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<h2><a name="top">Frequently Asked Questions about SLF4J</a></h2>
<p><b>Generalities</b></p>
<ol type="1">
<li><a href="#what_is">What is SLF4J?</a></li>
<li><a href="#when">When should SLF4J be used?</a></li>
<li><a href="#yet_another_facade"> Is SLF4J yet another loggingfacade?</a></li>
<li><a href="#why_new_project"> If SLF4J fixes JCL, then why
wasn't the fix made in JCL instead of creating a new project?
</a>
</li>
<li><a href="#need_to_recompile"> When using SLF4J, do I have to
recompile my application to switch to a different logging
system?
</a>
</li>
<li><a href="#requirements">What are SLF4J's requirements?</a></li>
<li><a href="#compatibility">Are SLF4J versions backward
compatible?</a></li>
<li><a href="#IllegalAccessError">I am getting
<code>IllegalAccessError</code> exceptions when using SLF4J. Why
is that?</a></li>
<li>
<a href="#license">Why is SLF4J licensed under X11 type
license instead of the Apache Software License?
</a>
</li>
<li>
<a href="#where_is_binding">Where can I get a particular
SLF4J binding?
</a>
</li>
<li>
<a href="#configure_logging">Should my library attempt to
configure logging?
</a>
</li>
<li>
<a href="#optional_dependency">In order to reduce the number of
dependencies of our software we would like to make SLF4J an
optional dependency. Is that a good idea?
</a>
</li>
<li>
<a href="#maven2">What about Maven transitive
dependencies?
</a>
</li>
<li>
<a href="#excludingJCL">How do I exclude commons-logging as a
Maven dependency?
</a>
</li>
</ol>
<b>About the SLF4J API</b>
<ol type="1">
<li>
<a href="#string_or_object"> Why don't the printing methods
in the Logger interface accept message of type Object, but only
messages of type String?
</a>
</li>
<li>
<a href="#exception_message">
Can I log an exception without an accompanying message?
</a>
</li>
<li>
<a href="#logging_performance"> What is the fastest way of
(not) logging?
</a>
</li>
<li>
<a href="#string_contents"> How can I log the string contents
of a single (possibly complex) object?
</a>
</li>
<li><a href="#fatal"> Why doesn't the
<code>org.slf4j.Logger</code> interface have methods for the
FATAL level? </a></li>
<li><a href="#trace">Why was the TRACE level introduced only in
SLF4J version 1.4.0? </a></li>
<li><a href="#i18n">Does the SLF4J logging API support I18N
(internationalization)? </a></li>
<li><a href="#noLoggerFactory">Is it possible to retrieve loggers
without going through the static methods in
<code>LoggerFactory</code>? </a></li>
<li><a href="#paramException">In the presence of an
exception/throwable, is it possible to parameterize a logging
statement?</a></li>
</ol>
<b>Implementing the SLF4J API</b>
<ol type="1">
<li><a href="#slf4j_compatible">How do I make my logging
framework SLF4J compatible? </a></li>
<li><a href="#marker_interface">How can my logging system add
support for the <code>Marker</code> interface? </a></li>
<li><a href="#version_checks">How does SLF4J's version check
mechanism work? </a></li>
</ol>
<b>General questions about logging</b>
<ol type="1">
<li><a href="#declared_static"> Should Logger members of a class
be declared as static? </a></li>
<li><a href="#declaration_pattern">Is there a recommended idiom
for declaring a loggers in a class?</a></li>
</ol>
<h2>Generalities</h2>
<dl>
<dt class="doAnchor" name="what_is">What is SLF4J?</dt>
<dd>
<p>SLF4J is a simple facade for logging systems allowing the
end-user to plug-in the desired logging system at deployment
time.
</p>
</dd>
<dt class="doAnchor" name="when">When should SLF4J be used?</dt>
<dd>
<p>In short, libraries and other embedded components should
consider SLF4J for their logging needs because libraries cannot
afford to impose their choice of logging framework on the
end-user. On the other hand, it does not necessarily make sense
for stand-alone applications to use SLF4J. Stand-alone
applications can invoke the logging framework of their choice
directly. In the case of logback, the question is moot because
logback exposes its logger API via SLF4J.
</p>
<p>SLF4J is only a facade, meaning that it does not provide a
complete logging solution. Operations such as configuring
appenders or setting logging levels cannot be performed with
SLF4J. Thus, at some point in time, any non-trivial
application will need to directly invoke the underlying
logging system. In other words, complete independence from the
API underlying logging system is not possible for a
stand-alone application. Nevertheless, SLF4J reduces the
impact of this dependence to near-painless levels.
</p>
<p>Suppose that your CRM application uses log4j for its
logging. However, one of your important clients request that
logging be performed through JDK 1.4 logging. If your
application is riddled with thousands of direct log4j calls,
migration to JDK 1.4 would be a relatively lengthy and
error-prone process. Even worse, you would potentially need to
maintain two versions of your CRM software. Had you been
invoking SLF4J API instead of log4j, the migration could be
completed in a matter of minutes by replacing one jar file with
another.
</p>
<p>SLF4J lets component developers to defer the choice of the
logging system to the end-user but eventually a choice needs
to be made.
</p>
</dd>
<dt class="doAnchor" name="yet_another_facade">Is SLF4J yet
another logging facade?</dt>
<dd>
<p>SLF4J is conceptually very similar to JCL. As such, it can
be thought of as yet another logging facade. However, SLF4J is
much simpler in design and arguably more robust. In a
nutshell, SLF4J avoid the class loader issues that plague JCL.
</p>
</dd>
<dt class="doAnchor" name="why_new_project">If SLF4J fixes JCL,
then why wasn't the fix made in JCL instead of creating a new
project?
</dt>
<dd>
<p>This is a very good question. First, SLF4J static binding
approach is very simple, perhaps even laughably so. It was
not easy to convince developers of the validity of that
approach. It is only after SLF4J was released and started to
become accepted did it gain respectability in the relevant
community.
</p>
<p>Second, SLF4J offers two enhancements which tend to be
underestimated. Parameterized log messages solve an important
problem associated with logging performance, in a pragmatic
way. Marker objects, which are supported by the
<code>org.slf4j.Logger</code> interface, pave the way for
adoption of advanced logging systems and still leave the door
open to switching back to more traditional logging systems if
need be.
