Chapter 3: Logback configuration

We start by presenting ways for configuring logback, with many example configuration scripts. Joran, the configuration framework upon which logback relies will be presented in a later chapter.

Configuration at initialization

Inserting log requests into the application code requires a fair amount of planning and effort. Observation shows that approximately four percent of code is dedicated to logging. Consequently, even a moderately sized application will contain thousands of logging statements embedded within its code. Given their number, we need tools to manage these log statements.

Logback can be configured programmatically or via a configuration script written in XML, Groovy, or as a serialized model. Existing log4j users can convert their log4j.properties files to logback.xml using the properties translator web application.

Let us begin by discussing the initialization steps that logback follows to try to configure itself:

1. Logback will search for any custom Configurator providers using service-provider loading facility. If any such custom provider is found, it takes precedence over logback's own configurators, e.g. DefaultJoranConfigurator (see below).

   A custom Configurator is an implementation of ch.qos.logback.​classic.spi.​Configurator interface. Custom configurators are searched by looking up file resources located under META-INF/services/​ch.qos.logback.​classic.spi.​Configurator. The contents of this file should specify the fully qualified class name of the desired Configurator implementation.

2. Nominal step If the previous configurators could not locate their required resources, then an instance of DefaultJoranConfigurator will be created and invoked.

   • If the system property "logback.configurationFile" is set, then DefaultJoranConfigurator will try to locate the file specified by the aforementioned system property. If this file can be located, it will be read and interpreted for configuration.

   • If the previous step fails, DefaultJoranConfigurator will try to locate the configuration file "logback-test.xml" on the classpath. If this file can be located, it will be read and interpreted for configuration.

   • If no such file is found, it will try to locate the configuration file "logback.xml" in the classpath. If this file can be located, it will be read and interpreted for configuration. Note that this is the nominal configuration step.

   since 1.5.8 PropertiesConfigurator, introduced in version 1.5.8, allows for setting logger levels via a properties file. The location of properties files can be specified as a file path as well as a URL via HTTP or HTTPS protocols. Watching files and reconfiguration upon change is also supported. Note that configuration files in properties format are intended to be part of a main configuration file in XML format or a configurator produced by logback-tyler.

   • If no configuration file could be located, DefaultJoranConfigurator will return with an execution status asking for the next available configurator, i.e. BasicConfigurator, to be invoked.

3. If none of the above succeeds, logback-classic will configure itself using the BasicConfigurator which will cause logging output to be directed to the console.

The last step is meant as a last-ditch effort to provide a default (but very basic) logging functionality in the absence of a configuration file.

If you are using a build tool according to Maven's folder structure, then if you place the logback-test.xml under the src/test/resources folder, Maven will ensure that it won't be included in the artifact produced. Thus, you can use a different configuration file, namely logback-test.xml during testing, and another file, namely, logback.xml, in production.

Fast start-up It takes about 100 milliseconds for Joran to parse a given logback configuration file. To shave off those milliseconds at application start up, you can use the service-provider loading facility (item 1 above) to load your own custom Configurator class with BasicConfigurator serving as a good starting point. Better yet, converting an XML file to Java using TylerConfigurator is a quick and quite convenient way of improving configuration times.

Groovy Given that Groovy is a full-fledged language, we have dropped support for logback.groovy in order to protect against potential attacks. Fortunately, groovy support was picked up at virtualdogbert/logback-groovy-config by Tucker Pelletier.

Automatically configuring logback

The simplest way to configure logback is by letting logback fall back to its default configuration. Let us give a taste of how this is done in an imaginary application called MyApp1.

Example: Simple example of BasicConfigurator usage (logback-examples/​src/main/​java/chapters/​configuration/MyApp1.java)

package chapters.configuration;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class MyApp1 {
  final static Logger logger = LoggerFactory.getLogger(​MyApp1.class);

  public static void main(String[] args) {
    logger.info("Entering application.");

    Foo foo = new Foo();
    foo.doIt();
    logger.info("Exiting application.");
  }
}

This class defines a static logger variable. It then instantiates a Foo object. The Foo class is listed below:

Example: Small class doing logging (logback-examples/src/​main/java/​chapters/​configuration/Foo.java)

package chapters.configuration;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class Foo {
  static final Logger logger = LoggerFactory.getLogger(​Foo.class);

  public void doIt() {
    logger.debug("Did it again!");
  }
}

In order to run the examples in this chapter, you need to make sure that certain jar files are present on the class path. Please refer to the setup page for further details.

Assuming the configuration files logback-test.xml or logback.xml are not present, logback will default to invoking BasicConfigurator which will set up a minimal configuration. This minimal configuration consists of a ConsoleAppender attached to the root logger. The output is formatted using a PatternLayoutEncoder set to the pattern %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} -%kvp- %msg%n. Moreover, by default the root logger is assigned the DEBUG level.

Thus, the output of the command java chapters.configuration.​MyApp1 should be similar to:

16:06:09.031 [main] INFO chapters.configuration.​MyApp1 -- Entering application. 16:06:09.046 [main] DEBUG chapters.configuration.Foo -- Did it again! 16:06:09.046 [main] INFO chapters.configuration.​MyApp1 -- Exiting application.

Except for code that configures logback (if such code exists) client code does not need to depend on logback. Applications that use logback as their logging framework will have a compile-time dependency on SLF4J but not logback.

The MyApp1 application links to logback via calls to org.slf4j.LoggerFactory and org.slf4j.Logger classes, retrieve the loggers it wishes to use, and chugs on. Note that the only dependencies of the Foo class on logback are through org.slf4j.LoggerFactory and org.slf4j.Logger imports. Except code that configures logback (if such code exists) client code does not need to depend on logback. Since SLF4J permits the use of any logging framework under its abstraction layer, it is easy to migrate large bodies of code from one logging framework to another.

Configuration with logback-test.xml or logback.xml

As mentioned earlier, logback will try to configure itself using the files logback-test.xml or logback.xml if found on the class path. Here is a configuration file equivalent to the one established by BasicConfigurator we've just seen.

