slf4j-site/src/site/pages/legacy.html - platform/external/slf4j - Git at Google

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     <h2>Bridging legacy APIs</h2>

     <p>Often, some of the components you depend on rely on a logging
     API other than SLF4J. You may also assume that these components
     will not switch to SLF4J in the immediate future. To deal with
     such circumstances, SLF4J ships with several bridging modules
     which redirect calls made to log4j, JCL and java.util.logging APIs
     to behave as if they were made to the SLF4J API instead. The
     figure below illustrates the idea.
     </p>

     <p>Please note that for source code under your control, you really
     should use the <a href="migrator.html">slf4j-migrator</a>. The
     binary-based solutions described in this page are appropriate for
     software beyond your control.
     </p>

     <p></p>
     <p></p>


     <p><a href="images/legacy.png">
     <img src="images/legacy.png" alt="click to enlarge" width="800"/>
     </a></p>

     <p>
     </p>

     <h2 class="doAnchor" name="jcl-over-slf4j">Gradual migration to
     SLF4J from Jakarta Commons Logging (JCL)</h2>

     <h4 class="doAnchor" name="jclOverSLF4J"><em>jcl-over-slf4j.jar</em></h4>

     <p>To ease migration to SLF4J from JCL, SLF4J distributions
     include the jar file <em>jcl-over-slf4j.jar</em>. This jar file is
     intended as a drop-in replacement for JCL version 1.1.1. It
     implements the public API of JCL but using SLF4J underneath, hence
     the name "JCL over SLF4J."
     </p>

     <p>Our JCL over SLF4J implementation will allow you to migrate to
     SLF4J gradually, especially if some of the libraries your software
     depends on continue to use JCL for the foreseeable future. You can
     immediately enjoy the benefits of SLF4J's reliability and preserve
     backward compatibility at the same time. Just replace
     <em>commons-logging.jar</em> with
     <em>jcl-over-slf4j.jar</em>. Subsequently, the selection of the
     underlying logging framework will be done by SLF4J instead of JCL
     <a href="http://articles.qos.ch/classloader.html">but without the
     class loader headaches plaguing JCL</a>. The underlying logging
     framework can be any of the frameworks supported by SLF4J. Often
     times, replacing <em>commons-logging.jar</em> with
     <em>jcl-over-slf4j.jar</em> will immediately and permanently solve
     class loader issues related to commons logging.
     </p>

     <h3  class="doAnchor" name="slf4jJCL"><em>slf4j-jcl.jar</em></h3>

     <p>Some of our users after having switched to SLF4J API realize that
     in some contexts the use of JCL is mandatory and their use of SLF4J
     can be a problem. For this uncommon but important case, SLF4J offers
     a JCL binding, found in the file <em>slf4j-jcl.jar</em>. The JCL
     binding will delegate all logging calls made through SLF4J API to
     JCL. Thus, if for some reason an existing application <em>must</em>
     use JCL, your part of that application can still code against the
     SLF4J API in a manner transparent to the larger application
     environment. Your choice of SLF4J API will be invisible to the rest
     of the application which can continue to use JCL.
     </p>

     <h3 class="doAnchor"
     name="jclRecursion"><em>jcl-over-slf4j.jar</em> should not be
     confused with <em>slf4j-jcl.jar</em></h3>


     <p>JCL-over-SLF4J, i.e. <em>jcl-over-slf4j.jar</em>, comes in handy
     in situations where JCL needs to be supported for backward
     compatibility reasons. It can be used to fix problems associated
     with JCL, without necessarily adopting the SLF4J API, a decision
     which can be deferred to a later time.
     </p>

     <p>On the other hand, <em>slf4j-jcl.jar</em> is useful
     <strong>after</strong> you have already adopted the SLF4J API for
     your component which needs to be embedded in a larger application
     environment where JCL is a formal requirement. Your software
     component can still use SLF4J API without disrupting the larger
     application. Indeed, <em>slf4j-jcl.jar</em> will delegate all
     logging decisions to JCL so that the dependency on SLF4J API by your
     component will be transparent to the larger whole.
     </p>

     <p>Please note that <em>jcl-over-slf4j.jar</em> and
     <em>slf4j-jcl.jar</em> cannot be deployed at the same time. The
     former jar file will cause JCL to delegate the choice of the
     logging system to SLF4J and the latter jar file will cause SLF4J
     to delegate the choice of the logging system to JCL, resulting in
     an <a href="codes.html#jclDelegationLoop">infinite loop</a>.
     </p>