</p>
</dd>
<dt class="doAnchor" name="need_to_recompile">When using SLF4J, do
I have to recompile my application to switch to a different
logging system?
</dt>
<dd>
<p>No, you do not need to recompile your application. You can
switch to a different logging system by removing the previous
SLF4J binding and replacing it with the binding of your choice.
</p>
<p>For example, if you were using the NOP implementation and
would like to switch to log4j version 1.2, simply replace
<em>slf4j-nop.jar</em> with <em>slf4j-log4j12.jar</em> on
your class path but do not forget to add log4j-1.2.x.jar as
well. Want to switch to JDK 1.4 logging? Just replace
<em>slf4j-log4j12.jar</em> with <em>slf4j-jdk14.jar</em>.
</p>
</dd>
<dt class="doAnchor" name="requirements">What are SLF4J's
requirements?
</dt>
<dd>
<p>As of version 1.7.0, SLF4J requires JDK 1.5 or later. Earlier
SLF4J versions, namely SLF4J 1.4, 1.5. and 1.6, required JDK
1.4.
</p>
<p>&nbsp;</p>
<table class="bodyTable striped">
<tr align="left">
<th>Binding</th>
<th>Requirements</th>
</tr>
<tr>
<td>slf4j-nop</td>
<td>JDK 1.5</td>
</tr>
<tr>
<td>slf4j-simple</td>
<td>JDK 1.5</td>
</tr>
<tr>
<td>slf4j-log4j12</td>
<td align="left">JDK 1.5, plus any other library
dependencies required by the log4j appenders in use</td>
</tr>
<tr>
<td>slf4j-jdk14</td>
<td>JDK 1.5 or later</td>
</tr>
<tr>
<td>logback-classic</td>
<td>JDK 1.5 or later, plus any other library dependencies
required by the logback appenders in use</td>
</tr>
</table>
</dd>
<!-- ==================================================== -->
<!-- entry has order dependees -->
<dt class="doAnchor" name="compatibility">Are SLF4J versions
backward compatible?
</dt>
<dd>
<p>From the clients perspective, the SLF4J API is backward
compatible for all versions. This means than you can upgrade
from SLF4J version 1.0 to any later version without
problems. Code compiled with <em>slf4j-api-versionN.jar</em>
will work with <em>slf4j-api-versionM.jar</em> for any versionN
and any versionM. <b>To date, binary compatibility in slf4j-api
has never been broken.</b></p>
<p>However, while the SLF4J API is very stable from the client's
perspective, SLF4J bindings, e.g. slf4j-simple.jar or
slf4j-log4j12.jar, may require a specific version of slf4j-api.
Mixing different versions of slf4j artifacts can be problematic
and is strongly discouraged. For instance, if you are using
slf4j-api-1.5.6.jar, then you should also use
slf4j-simple-1.5.6.jar, using slf4j-simple-1.4.2.jar will not
work.
</p>
<p>At initialization time, if SLF4J suspects that there may be a
version mismatch problem, it emits a warning about the said
mismatch.
</p>
</dd>
<!-- ==================================================== -->
<dt class="doAnchor" name="IllegalAccessError">I am getting
<code>IllegalAccessError</code> exceptions when using SLF4J. Why
is that?
</dt>
<dd>
<p>Here are the exception details.</p>
<pre class="source">Exception in thread "main" java.lang.IllegalAccessError: tried to access field
org.slf4j.impl.StaticLoggerBinder.SINGLETON from class org.slf4j.LoggerFactory
at org.slf4j.LoggerFactory.&lt;clinit>(LoggerFactory.java:60)</pre>
<p>This error is caused by the static initializer of the
<code>LoggerFactory</code> class attempting to directly access
the SINGLETON field of
<code>org.slf4j.impl.StaticLoggerBinder</code>. While this was
allowed in SLF4J 1.5.5 and earlier, in 1.5.6 and later the
SINGLETON field has been marked as private access.
</p>
<p>If you get the exception shown above, then you are using an
older version of slf4j-api, e.g. 1.4.3, with a new version of a
slf4j binding, e.g. 1.5.6. Typically, this occurs when your
Maven <em>pom.ml</em> file incorporates hibernate 3.3.0 which
declares a dependency on slf4j-api version 1.4.2. If your
<em>pom.xml</em> declares a dependency on an slf4j binding, say
slf4j-log4j12 version 1.5.6, then you will get illegal access
errors.
</p>
<p>To see which version of slf4j-api is pulled in by Maven, use
the maven dependency plugin as follows.</p>
<p class="source">mvn dependency:tree</p>
<p>If you are using Eclipse, please do not rely on the dependency
tree shown by <a
href="http://m2eclipse.codehaus.org/">m2eclipse</a>.</p>
<p>In your <em>pom.xml</em> file, explicitly declaring a
dependency on slf4j-api matching the version of the declared
binding will make the problem go away.
</p>
<p>Please also read the FAQ entry on <a
href="#compatibility">backward compatibility</a> for a more
general explanation.</p>
</dd>
<!-- ==================================================== -->
<dt class="doAnchor" name="license">Why is SLF4J licensed under
X11 type license instead of the Apache Software License?
</dt>
<dd>
<p>SLF4J is licensed under a permissive X11 type license
instead of the <a
href="http://www.apache.org/licenses/">ASL</a> or the <a
href="http://www.gnu.org/copyleft/lesser.html">LGPL</a>
because the X11 license is deemed by both the Apache Software
Foundation as well as the Free Software Foundation as
compatible with their respective licenses.
</p>
</dd>
<!-- ==================================================== -->
<dt class="doAnchor" name="where_is_binding">Where can I get a
particular SLF4J binding?
</dt>
<dd>
<p>SLF4J bindings for <a
href="api/org/slf4j/impl/SimpleLogger.html">SimpleLogger</a>,
<a href="api/org/slf4j/impl/NOPLogger.html">NOPLogger</a>, <a
href="api/org/slf4j/impl/Log4jLoggerAdapter.html">Log4jLoggerAdapter</a>
and <a
href="api/org/slf4j/impl/JDK14LoggerAdapter.html">JDK14LoggerAdapter</a>
are contained within the files <em>slf4j-nop.jar</em>,
<em>slf4j-simple.jar</em>, <em>slf4j-log4j12.jar</em>, and
<em>slf4j-jdk14.jar</em>. These files ship with the <a
href="download.html">official SLF4J distribution</a>. Please
note that all bindings depend on <em>slf4j-api.jar</em>.
