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<h1>
Users Guide : Jakarta Commons Logging</h1>
<i>1.0 Version</i>
<h3>
Table of Contents</h3>
<a href="#Introduction">Introduction</a>
<br><a href="#Users">Users Quick Start</a>
<br><a href="#Developers">Developers</a>
<br>&nbsp; <a href="#Best Practices">Best Practices</a>
<br><a href="#Integration">Integration</a>
<br>&nbsp; <a href="#Mechanism">Mechanism</a>
<br>&nbsp; <a href="#Configuring the Logger Implementation">Configuring the Logger Implementation</a>
<br><a href="#FAQ">Frequently Asked Questions</a>
<br>&nbsp; <a href="#Thread Safety">Is JCL Thread Safe?</a>
<h2>
<a NAME="Introduction"></a>Introduction</h2>
<p>The Jakarta Commons Logging (JCL) provides a Log interface that
is intended to be both light-weight and independent of numerous logging toolkits.
It provides the middleware/tooling developer a simple
logging abstraction, that allows the user (application developer) to plug in
a specific logging implementation.
</p>
<p>Familiarity with high-level details of various Logging implementations is presumed.
</p>
<p>The Jakarta Commons Logging provides a Log interface with thin-wrapper implementations for
other logging tools, including
<a href="http://jakarta.apache.org/log4j/docs/index.html">Log4J</a>,
<a href="http://jakarta.apache.org/avalon/logkit/index.html">Avalon LogKit</a>,
the Avalon Framework's logging infrastructure,
JDK 1.4, and an implementation of JDK 1.4 logging APIs (JSR-47) for pre-1.4
systems.
The interface maps closely to Log4J and LogKit.
</p>
<h2><a NAME="Users"></a>Users Quick Start</h2>
<p>
As far as possible, <em>Commons-Logging</em> tries to be as unobtrusive as possible.
In most cases, including the (full) <code>commons-logging.jar</code> in the classpath
should result in <em>Commons-Logging</em> configuring itself in a reasonable manner.
There's a good chance that it'll guess your preferred logging system and you won't
need to do any configuration at all!
</p>
<h3>Configuration</h3>
<p>
There are two base abstractions used by <em>Commons-Logging</em>: <code>Log</code>
(the basic logger) and <code>LogFactory</code> (which knows how to create <code>Log</code>
instances). Using <code>LogFactory</code> implementations other than the default is a
subject for advanced users only, so let's concentrate on configuring the default
implementation.
</p>
<p>
The default <code>LogFactory</code> implementation uses the following discovery process
to determine what type of <code>Log</code> implementation it should use
(the process terminates when the first positive match - in order - is found):
<ol>
<li>Look for a configuration attribute of this factory named
<code>org.apache.commons.logging.Log</code> (for backwards
compatibility to pre-1.0 versions of this API, an attribute
<code>org.apache.commons.logging.log</code> is also consulted).</li>
<li>Look for a system property named
<code>org.apache.commons.logging.Log</code> (for backwards
compatibility to pre-1.0 versions of this API, a system property
<code>org.apache.commons.logging.log</code> is also consulted).</li>
<li>If the Log4J logging system is available in the application
class path, use the corresponding wrapper class
(<a href="http://jakarta.apache.org/commons/logging/api/org/apache/commons/logging/impl/Log4JLogger.html">Log4JLogger</a>).</li>
<li>If the application is executing on a JDK 1.4 system, use
the corresponding wrapper class
(<a href="http://jakarta.apache.org/commons/logging/api/org/apache/commons/logging/impl/Jdk14Logger.html">Jdk14Logger</a>).</li>
<li>Fall back to the default simple logging wrapper
(<a href="http://jakarta.apache.org/commons/logging/api/org/apache/commons/logging/impl/SimpleLog.html">SimpleLog</a>).</li>
</ol>
Consult the <em>Commons-Logging</em> javadocs for details of the various <code>Log</code>
implementations that ship with the component. (The discovery process is also covered in more
detail there.)
</p>
<h2><a NAME="Developers"></a>Developers</h2>
<p>
To use the JCL SPI from a Java class,
include the following import statements:
<ul>
<code>
import org.apache.commons.logging.Log;
<br>
import org.apache.commons.logging.LogFactory;
<br>&nbsp;
</code>
</ul>
Note that some components using commons-logging may
either extend Log,
or provide a component-specific LogFactory implementation.
Review the component documentation for guidelines
on how commons-logging should be used in such components.
