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| <h2 id="usage">Usage</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Using the Kotlin API is as simple as mixing in the <a href="https://github.com/apache/logging-log4j-kotlin/blob/master/log4j-api-kotlin/src/main/kotlin/org/apache/logging/log4j/kotlin/Logging.kt"><code>Logging</code></a> interface to your class. Example:</p> |
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| <div class="content"> |
| <pre class="highlight"><code class="language-kotlin" data-lang="kotlin">import org.apache.logging.log4j.kotlin.Logging |
| |
| class MyClass: BaseClass, Logging { |
| fun doStuff() { |
| logger.info("Doing stuff") |
| } |
| fun doStuffWithUser(user: User) { |
| logger.info { "Doing stuff with ${user.name}." } |
| } |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The <code>Logging</code> interface can also be mixed into <code>object</code> declarations, including companions. This is generally preferable over the previous approach as there is a single logger created for every instance of the class.</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code class="language-kotlin" data-lang="kotlin">import org.apache.logging.log4j.kotlin.Logging |
| |
| class MyClass: BaseClass { |
| companion object : Logging |
| |
| ... |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Alternatively, a more traditional style can be used to instantiate a logger instance:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code class="language-kotlin" data-lang="kotlin">import org.apache.logging.log4j.kotlin |
| |
| class MyClass: BaseClass { |
| val logger = logger() |
| |
| ... |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The function <code>logger()</code> is an extension function on the <code>Any</code> type (or more specifically, any type <code>T</code> that extends <code>Any</code>).</p> |
| </div> |
| <div class="sect2"> |
| <h3 id="api_documentation">API Documentation</h3> |
| <div class="paragraph"> |
| <p>See <a href="https://logging.apache.org/TODO">KDocs</a>.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="configuration">Configuration</h3> |
| <div class="paragraph"> |
| <p>Log4j Kotlin API uses <a href="https://logging.apache.org/log4j/2.x/manual/configuration.html">Log4j configuration</a> by default.</p> |
| </div> |
| <div class="paragraph"> |
| <p>This supports XML, properties files, and Java-based builders, as well as JSON and YAML with additional dependencies.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="substituting_parameters">Substituting Parameters</h3> |
| <div class="paragraph"> |
| <p>Unlike Java, Kotlin provides native functionality for <a href="https://kotlinlang.org/docs/reference/basic-syntax.html#using-string-templates">string templates</a>.</p> |
| </div> |
| <div class="paragraph"> |
| <p>However, using a string template still incurs the message construction cost if the logger level is not enabled. To avoid this, prefer passing a lambda which won’t be evaluated until necessary.</p> |
| </div> |
| <div class="paragraph"> |
| <p>For example:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code class="language-kotlin" data-lang="kotlin">logger.debug { "Logging in user ${user.name} with birthday ${user.calcBirthday()}" }</code></pre> |
| </div> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="logger_names">Logger Names</h3> |
| <div class="paragraph"> |
| <p>Most logging implementations use a hierarchical scheme for matching logger names with logging configuration.</p> |
| </div> |
| <div class="paragraph"> |
| <p>In this scheme the logger name hierarchy is represented by '.' characters in the logger name, in a fashion very similar to the hierarchy used for Java/Kotlin package names.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The <code>Logger</code> property added by the <code>Logging</code> interface follows this convention: the interface ensures the <code>Logger</code> is automatically named according to the class it is being used in.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The value returned when calling the <code>logger()</code> extension method depends on the receiver of the extension. When called within an Object as shown above, the receiver is <code>this</code> and therefore the logger will again be named according to the class it is being used in. However, a logger named via another class can be obtained as well:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code class="language-kotlin" data-lang="kotlin">import org.apache.logging.log4j.kotlin |
| |
| class MyClass: BaseClass { |
| val logger = SomeOtherClass.logger() |
| |
| ... |
| }</code></pre> |
| </div> |
| </div> |
| <div class="sect3"> |
| <h4 id="explicitly_named_loggers">Explicitly Named Loggers</h4> |
| <div class="paragraph"> |
| <p>An explicitly-named logger may be obtained via the <code>logger</code> function that takes a <code>name</code> parameter:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code class="language-kotlin" data-lang="kotlin">import org.apache.logging.log4j.kotlin |
| |
| class MyClass: BaseClass { |
| val logger = logger("MyCustomLoggerName") |
| |
| ... |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>This is also needed in scopes that do not have a <code>this</code> Object, such as top-level functions.</p> |
| </div> |
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