</p>
<p>The binding for logback-classic ships with the <a
href="http://logback.qos.ch/download.html">logback
distribution</a>. However, as with all other bindings, the
logback-classic binding requires <em>slf4j-api.jar</em>.
</p>
</dd>
<dt class="doAnchor" name="configure_logging">Should my library
attempt to configure logging?
</dt>
<dd>
<p><b>Embedded components such as libraries not only do not need
to configure the underlying logging framework, they really
should not do so</b>. They should invoke SLF4J to log but should
let the end-user configure the logging environment. When
embedded components try to configure logging on their own, they
often override the end-user's wishes. At the end of the day, it
is the end-user who has to read the logs and process them. She
should be the person to decide how she wants her logging
configured.
</p>
</dd>
<!-- ======================================================= -->
<dt class="doAnchor" name="optional_dependency">In order to reduce
the number of dependencies of our software we would like to make
SLF4J an optional dependency. Is that a good idea?
</dt>
<dd>
<p><a
href="http://stackoverflow.com/questions/978670/is-it-worth-wrapping-a-logging-framework-in-an-additional-layer">This
question pops up</a> whenever a software project reaches a point
where it needs to devise a logging strategy.
</p>
<p>Let Wombat be a software library with very few
dependencies. If SLF4J is chosen as Wombat's logging API, then a
new dependency on <em>slf4j-api.jar</em> will be added to
Wombat's list of dependencies. Given that writing a logging
wrapper does not seem that hard, some developers will be tempted
to wrap SLF4J and link with it only if it is already present on
the classpath, making SLF4J an optional dependency of Wombat. In
addition to solving the dependency problem, the wrapper will
isolate Wombat from SLF4J's API ensuring that logging in Wombat
is future-proof.
</p>
<p>On the other hand, any SLF4J-wrapper by definition depends on
SLF4J. It is bound to have the same general API. If in the
future a new and significantly different logging API comes
along, code that uses the wrapper will be equally difficult to
migrate to the new API as code that used SLF4J directly. Thus,
the wrapper is not likely to future-proof your code, but to make
it more complex by adding an additional indirection on top of
SLF4J, which is an indirection in itself.
</p>
<p><span class="label">increased vulnerability</span> It is
actually worse than that. Wrappers will need to depend on
certain internal SLF4J interfaces which change from time to
time, contrary to the client-facing API which never
changes. Thus, wrappers are usually dependent on the major
version they were compiled with. A wrapper compiled against
SLF4J version 1.5.x will not work with SLF4J 1.6 whereas client
code using <code>org.slf4j.Logger</code>,
<code>LoggerFactory</code>, <code>MarkerFactory</code>,
<code>org.slf4j.Marker</code>, and <code>MDC</code> will work
fine with any SLF4J version from version 1.0 and onwards.
</p>
<p>It is reasonable to assume that in most projects Wombat will
be one dependency among many. If each library had its own
logging wrapper, then each wrapper would presumably need to be
configured separately. Thus, instead of having to deal with one
logging framework, namely SLF4J, the user of Wombat would have
to detail with Wombat's logging wrapper as well. The problem
will be compounded by each framework that comes up with its own
wrapper in order to make SLF4J optional. (Configuring or
dealing with the intricacies of five different logging wrappers
is not exactly exciting nor endearing.)
</p>
<p>The <a
href="http://velocity.apache.org/engine/devel/developer-guide.html#Configuring_Logging">logging
strategy adopted by the Velocity project</a> is a good example
of the "custom logging abstraction" anti-pattern. By adopting an
independent logging abstraction strategy, Velocity developers
have made life harder for themselves, but more importantly, they
made life harder for their users.
</p>
<p>Some projects try to detect the presence of SLF4J on the
class path and switch to it if present. While this approach
seems transparent enough, it will result in erroneous location
information. Underlying logging frameworks will print the
location (class name and line number) of the wrapper instead of
the real caller. Then there is the question of API coverage as
SLF4J support MDC and markers in addition to parameterized
logging. While one can come up with a seemingly working
SLF4J-wrapper within hours, many technical issues will emerge
over time which Wombat developers will have to deal with. Note
that SLF4J has evolved over several years and has 260 bug
reports filed against it.</p>
<p>For the above reasons, developers of frameworks should resist
the temptation to write their own logging wrapper. Not only is
it a waste of time of the developer, it will actually make life
more difficult for the users of said frameworks and make logging
code paradoxically more vulnerable to change.
</p>
</dd>
<!-- ======================================================= -->
<dt class="doAnchor" name="maven2">What about Maven transitive
dependencies?
</dt>
<dd>
<p>As an author of a library built with Maven, you might want to
test your application using a binding, say slf4j-log4j12 or
logback-classic, without forcing log4j or logback-classic as a
dependency upon your users. This is rather easy to accomplish.
</p>
<p>Given that your library's code depends on the SLF4J API, you
will need to declare slf4j-api as a compile-time (default scope)
dependency.
</p>
<pre class="prettyprint source">&lt;dependency&gt;
&lt;groupId&gt;org.slf4j&lt;/groupId&gt;
&lt;artifactId&gt;slf4j-api&lt;/artifactId&gt;
&lt;version&gt;${project.version}&lt;/version&gt;
&lt;/dependency&gt;</pre>
<p>Limiting the transitivity of the SLF4J binding used in your
tests can be accomplished by declaring the scope of the
SLF4J-binding dependency as "test". Here is an example:</p>
<pre class="prettyprint source">&lt;dependency&gt;
&lt;groupId&gt;org.slf4j&lt;/groupId&gt;
&lt;artifactId&gt;slf4j-log4j12&lt;/artifactId&gt;
&lt;version&gt;${project.version}&lt;/version&gt;
<b>&lt;scope&gt;test&lt;/scope&gt;</b>
&lt;/dependency&gt;</pre>
<p>Thus, as far as your users are concerned you are exporting
slf4j-api as a transitive dependency of your library, but not
any SLF4J-binding or any underlying logging system.