</p>
<p>
For each class definition, declare and initialize a
<code>log</code> attribute as follows:
<ul>
<code>
public class CLASS
<br>{
<br>&nbsp;&nbsp;&nbsp;private static Log log = LogFactory.getLog(CLASS.class);
<br>&nbsp;&nbsp;&nbsp;...
<br>&nbsp;
</code>
</ul>
<p>
Messages are logged to a <i>logger</i>, such as <code>log</code>
by invoking a method corresponding to <i>priority</i>.
The <code>org.apache.commons.logging.Log</code> interface defines the
following methods for use
in writing log/trace messages to the log:
<ul>
<code>
<br>log.fatal(Object message);
<br>log.fatal(Object message, Throwable t);
<br>log.error(Object message);
<br>log.error(Object message, Throwable t);
<br>log.warn(Object message);
<br>log.warn(Object message, Throwable t);
<br>log.info(Object message);
<br>log.info(Object message, Throwable t);
<br>log.debug(Object message);
<br>log.debug(Object message, Throwable t);
<br>log.trace(Object message);
<br>log.trace(Object message, Throwable t);
</code>
</ul>
<p>
Semantics for these methods are such that it is expected
that the severity, from highest to lowest, of messages is ordered as above.
</p>
<p>
In addition to the logging methods, the following are provided for code guards:
</p>
<ul>
<code>
log.isFatalEnabled();
<br>log.isErrorEnabled();
<br>log.isWarnEnabled();
<br>log.isInfoEnabled();
<br>log.isDebugEnabled();
<br>log.isTraceEnabled();
</code>
</ul>
<h3><a NAME="Best Practices">Best Practices</a></h3>
<p>Best practices for programming/planning are presented in two categories: General and Enterprise.
The general principles are fairly clear. Enterprise practices are a bit more involved
and it is not always as clear as to why they are important.
</p>
<p>
Enterprise best-practice principles apply to middleware components
and tooling that is expected to execute in an "Enterprise" level
environment.
These issues relate to Logging as Internationalization,
and fault detection.
Enterprise requires more effort and planning, but are strongly encouraged (if not required)
in production level systems. Different corporate enterprises/environments have different
requirements, so being flexible always helps.
</p>
<h4>General - Code Guards</h4>
<p>
Code guards are typically used to guard code that
only needs to execute in support of logging,
that otherwise introduces undesirable runtime overhead
in the general case (logging disabled).
Examples are multiple parameters, or expressions (i.e. string + " more") for parameters.
Use the guard methods of the form <code>log.is&lt;<i>Priority</i>>()</code> to verify
that logging should be performed, before incurring the overhead of the logging method call.
Yes, the logging methods will perform the same check, but only after resolving parameters.
</p>
<h4>General - Message Priorities/Levels</h4>
It is important to ensure that log message are
appropriate in content and severity.
The following guidelines are suggested:</a>
<ul>
<li><b>fatal</b> - Severe errors that cause premature termination.
Expect these to be immediately visible on a status console.
See also <a HREF="#Internationalization">Internationalization</a>.
</li>
<br>&nbsp;
<li><b>error</b> - Other runtime errors or unexpected conditions.
Expect these to be immediately visible on a status console.
See also <a HREF="#Internationalization">Internationalization</a>.
</li>
<br>&nbsp;
<li><b>warn</b> - Use of deprecated APIs, poor use of API, 'almost' errors,
other runtime situations that are undesirable or unexpected, but not
necessarily "wrong".
Expect these to be immediately visible on a status console.
See also <a HREF="#Internationalization">Internationalization</a>.
</li>
<br>&nbsp;
<li><b>info</b> - Interesting runtime events (startup/shutdown).
Expect these to be immediately visible on a console,
so be conservative and keep to a minimum.
See also <a HREF="#Internationalization">Internationalization</a>.
</li>
<br>&nbsp;
<li><b>debug</b> - detailed information on flow of through the system.
Expect these to be written to logs only.
</li>
<br>&nbsp;
<li><b>trace</b> - more detailed information.
Expect these to be written to logs only.
</li>
</ul>
<h4>General - Default Message Priority/Level</h4>
<p>
By default the message priority should be no lower than <b>info</b>.
That is, by default <b>debug</b> message should not be seen in the logs.
</p>
<h4>Enterprise - Logging Exceptions</h4>
The general rule in dealing with exceptions is to assume that
the user (developer using a tooling/middleware API) isn't going
to follow the rules.
