## Using Log4j 2 in Web Applications You must take particular care when using Log4j or any other logging framework within a Java EE web application. It's important for logging resources to be properly cleaned up (database connections closed, files closed, etc.) when the container shuts down or the web application is undeployed. Because of the nature of class loaders within web applications, Log4j resources cannot be cleaned up through normal means. Log4j must be "started" when the web application deploys and "shut down" when the web application undeploys. How this works varies depending on whether your application is a Servlet 3.0 or newer or Servlet 2.5 web application. In either case, you'll need to add the `log4j-web` module to your deployment as detailed in the Maven, Ivy, and Gradle Artifacts manual page. To avoid problems the Log4j shutdown hook will automatically be disabled when the log4j-web jar is included. ### Configuration Log4j allows the configuration file to be specified in web.xml using the `log4jConfiguration` context parameter. Log4j will search for configuration files by: If a location is provided it will be searched for as a servlet context resource. For example, if `log4jConfiguration` contains "logging.xml" then Log4j will look for a file with that name in the root directory of the web application. If no location is defined Log4j will search for a file that starts with "log4j2" in the WEB-INF directory. If more than one file is found, and if a file that starts with "log4j2-name" is present, where name is the name of the web application, then it will be used. Otherwise the first file will be used. The "normal" search sequence using the classpath and file URLs will be used to locate the configuration file. ### Servlet 3.0 and Newer Web Applications A Servlet 3.0 or newer web application is any `<web-app>` whose `version` attribute has a value of "3.0" or higher. Of course, the application must also be running in a compatible web container. Some examples are: Tomcat 7.0 and higher, GlassFish 3.0 and higher, JBoss 7.0 and higher, Oracle WebLogic 12c and higher, and IBM WebSphere 8.0 and higher. #### The Short Story Log4j 2 "just works" in Servlet 3.0 and newer web applications. It is capable of automatically starting when the application deploys and shutting down when the application undeploys. Thanks to the ServletContainerInitializer API added to Servlet 3.0, the relevant `Filter` and `ServletContextListener` classes can be registered dynamically on web application startup. Important Note! For performance reasons, containers often ignore certain JARs known not to contain TLDs or `ServletContainerInitializer`s and do not scan them for web-fragments and initializers. Importantly, Tomcat 7 <7.0.43 ignores all JAR files named log4j*.jar, which prevents this feature from working. This has been fixed in Tomcat 7.0.43, Tomcat 8, and later. In Tomcat 7 <7.0.43 you will need to change `catalina.properties` and remove "log4j*.jar" from the `jarsToSkip` property. You may need to do something similar on other containers if they skip scanning Log4j JAR files. #### The Long Story The Log4j 2 Web JAR file is a web-fragment configured to order before any other web fragments in your application. It contains a `ServletContainerInitializer` (Log4jServletContainerInitializer) that the container automatically discovers and initializes. This adds the Log4jServletContextListener and Log4jServletFilter to the `ServletContext`. These classes properly initialize and deinitialize the Log4j configuration. For some users, automatically starting Log4j is problematic or undesirable. You can easily disable this feature using the `isLog4jAutoInitializationDisabled` context parameter. Simply add it to your deployment descriptor with the value "true" to disable auto-initialization. You must define the context parameter in `web.xml`. If you set in programmatically, it will be too late for Log4j to detect the setting.
        <context-param>
            <param-name>isLog4jAutoInitializationDisabled</param-name>
            <param-value>true</param-value>
        </context-param>
Once you disable auto-initialization, you must initialize Log4j as you would a Servlet 2.5 web application. You must do so in a way that this initialization happens before any other application code (such as Spring Framework startup code) executes. You can customize the behavior of the listener and filter using the `log4jContextName`, `log4jConfiguration`, and/or `isLog4jContextSelectorNamed` context parameters. Read more about this in the Context Parameters section below. You must not manually configure the `Log4jServletContextListener` or `Log4jServletFilter` in your deployment descriptor (`web.xml`) or in another initializer or listener in a Servlet 3.0 or newer application unless you disable auto-initialization with `isLog4jAutoInitializationDisabled`. Doing so will result in startup errors and unspecified erroneous behavior. ### Servlet 2.5 Web Applications A Servlet 2.5 web application is any `<web-app>` whose `version` attribute has a value of "2.5." The `version` attribute is the only thing that matters; even if the web application is running in a Servlet 3.0 or newer container, it is a Servlet 2.5 web application if the `version` attribute is "2.5." Note that Log4j 2 does not support Servlet 2.4 and older web applications. If you are using Log4j in a Servlet 2.5 web application, or if you have disabled auto-initialization with the `isLog4jAutoInitializationDisabled` context parameter, you must configure the Log4jServletContextListener and Log4jServletFilter in the deployment descriptor or programmatically. The filter should match all requests of any type. The listener should be the very first listener defined in your application, and the filter should be the very first filter defined and mapped in your application. This is easily accomplished using the following `web.xml` code:
        <listener>
            <listener-class>org.apache.logging.log4j.web.Log4jServletContextListener</listener-class>
        </listener>

