SLF4J Fluent Logging API in Kotlin

2 minutes read in Kotlin Highlights

SLF4J 2.0.0 introduced new logging API:

logger.atInfo().log("Hello world")

New fluent api looks more verbose in simple cases like this. Let’s compare it to traditional API:

// traditional"Hello world")

// fluent
logger.atInfo().log("Hello world")

You may think that more advanced use-cases will be more readable or shorter. Let’s check:

logger.debug("Temperature set to {}. Old temperature was {}.", newT, oldT)

logger.atDebug().addArgument(newT).addArgument(oldT).log("Temperature set to {}. Old temperature was {}.")

logger.atDebug().log("Temperature set to {}. Old temperature was {}.", newT, oldT)

logger.atDebug().addArgument(newT).log("Temperature set to {}. Old temperature was {}.", oldT)

But actually not. Traditional API more readable and shorter than new one, so what the point of this shiny-new-fluent API?

First, it laziness, you can use Supplier to provide argument:

fun newT() = // costly computation
logger.atDebug().addArgument { newT() }.log("Temperature set to {}. Old temperature was {}.", oldT)

Second, now it’s much easier to extend API, for example, it’s possible to provide multiple Markers:

logger.atInfo().addMarker(DB).addMarker(SECURITY).log("SQL Injection attempt: '{}'", sql)

SLF4J and Kotlin

Yes, this new fluent API accommodates a variety of present and future features without combinatorial explosion, and without requiring separate logging facades. But currently I feel like Kotlin inline extension functions solve main problem (laziness) of traditional API in simple an elegant way, without any overhead:

// Lazy debug
logger.debug { "Temperature set to ${newT()}. Old temperature was $oldT." }

// vs fluent:
logger.atDebug().addArgument { newT() }.log("Temperature set to {}. Old temperature was {}.", oldT)

// first statement, decompiled to java:
if (logger.isDebugEnabled()) {
     String var6 = "Temperature set to " + newT() + ". Old temperature was " + oldT + '.';

// debug function source
inline fun Logger.debug(msg: () -> String) {
    if (isDebugEnabled) {


  1. New API enables new and future features, but sacrifices brevity and readability;
  2. Kotlin nicely solves existing API problems, without runtime overhead, thanks to inline functions.

See also:

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