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| |
| <section> |
| <h2><a name="Logging_Separation"></a>Logging Separation</h2> |
| |
| <p> |
| 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. |
| </p> |
| <section> |
| <h3><a name="Use_Cases"></a>Use Cases</h3> |
| <a name="UseCases"></a> |
| |
| <p> |
| This section describes some of the use cases where Log4j could be used and what its desired behavior |
| might be. |
| </p> |
| <section> |
| <h4><a name="Standalone_Application"></a>Standalone Application</h4> |
| |
| <p> |
| Standalone applications are usually relatively simple. They typically have one bundled executable |
| that requires only a single logging configuration. |
| </p> |
| </section><section> |
| <h4><a name="Web_Applications"></a>Web Applications</h4> |
| |
| <p> |
| 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 <a href="webapp.html">instructions to initialize Log4j 2 |
| in a web application</a>. |
| </p> |
| </section><section> |
| <h4><a name="Java_EE_Applications"></a>Java EE Applications</h4> |
| |
| <p> |
| 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 |
| <a href="webapp.html">instructions to initialize Log4j 2 in a web application</a>. |
| </p> |
| </section><section> |
| <h4><a name="a.22Shared.22_Web_Applications_and_REST_Service_Containers"></a>"Shared" Web Applications and REST Service Containers</h4> |
| |
| <p> |
| 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. |
| </p> |
| </section><section> |
| <h4><a name="OSGi_Applications"></a>OSGi Applications</h4> |
| |
| <p> |
| 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 |
| <a href="extending.html#ContextSelector">ContextSelector</a> should be overridden with |
| BasicContextSelector or JndiContextSelector. |
| </p> |
| </section></section> |
| <section> |
| <h3><a name="Approaches"></a>Approaches</h3> |
| <a name="Approaches"></a> |
| <section> |
| <h4><a name="The_Simple_Approach"></a>The Simple Approach</h4> |
| |
| <p> |
| 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. |
| </p> |
| |
| </section><section> |
| <h4><a name="Using_Context_Selectors"></a>Using Context Selectors</h4> |
| |
| <p> |
| There are a few patterns for achieving the desired state of logging separation using ContextSelectors: |
| </p> |
| |
| <ol style="list-style-type: decimal"> |
| |
| <li>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.</li> |
| |
| <li> |
| Place the logging jars in the container's classpath and use the default ClassLoaderContextSelector. |
| Follow the <a href="webapp.html">instructions to initialize Log4j 2 in a web application</a>. 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. |
| </li> |
| |
| <li> |
| Follow the <a href="webapp.html">instructions to initialize Log4j 2 in a web application</a> and set |
| the system property or servlet context parameter log4j2.contextSelector to |
| <kbd>org.apache.logging.log4j.core.selector.JndiContextSelector</kbd>. This will cause the container |
| to use JNDI to locate each web application's LoggerContext. Be sure to set the |
| isLog4jContextSelectorNamed context parameter to <kbd>true</kbd> and also set the |
| log4jContextName and log4jConfiguration context parameters. |
| </li> |
| </ol> |
| |
| <p> |
| 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. |
| </p> |
| </section></section> |
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