Example: Basic configuration file (logback-examples/​src/main/resources/​chapters/configuration/​sample0.xml)

<configuration>

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <!-- encoders are assigned the type
         ch.qos.logback.classic.encoder.PatternLayoutEncoder by default -->
    <encoder>
      <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} -%kvp- %msg%n</pattern>
    </encoder>
  </appender>

  <root level="debug">
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

After you have renamed sample0.xml as logback.xml (or logback-test.xml) place it into a directory accessible from the class path. Running the MyApp1 application should give identical results to its previous run.

Automatic printing of status messages in case of warnings or errors

When warnings or errors occur while parsing the configuration file, Logback prints its internal status messages to the console to aid diagnosis.

If warnings or errors occur during the parsing of the configuration file, logback will automatically print its internal status data on the console. Note that to avoid duplication, automatic status printing is disabled if the user explicitly registers a status listener (discussed below).

In the absence of warnings or errors, if you still wish to inspect logback's internal status, then you can instruct logback to print status data by invoking the print() of the StatusPrinter class. The MyApp2 application shown below is identical to MyApp1 except for the addition of two lines of code for printing internal status data.

Example: Print logback's internal status information (logback-examples/src/​main/java/​chapters/​configuration/MyApp2.java)

public static void main(String[] args) {
  // assume SLF4J is bound to logback in the current environment
  LoggerContext lc = (LoggerContext) LoggerFactory.getILoggerFactory(​);
  // print logback's internal status
  StatusPrinter.print(lc);
  ...
}

If everything goes well, you should see the following output on the console

At the end of this output, you can recognize the lines that were printed in the previous example. You should also notice logback's internal messages, a.k.a. Status objects, which allow convenient access to logback's internal state.

Status data

Enabling status output is often helpful when diagnosing issues with Logback.

Errors can also occur after configuration, for example when a disk is full or log files cannot be archived due to wrong permissions. Registering a StatusListener is recommended; it receives status events as they occur and facilitates troubleshooting.

The next example illustrates the installation of OnConsoleStatusListener.

Example: Registering a status listener (logback-examples/​src/main/resources/​chapters/configuration/​onConsoleStatusListener.​xml)

<configuration>
   <!-- Recommendation: place status listeners towards the the top of the configuration file -->
   <statusListener class="ch.qos.logback.​core.status.OnConsoleStatusListener" />

   <!-- ... the rest of the configuration file -->

</configuration>

A StatusListener can be installed using a configuration file assuming that:

1. the configuration file is found
2. the configuration file is well-formed XML.

If any of these two conditions is not fulfilled, Joran cannot interpret the configuration file and in particular the <statusListener/> element. If the configuration file is found but is malformed, then logback will detect the error condition and automatically print its internal status on the console. However, if the configuration file cannot be found, logback will not automatically print its status data, since this is not necessarily an error condition. Programmatically invoking StatusPrinter.print() as shown in the MyApp2 application above ensures that status information is printed in every case.

Forcing status output In the absence of status messages, tracking down a rogue logback.xml configuration file can be difficult, especially in production where the application source cannot be easily modified. To help identify the location of a rogue configuration file, you can set a StatusListener via the "logback.statusListenerClass" system property (discussed below) to force output of status messages. The "logback.statusListenerClass" system property can also be used to silence output automatically generated in case of errors.

Shorthand

As a shorthand, it is possible to register an OnConsoleStatusListener by setting the debug attribute to true within the <configuration/> element as shown below.

Example: Basic configuration file using debug mode (logback-examples/​src/main/resources/​chapters/configuration/​sample1.xml)

<configuration debug="true">

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <!-- encoders are  by default assigned the type
         ch.qos.logback.classic.encoder.PatternLayoutEncoder -->
    <encoder>
      <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} -%kvp- %msg%n</pattern>
    </encoder>
  </appender>

  <root level="debug">
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

By the way, setting debug="true" is strictly equivalent to installing an OnConsoleStatusListener as shown previously.

"logback.statusListenerClass" system property

One may also register a status listener by setting the "logback.statusListenerClass" Java system property to the name of the listener class you wish to register. For example,

java -Dlogback.statusListenerClass=ch.qos.logback.​core.status.OnConsoleStatusListener .​...

As a shorthand, the string "ch.qos.logback.​core.status.OnConsoleStatusListener" can be replaced by the case insensitive strings "STDOUT" or "SYSOUT". For example:

java -Dlogback.statusListenerClass=stdout ...

or equivalently:

java -Dlogback.statusListenerClass=sysout ...

Other StatusListener implementations

Logback ships with several status listener implementations. OnConsoleStatusListener prints incoming status messages on the console, i.e. on System.out. OnErrorConsoleStatusListener prints incoming status messages on System.err. NopStatusListener drops incoming status messages.

Note that automatic status printing (in case of errors) is disabled if any status listener is registered during configuration and in particular if the user specifies a status listener via the "logback.statusListenerClass" system property. Thus, by setting NopStatusListener as a status listener, you can silence internal status printing altogether.

java -Dlogback.statusListenerClass=ch.qos.logback.​core.status.NopStatusListener .​..

Viewing status messages

Logback collects its internal status data in a StatusManager object, accessible via the LoggerContext.

Given a StatusManager you can access all the status data associated with a logback context. To keep memory usage at reasonable levels, the default StatusManager implementation stores the status messages in two separate parts: the header part and the tail part. The header part stores the first H status messages whereas the tail part stores the last T messages. At present time H=T=150, although these values may change in future releases.

Logback-classic ships with a servlet called ViewStatusMessagesServlet. This servlet prints the contents of the StatusManager associated with the current LoggerContext as an HTML table. Here is sample output.

To add this servlet to your web-application, add the following lines to its WEB-INF/web.xml file.