        <filter>
            <filter-name>log4jServletFilter</filter-name>
            <filter-class>org.apache.logging.log4j.web.Log4jServletFilter</filter-class>
        </filter>
        <filter-mapping>
            <filter-name>log4jServletFilter</filter-name>
            <url-pattern>/*</url-pattern>
            <dispatcher>REQUEST</dispatcher>
            <dispatcher>FORWARD</dispatcher>
            <dispatcher>INCLUDE</dispatcher>
            <dispatcher>ERROR</dispatcher>
            <dispatcher>ASYNC</dispatcher><!-- Servlet 3.0 w/ disabled auto-initialization only; not supported in 2.5 -->
        </filter-mapping>
You can customize the behavior of the listener and filter using the `log4jContextName`, `log4jConfiguration`, and/or `isLog4jContextSelectorNamed` context parameters. Read more about this in the Context Parameters section below. ### Context Parameters By default, Log4j 2 uses the `ServletContext`'s context name as the `LoggerContext` name and uses the standard pattern for locating the Log4j configuration file. There are three context parameters that you can use to control this behavior. The first, `isLog4jContextSelectorNamed`, specifies whether the context should be selected using the JndiContextSelector. If `isLog4jContextSelectorNamed` is not specified or is anything other than `true`, it is assumed to be `false`. If `isLog4jContextSelectorNamed` is `true`, `log4jContextName` must be specified or `display-name` must be specified in `web.xml`; otherwise, the application will fail to start with an exception. `log4jConfiguration` should also be specified in this case, and must be a valid URI for the configuration file; however, this parameter is not required. If `isLog4jContextSelectorNamed` is not `true`, `log4jConfiguration` may optionally be specified and must be a valid URI or path to a configuration file or start with "classpath:" to denote a configuration file that can be found on the classpath. Without this parameter, Log4j will use the standard mechanisms for locating the configuration file. When specifying these context parameters, you must specify them in the deployment descriptor (`web.xml`) even in a Servlet 3.0 or never application. If you add them to the `ServletContext` within a listener, Log4j will initialize before the context parameters are available and they will have no effect. Here are some sample uses of these context parameters. #### Set the Logging Context Name to "myApplication"
        <context-param>
            <param-name>log4jContextName</param-name>
            <param-value>myApplication</param-value>
        </context-param>
#### Set the Configuration Path/File/URI to "/etc/myApp/myLogging.xml"
        <context-param>
            <param-name>log4jConfiguration</param-name>
            <param-value>file:///etc/myApp/myLogging.xml</param-value>
        </context-param>
#### Use the `JndiContextSelector`
        <context-param>
            <param-name>isLog4jContextSelectorNamed</param-name>
            <param-value>true</param-value>
        </context-param>
        <context-param>
            <param-name>log4jContextName</param-name>
            <param-value>appWithJndiSelector</param-value>
        </context-param>
        <context-param>
            <param-name>log4jConfiguration</param-name>
            <param-value>file:///D:/conf/myLogging.xml</param-value>
        </context-param>
Note that in this case you must also set the "Log4jContextSelector" system property to "`org.apache.logging.log4j.core.selector.JndiContextSelector`". ### Using Web Application Information During the Configuration You may want to use information about the web application during configuration. For example, you could embed the web application's context path in the name of a Rolling File Appender. See WebLookup in Lookups for more information. ### JavaServer Pages Logging You may use Log4j 2 within JSPs just as you would within any other Java code. Simple obtain a `Logger` and call its methods to log events. However, this requires you to use Java code within your JSPs, and some development teams rightly are not comfortable with doing this. If you have a dedicated user interface development team that is not familiar with using Java, you may even have Java code disabled in your JSPs. For this reason, Log4j 2 provides a JSP Tag Library that enables you to log events without using any Java code. To read more about using this tag library, read the Log4j Tag Library documentation. Important Note! As noted above, containers often ignore certain JARs known not to contain TLDs and do not scan them for TLD files. Importantly, Tomcat 7 <7.0.43 ignores all JAR files named log4j*.jar, which prevents the JSP tag library from being automatically discovered. This does not affect Tomcat 6.x and has been fixed in Tomcat 7.0.43, Tomcat 8, and later. In Tomcat 7 <7.0.43 you will need to change `catalina.properties` and remove "log4j*.jar" from the `jarsToSkip` property. You may need to do something similar on other containers if they skip scanning Log4j JAR files. ### Asynchronous Requests and Threads The handling of asynchronous requests is tricky, and regardless of Servlet container version or configuration Log4j cannot handle everything automatically. When standard requests, forwards, includes, and error resources are processed, the `Log4jServletFilter` binds the `LoggerContext` to the thread handling the request. After request processing completes, the filter unbinds the `LoggerContext` from the thread. Similarly, when an internal request is dispatched using a `javax.servlet.AsyncContext`, the `Log4jServletFilter` also binds the `LoggerContext` to the thread handling the request and unbinds it when request processing completes. However, this only happens for requests dispatched through the `AsyncContext`. There are other asynchronous activities that can take place other than internal dispatched requests. For example, after starting an `AsyncContext` you could start up a separate thread to process the request in the background, possibly writing the response with the `ServletOutputStream`. Filters cannot intercept the execution of this thread. Filters also cannot intercept threads that you start in the background during non-asynchronous requests. This is true whether you use a brand new thread or a thread borrowed from a thread pool. So what can you do for these special threads? You may not need to do anything. If you didn't use the `isLog4jContextSelectorNamed` context parameter, there is no need to bind the `LoggerContext` to the thread. Log4j can safely locate the `LoggerContext` on its own. In these cases, the filter provides only very modest performance gains, and only when creating new `Logger`s. However, if you did specify the `isLog4jContextSelectorNamed` context parameter with the value "true", you will need to manually bind the `LoggerContext` to asynchronous threads. Otherwise, Log4j will not be able to locate it. Thankfully, Log4j provides a simple mechanism for binding the `LoggerContext` to asynchronous threads in these special circumstances. The simplest way to do this is to wrap the `Runnable` instance that is passed to the `AsyncContext.start()` method.
    import java.io.IOException;
    import javax.servlet.AsyncContext;
    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;

