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.

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.

"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.

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 "Log4jContextSelector" 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. Include the Log4jContextListener in each web application. Each ContextListener can be configured to share the same configuration used at the container or they 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.
3. Use the JNDIContextFilter and set the system property "Log4jContextSelector" to "org.apache.logging.log4j.core.selector.JNDIContextSelector". This will cause the container to use JNDI to locate each's web application's LoggerContext.