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<document>
    <header>
        <title>Apache POI - Logging Framework</title>
        <authors>
            <person id="DS" name="Dominik Stadler" email="centic@apache.org"/>
            <person id="MV" name="Marius Volkhart" email="mariusvolkhart@apache.org"/>
        </authors>
    </header>

    <body>
        <section>
            <title>Introduction</title>
            <p>
                Logging in POI is used primarily as a debugging mechanism, not a normal runtime
                logging system. Logging at levels noisier than WARN is ONLY for autopsy type debugging, and should
                NEVER be enabled on a production system.
            </p>
        </section>
        <section>
            <title>POI 5.0.0 and above</title>
            <p>
                POI 5.0.0 switched to using <a href="https://www.slf4j.org/">SLF4J</a> for logging. If you want
                to enable logging, please read up on the various SLF4J compatible logging frameworks.
                <a href="https://logging.apache.org/log4j/2.x/">Apache Log4j v2</a> is a good choice.
                <a href="https://logback.qos.ch/">Logback</a> is also widely used.
            </p>
            <p>
                POI 5.1.0 goes further and specifically uses <a href="https://logging.apache.org/log4j/2.x/">Apache Log4j v2</a> directly.
                If you want to continue to use another SLF4J compatible logging framework, you can deploy the
                <a href="https://logging.apache.org/log4j/log4j-2.2/log4j-to-slf4j/index.html">log4j-to-slf4j</a> jar to
                facilitate this.
            </p>
            <p>
                POI tries to name loggers after the canonical name of the containing class. For example,
                <code>org.apache.poi.poifs.filesystem.POIFSFileSystem</code>. Use your logging framework's typical
                mechanisms for activating and deactivating logging for specific loggers.
            </p>
            <p>
                All loggers are named <code>com.apache.poi.*</code>, so rules applied to <code>com.apache.poi</code>
                will affect all POI loggers.
            </p>
        </section>
        <section>
            <title>Logging with Log4j 2 Core</title>
            <p>
                Capturing POI logs using Log4j 2 Core is as simple as including the
                <a href="https://logging.apache.org/log4j/2.x/maven-artifacts.html"><code>log4j-core</code></a> JAR in
                your project. POI also has dependencies on libraries that make use of the SLF4J and Apache Commons
                Logging APIs. Gather logs from these dependencies by adding the
                <a href="https://logging.apache.org/log4j/2.x/log4j-jcl/index.html">Commons Logging Bridge</a> and the
                the <a href="https://logging.apache.org/log4j/2.x/log4j-slf4j-impl/index.html">SLF4J Binding</a> to your
                project.
            </p>

            <p>
                The simplest configuration is to capture all POI logs at the same level as your application. You might
                want to collect all messages <code>INFO</code> and higher, and are OK with capturing POI messages as well.
            </p>
            <source>
                &lt;Configuration ...&gt;
                    &lt;Loggers&gt;
                        &lt;Root level="INFO"&gt;
                            ...
                        &lt;/Root&gt;
                    &lt;/Loggers&gt;
                &lt;/Configuration&gt;
            </source>

            <p>
                A more recommended configuration is to capture only messages from loggers you opt in to. For example,
                you might want to capture all messages from <code>com.example.myapplication</code> at <code>INFO</code>
                but only POI messages at <code>WARN</code> or more severe.
            </p>
            <source>
                &lt;Configuration ...&gt;
                    &lt;Loggers&gt;
                        &lt;Logger name="com.example.myapplication" level="INFO" /&gt;
                        &lt;Logger name="org.apache.poi" level="WARN" /&gt;

                        &lt;Root level="OFF"&gt;
                            ...
                        &lt;/Root&gt;
                    &lt;/Loggers&gt;
                &lt;/Configuration&gt;
            </source>

            <p>Another strategy you may decide to use is to capture all messages except those coming from POI.</p>
            <source>
                &lt;Configuration ...&gt;
                    &lt;Loggers&gt;
                        &lt;Logger name="org.apache.poi" level="OFF" /&gt;

                        &lt;Root level="INFO"&gt;
                            ...
                        &lt;/Root&gt;
                    &lt;/Loggers&gt;
                &lt;/Configuration&gt;
            </source>
        </section>
        <section>
            <title>Logging with Logback</title>
            <p>
                Capturing POI logs using Logback requires adding the
                <a href="https://logging.apache.org/log4j/2.x/log4j-to-slf4j/index.html">Log4j to SLF4J Adapter</a> to
                your project, along with the standard Logback dependencies. POI also has dependencies on libraries that
                make use of the SLF4J and Apache Commons Logging APIs. Gather logs from these dependencies by adding the
                <a href="https://www.slf4j.org/legacy.html#jcl-over-slf4j">Commons Logging Bridge</a> to your project.
            </p>