     <h2 class="doAnchor" name="log4j-over-slf4j">log4j-over-slf4j</h2>

     <p>SLF4J ship with a module called <em>log4j-over-slf4j</em>.  It
     allows log4j users to migrate existing applications to SLF4J without
     changing <em>a single line of code</em> but simply by replacing the
     <em>log4j.jar</em> file with <em>log4j-over-slf4j.jar</em>, as
     described below.
     </p>

     <h4 class="doAnchor" name="losHow">How does it work?</h4>

     <p>The log4j-over-slf4j module contains replacements of most widely
     used log4j classes, namely <code>org.apache.log4j.Category</code>,
     <code>org.apache.log4j.Logger</code>,
     <code>org.apache.log4j.Priority</code>,
     <code>org.apache.log4j.Level</code>,
     <code>org.apache.log4j.MDC</code>, and
     <code>org.apache.log4j.BasicConfigurator</code>. These replacement
     classes redirect all work to their corresponding SLF4J classes.
     </p>

     <p>To use log4j-over-slf4j in your own application, the first step
     is to locate and then to replace <em>log4j.jar</em> with
     <em>log4j-over-slf4j.jar</em>. Note that you still need an SLF4J
     binding and its dependencies for log4j-over-slf4j to work properly.
     </p>

     <p>In most situations, replacing a jar file is all it takes in
     order to migrate from log4j to SLF4J.
     </p>

     <p>Note that as a result of this migration, log4j configuration
     files will no longer be picked up. If you need to migrate your
     log4j.properties file to logback, the <a
     href="http://logback.qos.ch/translator/">log4j translator</a> might
     be of help. For configuring logback, please refer to <a
     href="http://logback.qos.ch/manual/index.html">its manual</a>.
     </p>

     <h4 class="doAnchor" name="losFail">When does it not work?</h4>

     <p>The <em>log4j-over-slf4j</em> module will not work when the
     application calls log4j components that are not present in the
     bridge.  For example, when application code directly references
     log4j appenders, filters or the PropertyConfigurator, then
     log4j-over-slf4j would be an insufficient replacement for
     log4j. However, when log4j is configured through a configuration
     file, be it <em>log4j.properties</em> or <em>log4j.xml</em>, the
     log4j-over-slf4j module should just work fine.
     </p>


     <h4 class="doAnchor" name="losOverhead">What about the
     overhead?</h4>

     <p>There overhead of using log4j-over-slf4j instead of log4j
     directly is relatively small. Given that log4j-over-slf4j
     immediately delegates all work to SLF4J, the CPU overhead should be
     negligible, in the order of a few <em>nanoseconds</em>. There is a
     memory overhead corresponding to an entry in a hashmap per logger,
     which should be usually acceptable even for very large applications
     consisting of several thousand loggers.  Moreover, if you choose
     logback as your underlying logging system, and given that logback is
     both much faster and more memory-efficient than log4j, the gains
     made by using logback should compensate for the overhead of using
     log4j-over-slf4j instead of log4j directly.
     </p>

     <h4 class="doAnchor" name="log4jRecursion">log4j-over-slf4j.jar
     and slf4j-log4j12.jar cannot be present simultaneously
     </h4>

     <p>The presence of <em>slf4j-log4j12.jar</em>, that is the log4j
     binding for SLF4J, will force all SLF4J calls to be delegated to
     log4j. The presence of <em>log4j-over-slf4j.jar</em> will in turn
     delegate all log4j API calls to their SLF4J equivalents. If both
     are present simultaneously, slf4j calls will be delegated to
     log4j, and log4j calls redirected to SLF4j, resulting in an <a
     href="codes.html#log4jDelegationLoop">endless loop</a>.
     </p>


     <h2 class="doAnchor" name="jul-to-slf4j">jul-to-slf4j bridge</h2>

     <p>The jul-to-slf4j module includes a java.util.logging (jul)
     handler, namely <code>SLF4JBridgeHandler</code>, which routes all
     incoming jul records to the SLF4j API. Please see <a
     href="api/org/slf4j/bridge/SLF4JBridgeHandler.html">SLF4JBridgeHandler
     javadocs</a> for usage instructions.
     </p>