  <servlet>
    <servlet-name>ViewStatusMessages</servlet-name>
    <servlet-class>ch.qos.logback.classic.ViewStatusMessagesServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>ViewStatusMessages</servlet-name>
    <url-pattern>/lbClassicStatus</url-pattern>
  </servlet-mapping>

The ViewStatusMessages servlet will be viewable at the URL http://host/yourWebapp/lbClassicStatus

Listening to status messages via code

You may also register a StatusListener via Java code. Here is sample code.

   LoggerContext lc = (LoggerContext) LoggerFactory.getILoggerFactory(​);
   StatusManager statusManager = lc.getStatusManager();
   OnConsoleStatusListener onConsoleListener = new OnConsoleStatusListener();
   statusManager.add(​onConsoleListener);

Note that the registered status listener will only receive status events subsequent to its registration. It will not receive prior messages. Thus, it is usually a good idea to place status listener registration directives at the top of the configuration file before other directives.

Setting the location of the configuration file via a system property

You may specify the location of the default configuration file with a system property named "logback.configurationFile". The value of this property can be a URL, a resource on the class path or a path to a file external to the application.

java -Dlogback.configurationFile=​/path/to/config.​xml chapters.configuration.​MyApp1

Note that the file extension must be ".xml" or ".groovy". Other extensions are ignored. Explicitly registering a status listener may help debugging issues with locating the configuration file.

Given that "logback.configurationFile" is a Java system property, it may be set within your application as well. However, the system property must be set before any logger instance is created.

import ch.qos.logback.classic.util.ContextInitializer;

public class ServerMain {
    public static void main(String args[]) throws IOException, InterruptedException {
       // must be set before the first call to  LoggerFactory.getLogger();
       // ContextInitializer.CONFIG_FILE_PROPERTY is set to "logback.configurationFile"
       System.setProperty(ContextInitializer.CONFIG_FILE_PROPERTY, "/path/to/config.xml");
       ...
    }
}

Automatically reloading configuration file upon modification

Logback-classic can scan for changes in its configuration file and automatically reconfigure itself when the configuration file changes.

If instructed to do so, logback-classic will scan for changes in its configuration file and automatically reconfigure itself when the configuration file changes. In order to instruct logback-classic to scan for changes in its configuration file and to automatically re-configure itself set the scan attribute of the <configuration> element to true, as shown next.

Example: Scanning for changes in configuration file and automatic re-configuration (logback-examples/​src/main/resources/​chapters/configuration/​scan1.xml)

<configuration scan="true">
  ...
</configuration>

By default, the configuration file will be scanned for changes once every minute. You can specify a different scanning period by setting the scanPeriod attribute of the <configuration> element. Values can be specified in units of milliseconds, seconds, minutes or hours. Here is an example:

Example: Specifying a different scanning period (logback-examples/​src/main/resources/​chapters/configuration/​scan2.xml)

<configuration scan="true" scanPeriod="30 seconds" >
  ...
</configuration> 

Note If no unit of time is specified, then the unit of time is assumed to be milliseconds, which is usually inappropriate. If you change the default scanning period, do not forget to specify a time unit.

Behind the scenes, when you set the scan attribute to true, a ReconfigureOnChangeTask will be installed. This task run in a separate thread and will check whether your configuration file has changed. ReconfigureOnChangeTask will automatically watch for any included files as well.

As it is easy to make errors while editing a configuration file, in case the latest version of the configuration file has XML syntax errors, it will fall back to a previous configuration file free of XML syntax errors.

Since 1.5.9 It is possible to watch for modifications in configuration files in properties format. See PropertiesConfigurator for further details.

Enabling packaging data in stack traces

While useful, packaging data is expensive to compute, especially in applications that frequently throw exceptions.

NOTE As of version 1.1.4, packaging data is disabled by default.

If instructed to do so, logback can include packaging data for each stack trace line it outputs. Packaging data consists of the name and version of the jar file whence the class of the stack trace line originated. Packaging data can be very useful in identifying software versioning issues. However, it is rather expensive to compute, especially in applications where exceptions are thrown frequently. Here is a sample output:

14:28:48.835 [btpool0-7] INFO  c.q.l.demo.prime.PrimeAction - 99 is not a valid value
java.lang.Exception: 99 is invalid
  at ch.qos.logback.demo.prime.PrimeAction.execute(PrimeAction.java:28) [classes/:na]
  at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:431) [struts-1.2.9.jar:1.2.9]
  at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236) [struts-1.2.9.jar:1.2.9]
  at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432) [struts-1.2.9.jar:1.2.9]
  at javax.servlet.http.HttpServlet.service(HttpServlet.java:820) [servlet-api-2.5-6.1.12.jar:6.1.12]
  at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:502) [jetty-6.1.12.jar:6.1.12]
  at ch.qos.logback.demo.UserServletFilter.doFilter(UserServletFilter.java:44) [classes/:na]
  at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1115) [jetty-6.1.12.jar:6.1.12]
  at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:361) [jetty-6.1.12.jar:6.1.12]
  at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:417) [jetty-6.1.12.jar:6.1.12]
  at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:230) [jetty-6.1.12.jar:6.1.12]

Packaging data is disabled by default but can be enabled by configuration:

<configuration packagingData="true">
  ...
</configuration>

Alternatively, packaging data can be enabled/disabled programmatically by invoking the setPackagingDataEnabled(boolean) method in LoggerContext, as shown next:

  LoggerContext lc = (LoggerContext) LoggerFactory.getILoggerFactory();
  lc.setPackagingDataEnabled(true);

Invoking JoranConfigurator directly

Logback relies on a configuration library called Joran, part of logback-core. Logback's default configuration mechanism invokes JoranConfigurator on the default configuration file it finds on the class path. If you wish to override logback's default configuration mechanism for whatever reason, you can do so by invoking JoranConfigurator directly. The next application, MyApp3, invokes JoranConfigurator on a configuration file passed as a parameter.

