HDFS-11333. Print a user friendly error message when plugins are not found. Contributed by Wei-Chiu Chuang.

(cherry picked from commit 859bd159ae554174200334b5eb1d7e8dbef958ad)
(cherry picked from commit a8531d5d52fb219c2adb5b6025cfc3fad264e8aa)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DataNode.java

@@ -913,7 +913,15 @@ public class DataNode extends ReconfigurableBase
   }
 
   private void startPlugins(Configuration conf) {
-    plugins = conf.getInstances(DFS_DATANODE_PLUGINS_KEY, ServicePlugin.class);
+    try {
+      plugins = conf.getInstances(DFS_DATANODE_PLUGINS_KEY,
+          ServicePlugin.class);
+    } catch (RuntimeException e) {
+      String pluginsValue = conf.get(DFS_DATANODE_PLUGINS_KEY);
+      LOG.error("Unable to load DataNode plugins. Specified list of plugins: " +
+          pluginsValue, e);
+      throw e;
+    }
     for (ServicePlugin p: plugins) {
       try {
         p.start(this);

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/NameNode.java

@@ -765,8 +765,15 @@ public class NameNode implements NameNodeStatusMXBean {
       httpServer.setFSImage(getFSImage());
     }
     rpcServer.start();
-    plugins = conf.getInstances(DFS_NAMENODE_PLUGINS_KEY,
-        ServicePlugin.class);
+    try {
+      plugins = conf.getInstances(DFS_NAMENODE_PLUGINS_KEY,
+          ServicePlugin.class);
+    } catch (RuntimeException e) {
+      String pluginsValue = conf.get(DFS_NAMENODE_PLUGINS_KEY);
+      LOG.error("Unable to load NameNode plugins. Specified list of plugins: " +
+          pluginsValue, e);
+      throw e;
+    }
     for (ServicePlugin p: plugins) {
       try {
         p.start(this);