Logging in Scala

Question

What is a good way to do logging in a Scala application  Something that is consistent with the language philosophy  does not clutter the code  and is low-maintenance and unobtrusive  Here s a basic requirement list    simple does not clutter the code  Scala is great for its brevity  I don t want half of my code to be logging statements log format can be changed to fit the rest of my enterprise logs and monitoring software supports levels of logging  ie debug  trace  error  can log to disk as well as other destinations  i e  socket  console  etc   minimum configuration  if any works in containers  ie  web server   optional  but nice to have  comes either as part of the language or as a maven artifact  so I don t have to hack my builds to use it   I know I can use the existing Java logging solutions  but they fail on at least two of the above  namely clutter and configuration   Thanks for your replies

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Quick and easy forms   Scala 2 10 and older   import com typesafe scalalogging slf4j Logger import org slf4j LoggerFactory val logger   Logger LoggerFactory getLogger  TheLoggerName    logger debug  Useful message         And build sbt   libraryDependencies     com typesafe      scalalogging-slf4j     1 1 0    Scala 2 11  and newer   import import com typesafe scalalogging Logger import org slf4j LoggerFactory val logger   Logger LoggerFactory getLogger  TheLoggerName    logger debug  Useful message         And build sbt   libraryDependencies     com typesafe scala-logging      scala-logging     3 1 0

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With Scala 2 10  Consider ScalaLogging by Typesafe   Uses macros to deliver a very clean API  https   github com typesafehub scala-logging  Quoting from their wiki      Fortunately Scala macros can be used to make our lives easier  ScalaLogging offers the class Logger with lightweight logging methods that will be expanded to the above idiom  So all we have to write is    logger debug s Some   expensiveExpression  message      After the macro has been applied  the code will have been transformed into the above described idiom   In addition ScalaLogging offers the trait Logging which conveniently provides a Logger instance initialized with the name of the class mixed into   import com typesafe scalalogging slf4j LazyLogging  class MyClass extends LazyLogging     logger debug  This is very convenient  -

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Using slf4j and a wrapper is nice but the use of it s built in interpolation breaks down when you have more than two values to interpolate  since then you need to create an Array of values to interpolate   A more Scala like solution is to use a thunk or cluster to delay the concatenation of the error message   A good example of this is Lift s logger  Log scala Slf4jLog scala  Which looks like this   class Log4JLogger val logger  Logger  extends LiftLogger     override def trace msg    gt  AnyRef    if  isTraceEnabled  logger trace msg      Note that msg is a call-by-name and won t be evaluated unless isTraceEnabled is true so there s no cost in generating a nice message string   This works around the slf4j s interpolation mechanism which requires parsing the error message   With this model  you can interpolate any number of values into the error message   If you have a separate trait that mixes this Log4JLogger into your class  then you can do  trace  The foobar from     a     doesn t match the foobar from           b     and you should reset the baz from     c     instead of  info  The foobar from  0  doesn t match the foobar from  1  and you should reset the baz from  c        Array a  b  c

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I find very convenient using some kind of java logger  sl4j for example  with simple scala wrapper  which brings me such syntax  val      new Logger        somewhere deep in dsl logging   object User with dsl logging            info message       dbg  debug message       trace  var a true       In my opinion very usefull mixin of java proven logging frameworks and scala s fancy syntax

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I pulled a bit of work form the Logging trait of scalax  and created a trait that also integrated a MessageFormat-based library   Then stuff kind of looks like this   class Foo extends Loggable       info   Dude  I m an  0  with  1 number       Log message   1234       We like the approach so far   Implementation   trait Loggable        val logger Logger   Logging getLogger this       def checkFormat msg String  refs Seq Any   String           if  refs size  gt  0  msgfmtSeq msg  refs  else msg       def trace msg String  refs Any     logger trace checkFormat msg  refs       def trace t Throwable  msg String  refs Any     logger trace  checkFormat msg  refs   t       def info msg String  refs Any     logger info checkFormat msg  refs       def info t Throwable  msg String  refs Any     logger info  checkFormat msg  refs   t       def warn msg String  refs Any     logger warn checkFormat msg  refs       def warn t Throwable  msg String  refs Any     logger warn  checkFormat msg  refs   t       def critical msg String  refs Any     logger error checkFormat msg  refs       def critical t Throwable  msg String  refs Any     logger error  checkFormat msg  refs   t             Note  implementation taken from scalax logging API     object Logging          def loggerNameForClass className  String                if  className endsWith      className substring 0  className length - 1            else className                def getLogger logging  AnyRef    LoggerFactory getLogger loggerNameForClass logging getClass getName