</p>
<p>Note that as of SLF4J version 1.6, in the absence of an SLF4J
binding, slf4j-api will default to a no-operation
implementation.
</p>
</dd>
<!-- ====================================================== -->
<dt class="doAnchor" name="excludingJCL">How do I exclude
commons-logging as a Maven dependency?
</dt>
<dd>
<p><b>alternative 1) explicit exclusion</b></p>
<p>Many software projects using Maven declare commons-logging as
a dependency. Therefore, if you wish to migrate to SLF4J or use
jcl-over-slf4j, you would need to exclude commons-logging in all
of your project's dependencies which transitively depend on
commons-logging. <a
href="http://maven.apache.org/guides/introduction/introduction-to-optional-and-excludes-dependencies.html">Dependency
exclusion</a> is described in the Maven documentation. Excluding
commons-logging explicitly for multiple dependencies distributed
on several <em>pom.xml</em> files can be a cumbersome and a
relatively error prone process.
</p>
<p><b>alternative 2) provided scope</b></p>
<p>Commons-logging can be rather simply and conveniently
excluded as a dependency by declaring it in the
<em>provided</em> scope within the pom.xml file of your
project. The actual commons-logging classes would be provided by
jcl-over-slf4j. This translates into the following pom file
snippet:</p>
<pre class="prettyprint source">&lt;dependency>
&lt;groupId>commons-logging&lt;/groupId>
&lt;artifactId>commons-logging&lt;/artifactId>
&lt;version>1.1.1&lt;/version>
&lt;scope>provided&lt;/scope>
&lt;/dependency>
&lt;dependency>
&lt;groupId>org.slf4j&lt;/groupId>
&lt;artifactId>jcl-over-slf4j&lt;/artifactId>
&lt;version>${project.version}&lt;/version>
&lt;/dependency></pre>
<p>The first dependency declaration essentially states that
commons-logging will be "somehow" provided by your
environment. The second declaration includes jcl-over-slf4j into
your project. As jcl-over-slf4j is a perfect binary-compatible
replacement for commons-logging, the first assertion becomes
true.
</p>
<p>Unfortunately, while declaring commons-logging in the
provided scope gets the job done, your IDE, e.g. Eclipse, will
still place <em>commons-logging.jar</em> on your project's class
path as seen by your IDE. You would need to make sure that
<em>jcl-over-slf4j.jar</em> is visible before
<em>commons-logging.jar</em> by your IDE.
</p>
<p><b>alternative 3) empty artifacts</b></p>
<p>An alternative approach is to depend on an <b>empty</b>
<em>commons-logging.jar</em> artifact. This clever <a
href="http://day-to-day-stuff.blogspot.com/2007/10/announcement-version-99-does-not-exist.html">approach
first was imagined</a> and initially supported by Erik van
Oosten.
</p>
<p>The empty artifact is available from a <a
href="http://version99.qos.ch">http://version99.qos.ch</a> a
high-availability Maven repository, replicated on several hosts
located in different geographical regions.</p>
<p>The following declaration adds the version99 repository to
the set of remote repositories searched by Maven. This
repository contains empty artifacts for commons-logging and
log4j. By the way, if you use the version99 repository, please
drop us a line at &lt;version99 AT qos.ch&gt;.
</p>
<pre class="prettyprint source">&lt;repositories>
&lt;repository>
&lt;id>version99&lt;/id>
&lt;!-- highly available repository serving empty artifacts --&gt;
&lt;url>http://version99.qos.ch/&lt;/url>
&lt;/repository>
&lt;/repositories></pre>
<p>Declaring version 99-empty of commons-logging in the
<code>&lt;dependencyManagement></code> section of your project
will direct all transitive dependencies for commons-logging to
import version 99-empty, thus nicely addressing the
commons-logging exclusion problem. The classes for commons-logging
will be provided by jcl-over-slf4j. The following lines declare
commons-logging version 99-empty (in the dependency management
section) and declare jcl-over-slf4j as a dependency.
</p>
<pre class="prettyprint source">&lt;dependencyManagement>
&lt;dependencies>
&lt;dependency>
&lt;groupId>commons-logging&lt;/groupId>
&lt;artifactId>commons-logging&lt;/artifactId>
&lt;version><b>99-empty</b>&lt;/version>
&lt;/dependency>
... other declarations...
&lt;/dependencies>
&lt;/dependencyManagement>
&lt;!-- Do not forget to declare a dependency on jcl-over-slf4j in the --&gt;
&lt;!-- dependencies section. Note that the dependency on commons-logging --&gt;
&lt;!-- will be imported transitively. You don't have to declare it yourself. --&gt;
&lt;dependencies>
&lt;dependency>
&lt;groupId>org.slf4j&lt;/groupId>
&lt;artifactId>jcl-over-slf4j&lt;/artifactId>
&lt;version>${project.version}&lt;/version>
&lt;/dependency>
... other dependency declarations
&lt;/dependencies>
</pre>
</dd>
</dl>
<h2>About the SLF4J API</h2>
<dl>
<dt class="doAnchor" name="string_or_object">Why don't the
printing methods in the Logger interface accept message of type
Object, but only messages of type String?
</dt>
<dd>
<p>In SLF4J 1.0beta4, the printing methods such as debug(),
info(), warn(), error() in the <a
href="api/org/slf4j/Logger.html">Logger interface</a> were
modified so as to accept only messages of type String
instead of Object.
</p>
<p>Thus, the set of printing methods for the DEBUG level
became:</p>
<pre class="prettyprint source">debug(String msg);
debug(String format, Object arg);
debug(String format, Object arg1, Object arg2);
debug(String msg, Throwable t);</pre>
<p>Previously, the first argument in the above methods was of
type <code>Object</code>.</p>
<p>This change enforces the notion that logging systems are
about decorating and handling messages of type String, and not
any arbitrary type (Object).
