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<h2>
<a name="top">Frequently Asked Questions</a>
</h2>
<h3>Logback project</h3>
<ol>
<li><a href="#why_lgpl">Why is logback distributed under LGPL
and not the Apache Software License?</a> </li>
<li><a href="#dependecy">What are logback's dependencies, i.e.
JDK version and third-party libraries?</a>
</li>
</ol>
<h3>Logback Classic</h3>
<ol type="1">
<li>
<a href="#logger_serialization">Are logback loggers
serializable?</a>
</li>
<li>
<a href="#auto_config">How does the automatic configuration
work?</a>
</li>
<li>
<a href="#configFileLocation">Where should the configuration
files such as <em>logback.groovy</em>,
<em>logback-test.xml</em> or <em>logback.xml</em> be located
on the classpath?</a> </li>
<li>
<a href="#sharedConfiguration">Is it possible for multiple JEE
applications to share the same configuration file but without
stepping on each other's toes?
</a>
</li>
<li>
<a href="#overrideFromCL">
How can I disable logging from the command line?
</a>
</li>
<li>
<a href="#setup_jetty">How can Jetty be instructed to use
logback-classic as its logging implementation?
</a>
</li>
</ol>
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<div class="section">
<h2>Logback project</h2>
<dl>
<dt>
<a name="why_lgpl" href="#why_lgpl">Why is logback
distributed under LGPL and not the Apache Software License
(ASL)?</a> </dt>
<dd>
<p>The logback project is dual licensed under the LGPL and
the EPL for two main reasons. For one, the different
license emphasizes that the fact that logback is a related
but <em>different</em> project than log4j.
</p>
<p>Both the LGPL and EPL are reasonable and widely-accepted
licenses. In contrast to the ASL, both the LGPL and he EPL
require that derivate work be licensed under the same
license. While there might be debate about the exact
definition of derivative work, we find such reciprocity both
justified and morally appealing -- that is the second
reason for our choice of the LGPL & EPL
dual-license. The subtly more liberal approach embodied in
the ASL is not necessarily wrong. It is the expression of a
different balance.
</p>
</dd>
</dl>
<hr/>
<dl>
<dt><a name="dependecy" href="#dependecy">What are logback's
dependencies, i.e. JDK version and third-party libraries?</a>
</dt>
<dd><p>This question is answered on a <a
href="dependencies.html">separate page</a> dedicated to the
question of dependencies.</p>
</dd>
</dl>
</div>
<div class="section">
<h2>Logback-classic</h2>
<dl>
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<dt>
<a name="auto_config" href="#auto_config">
How does the automatic configuration work?
</a>
</dt>
<dd>
<p>This question is answered in the <a
href="manual/configuration.html#auto_configuration">relevant
section</a> of the logback manual.
</p>
<hr/>
</dd>
<!-- ========================================================= -->
<dt>
<a name="configFileLocation"
href="#configFileLocation">Where should the configuration
files such as <em>logback.groovy</em>,
<em>logback-test.xml</em> or <em>logback.xml</em> be located
on the classpath?</a>
</dt>
<dd>
<p>Configuration files such as <em>logback.groovy</em>,
<em>logback-test.xml</em> or <em>logback.xml</em> can be
located <b>directly</b> under any folder declared in the
class path. For example, if the class path reads
"c:/java/jdk15/lib/rt.jar;c:/mylibs/" then the
<em>logback.xml</em> file should be located directly under
"c:/mylibs/", that is as "c:/mylibs/logback.xml". Placing it
under a sub-folder of c:/mylibs/, say, c:/mylibs/other/,
will not work.</p>
<p>For web-applications, configuration files can be placed
<b>directly</b> under <em>WEB-INF/classes/</em>.</p>
<hr/>
</dd>
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<dt>
<a name="logger_serialization" href="#logger_serialization">
Are logback loggers serializable?</a>
</dt>
<dd>
<p>Yes. A logback logger <em>is</em> an SLF4J logger and SLF4J
loggers are serializable. This means that an object
referencing a logger will be able to log after its
deserialization.
