only set if mdc is not null

Author: yifeih

launcher/pom.xml

@@ -58,7 +58,6 @@
     <dependency>
       <groupId>org.slf4j</groupId>
       <artifactId>slf4j-log4j12</artifactId>
-      <scope>test</scope>
     </dependency>
 
     <dependency>

launcher/src/main/java/org/apache/spark/launcher/InProcessAppHandle.java

@@ -63,7 +63,9 @@ synchronized void start(String appName, Method main, String[] args) {
 
     Map<String, String> mdcContextMap = MDC.getCopyOfContextMap();
     app = new Thread(() -> {
-      MDC.setContextMap(mdcContextMap);
+      if (mdcContextMap != null) {
+        MDC.setContextMap(mdcContextMap);
+      }
       try {
         main.invoke(null, (Object) args);
       } catch (Throwable t) {