Logger foundation

The declaration style determines call-site ergonomics, name stability, and how operators identify a logger.

Two entry points

Use KLogging when a class or companion object should share a log property. Kotlin lazy creates the logger once on first access.

class InvoiceService {
    companion object : KLogging()

    fun issue() {
        log.info { "Issuing invoice" }
    }
}

Use KotlinLogging for top-level code, an explicit category, or a KClass name.

private val auditLog = KotlinLogging.logger("billing.audit")
private val fileLog = KotlinLogging.logger { }
val serviceLog = KotlinLogging.logger(InvoiceService::class)

Naming contract

logger(name) rejects a blank name with IllegalArgumentException. The lambda entry point and KLogging delegate synthetic and companion-name normalization to KLoggerNameResolver. Prefer an explicit category when the name is a stable dashboard or routing key.

Provider boundary

The module returns SLF4J Logger; it does not install a backend. The application owns one provider such as Logback and its configuration. A library must not silently replace appenders or root levels.

Decision table

Situation	Choice
Every instance uses one category	companion KLogging()
File or top-level function	KotlinLogging.logger {}
Stable operational category	logger("stable.name")
Explicit type-based category	logger(Type::class)

Source and tests

• KLogging.kt
• KotlinLogging.kt
• KLoggerNameResolver.kt
• KLoggingTest.kt
• KotlinLoggingTest.kt

Next: control evaluation cost and supplier failure in Lazy messages.