The default Logger API has the same logger methods and (roughly) the type signature as SLF4J.

The biggest difference is that methods take a type class instance of Markers and Arguments, if you have them defined.


Markers work the same way, but must be an instance of Markers.

val marker = MarkerFactory.getDetachedMarker("foo"), "hello")

Using the DSL with marker enrichment is encouraged here, as it can make marker specification much easier:

import com.tersesystems.blindsight._
import com.tersesystems.blindsight.MarkersEnrichment._"markerKey" -> "markerValue").asMarkers, "marker and argument")

Generally, you should not need to use markers explicitly in messages, as they can be used with context more effectively.


Arguments in Blindsight are type checked, in constrast to the SLF4J API, which takes an Any. There must be a type class instance of ToArgument in scope. This is to prevent awkward toString matches on object instances, and ensure that structured logging is taking place.

// Will not compile, because no ToArgument[SomeRandomObject] is found in implicit scope!"one argument {}", new SomeRandomObject()) 

Default ToArgument are determined for the primitives (String, Int, etc):"one argument {}", 42) // works, because default

If you have more than two arguments, you will need to wrap them so they are provided as a single Arguments instance:"arg {}, arg {}, arg {}", Arguments(1, "2", false))

comes out as:

FgEddUhGnXE6O0Qbm7EAAA 17:36:55.142 [INFO ] e.s.Slf4jMain$ -  arg 1, arg 2, arg false

Exceptions come after arguments, and are not included in the list. For example:

val e = new Exception("something is horribly wrong")
logger.error("this is an error with argument {}", Arguments("a" -> "b"), e)

Lazy Blocks

There is a block oriented API available. This is useful for diagnostic logging and conditional logging to avoid unnecessary memory allocation. The block is only executed if the conditions for logging have been met, and returns a handle to the method itself.

// block only executed if logger is DEBUG or TRACE level.
logger.debug { debug =>
  val debugInfo = ...
  debug(st"I am a debugging statement with lots of extra $debugInfo")

Unchecked API

An unchecked API is available that does not use type class inference at all, and looks just like SLF4J with the additions of conditions and markers.

This is easier to use, but since Any cannot be type checked, it can output information you don’t expect. For example, it could output credit card information because calling toString exposes the case class data:

val logger = LoggerFactory.getLogger

val unchecked: SLF4JLogger[UncheckedSLF4JMethod] = logger.unchecked

// Uses Any, renders credit card as "toString"
val creditCard = CreditCard("4111111111111")

 // case class tostring renders CC number, which is unsafe!"this is risky unchecked {}", creditCard)

Where there are several arguments, the Arguments must be used rather than Seq or Array, as it is very awkward to use Seq and Array with varadic input:"this is risky unchecked {}, {}, {}", Arguments("1", 2, true))

In the unchecked API, you can set -Dblindsight.anywarn=true as a system property, and output will be written to System.out.error when calls to Any are made.

The source code for this page can be found here.