POI uses a custom logging framework which allows to configure where logs are sent to.

                POI uses the <a href="">Log4j 2</a> logging API.

                Logging in POI is used only as a debugging mechanism, not a normal runtime

                Logging in POI is used primarily as a debugging mechanism, not a normal runtime

                logging system.  Logging on level debug/info is ONLY for autopsie type debugging, and should

                logging system.  Logging at levels noisier than WARN is ONLY for autopsy type debugging, and should

        <section><title>Enable logging</title>

        <section><title>General logging behavior</title>

            By default, logging is disabled in POI. Sometimes it might be useful

            POI tries to name loggers after the canonical name of the containing class. For example,

            to enable logging to see some debug messages printed out which can

            <code>org.apache.poi.poifs.filesystem.POIFSFileSystem</code>. Use your logging framework's typical mechanisms

            help in analyzing problems.

            for activating and deactivating logging for specific loggers.

            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():

            All loggers are named <code>com.apache.poi.*</code>, so rules applied to <code>com.apache.poi</code> will

            affect all POI loggers.

        <section><title>Available logger implementations</title>

        <section><title>Log4j 2 Core Examples</title>

                The following logger implementations are provided by POI:

                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.

                You can send logs to other logging frameworks by implementing the interface <em>org.apache.poi.util.POILogger</em>.

                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>

                <Configuration ...>

                    <Loggers>

                        <Root level="INFO">

                            ...

                        </Root>

                    </Loggers>

                </Configuration>

            </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.

        <section><title>Implementation details</title>

        <section><title>Logback Examples</title>

                Every class uses a <code>POILogger</code> to log, and gets it using a static method

                Capturing POI logs using Logback requires adding the

                of the <code>POILogFactory</code> .

                <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.

                Each class in POI can log using a <code>POILogger</code>, which is an abstract class.

                The simplest configuration is to capture all POI logs at the same level as your application. You might

                We decided to make our own logging facade because:</p>

                want to collect all messages <code>INFO</code> and higher, and are OK with capturing POI messages as well.

            <ol>

            </p>

                <li>we need to log many values and we put many methods in this class to facilitate the

            <source>

                    programmer, without having him write string concatenations;</li>

                <configuration ...>

                <li>we need to be able to use POI without any logger package present.</li>

                    <root level="INFO">

            </ol>

                        ...

                    </root>

                </configuration>

            </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>

                <configuration ...>

                    <logger name="com.example.myapplication" level="INFO" />

                    <logger name="org.apache.poi" level="WARN" />

                    <root level="OFF">

                        ...

                    </root>

                </configuration>

            </source>

            <p>Another strategy you may decide to use is to capture all messages except those coming from POI.</p>

            <source>

                <configuration ...>

                    <logger name="org.apache.poi" level="OFF" />

                    <root level="INFO">

                        ...

                    </root>

                </configuration>

            </source>