Example: Invoking JoranConfigurator directly (logback-examples/src/​main/java/chapters/​configuration/MyApp3.java)

package chapters.configuration;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import ch.qos.logback.classic.LoggerContext;
import ch.qos.logback.classic.joran.JoranConfigurator;
import ch.qos.logback.core.joran.spi.JoranException;
import ch.qos.logback.core.util.StatusPrinter;

public class MyApp3 {
  final static Logger logger = LoggerFactory.getLogger(MyApp3.class);

  public static void main(String[] args) {
    // assume SLF4J is bound to logback in the current environment
    LoggerContext context = (LoggerContext) LoggerFactory.getILoggerFactory();

    try {
      JoranConfigurator configurator = new JoranConfigurator();
      configurator.setContext(context);
      // Call context.reset() to clear any previous configuration, e.g. default
      // configuration. For multi-step configuration, omit calling context.reset().
      context.reset();
      configurator.doConfigure(args[0]);
    } catch (JoranException je) {
      // StatusPrinter will handle this
    }
    StatusPrinter.printInCaseOfErrorsOrWarnings(context);

    logger.info("Entering application.");

    Foo foo = new Foo();
    foo.doIt();
    logger.info("Exiting application.");
  }
}

This application fetches the LoggerContext currently in effect, creates a new JoranConfigurator, sets the context on which it will operate, resets the logger context, and then finally asks the configurator to configure the context using the configuration file passed as a parameter to the application. Internal status data is printed in case of warnings or errors. Note that for multi-step configuration, context.reset() invocation should be omitted.

Stopping logback-classic

In order to release the resources used by logback-classic, it is always a good idea to stop the logback context. Stopping the context will close all appenders attached to loggers defined by the context and stop any active threads in an orderly way. Please also read the section on "shutdown hooks" just below.

import org.slf4j.LoggerFactory;
import ch.qos.logback.classic.LoggerContext;
...

// assume SLF4J is bound to logback-classic in the current environment
LoggerContext loggerContext = (LoggerContext) LoggerFactory.getILoggerFactory();
loggerContext.stop();

In web-applications the above code could be invoked from within the contextDestroyed method of Servlet​ContextListener in order to stop logback-classic and release resources. Starting with version 1.1.10, the appropriate ServletContextListener is installed automatically for you (see just below).

Stopping logback-classic via a shutdown hook

Installing a JVM shutdown hook is a convenient way for shutting down logback and releasing associated resources.

<configuration debug="true">
   <!-- in the absence of the class attribute, assume
   ch.qos.logback.core.hook.DefaultShutdownHook -->
   <shutdownHook/>

   <!-- rest of the config file.. -->

</configuration>

Note that you may install a shutdown hook of your own making by setting the class attribute to correspond to your shutdown hook's class name.

The default shutdown hook, namely DefaultShutdownHook, will stop the logback context after a specified delay (0 by default). Stopping the context will allow up to 30 seconds for any log file compression tasks running in the background to finish. In standalone Java applications, adding a <shutdownHook/> directive to your configuration file is an easy way to ensure that any ongoing compression tasks are allowed to finish before JVM exit. In applications within a Web server, webShutdownHook will be installed automatically making <shutdownHook/> directive quite redundant and unnecessary.

As of version 1.5.7, installing a shutdown hook replaces any previously installed hook. Therefore, at most one Logback shutdown hook can be installed via the usual logback.xml configuration.

WebShutdownHook or stopping logback-classic in web-applications

since 1.1.10 Logback-classic will automatically ask the web-server to install a LogbackServlet​ContainerInitializer implementing the ServletContainer​Initializer interface (available in servlet-api 3.x and later). This initializer will in turn install an instance of LogbackServlet​ContextListener.w This listener will stop the current logback-classic context when the web-app is stopped or reloaded.

You may disable automatic installation of LogbackServletContextListener by setting a <context-param> named logbackDisableServletContainerInitializer in your web application's WEB-INF/web.xml. Here is the relevant snippet.

<web-app>
    <context-param>
        <param-name>logbackDisableServletContainerInitializer</param-name>
        <param-value>true</param-value>
    </context-param>
    ....
</web-app>

Note that logbackDisableServlet​ContainerInitializer variable can also be set as a Java system property or an OS environment variable. The most local setting has priority, i.e. web-app first, system property second and OS environment last.

Configuration file syntax

As you have seen thus far in the manual with plenty of examples still to follow, logback allows you to redefine logging behavior without needing to recompile your code. Indeed, you can easily configure logback so as to disable logging for certain parts of your application, or direct output to a UNIX Syslog daemon, to a database, to a log visualizer, or forward logging events to a remote logback server, which would log according to local server policy, for example by forwarding the log event to a second logback server.

The remainder of this section presents the syntax of configuration files.

As will be demonstrated over and over, the syntax of logback configuration files is extremely flexible. As such, it is not possible to specify the allowed syntax with a DTD file or an XML schema. Nevertheless, the very basic structure of the configuration file can be described as, <configuration> element, containing zero or more <appender> elements, followed by zero or more <logger> elements, followed by at most one <root> element. The following diagram illustrates this basic structure.

If you are unsure which case to use for a given tag name, just follow the camelCase convention which is almost always the correct convention.

Case sensitivity of tag names

Since logback version 0.9.17, tag names pertaining to explicit rules are case-insensitive. For example, <logger>, <Logger> and <LOGGER> are valid configuration elements and will be interpreted in the same way. Note that XML well-formedness rules still apply, if you open a tag as <xyz> you must close it as </xyz>, </XyZ> will not work. As for implicit rules, tag names are case-sensitive except for the first letter. Thus, <xyz> and <Xyz> are equivalent but not <xYz>. Implicit rules usually follow the camelCase convention, common in the Java world. Since it is not easy to tell when a tag is associated with an explicit action and when it is associated with an implicit action, it is not trivial to say whether an XML tag is case-sensitive or insensitive with respect to the first letter. If you are unsure which case to use for a given tag name, just follow the camelCase convention which is almost always the correct convention.

Configuring loggers, or the <logger> element

At this point you should have at least some understanding of level inheritance and the basic selection rule. Otherwise, and unless you are an egyptologist, logback configuration will be no more meaningful to you than are hieroglyphics.

