jokade / slogging   0.6.2

MIT License GitHub

A Typesafe-logging (and slf4j) compatible logging library based on macros for Scala/JVM, Scala.js, and Scala Native

Scala versions: 2.13 2.12 2.11
Scala.js versions: 1.x
Scala Native versions: 0.3


A simple logging library for Scala, Scala.js, and Scala Native. Slogging is compatible to the scala-logging (and slf4j) API, and uses macros to check if logging statements should be executed.

News: Version 0.6.2 has been released (release notes)!


Getting Started

SBT Settings

Add one of the following lines to your build.sbt (depending on your target):

Scala/JVM with logging to stdout:

libraryDependencies += "biz.enef" %% "slogging" % "0.6.2"

with slf4j:

libraryDependencies ++= Seq(
  "biz.enef" %% "slogging-slf4j" % "0.6.2",
  "org.slf4j" % "slf4j-simple" % "1.7.+"  // or another slf4j implementation

Scala.js with logging to console:

libraryDependencies += "biz.enef" %%% "slogging" % "0.6.2"

with winston (Node.js):

libraryDependencies += "biz.enef" %%% "slogging-winston" % "0.6.2"

with remote logging via HTTP POST:

libraryDependencies += "biz.enef" %%% "slogging-http" % "0.6.2"

Scala Native with logging to stderr:

libraryDependencies += "biz.enef" %%% "slogging" % "0.6.2"

with logging to syslogd:

libraryDependencies += "biz.enef" %%% "slogging-syslog" % "0.6.2"

with logging to GLib:

libraryDependencies += "biz.enef" %%% "slogging-glib" % "0.6.2"

slogging 0.6.2 is published for both Scala 2.12.x and Scala 2.13.x, Scala.js 1.0, and Scala Native 0.3.9 / 0.4.0-M2.

Logging and Configuration

Add logging statements

Mix one of two traits into your class/object to give it logging capability:

  • StrictLogging: the logger is initialized when the instance is created
  • LazyLogging: the logger is initialized on first access


import slogging.LazyLogging

class Foo extends LazyLogging {
  def bar() {
    // log an error message
    // log a warning
    // log an info message"...")
    // log a debug message
    // log a tracing message
    // log a parameterized message"foo={}, bar={}",1,true)

Activate logging and set log level

The default logger is a NullLogger which simply ignores all logging statements. To activate logging output, assign a different logger factory to LoggerConfig.factory, or call the apply() method of the logger factory you want to use. Assign a new value to LoggerConfig.level to change the current log level. Example:

import slogging._

object Main extends App {
  // select logger backend, e.g. simple logging using println (supported by Scala/JVM, Scala.js, and Scala Native) 
  LoggerConfig.factory = PrintLoggerFactory()

  // set log level, e.g. to DEBUG
  LoggerConfig.level = LogLevel.DEBUG

Note: Some backends (slf4j, winston) have their own log level management, which needs to be adjusted as well.

Disable logging at compile time

It is possible to omit logging statements from the generated code altogether. To this end add the following flag to your build.sbt:

scalacOptions += "-Xmacro-settings:slogging.disable"


Sometimes it is useful to enable detailed logging (e.g. LogLevel.TRACE) only for some classes. This can be achieved using FilterLoggerFactory and setting FilterLogger.filter to a custom filter PartialFunction. The filter function receives a tuple (LogLevel.Value,String) that contains the log level and logger source name for every logging message; it must return the UnderlyingLogger instance to be used for this logging statement (and must be defined for every input, so make sure to provide a default case):

import slogging._

LoggerConfig.factory = FilterLoggerFactory()
LoggerConfig.level = LogLevel.TRACE

FilterLogger.filter = {
  // use PrintLogger for all trace statements from sources starting with ""
  case (LogLevel.TRACE,source) if source.startsWith("") => PrintLogger
  // ignore all other trace statements
  case (LogLevel.TRACE,_) => NullLogger
  // log all other levels
  case _ => PrintLogger

Note: The filter function is called after the current value of LoggerConfig.level has been checked. Hence, even if you want to log TRACE statements for a specific source using FilterLogger, you need to set FilterConfig.level = LogLevel.TRACE. This also means that all TRACE logging statements in the code are executed, even if they are subsequently discarded by NullLogger, which may have a serious impact on performance.



This simple backend prints all messages to stderr (or stdout) and can be used with JVM, JS an Scala Native projects.