</p>
<p>Just as importantly, the new set of method signatures offer
a clearer differentiation between the overloaded methods
whereas previously the choice of the invoked method due to
Java overloading rules were not always easy to follow.</p>
<p>It was also easy to make mistakes. For example, previously
it was legal to write:</p>
<pre class="prettyprint source">logger.debug(new Exception("some error"));</pre>
<p>Unfortunately, the above call did not print the stack trace
of the exception. Thus, a potentially crucial piece of
information could be lost. When the first parameter is
restricted to be of type String, then only the method
</p>
<pre class="prettyprint source">debug(String msg, Throwable t);</pre>
<p>can be used to log exceptions. Note that this method
ensures that every logged exception is accompanied with a
descriptive message.</p>
</dd>
<!-- ====================================================== -->
<dt class="doAnchor" name="exception_message">Can I log an
exception without an accompanying message?
</dt>
<dd>
<p>In short, no.</p>
<p>If <code>e</code> is an <code>Exception</code>, and you
would like to log an exception at the ERROR level, you must
add an accompanying message. For example,</p>
<pre class="prettyprint source">logger.error("some accompanying message", e);</pre>
<p>You might legitimately argue that not all exceptions have a
meaningful message to accompany them. Moreover, a good exception
should already contain a self explanatory description. The
accompanying message may therefore be considered redundant.
</p>
<p>While these are valid arguments, there are three opposing
arguments also worth considering. First, on many, albeit not
all occasions, the accompanying message can convey useful
information nicely complementing the description contained
in the exception. Frequently, at the point where the
exception is logged, the developer has access to more
contextual information than at the point where the exception
is thrown. Second, it is not difficult to imagine more or
less generic messages, e.g. "Exception caught", "Exception
follows", that can be used as the first argument for
<code>error(String msg, Throwable t)</code> invocations.
Third, most log output formats display the message on a
line, followed by the exception on a separate line. Thus,
the message line would look inconsistent without a message.
</p>
<p>In short, if the user were allowed to log an exception
without an accompanying message, it would be the job of the
logging system to invent a message. This is actually what
the <a href="http://tinyurl.com/cr9kg">throwing(String
sourceClass, String sourceMethod, Throwable thrown)</a>
method in java.util.logging package does. (It decides on its
own that accompanying message is the string "THROW".)
</p>
<p>It may initially appear strange to require an accompanying
message to log an exception. Nevertheless, this is common
practice in <em>all</em> log4j derived systems such as
java.util.logging, logkit, etc. and of course log4j itself. It
seems that the current consensus considers requiring an
accompanying message as a good a thing (TM).
</p>
</dd>
<!-- ====================================================== -->
<dt class="doAnchor" name="logging_performance">What is the
fastest way of (not) logging?
</dt>
<dd>
<p>SLF4J supports an advanced feature called parameterized
logging which can significantly boost logging performance for
<em>disabled</em> logging statement.</p>
<p> For some Logger <code>logger</code>, writing,</p>
<pre class="prettyprint source">logger.debug("Entry number: " + i + " is " + String.valueOf(entry[i]));</pre>
<p>incurs the cost of constructing the message parameter, that
is converting both integer <code>i</code> and
<code>entry[i]</code> to a String, and concatenating
intermediate strings. This, regardless of whether the message
will be logged or not.
</p>
<p>One possible way to avoid the cost of parameter
construction is by surrounding the log statement with a
test. Here is an example.</p>
<pre class="prettyprint source">if(logger.isDebugEnabled()) {
logger.debug("Entry number: " + i + " is " + String.valueOf(entry[i]));
}</pre>
<p>This way you will not incur the cost of parameter
construction if debugging is disabled for
<code>logger</code>. On the other hand, if the logger is
enabled for the DEBUG level, you will incur the cost of
evaluating whether the logger is enabled or not, twice: once in
<code>debugEnabled</code> and once in <code>debug</code>. This
is an insignificant overhead because evaluating a logger takes
less than 1% of the time it takes to actually log a statement.
</p>
<p><b>Better yet, use parameterized messages</b></p>
<p>There exists a very convenient alternative based on message
formats. Assuming <code>entry</code> is an object, you can write:
</p>
<pre class="prettyprint source">Object entry = new SomeObject();
logger.debug("The entry is {}.", entry);</pre>
<p>After evaluating whether to log or not, and only if the
decision is affirmative, will the logger implementation format
the message and replace the '{}' pair with the string value of
<code>entry</code>. In other words, this form does not incur
the cost of parameter construction in case the log statement is
disabled.
</p>
<p>The following two lines will yield the exact same
output. However, the second form will outperform the first
form by a factor of at least 30, in case of a
<em>disabled</em> logging statement.
</p>
<pre class="prettyprint source">logger.debug("The new entry is "+entry+".");
logger.debug("The new entry is {}.", entry);</pre>
<p>A <a
href="apidocs/org/slf4j/Logger.html#debug(java.lang.String,%20java.lang.Object%2C%20java.lang.Object)">two
argument</a> variant is also available. For example, you can
write:</p>
<pre class="prettyprint source">logger.debug("The new entry is {}. It replaces {}.", entry, oldEntry);</pre>
<p>If three or more arguments need to be passed, you can make
use of the <a
href="apidocs/org/slf4j/Logger.html#debug(java.lang.String%2C%20java.lang.Object...)"><code>Object...</code>
variant</a> of the printing methods. For example, you can
write:</p>
<pre class="prettyprint source">logger.debug("Value {} was inserted between {} and {}.", newVal, below, above);</pre>
<p>This form incurs the hidden cost of construction of an
Object[] (object array) which is usually very small. The one and
two argument variants do not incur this hidden cost and exist
solely for this reason (efficiency). The slf4j-api would be
smaller/cleaner with only the Object... variant.</p>
<p>Array type arguments, including multi-dimensional arrays,
are also supported.</p>
<p>SLF4J uses its own message formatting implementation which
differs from that of the Java platform. This is justified by
the fact that SLF4J's implementation performs about 10 times
faster but at the cost of being non-standard and less
flexible.
</p>
<p><b>Escaping the "{}" pair</b></p>
<p>The "{}" pair is called the <em>formatting anchor</em>. It
serves to designate the location where arguments need to be
substituted within the message pattern.