     <p><span class="label notice">Note on performance</span> Contrary
     to other bridging modules, namely jcl-over-slf4j and
     log4j-over-slf4j, which reimplement JCL and respectively log4j,
     the jul-to-slf4j module does not reimplement the java.util.logging
     because packages under the java.* namespace cannot be
     replaced. Instead, jul-to-slf4j translates <a
     href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/logging/LogRecord.html?is-external=true">LogRecord</a>
     objects into their SLF4J equivalent.  Please note this translation
     process incurs the cost of constructing a <code>LogRecord</code>
     instance regardless of whether the SLF4J logger is disabled for
     the given level or nor. <b>Consequently, j.u.l. to SLF4J
     translation can seriously increase the cost of disabled logging
     statements (60 fold or 6000%) and measurably impact the
     performance of enabled log statements (20% overall increase).</b>
     As of logback version 0.9.25, it is possible to completely
     eliminate the 60 fold translation overhead for disabled log
     statements with the help of <a
     href="http://logback.qos.ch/manual/configuration.html#LevelChangePropagator">LevelChangePropagator</a>.
     </p>

     <p>If you are concerned about application performance, then use of
     <code>SLF4JBridgeHandler</code> is appropriate only if any one of
     the following two conditions is true:
     </p>
     <ol>
       <li>few j.u.l. logging statements are in play </li>
       <li><code>LevelChangePropagator</code> has been installed</li>
     </ol>


     <h4 class="doAnchor" name="julRecursion">jul-to-slf4j.jar and slf4j-jdk14.jar cannot be present
     simultaneously
     </h4>

     <p>The presence of slf4j-jdk14.jar, that is the jul binding for
     SLF4J, will force SLF4J calls to be delegated to jul. On the other
     hand, the presence of jul-to-slf4j.jar, plus the installation of
     SLF4JBridgeHandler, by invoking "SLF4JBridgeHandler.install()" will
     route jul records to SLF4J. Thus, if both jar are present
     simultaneously (and SLF4JBridgeHandler is installed), slf4j calls
     will be delegated to jul and jul records will be routed to SLF4J,
     resulting in an endless loop.
     </p>


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	<h2>Bridging legacy APIs</h2>

	<p>Often, some of the components you depend on rely on a logging
	API other than SLF4J. You may also assume that these components
	will not switch to SLF4J in the immediate future. To deal with
	such circumstances, SLF4J ships with several bridging modules
	which redirect calls made to log4j, JCL and java.util.logging APIs
	to behave as if they were made to the SLF4J API instead. The
	figure below illustrates the idea.
	</p>

	<p>Please note that for source code under your control, you really
	should use the <a href="migrator.html">slf4j-migrator</a>. The
	binary-based solutions described in this page are appropriate for
	software beyond your control.
	</p>

	<p></p>
	<p></p>


	<p><a href="images/legacy.png">
	<img src="images/legacy.png" alt="click to enlarge" width="800"/>
	</a></p>

	<p>
	</p>

	<h2 class="doAnchor" name="jcl-over-slf4j">Gradual migration to
	SLF4J from Jakarta Commons Logging (JCL)</h2>

	<h4 class="doAnchor" name="jclOverSLF4J"><em>jcl-over-slf4j.jar</em></h4>

	<p>To ease migration to SLF4J from JCL, SLF4J distributions
	include the jar file <em>jcl-over-slf4j.jar</em>. This jar file is
	intended as a drop-in replacement for JCL version 1.1.1. It
	implements the public API of JCL but using SLF4J underneath, hence
	the name "JCL over SLF4J."
	</p>

	<p>Our JCL over SLF4J implementation will allow you to migrate to
	SLF4J gradually, especially if some of the libraries your software
	depends on continue to use JCL for the foreseeable future. You can
	immediately enjoy the benefits of SLF4J's reliability and preserve
	backward compatibility at the same time. Just replace
	<em>commons-logging.jar</em> with
	<em>jcl-over-slf4j.jar</em>. Subsequently, the selection of the
	underlying logging framework will be done by SLF4J instead of JCL
	<a href="http://articles.qos.ch/classloader.html">but without the
	class loader headaches plaguing JCL</a>. The underlying logging
	framework can be any of the frameworks supported by SLF4J. Often
	times, replacing <em>commons-logging.jar</em> with
	<em>jcl-over-slf4j.jar</em> will immediately and permanently solve
	class loader issues related to commons logging.
	</p>