A logger is configured using the <logger> element. A <logger> element takes exactly one mandatory name attribute, an optional level attribute, and an optional additivity attribute, admitting the values true or false. The value of the level attribute admitting one of the case-insensitive string values TRACE, DEBUG, INFO, WARN, ERROR, ALL or OFF. The special case-insensitive value INHERITED, or its synonym NULL, will force the level of the logger to be inherited from higher up in the hierarchy. This comes in handy if you set the level of a logger and later decide that it should inherit its level.

Note that unlike log4j, logback-classic does not close nor remove any previously referenced appenders when configuring a given logger.

The <logger> element may contain zero or more <appender-ref> elements; each appender thus referenced is added to the named logger. Note that unlike log4j, logback-classic does not close nor remove any previously referenced appenders when configuring a given logger.

Configuring the root logger, or the <root> element

The <root> element configures the root logger. It supports a single attribute, namely the level attribute. It does not allow any other attributes because the additivity flag does not apply to the root logger. Moreover, since the root logger is already named as "ROOT", it does not allow a name attribute either. The value of the level attribute can be one of the case-insensitive strings TRACE, DEBUG, INFO, WARN, ERROR, ALL or OFF. Note that the level of the root logger cannot be set to INHERITED or NULL.

Note that unlike log4j, logback-classic does not close nor remove any previously referenced appenders when configuring the root logger.

Similarly to the <logger> element, the <root> element may contain zero or more <appender-ref> elements; each appender thus referenced is added to the root logger. Note that unlike log4j, logback-classic does not close nor remove any previously referenced appenders when configuring the root logger.

Example

Setting the level of a logger or root logger is as simple as declaring it and setting its level, as the next example illustrates. Suppose we are no longer interested in seeing any DEBUG messages from any component belonging to the "chapters.configuration" package. The following configuration file shows how to achieve that.

Example: Setting the level of a logger (logback-examples/​src/main/resources/​chapters/configuration/​sample2.xml)

<configuration>

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <!-- encoders are assigned the type
         ch.qos.logback.classic.encoder.PatternLayoutEncoder by default -->
    <encoder>
      <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} -%kvp- %msg%n</pattern>
    </encoder>
  </appender>

  <logger name="chapters.configuration" level="INFO"/>

  <!-- Strictly speaking, the level attribute is not necessary since -->
  <!-- the level of the root level is set to DEBUG by default.       -->
  <root level="DEBUG">
    <appender-ref ref="STDOUT" />
  </root>

</configuration>

When the above configuration file is given as argument to the MyApp3 application, it will yield the following output:

17:34:07.578 [main] INFO  chapters.configuration.MyApp3 -- Entering application.
17:34:07.578 [main] INFO  chapters.configuration.MyApp3 -- Exiting application.

Note that the message of level DEBUG generated by the "chapters.configuration.Foo" logger has been suppressed. See also the Foo class.

You can configure the levels of as many loggers as you wish. In the next configuration file, we set the level of the chapters.configuration logger to INFO but at the same time set the level of the chapters.configuration.Foo logger to DEBUG.

Example: Setting the level of multiple loggers (logback-examples/​src/main/resources/​chapters/configuration/​sample3.xml)

<configuration>

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>
        %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} -%kvp- %msg%n
     </pattern>
    </encoder>
  </appender>

  <logger name="chapters.configuration" level="INFO" />
  <logger name="chapters.configuration.Foo" level="DEBUG" />

  <root level="DEBUG">
    <appender-ref ref="STDOUT" />
  </root>

</configuration>

Running MyApp3 with this configuration file will result in the following output on the console:

17:39:27.593 [main] INFO  chapters.configuration.MyApp3 -- Entering application.
17:39:27.593 [main] DEBUG chapters.configuration.Foo -- Did it again!
17:39:27.593 [main] INFO  chapters.configuration.MyApp3 -- Exiting application.

The table below list the loggers and their levels, after JoranConfigurator has configured logback with the sample3.xml configuration file.

It follows that the two logging statements of level INFO in the MyApp3 class as well as the DEBUG messages in Foo.doIt() are all enabled. Note that the level of the root logger is always set to a non-null value, DEBUG by default.

Let us note that the basic-selection rule depends on the effective level of the logger being invoked, not the level of the logger where appenders are attached. Logback will first determine whether a logging statement is enabled or not, and if enabled, it will invoke the appenders found in the logger hierarchy, regardless of their level. The configuration file sample4.xml is a case in point:

Example: Logger level sample (logback-examples/​src/main/resources/​chapters/configuration/​sample4.xml)

<configuration>

  <appender name="STDOUT"
   class="ch.qos.logback.core.ConsoleAppender">
   <encoder>
     <pattern>
        %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} -%kvp- %msg%n
      </pattern>
    </encoder>
  </appender>

  <logger name="chapters.configuration" level="INFO" />
 
  <!-- turn OFF all logging (children can override) -->
  <root level="OFF">
    <appender-ref ref="STDOUT" />
  </root>

</configuration>

The following table lists the loggers and their levels after applying the sample4.xml configuration file.

The ConsoleAppender named STDOUT, the only configured appender in sample4.xml, is attached to the root logger whose level is set to OFF. However, running MyApp3 with configuration script sample4.xml will yield:

17:52:23.609 [main] INFO chapters.configuration.MyApp3 - Entering application.
17:52:23.609 [main] INFO chapters.configuration.MyApp3 - Exiting application.

Thus, the level of the root logger has no apparent effect because the loggers in chapters.configuration.​MyApp3 and chapters.configuration.Foo classes are all enabled for the INFO level. As a side note, the chapters.configuration logger exists by virtue of its declaration in the configuration file - even if the Java source code does not directly refer to it.