</p>
<p>The deserialized logger instance will be generated by
<code>org.slf4j.LoggerFactory</code>. Thus, it is possible
for a logback logger to be deserialized as a log4j or j.u.l.
logger, depending on the deserialization environment.</p>
<hr/>
</dd>
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<dt>
<a name="sharedConfiguration" href="#sharedConfiguration">
Is it possible for multiple JEE applications to share the
same configuration file but without stepping on each other's
toes?</a>
</dt>
<dd>
<p>Yes, it is. Using <a
href="manual/configuration.html#variableSubstitution">variable
substitution</a>, it is possible to have a single
configuration file to output logs to different destinations
for each JEE application. Here is a sample
configuration file designed for this purpose.</p>
<p class="source"><configuration>
<appender name="FILE" class="ch.qos.logback.core.FileAppender">
<!-- "application-name" is a variable -->
<File><b>c:/logs/${application-name}.log</b></File>
<layout class="ch.qos.logback.classic.PatternLayout">
<Pattern>%d %p %t %c - %m%n</Pattern>
</layout>
</appender>
<root level="debug">
<appender-ref ref="FILE"/>
</root>
</configuration> </p>
<p>Assuming each JEE application loads a different copy of
logback classes into memory, if we can somehow inject a
different value for <code><em>application-name</em></code>
each time an application starts, logs will be output to
different files. We just need to initialize logback with the
above configuration file while injecting a different value
for <code><em>application-name</em></code> variable. Here is
sample code that programmatically configures logback. It
should be invoked during the initialization of your JEE
applications.
</p>
<p class="source"> LoggerContext context = (LoggerContext) LoggerFactory.getILoggerFactory();
JoranConfigurator jc = new JoranConfigurator();
jc.setContext(context);
context.reset(); // override default configuration
// inject the name of the current application as "application-name"
// property of the LoggerContext
<b>context.putProperty("application-name", NAME_OF_CURRENT_APPLICATION);</b>
jc.doConfigure("/path/to/the/above/configuration/file.xml"); </p>
<hr/>
</dd>
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<dt>
<a name="overrideFromCL" href="#overrideFromCL">
How can I disable logging from the command line?
</a>
</dt>
<dd>
<p>Logback does not allow logging to be disabled from the command
line. However, if the configuration file allows it, you can
set the level of loggers on the command line via a Java
system property. Here is such a configuration file.</p>
<p class="source"><configuration>
<appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
<layout class="ch.qos.logback.classic.PatternLayout">
<Pattern>%d [%thread] %level %logger - %m%n</Pattern>
</layout>
</appender>
<root level="<b>${root-level:-INFO}</b>">
<appender-ref ref="CONSOLE"/>
</root>
</configuration></p>
<p>Making use of <a
href="manual/configuration.html#variableSubstitution">variable
substitution</a> as well as <a
href="manual/configuration.html#defaultValuesForVariables">default
values for variables</a>, if the <span
class="variable">root-level</span> system property is set to
<code>OFF</code>, then all logging will be turned
off. However, if it is not set, it will assume the default
value of <code>INFO</code>. Note that you can set the <span
class="variable">root-level</span> system property to any
level value of your choice. The value <code>OFF</code> is
just an example.
</p>
<hr/>
</dd>
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<dt>
<a name="setup_jetty" href="#">
How can Jetty be instructed to use logback-classic as its
logging implementataion?
</a>
</dt>
<dd>
<p>The Jetty application server uses SLF4J for its internal
logging.
</p>
<p>Logback jar files must be present on Jetty's class
path. These files are
<em>logback-core-1.0.9.jar</em> and
<em>logback-classic-1.0.9.jar</em>. These files
should be placed under the <em>$JETTY_HOME/lib</em>
directory.
</p>
<p>Since Jetty uses an older version of SLF4J internally,
we recommend that the old version be replaced by
<em>slf4j-api-1.7.2.jar</em>. This file can be
downloaded from the <a
href="http://www.slf4j.org/download.html">SLF4J</a> project.
</p>
<p>For automatically configuring logback-classic, you can
place the file <em>logback.xml</em> under the
<em>$JETTY_HOME/resources</em> directory. You can find
sample configuration files in the
<em>logback-examples/src/main/java/chapters/appenders/conf/</em>
folder shipping within the logback distribution.
</p>
<hr/>
</dd>
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