Since any problems that result are going to be assigned to you,
it's in your best interest to be prepared with the proactive
tools necessary to demonstrate that your component works correctly,
or at worst that the problem analyzed from your logs.
For this discussion, we must make a distinction between different types of exceptions
based on what kind of boundaries they cross:
<ul>
<li><p><b>External Boundaries - Expected Exceptions</b>.
This classification includes exceptions such as <code>FileNotFoundException</code>
that cross API/SPI boundaries, and are exposed to the user of a component/toolkit.
These are listed in the 'throws' clause of a method signature.
</p>
<p>
Appropriate handling of these exceptions depends upon the type
of code you are developing.
API's for utility functions and tools should log these at the <b>debug</b> level,
if they are caught at all by internal code.
</p>
<p>For higher level frameworks and middleware components,
these exceptions should be caught immediatly prior to crossing
the API/SPI interface back to user code-space,
logged with full stack trace at <b>info</b> level,
and rethrown.
The assures that the log contains a record of the root cause for
future analysis <i>in the event that the exception is not caught and resolved
as expected by the user's code</i>.
</p>
</li>
<li><p><b>External Boundaries - Unexpected Exceptions</b>.
This classification includes exceptions such as <code>NullPointerException</code>
that cross API/SPI boundaries, and are exposed to the user of a component/toolkit.
These are runtime exceptions/error that are NOT
listed in the 'throws' clause of a method signature.
</p>
<p>
Appropriate handling of these exceptions depends upon the type
of code you are developing.
API's for utility functions and tools should log these at the <b>debug</b> level,
if they are caught at all.
</p>
<p>For higher level frameworks and middleware components,
these exceptions should be caught immediatly prior to crossing
the API/SPI interface back to user code-space,
logged with full stack trace at <b>info</b> level,
and rethrown/wrapped as <code><i>Component</i>InternalError</code>.
The assures that the log contains a record of the root cause for
future analysis <i>in the event that the exception is not caught and
logged/reported as expected by the user's code</i>.
</p></li>
<li><p><b>Internal Boundaries</b>.
Exceptions that occur internally and are resolved internally.
These should be logged when caught as <b>debug</b> or <b>info</b> messages,
at the programmer's discretion.
</p></li>
<li><p><b>Significant Internal Boundaries</b>.
This typically only applies to middleware components
that span networks or runtime processes.
Exceptions that cross over significant internal component boundaries, such as networks.
These should be logged when caught as <b>info</b> messages.
Do not assume that such a (process/network) boundary will deliver exceptions to the 'other side'.
</p></li>
</ul>
<h5>Why info level instead of debug?</h5>
You want to have exception/problem information available for
first-pass problem determination in a production level
enterprise application without turning on <b>debug</b>
as a default log level. There is simply too much information
in <b>debug</b> to be appropriate for day-to-day operations.
<h5>More Control of Enterprise Exception Logging</h5>
<p>If more control is desired for the level of detail of these
'enterprise' exceptions, then consider creating a special
logger just for these exceptions:
<ul>
<code>
&nbsp;&nbsp;&nbsp;Log log = LogFactory.getLog("org.apache.<i>component</i>.enterprise");
</code>
</ul>
This allows the 'enterprise' level information to be turned on/off explicitly
by most logger implementations.
</p>
<a NAME="Internationalization"></a>
<h4>Enterprise - National Language Support - Internationalization</h4>
<p>NLS internationalization involves looking up messages from
a message file by a message key, and using that message for logging.
There are various tools in Java, and provided by other components,
for working with NLS messages.
</p>
<p>
NLS enabled components are particularly appreciated
(thats an open-source-correct term for 'required by corporate end-users' :-)
for <strong>tooling</strong> and <strong>middleware</strong> components.
</p>
<p>NLS internationalization SHOULD be strongly considered for used for
<b>fatal</b>, <b>error</b>, <b>warn</b>, and <b>info</b> messages.
It is generally considered optional for <b>debug</b> and <b>trace</b> messages.
</p>
<p>Perhaps more direct support for internationalizing log messages
can be introduced in a future or alternate version of the <code>Log</code> interface.
</p>
<h2><a NAME="Integration"></a>Integration</h2>
<p>The minimum requirement to integrate with another logger
is to provide an implementation of the
<code>org.apache.commons.logging.Log</code> interface.
In addition, an implementation of the
<code>org.apache.commons.logging.LogFactory</code> interface
can be provided to meet
specific requirements for connecting to, or instantiating, a logger.