Configuring Appenders

An appender is configured with the <appender> element, which takes two mandatory attributes name and class. The name attribute specifies the name of the appender whereas the class attribute specifies the fully qualified name of the appender class to instantiate. The <appender> element may contain zero or one <layout> elements, zero or more <encoder> elements and zero or more <filter> elements. Apart from these three common elements, <appender> elements may contain any number of elements corresponding to JavaBean properties of the appender class. Seamlessly supporting any property of a given logback component is one of the major strengths of Joran as discussed in a later chapter. The following diagram illustrates the common structure. Note that support for properties is not shown in the diagram below.

The <layout> element takes a mandatory class attribute specifying the fully qualified name of the layout class to instantiate. As with the <appender> element, <layout> may contain other elements corresponding to properties of the layout instance. Since it's such a common case, if the layout class is PatternLayout, then the class attribute can be omitted as specified by default class mapping rules.

The <encoder> element takes a mandatory class attribute specifying the fully qualified name of the encoder class to instantiate. Since it's such a common case, if the encoder class is PatternLayoutEncoder, then the class attribute can be omitted as specified by default class mapping rules.

Logging to multiple appenders is as easy as defining the various appenders and referencing them in a logger, as the next configuration file illustrates:

Example: Multiple loggers (logback-examples/​src/main/resources/​chapters/configuration/​multiple.xml)

<configuration>

  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>myApp.log</file>

    <encoder>
      <pattern>%date %level [%thread] %logger{10} [%file:%line] -%kvp- %msg%n</pattern>
    </encoder>
  </appender>

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%kvp %msg%n</pattern>
    </encoder>
  </appender>

  <root level="debug">
    <appender-ref ref="FILE" />
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

These configuration scripts define two appenders called FILE and STDOUT. The FILE appender logs to a file called myApp.log. The encoder for this appender is a PatternLayoutEncoder that outputs the date, level, thread name, logger name, file name and line number where the log request is located, the message and line separator character(s). The second appender called STDOUT outputs to the console. The encoder for this appender outputs only the message string followed by a line separator.

The appenders are attached to the root logger by referencing them by name within an appender-ref element. Note that each appender has its own encoder. Encoders are usually not designed to be shared by multiple appenders. The same is true for layouts. As such, logback configuration files do not provide any syntactical means for sharing encoders or layouts.

Appenders accumulate

By default, appenders are cumulative: a logger will log to the appenders attached to itself (if any) as well as all the appenders attached to its ancestors. Thus, attaching the same appender to multiple loggers will cause logging output to be duplicated.

Example: Duplicate appender (logback-examples/​src/main/resources/​chapters/configuration/​duplicate.xml)

<configuration>

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} -%kvp- %msg%n</pattern>
    </encoder>
  </appender>

  <logger name="chapters.configuration">
    <appender-ref ref="STDOUT" />
  </logger>

  <root level="debug">
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

Running MyApp3 with duplicate.xml will yield the following output:

14:25:36.343 [main] INFO chapters.configuration.MyApp3 -- Entering application. 14:25:36.343 [main] INFO chapters.configuration.MyApp3 -- Entering application. 14:25:36.359 [main] DEBUG chapters.configuration.Foo -- Did it again! 14:25:36.359 [main] DEBUG chapters.configuration.Foo -- Did it again! 14:25:36.359 [main] INFO chapters.configuration.MyApp3 -- Exiting application. 14:25:36.359 [main] INFO chapters.configuration.MyApp3 -- Exiting application.

Notice the duplicated output. The appender named STDOUT is attached to two loggers, to root and to chapters.configuration. Since the root logger is the ancestor of all loggers and chapters.configuration is the parent of both chapters.configuration.​MyApp3 and chapters.configuration.Foo, each logging request made with these two loggers will be output twice, once because STDOUT is attached to chapters.configuration and once because it is attached to root.

Appender additivity is not intended as a trap for new users. It is quite a convenient logback feature. For instance, you can configure logging such that log messages appear on the console (for all loggers in the system) while messages only from some specific set of loggers flow into a specific appender.

Example: Multiple appender (logback-examples/​src/main/resources/​chapters/​configuration/​restricted.xml)

<configuration>

  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>myApp.log</file>
    <encoder>
      <pattern>%date %level [%thread] %logger{10} [%file:%line] -%kvp- %msg%n</pattern>
    </encoder>
  </appender>

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%kvp %msg%n</pattern>
    </encoder>
  </appender>

  <logger name="chapters.configuration">
    <appender-ref ref="FILE" />
  </logger>

  <root level="debug">
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

In this example, the console appender will log all the messages (for all loggers in the system) whereas only logging requests originating from the chapters.configuration logger and its children will go into the myApp.log file.

Overriding the default cumulative behaviour

In case the default cumulative behavior turns out to be unsuitable for your needs, you can override it by setting the additivity flag to false. Thus, a branch in your logger tree may direct output to a set of appenders different from those of the rest of the tree.

Example: Additivity flag (logback-examples/​src/main/resources/​chapters/configuration/​additivityFlag.​xml)

<configuration>

  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>foo.log</file>
    <encoder>
      <pattern>%date %level [%thread] %logger{10} [%file : %line] -%kvp- %msg%n</pattern>
    </encoder> 
  </appender>

  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%msg%n</pattern>
    </encoder>
  </appender>

  <logger name="chapters.configuration.Foo" additivity="false">
    <appender-ref ref="FILE" />
  </logger>

  <root level="debug">
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

This example, the appender named FILE is attached to the chapters.configuration.Foo logger. Moreover, the chapters.configuration.Foo logger has its additivity flag set to false such that its logging output will be sent to the appender named FILE but not to any appender attached higher in the hierarchy. Other loggers remain oblivious to the additivity setting of the chapters.configuration.Foo logger. Running the MyApp3 application with the additivityFlag.xml configuration file will output results on the console from the chapters.configuration.​MyApp3 logger. However, output from the chapters.configuration.Foo logger will appear in the foo.log file and only in that file.

