Add mdc() DSL

build.gradle.kts

@@ -40,7 +40,7 @@ dependencies {
     testImplementation(libs.mockk)
 
     testRuntimeOnly(libs.junit.jupiter.engine)
-    testRuntimeOnly(libs.slf4j.simple)
+    testRuntimeOnly(libs.logback)
 }
 
 val gitVersion: Closure<String> by extra
@@ -51,7 +51,7 @@ version = gitVersion()
 
 java {
     toolchain {
-        languageVersion.set(JavaLanguageVersion.of(8))
+        languageVersion.set(JavaLanguageVersion.of(11))
     }
 
     withSourcesJar()

gradle/libs.versions.toml

@@ -2,15 +2,16 @@
 dokka = "1.8.20"
 git-version = "3.0.0"
 junit = "5.10.0"
+logback = "1.4.8"
 mockk = "1.13.5"
 slf4j = "2.0.7"
 
 [libraries]
 junit-jupiter-api = { group = "org.junit.jupiter", name = "junit-jupiter-api", version.ref = "junit" }
 junit-jupiter-engine = { group = "org.junit.jupiter", name = "junit-jupiter-engine", version.ref = "junit" }
+logback = { group = "ch.qos.logback", name = "logback-classic", version.ref = "logback" }
 mockk = { group = "io.mockk", name = "mockk", version.ref = "mockk" }
 slf4j-api = { group = "org.slf4j", name = "slf4j-api", version.ref = "slf4j" }
-slf4j-simple = { group = "org.slf4j", name = "slf4j-simple", version.ref = "slf4j" }
 
 [plugins]
 dokka = { id = "org.jetbrains.dokka", version.ref = "dokka" }

src/main/kotlin/ch/leadrian/slf4k/Slf4k.kt

@@ -19,6 +19,7 @@ package ch.leadrian.slf4k
 
 import org.slf4j.Logger
 import org.slf4j.LoggerFactory
+import org.slf4j.MDC
 import org.slf4j.Marker
 
 /**
@@ -220,4 +221,34 @@ inline fun Logger.error(marker: Marker, throwable: Throwable?, message: () -> St
     if (isErrorEnabled) {
         error(marker, message(), throwable)
     }
+}
+
+/**
+ * Execute the given [action] in an MDC context where the [key] is assigned [value].
+ *
+ * @return the result of [action], may be [Unit]
+ */
+inline fun <T> mdc(key: String, value: String?, action: () -> T): T {
+    putCloseable(key, value).use {
+        return action()
+    }
+}
+
+@PublishedApi
+internal fun putCloseable(key: String, value: String?): AutoCloseable {
+    val previousValue = MDC.get(key)
+    return if (previousValue != null) {
+        val closeable = RestoringMDCCloseable(key, previousValue)
+        MDC.put(key, value)
+        closeable
+    } else {
+        MDC.putCloseable(key, value)
+    }
+}
+
+private class RestoringMDCCloseable(private val key: String, private val previousValue: String) : AutoCloseable {
+
+    override fun close() {
+        MDC.put(key, previousValue)
+    }
 }

src/test/kotlin/ch/leadrian/slf4k/Slf4kTest.kt

@@ -23,8 +23,11 @@ import io.mockk.just
 import io.mockk.mockk
 import io.mockk.verify
 import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertNull
 import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.assertAll
 import org.slf4j.Logger
+import org.slf4j.MDC
 import org.slf4j.Marker
 
 internal class Slf4kTest {
@@ -538,5 +541,20 @@ internal class Slf4kTest {
 
         assertEquals("TestLogger", logger.name)
     }
+
+    @Test
+    fun `should add MDC context`() {
+        val valueBefore = MDC.get("foo")
+        val valueDuring = mdc("foo", "bar") {
+            MDC.get("foo")
+        }
+        val valueAfter = MDC.get("foo")
+
+        assertAll(
+            { assertNull(valueBefore) },
+            { assertEquals("bar", valueDuring) },
+            { assertNull(valueAfter) },
+        )
+    }
 }