</p>
<ul>
<li><h4>org.apache.commons.logging.Log</h4></li>
<p>
The default <code>LogFactory</code> provided by JCL
can be configured to instantiate a specific implementation of the
<code>org.apache.commons.logging.Log</code> interface
by setting the property of the same name (<code>org.apache.commons.logging.Log</code>).
This property can be specified as a system property,
or in the <code>commons-logging.properties</code> file,
which must exist in the CLASSPATH.
</p>
<h4>Default logger if not plugged</h4>
<p>
The Jakarta Commons Logging SPI uses the
implementation of the <code>org.apache.commons.logging.Log</code>
interface specified by the system property
<code>org.apache.commons.logging.Log</code>.
If the property is not specified or the class is not available then the JCL
provides access to a default logging toolkit by searching the CLASSPATH
for the following toolkits, in order of preference:
<ul>
<li><a href="http://jakarta.apache.org/log4j/docs/index.html">Log4J</a></li>
<li>JDK 1.4</li>
<li>JCL SimpleLog</li>
</ul>
</p>
<li><h4>org.apache.commons.logging.LogFactory</h4></li>
<p>
If desired, the default implementation of the
<code>org.apache.commons.logging.LogFactory</code>
interface can be overridden,
allowing the JDK 1.3 Service Provider discovery process
to locate and create a LogFactory specific to the needs of the application.
Review the Javadoc for the <code>LogFactoryImpl.java</code>
for details.
<p>
</ul>
<h3><a NAME="Mechanism">Mechanism</a></h3>
<ul>
<li><h5>Life cycle</h5></li>
The JCL LogFactory implementation must assume responsibility for
either connecting/disconnecting to a logging toolkit,
or instantiating/initializing/destroying a logging toolkit.
<br>&nbsp;
<li><h5>Exception handling</h5></li>
The JCL Log interface doesn't specify any exceptions to be handled,
the implementation must catch any exceptions.
<br>&nbsp;
<li><h5>Multiple threads</h5></li>
The JCL Log and LogFactory implementations must ensure
that any synchronization required by the logging toolkit
is met.
</ul>
<h3><a NAME="Configuring the Logger Implementation">Configuring the Logger Implementation</a></h3>
The Jakarta Commons Logging (JCL) SPI
can be configured to use different logging toolkits.
<p>
Configuration of the behavior of the JCL ultimately depends upon the
logging toolkit being used.
The JCL SPI uses
<a href="http://jakarta.apache.org/log4j/docs/index.html">Log4J</a>
by default if it is available (in the CLASSPATH).
<h4>Log4J</h4>
As
<a href="http://jakarta.apache.org/log4j/docs/index.html">Log4J</a>
is the default logger,
a <i>few</i> details are presented herein to get the developer/integrator going.
<p>
Configure Log4J using system properties and/or a properties file:
<ul>
<li><h5>log4j.configuration=<i>log4j.properties</i></h5></li>
Use this system property to specify the name of a Log4J configuration file.
If not specified, the default configuration file is <i>log4j.properties</i>.
<br>&nbsp;
<li><h5>log4j.rootCategory=<i>priority</i> [, <i>appender</i>]*</h5></li>
Set the default (root) logger priority.
<br>&nbsp;
<li><h5>log4j.logger.<i>logger.name</i>=<i>priority</i></h5></li>
Set the priority for the named logger
and all loggers hierarchically lower than, or below, the
named logger.
<i>logger.name</i> corresponds to the parameter of
<code>LogFactory.getLog(<i>logger.name</i>)</code>,
used to create the logger instance. Priorities are:
<code>DEBUG</code>,
<code>INFO</code>,
<code>WARN</code>,
<code>ERROR</code>,
or <code>FATAL</code>.
<p>
Log4J understands hierarchical names,
enabling control by package or high-level qualifiers:
<code>log4j.logger.org.apache.component=DEBUG</code>
will enable debug messages for all classes in both
<code>org.apache.component</code>
and
<code>org.apache.component.sub</code>.
Likewise, setting
<code>log4j.logger.org.apache.component=DEBUG</code>
will enable debug message for all 'component' classes,
but not for other Jakarta projects.
<br>&nbsp;
<li><h5>log4j.appender.<i>appender</i>.Threshold=<i>priority</i></h5></li>
Log4J <i>appenders</i> correspond to different output devices:
console, files, sockets, and others.
If appender's <i>threshold</i>
is less than or equal to the message priority then
the message is written by that appender.
