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Version: ZIO 2.x

SLF4J v2

The Simple Logging Facade for Java (SLF4J v2 - using JDK9+ module system JPMS) serves as a simple facade or abstraction for various logging frameworks (e.g. java.util.logging, logback, log4j).

In order to use this logging backend, we need to add the following line in our build.sbt file:

libraryDependencies += "dev.zio" %% "zio-logging-slf4j2" % "2.1.11"

NOTE: SLF4J v2 implementation is similar to v1, however there are some differences, v1 using MDC context, working with JDK8, v2 using key-value pairs, working with JDK9+.

Logger layer:

import zio.logging.backend.SLF4J

val logger = Runtime.removeDefaultLoggers >>> SLF4J.slf4j

Default SLF4J logger setup:

  • logger name (by default) is extracted from zio.Trace
    • for example, trace will have zio.logging.example.Slf4jSimpleApp as logger name
    • NOTE: custom logger name may be set by zio.logging.loggerName aspect
  • all annotations (logger name and log marker name annotations are excluded) are passed like key-value pairs
  • cause is logged as throwable

See also LogFormat and LogAppender

Custom logger name set by aspect:

ZIO.logInfo("Starting user operation") @@ zio.logging.loggerName("zio.logging.example.UserOperation")

Log marker name set by aspect:

ZIO.logInfo("Confidential user operation") @@ SLF4J.logMarkerName("CONFIDENTIAL")


You can find the source code here

SLF4J logger name and annotations

package zio.logging.example

import zio.logging.LogAnnotation
import zio.logging.backend.SLF4J
import zio.{ ExitCode, Runtime, Scope, ZIO, ZIOAppDefault, _ }

import java.util.UUID

object Slf4jSimpleApp extends ZIOAppDefault {

override val bootstrap: ZLayer[ZIOAppArgs, Any, Any] = Runtime.removeDefaultLoggers >>> SLF4J.slf4j

private val users = List.fill(2)(UUID.randomUUID())

override def run: ZIO[Scope, Any, ExitCode] =
for {
_ <- ZIO.logInfo("Start")
traceId <- ZIO.succeed(UUID.randomUUID())
_ <- ZIO.foreachPar(users) { uId =>
ZIO.logInfo("Starting user operation") *>
ZIO.logInfo("Confidential user operation") @@ SLF4J.logMarkerName("CONFIDENTIAL") *>
ZIO.sleep(500.millis) *>
ZIO.logInfo("Stopping user operation")
} @@ ZIOAspect.annotated("user", uId.toString)
} @@ LogAnnotation.TraceId(traceId) @@ zio.logging.loggerName("zio.logging.example.UserOperation")
_ <- ZIO.logInfo("Done")
} yield ExitCode.success


Logback configuration:

<?xml version="1.0" encoding="UTF-8"?>
<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
<layout class="ch.qos.logback.classic.PatternLayout">
<Pattern>%d{HH:mm:ss.SSS} [%thread] [%kvp] %-5level %logger{36} %msg%n</Pattern>
<turboFilter class="ch.qos.logback.classic.turbo.MarkerFilter">
<root level="debug">
<appender-ref ref="STDOUT" />

Expected Console Output:

12:13:17.495 [ZScheduler-Worker-8] [] INFO  zio.logging.example.Slf4j2SimpleApp Start
12:13:17.601 [ZScheduler-Worker-11] [trace_id="7826dd28-7e37-4b54-b84d-05399bb6cbeb" user="7b6a1af5-bad2-4846-aa49-138d31b40a24"] INFO zio.logging.example.UserOperation Starting user operation
12:13:17.601 [ZScheduler-Worker-10] [trace_id="7826dd28-7e37-4b54-b84d-05399bb6cbeb" user="4df9cdbc-e770-4bc9-b884-756e671a6435"] INFO zio.logging.example.UserOperation Starting user operation
12:13:18.167 [ZScheduler-Worker-13] [trace_id="7826dd28-7e37-4b54-b84d-05399bb6cbeb" user="7b6a1af5-bad2-4846-aa49-138d31b40a24"] INFO zio.logging.example.UserOperation Stopping user operation
12:13:18.167 [ZScheduler-Worker-15] [trace_id="7826dd28-7e37-4b54-b84d-05399bb6cbeb" user="4df9cdbc-e770-4bc9-b884-756e671a6435"] INFO zio.logging.example.UserOperation Stopping user operation
12:13:18.173 [ZScheduler-Worker-13] [] INFO zio.logging.example.Slf4j2SimpleApp Done