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.
This section describes some of the use cases where Log4j could be used and what its desired behavior might be.
Standalone applications are usually relatively simple. They typically have one bundled executable that requires only a single logging configuration.
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 to initialize Log4j 2 in a web application.
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 to initialize Log4j 2 in a web application.
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.
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 ContextSelector should be overridden with BasicContextSelector or JndiContextSelector.
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.
There are a few patterns for achieving the desired state of logging separation using ContextSelectors:
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.
Place the logging jars in the container's classpath and use the default ClassLoaderContextSelector. Follow the 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.
Follow the instructions to initialize Log4j 2 in a web application and set the system property or servlet context parameter log4j2.contextSelector to org.apache.logging.log4j.core.selector.JndiContextSelector. This will cause the container to use JNDI to locate each web application's LoggerContext. Be sure to set the isLog4jContextSelectorNamed context parameter to true and also set the log4jContextName and log4jConfiguration context parameters.
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.