Kotlin Logging Facade (klf)

Kotlin (Multiplatform) logging facade for idiomatic logging in Kotlin with appenders for all supported KMP platforms.

Setup

Gradle

implementation("net.codinux.log:klf:1.8.0")

Native Images (e.g. Quarkus native)

For native images use

implementation("net.codinux.log:klf-graal:1.8.0")

which substitutes calls to reflection. (As otherwise each class for which a Logger gets created would need to be annotated with @RegisterForReflection.)

Compatibility note

With version 1.6.0 the project has been renamed from kmp-log to klf and the Kotlin version changed from 1.8 to 1.9. Projects targeting Kotlin 1.8 therefore should use net.codinux.log:kmp-log:1.5.1.

Usage

Kotlin style

    package com.example.service
    
    import net.codinux.log.logger
    
    class OrderService {
        private val log by logger() // automatically uses declaring class / OrderService::class.name as logger name
      
        fun showUsage() {
            log.info { "Message with ${heavyCalculation()}" } // lambda and therefor heavyCalculation() gets only called if INFO level is enabled
    
            log.error(e) { "An error occurred" } // e is a throwable
        }
    }

The log lambdas like info { }, warn { }, ... are only called if the according log level is enabled.
Therefor there's no need for String formats like log.info("Message with %s", heavyCalculation()) as "Message with ${heavyCalculation()}" of example above gets only called if INFO level is enabled.

With by logger() the logger name gets automatically derived from declaring class. In above example it would by "com.example.service.OrderService".
And the logger instance gets instantiated lazily on its first usage (or never if it never gets called).

If the logger is declared in a companion object automatically the enclosing class gets used as logger name (like in the following example OrderService instead of OrderService.Companion).
Except on JavaScript, there the logger name will always be "Companion".

    import net.codinux.log.logger
    
    class OrderService {
      companion object {
        private val log by logger() // automatically uses enclosing class = OrderService::class.name as logger name (except on JavaScript)
      }
    }

Classically:

package com.example.service

import net.codinux.log.LoggerFactory

class OrderService {
    private val log = LoggerFactory.getLogger(OrderService::class)
    // or by setting logger name (tag) via a string
    private val logByName = LoggerFactory.getLogger("OrderService")
  
    fun showUsage() {
        log.info("An info message: $detailedMessage") // string get directly concatenated whether INFO level is enabled or not

        logByName.error("An error occurred", e) // e is a throwable
    }
}

You might ask why is there no overload with format arguments like fun info(message: String, exception: Throwable? = null, vararg arguments: Any).
This is due to the restrictions of Kotlin multiplatform as there's no String.format() available (also the format specifier differ, e.g. %s on the JVM and %@ on iOS and macOS).
So if you like to format the log message, please use lambda variants.

Static Loggers

Since version 1.5.0 klf also supports static Loggers:

package com.example.service

import net.codinux.log.LoggerFactory

class OrderService {
  
    fun showUsage() {
        Log.info<OrderService> { "Message with ${heavyCalculation()}" }

        Log.error<OrderService>(e) { "An error occurred" } // e is a throwable

        // or set the logger tag via String:
        Log.info(loggerName = "OrderService") { "Message with ${heavyCalculation()}" }
        
        // The logger tag can also be omitted; in this case, the default logger name resolution will apply, as described below.
        Log.info { "Message with ${heavyCalculation()}" }
    }
}

Logger name resolution if no logger tag is provided

If no logger tag is provided, e.g. with Log.info { "Info" }, the following logger name resolution will be applied:

If LoggerFactory.config.useCallerMethodIfLoggerNameNotSet or in debug mode LoggerFactory.debugConfig.useMethodNameIfLogTagIsNotSet is set to true, then the method that executed the log statement will be used as logger name (which is quite handy e.g. for @Composable functions).
It's a bit resource intensive, see below.
Currently only works on JVM and Android.
Otherwise if set, the value of LoggerFactory.defaultLoggerName will be used.
If defaultLoggerName is not set, we try to determine the app name (e.g. Main Bundle name on iOS, app's package name and name on Android (but see Configuration -> Android), jar name on JVM and URL's last path name on JavaScript).
If this does not work, "net.codinux.log.klf" will be used as logger name.

Configuration

Android

For some features to work on Android, like logging the app name or determining if running in debug mode, Android Context is required. Set it at app start, e.g. in MainActivity:

class MainActivity : ComponentActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        net.codinux.log.android.AndroidContext.applicationContext = this.applicationContext

        // ...
    }
}

Options

rootLevel

Experimental: Sets the default log level for all loggers that will be used if no logger specific level is set with [Logger.level].

Be aware, this does not work reliably for all logging backends, e.g. we don't have implementations for all slf4j logging backends and the Android min log level is unchangeable.

If slf4j is on the classpath, configure log level via logging backend (logback, log4j, ...).

Defaults to LogLevel.Info and in debug mode to LogLevel.Debug.

useCallerMethodIfLoggerNameNotSet

If no logger tag is passed to log statement, e.g. with Log.info { ".." }, and LoggerFactory.config.useCallerMethodIfLoggerNameNotSet or in debug mode LoggerFactory.debugConfig.useMethodNameIfLogTagIsNotSet is set to true, then the method that executed the log statement will be used as logger name, in the format <class name>.<method name>.

This comes in handy in Compose as in composable functions there's (usually) no class to reference with by logger() or Log.info<ClassName> { }.