</p>
<p>SLF4J only cares about the <em>formatting anchor</em>, that
is the '{' character immediately followed by '}'. Thus, in
case your message contains the '{' or the '}' character, you
do not have to do anything special unless the '}' character
immediately follows '}'. For example,
</p>
<pre class="prettyprint source">logger.debug("Set {1,2} differs from {}", "3");</pre>
<p>which will print as "Set {1,2} differs from 3". </p>
<p>You could have even written,</p>
<pre class="prettyprint source">logger.debug("Set {1,2} differs from {{}}", "3");</pre>
<p>which would have printed as "Set {1,2} differs from {3}". </p>
<p>In the extremely rare case where the the "{}" pair occurs
naturally within your text and you wish to disable the special
meaning of the formatting anchor, then you need to escape the
'{' character with '\', that is the backslash character. Only
the '{' character should be escaped. There is no need to
escape the '}' character. For example,
</p>
<pre class="prettyprint source">logger.debug("Set \\{} differs from {}", "3");</pre>
<p>will print as "Set {} differs from 3". Note that within
Java code, the backslash character needs to be written as
'\\'.</p>
<p>In the rare case where the "\{}" occurs naturally in the
message, you can double escape the formatting anchor so that
it retains its original meaning. For example,</p>
<pre class="prettyprint source">logger.debug("File name is C:\\\\{}.", "file.zip");</pre>
<p>will print as "File name is C:\file.zip".</p>
</dd>
<!-- ================================================= -->
<dt class="doAnchor" name="string_contents">How can I log the
string contents of a single (possibly complex) object?</dt>
<dd>
<p> In relatively rare cases where the message to be logged
is the string form of an object, then the parameterized
printing method of the appropriate level can be
used. Assuming <code>complexObject</code> is an object of
certain complexity, for a log statement of level DEBUG, you
can write:
</p>
<pre class="prettyprint source">logger.debug("{}", complexObject);</pre>
<p>The logging system will invoke
<code>complexObject.toString()</code> method only after it
has ascertained that the log statement was
enabled. Otherwise, the cost of
<code>complexObject.toString()</code> conversion will be
advantageously avoided.
</p>
</dd>
<!-- ================================================= -->
<dt class="doAnchor" name="fatal">Why doesn't the
<code>org.slf4j.Logger</code> interface have methods for the FATAL
level?
</dt>
<dd>
<p>The <a href="apidocs/org/slf4j/Marker.html">Marker</a>
interface, part of the <code>org.slf4j</code> package, renders
the FATAL level largely redundant. If a given error requires
attention beyond that allocated for ordinary errors, simply mark
the logging statement with a specially designated marker which
can be named "FATAL" or any other name to your liking.
</p>
<p>Here is an example,</p>
<pre class="prettyprint">import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.slf4j.Marker;
import org.slf4j.MarkerFactory;
class Bar {
void foo() {
<b>Marker fatal = MarkerFactory.getMarker("FATAL");</b>
Logger logger = LoggerFactory.getLogger("aLogger");
try {
... obtain a JDBC connection
} catch (JDBException e) {
logger.error(<b>fatal</b>, "Failed to obtain JDBC connection", e);
}
}
}</pre>
<p>While markers are part of the SLF4J API, only logback
supports markers off the shelf. For example, if you add the
<code>%marker</code> conversion word to its pattern, logback's
<code>PatternLayout</code> will add marker data to its
output. Marker data can be used to <a
href="http://logback.qos.ch/manual/filters.html">filter
messages</a> or even <a
href="http://logback.qos.ch/manual/appenders.html#OnMarkerEvaluator">trigger</a>
an outgoing email <a
href="http://logback.qos.ch/recipes/emailPerTransaction.html">at
the end of an individual transaction</a>.
</p>
<p>In combination with logging frameworks such as log4j and
java.util.logging which do not support markers, marker data will
be silently ignored.</p>
<p>Markers add a new dimension with infinite possible values for
processing log statements compared to five values, namely ERROR,
WARN, INFO, DEBUG and TRACE, allowed by levels. At present time,
only logback supports marker data. However, nothing prevents
other logging frameworks from making use of marker data.
</p>
</dd>
<!-- ======================================================= -->
<dt class="doAnchor" name="trace">Why was the TRACE level
introduced only in SLF4J version 1.4.0?</dt>
<dd>
<p>The addition of the TRACE level has been frequently and
hotly debated request. By studying various projects, we
observed that the TRACE level was used to disable logging
output from certain classes <em>without</em> needing to
configure logging for those classes. Indeed, the TRACE level
is by default disabled in log4j and logback as well most other
logging systems. The same result can be achieved by adding the
appropriate directives in configuration files.
</p>
<p>Thus, in many of cases the TRACE level carried the same
semantic meaning as DEBUG. In such cases, the TRACE level
merely saves a few configuration directives. In other, more
interesting occasions, where TRACE carries a different meaning
than DEBUG, <a href="api/org/slf4j/Marker.html">Marker</a>
objects can be put to use to convey the desired
meaning. However, if you can't be bothered with markers and
wish to use a logging level lower than DEBUG, the TRACE level
can get the job done.
</p>
<p>Note that while the cost of evaluating a disabled log
request is in the order of a few <code>nanoseconds</code>, the
use of the TRACE level (or any other level for that matter) is
discouraged in tight loops where the log request might be
evaluated millions of times. If the log request is enabled,
then it will overwhelm the target destination with massive
output. If the request is disabled, it will waste resources.
</p>
<p>In short, although we still discourage the use of the TRACE
level because alternatives exist or because in many cases log
requests of level TRACE are wasteful, given that people kept
asking for it, we decided to bow to popular demand.
</p>
</dd>
<!-- ================================================= -->
<dt class="doAnchor" name="i18n">Does the SLF4J logging API
support I18N (internationalization)?
</dt>
<dd>
<p>Yes, as of version 1.5.9, SLF4J ships with a package called
<code>org.slf4j.cal10n</code> which adds <a
href="localization.html">localized/internationalized logging</a>
support as a thin layer built upon the <a
href="http://cal10n.qos.ch">CAL10N API</a>.</p>
</dd>
<!-- ================================================= -->
<dt class="doAnchor" name="noLoggerFactory">Is it possible to
retrieve loggers without going through the static methods in
<code>LoggerFactory</code>?
</dt>
<dd>
<p>Yes. <code>LoggerFactory</code> is essentially a wrapper
around an <a
href="xref/org/slf4j/ILoggerFactory.html"><code>ILoggerFactory</code></a>
instance. The <code>ILoggerFactory</code> instance in use is
determined according to the static binding conventions of the
SLF4J framework. See the <a
href="xref/org/slf4j/LoggerFactory.html#217">getSingleton()</a>
method in <code>LoggerFactory</code> for details.