            <p>
                The simplest configuration is to capture all POI logs at the same level as your application. You might
                want to collect all messages <code>INFO</code> and higher, and are OK with capturing POI messages as well.
            </p>
            <source>
                &lt;configuration ...&gt;
                    &lt;root level="INFO"&gt;
                        ...
                    &lt;/root&gt;
                &lt;/configuration&gt;
            </source>

            <p>
                A more recommended configuration is to capture only messages from loggers you opt in to. For example,
                you might want to capture all messages from <code>com.example.myapplication</code> at <code>INFO</code>
                but only POI messages at <code>WARN</code> or more severe.
            </p>
            <source>
                &lt;configuration ...&gt;
                    &lt;logger name="com.example.myapplication" level="INFO" /&gt;
                    &lt;logger name="org.apache.poi" level="WARN" /&gt;

                    &lt;root level="OFF"&gt;
                        ...
                    &lt;/root&gt;
                &lt;/configuration&gt;
            </source>

            <p>Another strategy you may decide to use is to capture all messages except those coming from POI.</p>
            <source>
                &lt;configuration ...&gt;
                    &lt;logger name="org.apache.poi" level="OFF" /&gt;

                    &lt;root level="INFO"&gt;
                        ...
                    &lt;/root&gt;
                &lt;/configuration&gt;
            </source>
        </section>
        <section>
            <title>Legacy POI Logging Framework (no longer supported in POI 5.0.0 and above)</title>
            <p>
                Prior to POI 5.0.0, POI used a custom logging framework which allows to configure where logs are sent to.
            </p>
            <p>
                Logging in POI 3 and 4 is used only as a debugging mechanism, not as a normal runtime
                logging system.  Logging at level debug/info is ONLY for debugging, and should
                NEVER be enabled on a production system.
            </p>
            <p>
                The framework is extensible so that you can send log messages to any logging framework
                that your application uses.
            </p>
            <p>
                A number of default logging implementations are supported by POI out-of-the-box and can be selected via a
                system property.
            </p>
        </section>
        <section><title>POI 4.x and before: Enable Legacy POI Logging Framework</title>
            <p>
                By default, logging is disabled in POI 3 and 4. Sometimes, it might be useful
                to enable logging to see some debug messages printed out which can
                help in analyzing problems.
            </p>
            <p>
                You can select the logging framework by setting the system property <em>org.apache.poi.util.POILogger</em> during application startup or by calling System.setProperty():
            </p>
            <source>
                System.setProperty("org.apache.poi.util.POILogger", "org.apache.poi.util.CommonsLogger" );
            </source>
            <p>
                Note: You need to call <em>setProperty()</em> before any POI functionality is invoked as the logger is only initialized during startup.
            </p>
        </section>
        <section><title>POI 4.x and before: Available Legacy POI Logging Framework implementations</title>
            <p>
                The following logger implementations are provided by POI 3 and 4:
            </p>
            <table>
                <tr>
                    <th>Class</th>
                    <th>Type</th>
                </tr>
                <tr>
                    <td>org.apache.poi.util.SystemOutLogger</td>
                    <td>Sends log output to the system console</td>
                </tr>
                <tr>
                    <td>org.apache.poi.util.NullLogger</td>
                    <td>Default logger, does not log anything</td>
                </tr>
                <tr>
                    <td>org.apache.poi.util.CommonsLogger</td>
                    <td>Allows to use <a href="https://commons.apache.org/proper/commons-logging/">Apache Commons Logging</a> for logging. This can use JDK1.4 logging,
                        log4j, logkit, etc. The log4j dependency was removed in POI 5.0.0, so you will need to include this dependency yourself if you need it.</td>
                </tr>
                <tr>
                    <td>org.apache.poi.util.DummyPOILogger</td>
                    <td>Simple logger which will keep all log-lines in memory for later analysis (this class is not in the jar, just in the test source).
                        Used primarily for testing. Note: this may cause a memory leak if used in production application!</td>
                </tr>
            </table>
        </section>
        <section><title>POI 4.x and before: Sending logs to a different log framework</title>
            <p>
                You can send logs to other logging frameworks by implementing the interface <em>org.apache.poi.util.POILogger</em>.
            </p>
        </section>
        <section><title>POI 4.x and before: Implementation details</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>
                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>
        </section>
    </body>
    <footer>
        <legal>
            Copyright (c) @year@ The Apache Software Foundation All rights reserved.
            <br />
            Apache POI, POI, Apache, the Apache feather logo, and the Apache
            POI project logo are trademarks of The Apache Software Foundation.
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