// build.sbt
libraryDependencies += "biz.enef" %% "slogging" % "VERSION"
import slogging._

// activate PrintLogger
LoggerConfig.factory = PrintLoggerFactory()

// set this to activate timestamp logging
PrintLogger.printTimestamp = true

You can change the output stream to which messages are logged for each level, e.g.:

// use stderr for ERROR and WARN
PrintLoggerFactory.errorStream = System.err
PrintLoggerFactory.warnStream = System.err

// use stdout for all other levels
PrintLoggerFactory.infoStream = System.out
PrintLoggerFactory.debugStream = System.out
PrintLoggerFactory.traceStream = System.out

Furthermore, you may also change the format of logging message by providing a custom MessageFormatter:

object CustomFormatter extends MessageFormater {
  def formatMessage(level: MessageLevel, name: String, msg: String, cause: Option[Throwable]): String = {
    /* ... */

PrintLoggerFactory.formatter = CustomFormatter

Scala / JVM

slogging supports the following logger backends on Scala (JVM):


This backend is just a wrapper around slf4j.


// build.sbt
libraryDependencies ++= Seq(
  "biz.enef" %% "slogging-slf4j" % "VERSION",
  "org.slf4j" % "slf4j-simple" % "1.7.+"  // or another slf4j implementation
import slogging._

// activate SLF4J backend
LoggerConfig.factory = SLF4JFactory()


slogging support the following logger backends for Scala.js:


Similar to PrintLoggerFactory, but uses console.log instead of println().


// build.sbt
libraryDependencies += "biz.enef" %%% "slogging" % "VERSION"
import slogging._

// activate ConsoleLogger; no additional configuration required
LoggerConfig.factory = ConsoleLoggerFactory()


A wrapper around the winston logging library for Node.js.


// build.sbt
libraryDependencies += "biz.enef" %%% "slogging-winston" % "VERSION"

// activate module support to load winston module
scalaJSModuleKind := ModuleKind.CommonJSModule
import slogging._

// use default winston logger
LoggerConfig.factory = WinstonLoggerFactory()

// use a custom winston logger
import WinstonLoggerFactory.{WinstonLogger, winston}

LoggerConfig.factory = WinstonLoggerFactory(WinstonLogger(literal(
  levels = js.Dictionary(
    "trace" -> 0,
    "debug" -> 1,
    "info"  -> 2,
    "warn"  -> 3,
    "error" -> 4
  transports = js.Array(
      level = "debug"


This backend sends log messages to a HTTP server via Ajax POST requests. Note that the same origin policy usually requires that the server to which the messages are sent must be the same server from which the javascript source was loaded.


import slogging._

// function that assembles the JSON object to be sent
// (only required if you want to override the default formatter)
val fmt: HttpLoggerFactory.MessageFormatter = (clientId,level,name,msg,cause) => js.Dynamic.literal(
  id = clientId,
  loggerName = name,
  logMessage = msg

// configure & activate remote HTTP logging 
LoggerConfig.factory = 
  HttpLoggerFactory("/logging", // target URL for log messages
                    "client1",  // ID of the client that sends the messages (optional)
                    fmt         // message formatting function (optional)

Scala Native

slogging supports the following logger backends on Scala Native:


Logs all messages to stderr using fprintf. Messages are enclosed in ANSI terminal control codes, which can be configured separately for every level.


// build.sbt
libraryDependencies += "biz.enef" %%% "slogging" % "VERSION"
// use default TerminalLogger
// (error messages are printed in red, warnings in yellow)
LoggerConfig.factory = TerminalLoggerFactory()

// print INFO messages in blue
TerminalLoggerFactory.infoCode =

// change the message formatter
TerminalLoggerFactory.formatter = ...


This backend uses the standard syslog facility.


// build.sbt
libraryDependencies += "biz.enef" %%% "slogging-syslog" % "VERSION"
LoggerConfig.factory = SyslogLoggerFactory()


This backend uses GLib's g_log() to log messages, e.g. for use with scalanative-gtk.


// build.sbt
libraryDependencies += "biz.enef" %%% "slogging-glib" % "VERSION"

nativeLinkingOptions += "-lglib-2.0" // you may need to change this depending on your glib installation
LoggerConfig.factory = GLibLoggerFactory()

// optionally change message formatter
GLibLoggerFactory.formatter = ...


This code is open source software licensed under the MIT License