Setting the context name

As mentioned in an earlier chapter, every logger is attached to a logger context. By default, the logger context is called "default". However, you can set a different name with the help of the <contextName> configuration directive. Note that once set, the logger context name cannot be changed. Setting the context name is a simple and straightforward method in order to distinguish between multiple applications logging to the same target.

Example: Set the context name and display it (logback-examples/​src/main/resources/​chapters/configuration/​contextName.xml)

<configuration>
  <contextName>myAppName</contextName>
  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%d %contextName [%t] %level %logger{36} -%kvp- %msg%n</pattern>
    </encoder>
  </appender>

  <root level="debug">
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

This last example illustrates naming of the logger context. Adding the contextName conversion word in layout's pattern will output the said name.

Variable substitution

Note Earlier versions of this document used the term "property substitution" instead of the term "variable". Please consider both terms interchangeable although the latter term conveys a clearer meaning.

As in many scripting languages, logback configuration files support definition and substitution of variables. Variables have a scope (see below). Moreover, variables can be defined within the configuration file itself, in an external file, in an external resource or even computed and defined on the fly.

Variable substitution can occur at any point in a configuration file where a value can be specified.

Variable substitution can occur at any point in a configuration file where a value can be specified. The syntax of variable substitution is similar to that of Unix shells. The string between an opening ${ and closing } is interpreted as a reference to the value of the property. For property aName, the string "${aName}" will be replaced with the value held by the aName property.

As they often come in handy, the HOSTNAME and CONTEXT_NAME variables are automatically defined and have context scope. Given that in some environments it may take some time to compute the hostname, its value is computed lazily (only when needed). Moreover, HOSTNAME can be set from within the configuration directly.

Defining variables

Variables can be defined one at a time in the configuration file itself or loaded wholesale from an external properties file or an external resource. For historical reasons, the XML element for defining variables is <property> although in logback 1.0.7 and later the element <variable> can be used interchangeably.

The next example shows a variable declared at the beginning of the configuration file. It is then used further down the file to specify the location of the output file.

Example: Simple Variable substitution (logback-examples/​src/main/resources/​chapters/configuration/​variableSubstitution1.​xml)

<configuration>

  <variable name="USER_HOME" value="/home/sebastien" />

  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>${USER_HOME}/myApp.log</file>
    <encoder>
      <pattern>%kvp %msg%n</pattern>
    </encoder>
  </appender>

  <root level="debug">
    <appender-ref ref="FILE" />
  </root>
</configuration>

The next example shows the use of a System property to achieve the same result. The property is not declared in the configuration file, thus logback will look for it in the System properties. Java system properties can be set on the command line as shown next:

java -DUSER_HOME="/home/sebastien" MyApp2

Example: System Variable substitution (logback-examples/​src/main/resources/​chapters/configuration/​variableSubstitution2.​xml)

<configuration>

  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>${USER_HOME}/myApp.log</file>
    <encoder>
      <pattern>%kvp %msg%n</pattern>
    </encoder>
  </appender>

  <root level="debug">
    <appender-ref ref="FILE" />
  </root>
</configuration>

When multiple variables are needed, it may be more convenient to create a separate file that will contain all the variables. Here is how one can do such a setup.

Example: Variable substitution using a separate file (logback-examples/​src/main/resources/​chapters/configuration/​variableSubstitution3.​xml)

<configuration>

  <variable file="src/main/java/chapters/configuration/variables1.properties" />

  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
     <file>${USER_HOME}/myApp.log</file>
     <encoder>
       <pattern>%kvp %msg%n</pattern>
     </encoder>
   </appender>

   <root level="debug">
     <appender-ref ref="FILE" />
   </root>
</configuration>

This configuration file contains a reference to a file named variables1.properties. The variables contained in that file will be read and then defined within local scope. Here is what the variable.properties file might look like.

USER_HOME=/home/sebastien

You may also reference a resource on the class path instead of a file.

<configuration>

  <variable resource="resource1.properties" />

  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
     <file>${USER_HOME}/myApp.log</file>
     <encoder>
       <pattern>%kvp %msg%n</pattern>
     </encoder>
   </appender>

   <root level="debug">
     <appender-ref ref="FILE" />
   </root>
</configuration>

Scopes

A property can be defined for insertion in local scope, in context scope, or in system scope. Local scope is the default. Although it is possible to read variables from the OS environment, it is not possible to write into the OS environment.

local scope A property with local scope exists from the point of its definition in a configuration file until the end of interpretation/execution of said configuration file. As a corollary, each time a configuration file is parsed and executed, variables in local scope are defined anew.

context scope A property with context scope is inserted into the context and lasts as long as the context or until it is cleared. Once defined, a property in context scope is part of the context. As such, it is available in all logging events, including those sent to remote hosts via serialization.

system scope A property with system scope is inserted into the JVM's system properties and lasts as long as the JVM or until it is cleared.

Properties are looked up in the local scope first, in the context scope second, in the system properties scope third, and in the OS environment last.

During substitution, properties are looked up in the local scope first, in the context scope second, in the system properties scope third, and in the OS environment fourth and last.

The scope attribute of the <property> element, <define> element or the <insertFromJNDI> element can be used to set the scope of a property. The scope attribute admits "local", "context" and "system" strings as possible values. If not specified, the scope is always assumed to be "local".

Example: A variable defined in "context" scope (logback-examples/​src/main/resources/​chapters/configuration/​contextScopedVariable.​xml)

<configuration>

  <variable scope="context" name="nodeId" value="firstNode" />

  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>/opt/${nodeId}/myApp.log</file>
    <encoder>
      <pattern>%kvp %msg%n</pattern>
    </encoder>
  </appender>

  <root level="debug">
    <appender-ref ref="FILE" />
  </root>
</configuration>

In the above example, given that the nodeId property is defined in the context scope, it will be available in every logging event, even those sent to remote hosts via serialization.

Default values for variables

Under certain circumstances, it may be desirable for a variable to have a default value if it is not declared or its value is null. As in the Bash shell, default values can be specified using the ":-" operator. For example, assuming the variable named aName is not defined, "${aName:-golden}" will be interpreted as "golden".