</p>
<p>However, nothing prevents you from using your own
<code>ILoggerFactory</code> instance. Note that you can also
obtain a reference to the <code>ILoggerFactory</code> that the
<code>LoggerFactory</code> class is using by invoking the <a
href="apidocs/org/slf4j/LoggerFactory.html#getILoggerFactory()">
<code>LoggerFactory.getILoggerFactory()</code></a> method.
</p>
<p>Thus, if SLF4J binding conventions do not fit your needs, or
if you need additional flexibility, then do consider using the
<code>ILoggerFactory</code> interface as an alternative to
inventing your own logging API.</p>
</dd>
<!-- ================================================= -->
<dt class="doAnchor" name="paramException">In the presence of an
exception/throwable, is it possible to parameterize a logging
statement?</dt>
<dd>
<p>Yes, as of SLF4J 1.6.0, but not in previous versions. The
SLF4J API supports parametrization in the presence of an
exception, assuming the exception is the last parameter. Thus,
</p>
<pre class="prettyprint">String s = "Hello world";
try {
Integer i = Integer.valueOf(s);
} catch (NumberFormatException e) {
logger.error("Failed to format {}", s, e);
}</pre>
<p>will print the <code>NumberFormatException</code> with its
stack trace as expected. The java compiler will invoke the <a
href="http://www.slf4j.org/apidocs/org/slf4j/Logger.html#error%28java.lang.String,%20java.lang.Object,%20java.lang.Object%29">error
method taking a String and two Object arguments</a>. SLF4J, in
accordance with the programmer's most probable intention, will
interpret <code>NumberFormatException</code> instance as a
throwable instead of an unused <code>Object</code> parameter. In
SLF4J versions prior to 1.6.0, the
<code>NumberFormatException</code> instance was simply ignored.
</p>
<p>If the exception is not the last argument, it will be treated
as a plain object and its stack trace will NOT be printed.
However, such situations should not occur in practice.
</p>
</dd>
</dl>
<h2>Implementing the SLF4J API</h2>
<dl>
<!-- ============================================================= -->
<dt class="doAnchor" name="slf4j_compatible">How do I make my
logging framework SLF4J compatible?
</dt>
<dd>
<p>Adding supporting for the SLF4J is surprisingly
easy. Essentially, you coping an existing binding and tailoring
it a little (as explained below) does the trick.
</p>
<p>Assuming your logging system has notion of a
logger, called say <code>MyLogger</code>, you need to provide
an adapter for <code>MyLogger</code> to
<code>org.slf4j.Logger</code> interface. Refer to slf4j-jcl,
slf4j-jdk14, and slf4j-log4j12 modules for examples of
adapters.
</p>
<p>Once you have written an appropriate adapter, say
<code>MyLoggerAdapter</code>, you need to provide a factory
class implementing the <code>org.slf4j.ILoggerFactory</code>
interface. This factory should return instances
<code>MyLoggerAdapter</code>. Let <code>MyLoggerFactory</code>
be the name of your factory class.
</p>
<p>Once you have the adapter, namely
<code>MyLoggerAdapter</code>, and a factory, namely
<code>MyLoggerFactory</code>, the last remaining step is to
modify the <code>StaticLoggerBinder</code> class so that it
returns an new instance of <code>MyLoggerFactory</code>. You
will also need to modify the
<code>loggerFactoryClassStr</code> variable.
</p>
<p>For Marker or MDC support, you could use the one of the
existing NOP implementations.
</p>
<p>In summary, to create an SLF4J binding for your logging
system, follow these steps:</p>
<ol>
<li>start with a copy of an existing module,</li>
<li>create an adapter between your logging system and
<code>org.slf4j.Logger</code> interface
</li>
<li>create a factory for the adapter created in the previous step,</li>
<li>modify <code>StaticLoggerBinder</code> class to use the
factory you created in the previous step</li>
</ol>
</dd>
<!-- ============================================================= -->
<dt class="doAnchor" name="marker_interface">How can my logging
system add support for the <code>Marker</code> interface?
</dt>
<dd>
<p>Markers constitute a revolutionary concept which is
supported by logback but not other existing logging
systems. Consequently, SLF4J conforming logging systems are
allowed to ignore marker data passed by the user.
</p>
<p>However, even though marker data may be ignored, the user
must still be allowed to specify marker data. Otherwise, users
would not be able to switch between logging systems that
support markers and those that do not.
</p>
<p>The <code>MarkerIgnoringBase</code> class can serve as a
base for adapters or native implementations of logging
systems lacking marker support. In
<code>MarkerIgnoringBase</code>, methods taking marker data
simply invoke the corresponding method without the Marker
argument, discarding any Marker data passed as
argument. Your SLF4J adapters can extend
<code>MarkerIgnoringBase</code> to quickly implement the
methods in <code>org.slf4j.Logger</code> which take a
<code>Marker</code> as the first argument.
</p>
</dd>
<!-- ============================================================= -->
<dt class="doAnchor" name="version_checks">How does SLF4J's
version check mechanism work?
</dt>
<dd>
<p>The version check performed by SLF4J API during its
initialization is an <em>elective</em> process. Conforming SLF4J
implementations may choose <em>not</em> to participate, in which
case, no version check will be performed.
</p>
<p>However, if an SLF4J implementation decides to participate,
than it needs to declare a variable called
REQUESTED_API_VERSION within its copy of the
<code>StaticLoggerBinder</code> class. The value of this
variable should be equal to the version of the slf4j-api.jar
it is compiled with. If the implementation is upgraded to a
newer version of slf4j-api, than you also need to update the
value of REQUESTED_API_VERSION.
