YARN-9813. RM does not start on JDK11 when UIv2 is enabled. (Adam Antal/Eric Yang via wangda)

Change-Id: I18b8edc930b2efa0652f59c246931ad0d46827f3
(cherry picked from commit 34b82e6da0a471010cdae613ba39487889d79369)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/ResourceManager.java

@@ -1178,8 +1178,7 @@ public class ResourceManager extends CompositeService
 
       if (null == onDiskPath) {
         String war = "hadoop-yarn-ui-" + VersionInfo.getVersion() + ".war";
-        URLClassLoader cl = (URLClassLoader) ClassLoader.getSystemClassLoader();
-        URL url = cl.findResource(war);
+        URL url = getClass().getClassLoader().getResource(war);
 
         if (null == url) {
           onDiskPath = getWebAppsPath("ui2");