This allows different levels of detail to be appear
at different log destinations.
<p>
For example: one can capture DEBUG (and higher) level information in a logfile,
while limiting console output to INFO (and higher).
</ul>
<h2><a NAME="FAQ">Frequently Asked Questions</a></h2>
<h3><a NAME="Thread Safety">Is JCL Thread Safe?</a></h3>
<p> JCL doesn't (and cannot) impose any requirement on thread safety on the underlying implementation and thus its SPI contract doesn't guarantee thread safety. However, JCL can be safely used a multi-threaded environment as long as the underlying implementation is thread-safe.</p>
<p>It would be very unusual for a logging system to be thread unsafe. Certainly, JCL is thread safe when used with the distributed Log implementations.</p>
</body>
</html>

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@@ -181,7 +181,8 @@ In addition to the logging methods, the following are provided for code guards:
log.isTraceEnabled();
</source>
</ul>
<subsection name='Best Practices'>
</section>
<section name='Best Practices'>
<p>
Best practices for programming/planning are presented in two categories:
General and Enterprise.
@@ -198,7 +199,8 @@ Enterprise requires more effort and planning, but are strongly encouraged (if no
in production level systems. Different corporate enterprises/environments have different
requirements, so being flexible always helps.
</p>
<subsection name='General - Code Guards'>
<subsection name='General'>
<subsection name='Code Guards'>
<p>
Code guards are typically used to guard code that
only needs to execute in support of logging,
@@ -210,7 +212,7 @@ that logging should be performed, before incurring the overhead of the logging m
Yes, the logging methods will perform the same check, but only after resolving parameters.
</p>
</subsection>
<subsection name='General - Message Priorities/Levels'>
<subsection name='Message Priorities/Levels'>
<p>
It is important to ensure that log message are
appropriate in content and severity.
@@ -220,25 +222,29 @@ The following guidelines are suggested:
<li>
<b>fatal</b> - Severe errors that cause premature termination.
Expect these to be immediately visible on a status console.
See also <a HREF="#Internationalization">Internationalization</a>.
See also <a HREF="#National%20Language%20Support%20And%20Internationalization">
Internationalization</a>.
</li>
<li>
<b>error</b> - Other runtime errors or unexpected conditions.
Expect these to be immediately visible on a status console.
See also <a HREF="#Internationalization">Internationalization</a>.
See also <a HREF="#National%20Language%20Support%20And%20Internationalization">
Internationalization</a>.
</li>
<li>
<b>warn</b> - Use of deprecated APIs, poor use of API, 'almost' errors,
other runtime situations that are undesirable or unexpected, but not
necessarily "wrong".
Expect these to be immediately visible on a status console.
See also <a HREF="#Internationalization">Internationalization</a>.
See also <a HREF="#National%20Language%20Support%20And%20Internationalization">
Internationalization</a>.
</li>
<li>
<b>info</b> - Interesting runtime events (startup/shutdown).
Expect these to be immediately visible on a console,
so be conservative and keep to a minimum.
See also <a HREF="#Internationalization">Internationalization</a>.
See also <a HREF="#National%20Language%20Support%20And%20Internationalization">
Internationalization</a>.
</li>
<li>
<b>debug</b> - detailed information on the flow through the system.
@@ -250,13 +256,15 @@ Expect these to be written to logs only.
</li>
</ul>
</subsection>
<subsection name='General - Default Message Priority/Level'>
<subsection name='Default Message Priority/Level'>
<p>
By default the message priority should be no lower than <b>info</b>.
That is, by default <b>debug</b> message should not be seen in the logs.
</p>
</subsection>
<subsection name='Enterprise - Logging Exceptions'>
</subsection>
<subsection name='Enterprise'>
<subsection name='Logging Exceptions'>
<p>
The general rule in dealing with exceptions is to assume that
the user (developer using a tooling/middleware API) isn't going
@@ -352,8 +360,7 @@ This allows the 'enterprise' level information to be turned on/off explicitly
by most logger implementations.
</p>
</subsection>
</subsection>
<subsection name='Enterprise - National Language Support - Internationalization'>
<subsection name='National Language Support And Internationalization'>
<p>
NLS internationalization involves looking up messages from
a message file by a message key, and using that message for logging.
@@ -375,6 +382,7 @@ Perhaps more direct support for internationalizing log messages
can be introduced in a future or alternate version of the <code>Log</code> interface.
</p>
</subsection>
</subsection>
</section>
<section name='Integration'>
<p>