    import org.apache.logging.log4j.LogManager;
    import org.apache.logging.log4j.Logger;
    import org.apache.logging.log4j.web.WebLoggerContextUtils;

    public class TestAsyncServlet extends HttpServlet {

        @Override
        protected void doGet(final HttpServletRequest req, final HttpServletResponse resp) throws ServletException, IOException {
            final AsyncContext asyncContext = req.startAsync();
            asyncContext.start(WebLoggerContextUtils.wrapExecutionContext(this.getServletContext(), new Runnable() {
                @Override
                public void run() {
                    final Logger logger = LogManager.getLogger(TestAsyncServlet.class);
                    logger.info("Hello, servlet!");
                }
            }));
        }

        @Override
        protected void doPost(final HttpServletRequest req, final HttpServletResponse resp) throws ServletException, IOException {
            final AsyncContext asyncContext = req.startAsync();
            asyncContext.start(new Runnable() {
                @Override
                public void run() {
                    final Log4jWebSupport webSupport =
                        WebLoggerContextUtils.getWebLifeCycle(TestAsyncServlet.this.getServletContext());
                    webSupport.setLoggerContext();
                    // do stuff
                    webSupport.clearLoggerContext();
                }
            });
        }
    }
This can be slightly more convenient when using Java 1.8 and lambda functions as demonstrated below.
    import java.io.IOException;
    import javax.servlet.AsyncContext;
    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;

    import org.apache.logging.log4j.LogManager;
    import org.apache.logging.log4j.Logger;
    import org.apache.logging.log4j.web.WebLoggerContextUtils;

    public class TestAsyncServlet extends HttpServlet {
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            final AsyncContext asyncContext = req.startAsync();
            asyncContext.start(WebLoggerContextUtils.wrapExecutionContext(this.getServletContext(), () -> {
                final Logger logger = LogManager.getLogger(TestAsyncServlet.class);
                logger.info("Hello, servlet!");
            }));
        }
    }
Alternatively, you can obtain the Log4jWebLifeCycle instance from the `ServletContext` attributes, call its `setLoggerContext` method as the very first line of code in your asynchronous thread, and call its `clearLoggerContext` method as the very last line of code in your asynchronous thread. The following code demonstrates this. It uses the container thread pool to execute asynchronous request processing, passing an anonymous inner `Runnable` to the `start` method.
    import java.io.IOException;
    import javax.servlet.AsyncContext;
    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;

    import org.apache.logging.log4j.LogManager;
    import org.apache.logging.log4j.Logger;
    import org.apache.logging.log4j.web.Log4jWebLifeCycle;
    import org.apache.logging.log4j.web.WebLoggerContextUtils;

    public class TestAsyncServlet extends HttpServlet {
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
             final AsyncContext asyncContext = req.startAsync();
            asyncContext.start(new Runnable() {
                @Override
                public void run() {
                    final Log4jWebLifeCycle webLifeCycle =
                        WebLoggerContextUtils.getWebLifeCycle(TestAsyncServlet.this.getServletContext());
                    webLifeCycle.setLoggerContext();
                    try {
                        final Logger logger = LogManager.getLogger(TestAsyncServlet.class);
                        logger.info("Hello, servlet!");
                    } finally {
                        webLifeCycle.clearLoggerContext();
                    }
                }
            });
       }
    }
Note that you must call `clearLoggerContext` once your thread is finished processing. Failing to do so will result in memory leaks. If using a thread pool, it can even disrupt the logging of other web applications in your container. For that reason, the example here shows clearing the context in a `finally` block, which will always execute.