, here are my recommendations:

* Get rid of the log4j-web module and move this stuff into log4j-core. We'll need [~timothyjward]'s
input on how this could impact OSGi support. In theory, I don't think we should have a problem.
No other Log4j classes will refer to the listener or filter. These classes will only ever
get loaded BY the Servlet container, in which case the Servlet API will already be loaded,
so I don't think it would be possible to get {{NoClassDefFoundError}} s.
* Create a {{/META-INF/web-fragment.xml}} file with the contents noted below. This will ensure
that the Log4j fragment is loaded before any other fragments.
* Create a {{Log4jWebInitializer}} class (doesn't implement any interfaces, package-private)
to properly initialize and de-initialize Log4j in a Servlet container. It works like this:
If the context parameters currently required by the {{JNDIContextFilter}} exist, it initializes
Log4j the way {{JNDIContextFilter}} currently does. If they don't exist, it initializes Log4j
the way {{Log4jContextListener}} currently does.
* Create a {{Log4jServletContainerInitializer implements ServletContainerInitializer}} and
a {{/META-INF/services/javax.servlet.ServletContainerInitializer}} file to go along with it.
This initializer:
** Initializes Log4j properly using the {{Log4jWebInitializer}}.
** Installs a private listener to de-initialize Log4j when the container shuts down the application
using the {{Log4jWebInitializer}}.
** Installs the {{NamedContextFilter}} (renamed from {{JNDIContextFilter}}), which doesn't
do any initialization/deinitialization, it just does what it currently does for {{doFilter}}.
* Change the existing {{Log4jContextListener}} to use the {{Log4jWebInitializer}}.
* Create a new manual page directly under the "MANUAL" menu heading on the homepage that explains
how to use Log4j in a Servlet container:
** If you're using Servlet 3.0 or higher, it "just works," but if you're also using JNDI you
need to create the proper context parameters.
** If you're using Servlet 2.5, you MUST add the {{Log4jContextListener}} (and the {{NamedContextFilter}}
and context parameters if you're using JNDI) to your deployment descriptor.

Now, to address some questions:

# "What to do about JBoss 5, which doesn't support web-fragment.xml?" JBoss 5 doesn't support
web-fragment.xml because it's a Servlet 2.5 container, not a Servlet 3.0 container. Users
of JBoss 5 would follow the Servlet 2.5 instructions just like all other users. Users of JBoss
6, 7, 8, etc., would follow the Servlet 3.0 instructions ("don't do anything").
# "A container with multiple web applications. The Log4j jars are in the Tomcat classpath
and they all share the same configuration file." versus "A container with multiple web applications.
Each has their own copy of the log4j jars and each has their own configuration file." Easy!
The {{Log4jServletContainerInitializer}} will get executed for every application, whether
the Log4j JARs are in the Tomcat classpath or in /WEB-INF/lib (this is per the Servlet specification).
Each application will get its own context. Regardless of where the JARs are, that context
will load the application's configuration file if it has its own, and will load the shared
configuration if it doesn't have its own.

Thoughts?

{code:xml|title=web-fragment.xml}<web-fragment ... metadata-complete="true">
    <name>log4j</name>
    <distributable/>
    <ordering>
        <before>
            <others/>
        </before>
    </ordering>
</web-fragment>{code}

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