Log4j_Kotlin

Usage

Using the Kotlin API is as simple as mixing in the Logging interface to your class. Example:

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.

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:

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 KDocs.

Configuration

Log4j Kotlin API uses 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 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:

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:

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:

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.