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= Logging Separation
Ralph Goers <rgoers@apache.org>

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There are many well known use cases where applications may share an
environment with other applications and each has a need to have its own,
separate logging environment. This purpose of this section is to discuss
some of these cases and ways to accomplish this.

[#UseCases]
== Use Cases

This section describes some of the use cases where Log4j could be used
and what its desired behavior might be.

=== Standalone Application

Standalone applications are usually relatively simple. They typically
have one bundled executable that requires only a single logging
configuration.

=== Web Applications

A typical web application will be packaged as a WAR file and will
include all of its dependencies in WEB-INF/lib and will have its
configuration file located in the class path or in a location configured
in the web.xml. Be sure to follow the instructions in http://logging.apache.org/log4j/jakarta[Log4j Jakarta EE] page.

=== Java EE Applications

A Java EE application will consist of one or more WAR files and possible
some EJBs, typically all packaged in an EAR file. Usually, it is
desirable to have a single configuration that applies to all the
components in the EAR. The logging classes will generally be placed in a
location shared across all the components and the configuration needs to
also be shareable. Be sure to follow the instructions in the
http://logging.apache.org/log4j/jakarta[Log4j Jakarta EE] page.

=== "Shared" Web Applications and REST Service Containers

In this scenario there are multiple WAR files deployed into a single
container. Each of the applications should use the same logging
configuration and share the same logging implementation across each of
the web applications. When writing to files and streams each of the
applications should share them to avoid the issues that can occur when
multiple components try to write to the same file(s) through different
File objects, channels, etc.

=== OSGi Applications

An OSGi container physically separates each JAR into its own
ClassLoader, thus enforcing modularity of JARs as well as providing
standardized ways for JARs to share code based on version numbers.
Suffice to say, the OSGi framework is beyond the scope of this manual.
There are some differences when using Log4j in an OSGi container. By
default, each JAR bundle is scanned for its own Log4j configuration
file. Similar to the web application paradigm, every bundle has its own
LoggerContext. As this may be undesirable when a global Log4j
configuration is wanted, then the
link:extending.html#ContextSelector[ContextSelector] should be
overridden with `BasicContextSelector` or `JndiContextSelector`.

[#Approaches]
== Approaches

=== The Simple Approach

The simplest approach for separating logging within applications is to
package each application with its own copy of Log4j and to use the
BasicContextSelector. While this works for standalone applications and
may work for web applications and possibly Java EE applications, it does
not work at all in the last case. However, when this approach does work
it should be used as it is ultimately the simplest and most
straightforward way of implementing logging.

=== Using Context Selectors

There are a few patterns for achieving the desired state of logging
separation using ContextSelectors:

1.  Place the logging jars in the container's classpath and set the
system property `log4j2.contextSelector` to
`org.apache.logging.log4j.core.selector.BasicContextSelector`. This will
create a single LoggerContext using a single configuration that will be
shared across all applications.
2.  Place the logging jars in the container's classpath and use the
default ClassLoaderContextSelector. Follow the
link:webapp.html[instructions to initialize Log4j 2 in a web
application]. Each application can be configured to share the same
configuration used at the container or can be individually configured.
If status logging is set to debug in the configuration there will be
output from when logging is initialized in the container and then again
in each web application.

The `JndiContextSelector` will not work unless `log4j2.enableJndi=true` is set as a system property
or environment variable. See the
link:./configuration.html#enableJndi[log4j2.enableJndi] system property.

The exact method for setting system properties depends on the container.
For Tomcat, edit `$CATALINA_HOME/conf/catalina.properties`. Consult the
documentation for other web containers.