It's also trying to remove auto generated class name parts like they are applied to Coroutine functions (like "org.example.AppKt$App$2$1$2.invokeSuspend" -> "org.example.App"). This may not work reliably under all circumstances as we have to extract the class name and method name from strings and we may are not aware of all possible formats yet.

But be aware that this is a bit resource intensive as it walks up the call stack for each log call to find the calling method, so preferably only enable it in debug mode.
For UI applications this should be fine anyway, just don't enable it on high load servers.

This currently works only on JVM and Android.

defaultLoggerName

If no logger tag is passed to log statement, e.g. with Log.info { ".." }, and useCallerMethodIfLoggerNameNotSet is set to false, then the value configured with defaultLoggerName will be used.

If also defaultLoggerName is not set, we are trying to determine the app name and use that as logger name.
If this does not work, "net.codinux.log.klf" will be used as logger name.

Debug mode vs. normal / release mode

For all options in LoggerFactory.config there's a same named option in LoggerFactory.debugConfig.

Values from debugConfig will only be applied when running in debug mode / a debugger is attached.
So you can set values that only get applied during development / when running in debug mode and don't touch your normal user's configuration like:

LoggerFactory.debugConfig.useCallerMethodIfLoggerNameNotSet = true

This does currently not work on JS and WASM (hints how to detect attached debugger in JS are welcome).
For Android it means that the app is compiled in debug build variant.

Log appenders

klf by ships with a lot of log appenders. Depending on the platform by default these appenders are used:

JVM

If slf4j is on the classpath: slf4j.

Otherwise: Console (println())

Android

Logcat

Apple system (macOS, iOS, iPadOS, watchOS, tvOS)

If available (iOS 10.0+, iPadOS 10.0+, macOS 10.12+, Mac Catalyst 13.1+, tvOS 10.0+, watchOS 3.0+): OSLog

Otherwise: NSLog

JavaScript

JavaScript Console (console.log() / console.error() / ...)

Native

Console (println())

Getting notified about log events

If you want to get notified about each log event, add a NotifyAboutLogEventsAppender:

LoggerFactory.addAppender(NotifyAboutLogEventsAppender(includeThreadName = true, includeException = true) { event ->
    // handle received log event
})

For includeException true is the default value, for includeThreadName false.

Loki

If you want to apply centralized logging, you can push all logs collected with klf directly to Loki with klf-loki-appender:

Add to Gradle:

implementation("net.codinux.log:klf-loki-appender:1.8.0")

And on app start add LokiAppender:

// configure Loki url and credentials:
val config = LokiLogAppenderConfig(writer = WriterConfig("http://192.168.0.27:3100", username = null, password = null))

// then add LokiAppender to klf LoggerFactory
LoggerFactory.addAppender(LokiAppender(config))

If using http (e.g. for tests in local network) on Android add android:usesCleartextTraffic="true" to AndroidManifest.xml -> tag.

Custom

You can also use add your custom log appender by implementing Appender interface:

  LoggerFactory.addAppender(CustomAppender()) // CustomAppender implements Appender interface

If you dislike above default appenders and want to control logger creation entirely implement ILoggerFactory and call:

  LoggerFactory.setLoggerFactory(MyCustomLoggerFactory())

WebAssembly (experimental)

Version 1.1.3 added experimental support for WASM Browser. While it should work in most circumstances, there is explicitly currently no support for multithreaded applications due to lack of synchronization mechanisms in Kotlin WASM.

MDC (Java only)

klf has some convenience functions if values should get added to MDC (Mapped Diagnostic Context) only for the next log message and should then automatically get cleared again:

  log.withMdc("key1" to "value1", "key2" to "value2").info { "Info" }

  runWithMdc("key1" to "value1") {
    log.info { "Info" }
  }

Terminology

The terminology is mostly identical with slf4j / logback.

LoggerFactory

Primary facade to create a Logger, e.g.
private val log = LoggerFactory.getLogger(OrderService::class).

klf ships with two LoggerFactories:

Slf4jLoggerFactory that delegates logging to slf4j, so that on JVM any logging framework that implements slf4j can be used. Gets automatically used if slf4j is on the classpath and its ILoggerFactory implementation is any other than NOPLoggerFactory.
DefaultLoggerFactory that delegates logging to system's default log appender (see Log appenders).

Logger

Interface to log messages at different levels (Info, Error, ...) , e.g.
log.error(e) { "Calculating Fibonacci numbers for $number failed" }.

Appender

If a messages gets accepted by a Logger, Appender do the actual work of 'writing' log messages.
Multiple appender can be defined so that one logged messages e.g. gets written to console, to a file (only available via a slf4j compatible logging framework), to Elasticsearch, Loki, ...
All Loggers use the same appenders. (It's not possible to set an appender for a specific Logger like as logback.)
Except ConsoleAppender klf doesn't implement any Appender itself but delegates on to native loggers like Logcat, OSLog, JavaScript Console, slf4j, ...

License

Copyright 2023 codinux GmbH & Co. KG

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

codinux-gmbh/klf

Kotlin Logging Facade (klf)

Setup

Gradle

Native Images (e.g. Quarkus native)

Compatibility note

Usage

Kotlin style

Classically:

Static Loggers

Logger name resolution if no logger tag is provided

Configuration

Android

Options

rootLevel

useCallerMethodIfLoggerNameNotSet

defaultLoggerName

Debug mode vs. normal / release mode

Log appenders

JVM

Android

Apple system (macOS, iOS, iPadOS, watchOS, tvOS)

JavaScript

Native

Getting notified about log events

Loki

Custom

WebAssembly (experimental)

MDC (Java only)

Terminology

LoggerFactory

Logger

Appender

License