</p>
<p>For each version, SLF4J API maintains a list of compatible
versions. SLF4J will emit a version mismatch warning only if
the requested version is not found in the compatibility
list. So even if your SLF4J binding has a different release
schedule than SLF4J, assuming you update the SLF4J version you
use every 6 to 12 months, you can still participate in the
version check without incurring a mismatch warning. For
example, logback has a different release schedule but still
participates in version checks.</p>
<p><b>As of SLF4J 1.5.5</b>, all bindings shipped within the
SLF4J distribution, e.g. slf4j-log4j12, slf4j-simple and
slf4j-jdk14, declare the REQUESTED_API_VERSION field with a
value equal to their SLF4J version. It follows that, for example
if slf4j-simple-1.5.8.jar is mixed with slf4j-api-1.6.0.jar,
given that 1.5.8 is not on the compatibility list of SLF4J
version 1.6.x, a version mismatch warning will be issued.
</p>
<p>Note that SLF4J versions prior to 1.5.5 did not have a
version check mechanism. Only slf4j-api-1.5.5.jar and later can
emit version mismatch warnings.
</p>
</dd>
</dl>
<h2>General questions about logging</h2>
<dl>
<!-- ============================================================= -->
<dt class="doAnchor" name="declared_static">Should Logger members
of a class be declared as static?
</dt>
<dd>
<p>We <code>used</code> to recommend that loggers members be
declared as instance variables instead of static. After further
analysis, <b>we no longer recommend one approach over the
other.</b>
</p>
<p>Here is a summary of the pros and cons of each approach.
</p>
<table class="bodyTable">
<tr valign="top">
<th width="50%">Advantages for declaring loggers as static</th>
<th width="50%">Disadvantages for declaring loggers as static</th>
</tr>
<tr valign="top" class="alt">
<td>
<ol>
<li>common and well-established idiom</li>
<li>less CPU overhead: loggers are retrieved and
assigned only once, at hosting class
initialization</li>
<li>less memory overhead: logger declaration will
consume one reference per class</li>
</ol>
</td>
<td> <!-- static con -->
<ol>
<li>For libraries shared between applications, not
possible to take advantage of repository selectors. It
should be noted that if the SLF4J binding and the
underlying API ships with each application (not shared
between applications), then each application will still
have its own logging environment.
</li>
<li>not IOC-friendly</li>
</ol>
</td>
</tr>
<tr>
<th width="50%">Advantages for declaring loggers as instance variables</th>
<th width="50%">Disadvantages for declaring loggers as
instance variables</th>
</tr>
<tr class="alt" valign="top">
<td> <!-- instance pros -->
<ol>
<li>Possible to take advantage of repository selectors
even for libraries shared between applications. However,
repository selectors only work if the underlying logging
system is logback-classic. Repository selectors do not
work for the SLF4J+log4j combination.
</li>
<li>IOC-friendly</li>
</ol>
</td>
<td> <!-- instance cons -->
<ol>
<li>Less common idiom than declaring loggers as static
variables</li>
<li>higher CPU overhead: loggers are retrieved and
assigned for each instance of the hosting class</li>
<li>higher memory overhead: logger declaration will
consume one reference per instance of the hosting class</li>
</ol>
</td>
</tr>
</table>
<h3>Explanation</h3>
<p>Static logger members cost a single variable reference for
all instances of the class whereas an instance logger member
will cost a variable reference for every instance of the
class. For simple classes instantiated thousands of times
there might be a noticeable difference.
</p>
<p>However, more recent logging systems, e.g log4j or logback,
support a distinct logger context for each application running
in the application server. Thus, even if a single copy of
<em>log4j.jar</em> or <em>logback-classic.jar</em> is deployed
in the server, the logging system will be able to differentiate
between applications and offer a distinct logging environment
for each application.
</p>
<p>More specifically, each time a logger is retrieved by
invoking <code>LoggerFactory.getLogger()</code> method, the
underlying logging system will return an instance appropriate
for the current application. Please note that within the
<em>same</em> application retrieving a logger by a given name
will always return the same logger. For a given name, a
different logger will be returned only for different
applications.
</p>
<p>If the logger is static, then it will only be retrieved once
when the hosting class is loaded into memory. If the hosting
class is used in only in one application, there is not much to
be concerned about. However, if the hosting class is shared
between several applications, then all instances of the shared
class will log into the context of the application which
happened to first load the shared class into memory - hardly the
behavior expected by the user.
</p>
<p>Unfortunately, for non-native implementations of the SLF4J
API, namely with slf4j-log4j12, log4j's repository selector will
not be able to do its job properly because slf4j-log4j12, a
non-native SLF4J binding, will store logger instances in a map,
short-circuiting context-dependent logger retrieval. For native
SLF4J implementations, such as logback-classic, repository
selectors will work as expected.
</p>
<p>The Apache Commons wiki contains an <a
href="http://wiki.apache.org/jakarta-commons/Logging/StaticLog">informative
article</a> covering the same question.</p>
<p><b>Logger serialization</b></p>
<p>Contrary to static variables, instance variables are
serialized by default. As of SLF4J version 1.5.3, logger
instances survive serialization. Thus, serialization of the host
class no longer requires any special action, even when loggers
are declared as instance variables. In previous versions, logger
instances needed to be declared as <code>transient</code> in the
host class. </p>
<p><b>Summary</b></p>
<p>In summary, declaring logger members as static variables
requires less CPU time and have a slightly smaller memory
footprint. On the other hand, declaring logger members as
instance variables requires more CPU time and have a slightly
higher memory overhead. However, instance variables make it
possible to create a distinct logger environment for each
application, even for loggers declared in shared
libraries. Perhaps more important than previously mentioned
considerations, instance variables are IOC-friendly whereas
static variables are not.
</p>
<p>See also <a
href="http://wiki.apache.org/jakarta-commons/Logging/StaticLog">related
discussion</a> in the commons-logging wiki.
</p>
</dd>
</dl>
<!-- ============================================================= -->
<dl>
<dt class="doAnchor" name="declaration_pattern">Is there a
recommended idiom for declaring a logger in a class?
</dt>
<dd>
<p>The following is the recommended logger declaration
idiom. For reasons <a href="#declared_static">explained
above</a>, it is left to the user to determine whether loggers
are declared as static variables or not.</p>
<pre class="prettyprint source">package some.package;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class MyClass {
<b>final (static) Logger logger = LoggerFactory.getLogger(MyClass.class);</b>
... etc
}</pre>
<p>Unfortunately, give that the name of the hosting class is
part of the logger declaration, the above logger declaration
idiom is not is <em>not</em> resistant to cut-and-pasting
between classes.
</p>
</dd>
</dl>
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