Use concurrent collection for the thread safety.

Reviewed in https://codereview.appspot.com/10715045/

src/main/java/com/google/appengine/demos/websocketchat/server/ChatSocketServer.java

@@ -24,13 +24,13 @@
 import java.net.URI;
 import java.net.URISyntaxException;
 import java.net.URL;
-import java.util.ArrayList;
 import java.util.Collection;
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.logging.Logger;
 
 import com.google.appengine.api.NamespaceManager;
@@ -123,12 +123,12 @@ public static class ChatServerBridge implements Runnable {
 
     private Map<String, Set<String>> globalParticipantsMap;
 
-    private List<Key<ChatRoomParticipants>> chatRoomParticipantsKeyList;
+    private CopyOnWriteArrayList<Key<ChatRoomParticipants>> chatRoomParticipantsKeyList;
 
     private ChatServerBridge() {
       namespace = NamespaceManager.get();
-      globalParticipantsMap = new HashMap<>();
-      chatRoomParticipantsKeyList = new ArrayList<>();
+      globalParticipantsMap = new ConcurrentHashMap<>();
+      chatRoomParticipantsKeyList = new CopyOnWriteArrayList<>();
     }
 
     /**
@@ -191,8 +191,8 @@ public void stop() {
         // delete participant list in the datastore
         ofy().delete().keys(chatRoomParticipantsKeyList).now();
         // initialize variables
-        globalParticipantsMap = new HashMap<>();
-        chatRoomParticipantsKeyList = new ArrayList<>();
+        globalParticipantsMap = new ConcurrentHashMap<>();
+        chatRoomParticipantsKeyList = new CopyOnWriteArrayList<>();
         chatSocketServer = null;
       } catch (IOException|InterruptedException e) {
         LOG.warning(Throwables.getStackTraceAsString(e));

src/main/java/com/google/appengine/demos/websocketchat/server/MetaInfoManager.java

@@ -19,7 +19,11 @@
 import com.google.common.collect.ImmutableSet;
 import org.java_websocket.WebSocket;
 
-import java.util.*;
+import java.util.Map;
+import java.util.Set;
+import java.util.TreeSet;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
 
 /**
  * A class that manages connections from the clients.
@@ -80,8 +84,8 @@ private static String createIdFromConnection(WebSocket connection) {
    * Creates a MetaInfoManager with the initialized map objects.
    */
   public MetaInfoManager() {
-    connectionMap = new HashMap<>();
-    participantMap = new HashMap<>();
+    connectionMap = new ConcurrentHashMap<>();
+    participantMap = new ConcurrentHashMap<>();
   }
 
   /**