Logging in Kotlin: Best Libraries and Practices

Logging for Kotlin: Best Solutions

Logging is an integral part of developing any serious application. In the Kotlin ecosystem, thanks to full compatibility with Java and the availability of modern DSL approaches, there are several proven solutions. In this article, we will break down the best libraries for logging in Kotlin: from classic Java frameworks to specialized Kotlin wrappers. You will learn how to set up effective logging for server-side (Ktor, Spring Boot) and mobile (Android) projects.

1. SLF4J + Logback — The Classic Choice

SLF4J (Simple Logging Facade for Java) is a facade (abstraction) over various logging frameworks. Paired with Logback (an implementation created by the same author), this is the de facto standard for JVM projects, including Kotlin.

Features:

Time-tested stability and performance.
Flexible configuration via XML or Groovy/DSL.
Support for asynchronous logging (AsyncAppender).
Automatic log compression and rotation.

Configuration example in Kotlin:

// build.gradle.ktsdependencies {    implementation("ch.qos.logback:logback-classic:1.4.14")    implementation("org.slf4j:slf4j-api:2.0.9")}

// Main.ktimport org.slf4j.LoggerFactory

private val logger = LoggerFactory.getLogger("MyApp")

fun main() {    logger.info("Application started")    try {        // business logic    } catch (e: Exception) {        logger.error("Execution error: {}", e.message, e)    }}

Logback automatically looks for the logback.xml file in the classpath. Example of a simple config:

<configuration>    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">        <encoder>            <pattern>%d{HH:mm:ss} [%thread] %-5level %logger{36} - %msg%n</pattern>        </encoder>    </appender>

    <root level="debug">        <appender-ref ref="STDOUT" />    </root></configuration>

2. kotlin-logging — Lightweight Kotlin Wrapper

The kotlin-logging library (by author Marcin Zajączkowski) provides a minimalistic Kotlin interface on top of SLF4J. It adds support for lambda expressions and inline functions, which helps avoid the overhead of string formatting if the logging level is disabled.

Advantages:

Lazy message evaluation: logger.debug { "Complex ${expensive()}" }.
No need to declare val logger manually (via the logger() delegate).
Full compatibility with any SLF4J implementation (Logback, Log4j2).

Usage example:

// build.gradle.ktsdependencies {    implementation("io.github.microutils:kotlin-logging-jvm:3.0.5")}

// Main.ktimport mu.KotlinLogging

private val logger = KotlinLogging.logger {}

fun main() {    val userId = 42    logger.info { "User $userId logged in" }    logger.debug { "Detailed report: ${generateReport()}" } // won't execute if debug is disabled}

fun generateReport(): String = "very long report"

3. Timber — For Android Development

Timber is a lightweight library by Jake Wharton, created specifically for Android. It extends the standard android.util.Log, adding automatic class detection, stack traces, and the ability to "plant" trees for different logging levels.