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Welcome to the kotlin-logging wiki!
kotlin-logging is a lightweight logging framework for Kotlin. A convenient and performant logging library wrapping slf4j with Kotlin extensions kotlin-logging provides both:
- Idiomatic way to obtain a logger without the need to specify class name
- Enhanced logging api that integrates fluently with Kotlin
Important note: kotlin-logging depends on slf4j-api. In runtime, it is also required to depend on a logging implementation. More details here.
<dependency>
<groupId>io.github.microutils</groupId>
<artifactId>kotlin-logging</artifactId>
<version>1.4.9</version>
</dependency>
See full example in kotlin-logging-example-maven.
compile 'io.github.microutils:kotlin-logging:1.4.9'
Or alternatively, download jar from github or bintray or maven-central.
The preferred way to obtain a logger is as follows:
// Place definition above class declaration, below imports,
// to make field static and accessible only within the file
private val logger = KotlinLogging.logger {}
Another option (less recommended) is to is to have the Companion
object extends KLogging()
:
companion object: KLogging()
In cases the Companion
object already extending other class it is recommend to implement the KLoggable
interface:
companion object: Any(), KLoggable {
override val logger = logger()
...
}
Other (less recommended) alternatives are:
companion object: Any(), KLoggable by NamedKLogging("com.MyClass")
Or implementing it as a non static member:
class ClassHasLogging: KLoggable {
override val logger = logger()
}
all examples also available in tests.
The most simple use is the same as with java logger:
logger.info("hello message")
For sequences that are expected to be frequently used or costly prefer lazily evaluated messages:
logger.debug { "lazy evaluated $hello message" }
(Message is in lambda and gets evaluated only if debug log level is enabled at runtime)
When logging exceptions, it is also possible to use a lambda expression to calculate the attached message:
logger.error(exception) { "a $fancy message about the $exception" }
The complete list of methods can be found in KLogger source code.
MDC allows adding implicit context data such as user id to all log messages base on the caller thread. More details can be found on https://www.slf4j.org/manual.html#mdc.
In kotlin-logging
it is possible to set MDC properties to be used later in lambda that way:
import mu.withLoggingContext
...
withLoggingContext("userId" to userId) {
doSomething()
}
Providing multiple values is also supported. See: KotlinLoggingMDC source code.
A: Yes. More details here: https://github.com/MicroUtils/kotlin-logging/issues/11
A: As with other framework specific features it is supported and should be configured in the logging framework itself. More details here: https://github.com/MicroUtils/kotlin-logging/issues/24