////
    Licensed to the Apache Software Foundation (ASF) under one or more
    contributor license agreements.  See the NOTICE file distributed with
    this work for additional information regarding copyright ownership.
    The ASF licenses this file to You under the Apache License, Version 2.0
    (the "License"); you may not use this file except in compliance with
    the License.  You may obtain a copy of the License at

         http://www.apache.org/licenses/LICENSE-2.0

    Unless required by applicable law or agreed to in writing, software
    distributed under the License is distributed on an "AS IS" BASIS,
    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    See the License for the specific language governing permissions and
    limitations under the License.
////
== Usage

Using the Kotlin API is as simple as mixing in the https://github.com/apache/logging-log4j-kotlin/blob/master/log4j-api-kotlin/src/main/kotlin/org/apache/logging/log4j/kotlin/Logging.kt[`Logging`] interface to your class. Example:

[source,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}." }
  }
}
----

The `Logging` interface can also be mixed into `object` declarations, including companions. This is generally preferable over the previous approach as there is a single logger created for every instance of the class.

[source,kotlin]
----
import org.apache.logging.log4j.kotlin.Logging

class MyClass: BaseClass {
  companion object : Logging

  ...
}
----

Alternatively, a more traditional style can be used to instantiate a logger instance:

[source,kotlin]
----
import org.apache.logging.log4j.kotlin

class MyClass: BaseClass {
  val logger = logger()

  ...
}
----

The function `logger()` is an extension function on the `Any` type (or more specifically, any type `T` that extends `Any`).

=== API Documentation

See https://logging.apache.org/TODO[KDocs].

=== Configuration

Log4j Kotlin API uses https://logging.apache.org/log4j/2.x/manual/configuration.html[Log4j configuration] by default.

This supports XML, properties files, and Java-based builders, as well as JSON and YAML with additional dependencies.

=== Substituting Parameters

Unlike Java, Kotlin provides native functionality for https://kotlinlang.org/docs/reference/basic-syntax.html#using-string-templates[string templates].

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.

For example:

[source,kotlin]
----
logger.debug { "Logging in user ${user.name} with birthday ${user.calcBirthday()}" }
----

=== Logger Names

Most logging implementations use a hierarchical scheme for matching logger names with logging configuration.

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.

The `Logger` property added by the `Logging` interface follows this convention: the interface ensures the `Logger` is automatically named according to the class it is being used in.

The value returned when calling the `logger()` extension method depends on the receiver of the extension. When called within an Object as shown above, the receiver is `this` 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:

[source,kotlin]
----
import org.apache.logging.log4j.kotlin

class MyClass: BaseClass {
  val logger = SomeOtherClass.logger()

  ...
}
----

==== Explicitly Named Loggers

An explicitly-named logger may be obtained via the `logger` function that takes a `name` parameter:

[source,kotlin]
----
import org.apache.logging.log4j.kotlin

class MyClass: BaseClass {
  val logger = logger("MyCustomLoggerName")

  ...
}
----

This is also needed in scopes that do not have a `this` Object, such as top-level functions.