	<h3 class="doAnchor" name="slf4jJCL"><em>slf4j-jcl.jar</em></h3>

	<p>Some of our users after having switched to SLF4J API realize that
	in some contexts the use of JCL is mandatory and their use of SLF4J
	can be a problem. For this uncommon but important case, SLF4J offers
	a JCL binding, found in the file <em>slf4j-jcl.jar</em>. The JCL
	binding will delegate all logging calls made through SLF4J API to
	JCL. Thus, if for some reason an existing application <em>must</em>
	use JCL, your part of that application can still code against the
	SLF4J API in a manner transparent to the larger application
	environment. Your choice of SLF4J API will be invisible to the rest
	of the application which can continue to use JCL.
	</p>

	<h3 class="doAnchor"
	name="jclRecursion"><em>jcl-over-slf4j.jar</em> should not be
	confused with <em>slf4j-jcl.jar</em></h3>


	<p>JCL-over-SLF4J, i.e. <em>jcl-over-slf4j.jar</em>, comes in handy
	in situations where JCL needs to be supported for backward
	compatibility reasons. It can be used to fix problems associated
	with JCL, without necessarily adopting the SLF4J API, a decision
	which can be deferred to a later time.
	</p>

	<p>On the other hand, <em>slf4j-jcl.jar</em> is useful
	<strong>after</strong> you have already adopted the SLF4J API for
	your component which needs to be embedded in a larger application
	environment where JCL is a formal requirement. Your software
	component can still use SLF4J API without disrupting the larger
	application. Indeed, <em>slf4j-jcl.jar</em> will delegate all
	logging decisions to JCL so that the dependency on SLF4J API by your
	component will be transparent to the larger whole.
	</p>

	<p>Please note that <em>jcl-over-slf4j.jar</em> and
	<em>slf4j-jcl.jar</em> cannot be deployed at the same time. The
	former jar file will cause JCL to delegate the choice of the
	logging system to SLF4J and the latter jar file will cause SLF4J
	to delegate the choice of the logging system to JCL, resulting in
	an <a href="codes.html#jclDelegationLoop">infinite loop</a>.
	</p>


	<h2 class="doAnchor" name="log4j-over-slf4j">log4j-over-slf4j</h2>

	<p>SLF4J ship with a module called <em>log4j-over-slf4j</em>. It
	allows log4j users to migrate existing applications to SLF4J without
	changing <em>a single line of code</em> but simply by replacing the
	<em>log4j.jar</em> file with <em>log4j-over-slf4j.jar</em>, as
	described below.
	</p>

	<h4 class="doAnchor" name="losHow">How does it work?</h4>

	<p>The log4j-over-slf4j module contains replacements of most widely
	used log4j classes, namely <code>org.apache.log4j.Category</code>,
	<code>org.apache.log4j.Logger</code>,
	<code>org.apache.log4j.Priority</code>,
	<code>org.apache.log4j.Level</code>,
	<code>org.apache.log4j.MDC</code>, and
	<code>org.apache.log4j.BasicConfigurator</code>. These replacement
	classes redirect all work to their corresponding SLF4J classes.
	</p>

	<p>To use log4j-over-slf4j in your own application, the first step
	is to locate and then to replace <em>log4j.jar</em> with
	<em>log4j-over-slf4j.jar</em>. Note that you still need an SLF4J
	binding and its dependencies for log4j-over-slf4j to work properly.
	</p>

	<p>In most situations, replacing a jar file is all it takes in
	order to migrate from log4j to SLF4J.
	</p>

	<p>Note that as a result of this migration, log4j configuration
	files will no longer be picked up. If you need to migrate your
	log4j.properties file to logback, the <a
	href="http://logback.qos.ch/translator/">log4j translator</a> might
	be of help. For configuring logback, please refer to <a
	href="http://logback.qos.ch/manual/index.html">its manual</a>.
	</p>

