Kotlin#119: first implementation of a SLF4J MDC Context

Author: Alphonse Bendt

integration/kotlinx-coroutines-slf4j/src/main/kotlin/kotlinx/coroutines/experimental/slf4j/MDCContext.kt

@@ -8,7 +8,7 @@ import kotlin.coroutines.experimental.CoroutineContext
 
 class MDCContext(
         private val context: CoroutineContext,
-        private val contextMap: Map<String, String> = MDC.getCopyOfContextMap()
+        private val contextMap: Map<String, String>? = MDC.getCopyOfContextMap()
 ) : AbstractCoroutineContextElement(ContinuationInterceptor), ContinuationInterceptor {
 
     override fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T> {
@@ -25,7 +25,10 @@ class MDCContext(
     private inner class Wrapper<T>(private val continuation: Continuation<T>) : Continuation<T> {
         private inline fun wrap(block: () -> Unit) {
             try {
-                MDC.setContextMap(contextMap)
+                contextMap?.let {
+                    MDC.setContextMap(contextMap)
+                }
+
                 block()
             } finally {
                 MDC.clear()

integration/kotlinx-coroutines-slf4j/src/test/kotlin/kotlinx/coroutines/experimental/slf4j/MDCContextTest.kt

@@ -1,12 +1,25 @@
 package kotlinx.coroutines.experimental.slf4j
 
 import kotlinx.coroutines.experimental.*
+import org.junit.After
+import org.junit.Before
 import org.junit.Test
 import org.slf4j.MDC
 import kotlin.test.assertEquals
+import kotlin.test.assertNull
 
 class MDCContextTest : TestBase() {
 
+    @Before
+    fun setUp() {
+        MDC.clear()
+    }
+
+    @After
+    fun tearDown() {
+        MDC.clear()
+    }
+
     @Test
     fun mdcContextIsNotPassedByDefaultBetweenCoRoutines() = runTest {
         expect(1)
@@ -60,4 +73,13 @@ class MDCContextTest : TestBase() {
             assertEquals(mdcValue, "myValue")
         }
     }
+
+    @Test
+    fun mdcContextMayBeEmpty() {
+        runBlocking(MDCContext(DefaultDispatcher)) {
+            val mdcValue = MDC.get("myKey")
+
+            assertNull(mdcValue)
+        }
+    }
 }

integration/kotlinx-coroutines-slf4j/src/test/resources/logback-test.xml

@@ -1,5 +1,15 @@
-<appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
-    <layout>
-        <Pattern>%X{first} %X{last} - %m%n</Pattern>
-    </layout>
-</appender>
+<?xml version="1.0" encoding="UTF-8"?>
+<configuration debug="false">
+
+    <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
+        <layout>
+            <Pattern>%X{first} %X{last} - %m%n</Pattern>
+        </layout>
+    </appender>
+
+    <root level="DEBUG">
+        <appender-ref ref="CONSOLE" />
+    </root>
+</configuration>
+
+