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Don t use Logula I ve actually followed the recommendation of Eugene and tried it and found out that it has a clumsy configuration and is subjected to bugs  which don t get fixed  such as this one   It doesn t look to be well maintained and it doesn t support Scala 2 10  Use slf4s   slf4j-simple Key benefits   Supports latest Scala 2 10  to date it s M7  Configuration is versatile but couldn t be simpler  It s done with system properties  which you can set either by appending something like -Dorg slf4j simplelogger defaultlog trace to execution command or hardcode in your script  System setProperty  quot org slf4j simplelogger defaultlog quot    quot trace quot    No need to manage trashy config files  Fits nicely with IDEs  For instance to set the logging level to  quot trace quot  in a specific run configuration in IDEA just go to Run Debug Configurations and add -Dorg slf4j simplelogger defaultlog trace to VM options  Easy setup  just drop in the dependencies from the bottom of this answer  Here s what you need to be running it with Maven   lt dependency gt     lt groupId gt com weiglewilczek slf4s lt  groupId gt     lt artifactId gt slf4s 2 9 1 lt  artifactId gt     lt version gt 1 0 7 lt  version gt   lt  dependency gt   lt dependency gt     lt groupId gt org slf4j lt  groupId gt     lt artifactId gt slf4j-simple lt  artifactId gt     lt version gt 1 6 6 lt  version gt   lt  dependency gt

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Haven t tried it yet  but Configgy looks promising for both configuration and logging    http   github com robey configgy tree master

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You should have a look at the scalax library   http   scalax scalaforge org  In this library  there is a Logging trait  using sl4j as backend  By using this trait  you can log quite easily  just use the logger field  in the class inheriting the trait

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Writer  Monoid and a Monad implementation

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I use SLF4J   Logback classic and apply it like this   trait Logging     lazy val logger   LoggerFactory getLogger getClass     implicit def logging2Logger anything  Logging   Logger   anything logger     Then you can use it whichever fits your style better   class X with Logging       logger debug  foo       debug  bar       but this approach of course uses a logger instance per class instance

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Logging in 2020 I was really surprised that Scribe logging framework that I use at work isn t even mentioned here  What is more  it doesn t even appear on the first page in Google after searching  quot scala logging quot   But this page appears when googling it  So let me leave that here  Main advantages of Scribe   it is NOT a slf4j wrapper supports also Scala js and Scala Native it is really fast  check comparison here  https   www matthicks com 2018 02 scribe-2-fastest-jvm-logger-in-world html

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This is how I got Scala Logging working for me   Put this in your build sbt   libraryDependencies     com typesafe scala-logging      scala-logging     3 7 2   libraryDependencies     ch qos logback     logback-classic     1 2 3    Then  after doing an sbt update  this prints out a friendly log message   import com typesafe scalalogging   object MyApp extends App with LazyLogging     logger info  Hello there       If you are using Play  you can of course simply import play api Logger for writing log messages  Logger debug  Hi     See the docs for more info

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slf4j wrappers  Most of Scala s logging libraries have been some wrappers around a Java logging framework  slf4j  log4j etc   but as of March 2015  the surviving log libraries are all slf4j  These log libraries provide some sort of log object to which you can call info       debug       etc  I m not a big fan of slf4j  but it now seems to be the predominant logging framework  Here s the description of SLF4J      The Simple Logging Facade for Java or  SLF4J  serves as a simple facade or abstraction for various logging frameworks  e g  java util logging  log4j and logback  allowing the end user to plug in the desired logging framework at deployment time    The ability to change underlying log library at deployment time brings in unique characteristic to the entire slf4j family of loggers  which you need to be aware of    classpath as configuration approach  The way slf4j knows which underlying logging library you are using is by loading a class by some name  I ve had issues in which slf4j not recognizing my logger when classloader was customized  Because the simple facade tries to be the common denominator  it s limited only to actual log calls  In other words  the configuration cannot be done via the code    In a large project  it could actually be convenient to be able to control the logging behavior of transitive dependencies if everyone used slf4j   Scala Logging  Scala Logging is written by Heiko Seeberger as a successor to his slf4s  It uses macro to expand calls into if expression to avoid potentially expensive log call      Scala Logging is a convenient and performant logging library wrapping logging libraries like SLF4J and potentially others    Historical loggers   Logula  a Log4J wrapper written by Coda Hale  Used to like this one  but now it s abandoned  configgy  a java util logging wrapper that used to be popular in the earlier days of Scala  Now abandoned

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After using slf4s and logula for a while  I wrote loglady  a simple logging trait wrapping slf4j    It offers an API similar to that of Python s logging library  which makes the common cases  basic string  simple formatting  trivial and avoids formatting boilerplate   http   github com dln loglady

[logging] Logging in Scala

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