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Version: ZIO 2.x

SLF4J v2 bridge

It is possible to use zio-logging for SLF4J loggers, usually third-party non-ZIO libraries. To do so, import the zio-logging-slf4j2-bridge module for SLF4J v2 (using JDK9+ module system (JPMS))

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

and use the Slf4jBridge.initialize layer when setting up logging:

import zio.logging.slf4j.Slf4jBridge


SLF4J logger name is stored in log annotation with key logger_name (zio.logging.loggerNameAnnotationKey), following log format

import zio.logging.slf4j.Slf4jBridge
import zio.logging.LoggerNameExtractor

val loggerName = LoggerNameExtractor.loggerNameAnnotationOrTrace
val loggerNameFormat = loggerName.toLogFormat()

may be used to get logger name from log annotation or ZIO Trace.

This logger name extractor is used by default in log filter, which applying log filtering by defined logger name and level:

val logFilter: LogFilter[String] = LogFilter.logLevelByName(
"zio.logging.slf4j" -> LogLevel.Debug,
"SLF4J-LOGGER" -> LogLevel.Warning

SLF4J bridge with custom logger can be setup:

import zio.logging.slf4j.Slf4jBridge
import zio.logging.{ ConsoleLoggerConfig, consoleJsonLogger }

val logger = Runtime.removeDefaultLoggers >>> consoleJsonLogger(ConsoleLoggerConfig.default) >+> Slf4jBridge.initialize

NOTE You should either use zio-logging-slf4j to send all ZIO logs to an SLF4j provider (such as logback, log4j etc) OR zio-logging-slf4j-bridge to send all SLF4j logs to ZIO logging. Enabling both causes circular logging and makes no sense.


You can find the source code here

SLF4J bridge with JSON console logger

package zio.logging.slf4j.bridge

import zio.logging.slf4j.bridge.Slf4jBridge
import zio.logging.{
import zio.{ ExitCode, LogLevel, Runtime, Scope, ZIO, ZIOAppArgs, ZIOAppDefault, ZLayer }

object Slf4jBridgeExampleApp extends ZIOAppDefault {

private val slf4jLogger = org.slf4j.LoggerFactory.getLogger("SLF4J-LOGGER")

private val logFilter: LogFilter[String] = LogFilter.logLevelByName(
"zio.logging.slf4j" -> LogLevel.Debug,
"SLF4J-LOGGER" -> LogLevel.Warning

override val bootstrap: ZLayer[ZIOAppArgs, Any, Any] =
Runtime.removeDefaultLoggers >>> consoleJsonLogger(
LogFormat.label("name", LoggerNameExtractor.loggerNameAnnotationOrTrace.toLogFormat()) + LogFormat.default,
) >+> Slf4jBridge.initialize

override def run: ZIO[Scope, Any, ExitCode] =
for {
_ <- ZIO.logDebug("Start")
_ <- ZIO.succeed(slf4jLogger.debug("Test {}!", "DEBUG"))
_ <- ZIO.succeed(slf4jLogger.warn("Test {}!", "WARNING"))
_ <- ZIO.logInfo("Done")
} yield ExitCode.success


Expected Console Output:

{"name":"SLF4J-LOGGER","timestamp":"2023-01-07T18:25:40.416612+01:00","level":"WARN","thread":"zio-fiber-6","message":"Test WARNING!"}
{"name":"zio.logging.slf4j.bridge.Slf4jBridgeExampleApp","timestamp":"2023-01-07T18:25:40.42043+01:00 ","level":"INFO","thread":"zio-fiber-4","message":"Done"}