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<!DOCTYPE document PUBLIC "-//APACHE//DTD Documentation V1.1//EN" "../dtd/document-v11.dtd">
<document>
<header>
<title>POI Utils</title>
<subtitle>Overview</subtitle>
<authors>
<person name="Nicola Ken Barozzi" email="nicolaken@apache.org"/>
<person name="Andrew C. Oliver" email="acoliver@apache.org"/>
</authors>
</header>
<body>
<section><title>Logging</title>
<p>
Logging in POI is used only as a debugging mechanism, not a normal runtime
logging system. Logging is ONLY for autopsie type debugging, and should
NEVER be enabled on a production system. Enabling logging will reduce
performance by at least a factor of 100. If you are not developing
POI or trying to debug why POI isn't reading a file correctly, then DO
NOT enable logging. You've been warned.
</p>
<p>
Hence, we need to be able to easily disable it entirely and make POI not dependent
on any logging package.
</p>
<warning>
POI is not dependent on commons-logging for running, but not for compiling.
</warning>
<section><title>Logging Overview</title>
<p>
Every class uses a <code>POILogger</code> to log, and gets it using a static method
of the <code>POILogFactory</code> .
</p>
<p>
The <code>POILogFactory</code> uses the <code>NullLogger</code> by default;
it can be instructed to use any other <code>POILogger</code> implementation
by setting the system property <code>org.apache.poi.util.POILogger</code>.
</p>
<note> java -Dorg.apache.poi.util.POILogger=the.package.of.MyPoiLoggerImpl ProgramThatUsesPoi
</note>
<fixme author="nicolaken"> Still needs testing.
</fixme>
</section>
<section><title>POILogFactory</title>
<p>
Each class in POI can get its <code>POILogger</code> by calling a static method
of the <code>POILogFactory</code> .
</p>
</section>
<section><title>POILogger</title>
<p>
Each class in POI can log using a <code>POILogger</code>, which is an abstract class.
We decided to make our own logging facade because:</p>
<ol>
<li>we need to log many values and we put many methods in this class to facilitate the
programmer, without having him write string concatenations;</li>
<li>we need to be able to use POI without any logger package present.</li>
</ol>
<p>There are three implementations available, and you can roll out your own, just
extend <code>org.apache.poi.util.POILogger</code>.
</p>
<section><title>NullLogger</title>
<p>Discards every logging request.</p>
</section>
<section><title>SystemOutLogger</title>
<p>Sends every logging request to System.out.</p>
</section>
<section><title>CommonsLogger</title>
<p>Sends every logging request to the Commons Logging package. This can use JDK1.4 logging,
log4j, logkit, and is an actively maintained Jakarta Project.</p>
</section>
</section>
</section>
</body>
</document>