Nested variables

Variable nesting is fully supported. Both the name, default-value and value definition of a variable can reference other variables.

value nesting

The value definition of a variable can contain references to other variables. Suppose you wish to use variables to specify not only the destination directory but also the file name, and combine those two variables in a third variable called "destination". The properties file shown below gives an example.

Example: Nested variable references (logback-examples/​src/main/resources/​chapters/configuration/​variables2.properties)

USER_HOME=/home/sebastien
fileName=myApp.log
destination=${USER_HOME}/${fileName}

Note that in the properties file above, "destination" is composed from two other variables, namely "USER_HOME" and "fileName".

<configuration>

  <variable file="variables2.properties" />

  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>${destination}</file>
    <encoder>
      <pattern>%kvp %msg%n</pattern>
    </encoder>
  </appender>

  <root level="debug">
    <appender-ref ref="FILE" />
  </root>
</configuration>

name nesting

When referencing a variable, the variable name may contain a reference to another variable. For example, if the variable named "userid" is assigned the value "alice", then "${${userid}.password}" references the variable with the name "alice.password".

default value nesting

The default value of a variable can reference another variable. For example, assuming the variable 'id' is unassigned and the variable 'userid' is assigned the value "alice", then the expression "${id:-${userid}}" will return "alice".

HOSTNAME property

As it often comes in handy, the HOSTNAME property is defined automatically during configuration with context scope.

CONTEXT_NAME property

As its name indicates, the CONTEXT_NAME property corresponds to the name of the current logging context.

Setting a timestamp

The timestamp element can define a property according to current date and time. The timestamp element is explained in a subsequent chapter.

Defining variables, aka properties, on the fly

You may define properties dynamically using the <define> element. The define element takes two mandatory attributes: name and class. The name attribute designates the name of the property to set whereas the class attribute designates any class implementing the PropertyDefiner interface. The value returned by the getPropertyValue() method of the PropertyDefiner instance will be the value of the named property. You may also specify a scope for the named property by specifying a scope attribute.

Here is an example.

<configuration>

  <define name="rootLevel" class="a.class.implementing.PropertyDefiner">
    <shape>round</shape>
    <color>brown</color>
    <size>24</size>
  </define>

  <root level="${rootLevel}"/>
</configuration>

In the above example, shape, color and size are properties of "a.class.implementing.​PropertyDefiner". As long as there is a setter for a given property in your implementation of the PropertyDefiner instance, logback will inject the appropriate values as specified in the configuration file.

At the present time, logback ships with some fairly simple implementations of PropertyDefiner.

Conditional processing of configuration files

Developers often need to juggle between several logback configuration files targeting different environments such as development, testing and production. These configuration files have substantial parts in common differing only in a few places. To avoid duplication, logback supports conditional processing of configuration files with the help of <condition>, <if>, <then> and <else> elements so that a single configuration file can adequately target several environments.

The general format for conditional statements is shown below.

   <!-- condition-if-then form -->
   <condition class="a.class.implementing.PropertyCondition">
      <!-- additional parameters if needed -->
   </condition> 
   <!-- if-then form -->
   <if>
    <then>
      ...
    </then>
  </if>

  <!-- condition-if-then-else form -->
  <condition class="a.class.implementing.ProppertyCondition">
      <!-- additional parameters if needed -->
  </condition> 
  <if>
    <then>
      ...
    </then>
    <else>
      ...
    </else>
  </if>

Since 1.5.20 Note that the <condition> element precedes the <if>/<then>/<else> elements. This syntax is similar to conditional expressions in Smalltalk. Conditional processing using the <condition> element was introduced in Logback version 1.5.20.

If the <condition> evaluates to true, the <then> part is activated and if false, the <else> part is activated, if present.

Conditional processing is supported anywhere within the <configuration> element. Nested condition-if-then-else statements are also supported. However, XML syntax is cumbersome and is ill-suited as the foundation of a general purpose programming language. Thus, conditionals should be used sparingly.

The class passed to the <condition> element must implement the PropertyCondition interface.

Logback-core ships with basic condition implementations derived from PropertyConditionBase in the ch.qos.logback.core.boolex package.

Here is an example which checks if the HOSTNAME property equals "torino".

<configuration debug="true">

  <condition class="ch.qos.logback.core.boolex.PropertyEqualityCondition">
    <key>HOSTNAME</key>
    <value>torino</value>
  </condition>
  <if>
    <then>
      <appender name="CON" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
          <pattern>%d %-5level %logger{35} -%kvp- %msg %n</pattern>
        </encoder>
      </appender>
      <root>
        <appender-ref ref="CON" />
      </root>
    </then>
  </if>

  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>${randomOutputDir}/conditional.log</file>
    <encoder>
      <pattern>%d %-5level %logger{35} -%kvp- %msg %n</pattern>
   </encoder>
  </appender>

  <root level="ERROR">
     <appender-ref ref="FILE" />
  </root>
</configuration>

ProppertyConditionBase class defines the following helper methods property(String key), p(String key), isDefined(String key), isDefined(String key), isNull(String key).

For a key passed as argument, the property(String key) or its shorter equivalent p(String key) methods return the value of the property corresponding to key. For example, to access the value of a property with key "k", you would write property("k") or equivalently p("k"). If the property with key "k" is undefined, the property method will return the empty string and not null. This avoids the need to check for null values.

The isDefined(String key) method can be used to check whether a property is defined. For example, to check whether the property "k" is defined you would write isDefined("k") Similarly, if you need to check whether a property is null, the isNull() method is provided. Example: isNull("k").

SequenceNumberGenerator

Since 1.3.0Logback supports a sequence number field which is automatically populated at event creation. This field is fed by a sequence number generator attached to the logging context.

A sequence number generator is set as follows:

<configuration>
    <sequenceNumberGenerator class="ch.qos.logback.core.spi.BasicSequenceNumberGenerator"/>

     <!-- rest of the configuration file .... -->
</configuration>