	<h4 class="doAnchor" name="losFail">When does it not work?</h4>

	<p>The <em>log4j-over-slf4j</em> module will not work when the
	application calls log4j components that are not present in the
	bridge. For example, when application code directly references
	log4j appenders, filters or the PropertyConfigurator, then
	log4j-over-slf4j would be an insufficient replacement for
	log4j. However, when log4j is configured through a configuration
	file, be it <em>log4j.properties</em> or <em>log4j.xml</em>, the
	log4j-over-slf4j module should just work fine.
	</p>



	<h4 class="doAnchor" name="losOverhead">What about the
	overhead?</h4>

	<p>There overhead of using log4j-over-slf4j instead of log4j
	directly is relatively small. Given that log4j-over-slf4j
	immediately delegates all work to SLF4J, the CPU overhead should be
	negligible, in the order of a few <em>nanoseconds</em>. There is a
	memory overhead corresponding to an entry in a hashmap per logger,
	which should be usually acceptable even for very large applications
	consisting of several thousand loggers. Moreover, if you choose
	logback as your underlying logging system, and given that logback is
	both much faster and more memory-efficient than log4j, the gains
	made by using logback should compensate for the overhead of using
	log4j-over-slf4j instead of log4j directly.
	</p>

	<h4 class="doAnchor" name="log4jRecursion">log4j-over-slf4j.jar
	and slf4j-log4j12.jar cannot be present simultaneously
	</h4>

	<p>The presence of <em>slf4j-log4j12.jar</em>, that is the log4j
	binding for SLF4J, will force all SLF4J calls to be delegated to
	log4j. The presence of <em>log4j-over-slf4j.jar</em> will in turn
	delegate all log4j API calls to their SLF4J equivalents. If both
	are present simultaneously, slf4j calls will be delegated to
	log4j, and log4j calls redirected to SLF4j, resulting in an <a
	href="codes.html#log4jDelegationLoop">endless loop</a>.
	</p>



	<h2 class="doAnchor" name="jul-to-slf4j">jul-to-slf4j bridge</h2>

	<p>The jul-to-slf4j module includes a java.util.logging (jul)
	handler, namely <code>SLF4JBridgeHandler</code>, which routes all
	incoming jul records to the SLF4j API. Please see <a
	href="api/org/slf4j/bridge/SLF4JBridgeHandler.html">SLF4JBridgeHandler
	javadocs</a> for usage instructions.
	</p>

	<p><span class="label notice">Note on performance</span> Contrary
	to other bridging modules, namely jcl-over-slf4j and
	log4j-over-slf4j, which reimplement JCL and respectively log4j,
	the jul-to-slf4j module does not reimplement the java.util.logging
	because packages under the java.* namespace cannot be
	replaced. Instead, jul-to-slf4j translates <a
	href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/logging/LogRecord.html?is-external=true">LogRecord</a>
	objects into their SLF4J equivalent. Please note this translation
	process incurs the cost of constructing a <code>LogRecord</code>
	instance regardless of whether the SLF4J logger is disabled for
	the given level or nor. <b>Consequently, j.u.l. to SLF4J
	translation can seriously increase the cost of disabled logging
	statements (60 fold or 6000%) and measurably impact the
	performance of enabled log statements (20% overall increase).</b>
	As of logback version 0.9.25, it is possible to completely
	eliminate the 60 fold translation overhead for disabled log
	statements with the help of <a
	href="http://logback.qos.ch/manual/configuration.html#LevelChangePropagator">LevelChangePropagator</a>.
	</p>

	<p>If you are concerned about application performance, then use of
	<code>SLF4JBridgeHandler</code> is appropriate only if any one of
	the following two conditions is true:
	</p>
	<ol>
	<li>few j.u.l. logging statements are in play </li>
	<li><code>LevelChangePropagator</code> has been installed</li>
	</ol>


	<h4 class="doAnchor" name="julRecursion">jul-to-slf4j.jar and slf4j-jdk14.jar cannot be present
	simultaneously
	</h4>

	<p>The presence of slf4j-jdk14.jar, that is the jul binding for
	SLF4J, will force SLF4J calls to be delegated to jul. On the other
	hand, the presence of jul-to-slf4j.jar, plus the installation of
	SLF4JBridgeHandler, by invoking "SLF4JBridgeHandler.install()" will
	route jul records to SLF4J. Thus, if both jar are present
	simultaneously (and SLF4JBridgeHandler is installed), slf4j calls
	will be delegated to jul and jul records will be routed to SLF4J,
	resulting in an endless loop.
	</p>


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