Index: solr/core/src/java/org/apache/solr/cloud/CreateAliasCmd.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- solr/core/src/java/org/apache/solr/cloud/CreateAliasCmd.java	(revision cbae3fb15e748bafd3087397e8ee0a666425fa63)
+++ solr/core/src/java/org/apache/solr/cloud/CreateAliasCmd.java	(revision )
@@ -21,7 +21,9 @@
 import java.util.List;
 import java.util.Locale;
 import java.util.Set;
+import java.util.stream.Collectors;
 
+import org.apache.commons.lang3.StringUtils;
 import org.apache.solr.cloud.OverseerCollectionMessageHandler.Cmd;
 import org.apache.solr.common.SolrException;
 import org.apache.solr.common.cloud.ClusterState;
@@ -44,12 +46,14 @@
   public void call(ClusterState state, ZkNodeProps message, NamedList results)
       throws Exception {
     String aliasName = message.getStr(NAME);
-    String collections = message.getStr("collections"); // could be comma delimited list
+//    String collections = message.get("collections"); // could be comma delimited list
+    final List<String> canonicalCollectionList = parseCollectionsParameter(message.get("collections"));
+    final String canonicalCollectionsString = StringUtils.join(canonicalCollectionList, ",");
 
     ZkStateReader zkStateReader = ocmh.zkStateReader;
-    validateAllCollectionsExistAndNoDups(collections, zkStateReader);
+    validateAllCollectionsExistAndNoDups(canonicalCollectionList, zkStateReader);
 
-    zkStateReader.aliasesHolder.applyModificationAndExportToZk(aliases -> aliases.cloneWithCollectionAlias(aliasName, collections));
+    zkStateReader.aliasesHolder.applyModificationAndExportToZk(aliases -> aliases.cloneWithCollectionAlias(aliasName, canonicalCollectionsString));
 
     // Sleep a bit to allow ZooKeeper state propagation.
     //
@@ -66,20 +70,32 @@
     Thread.sleep(100);
   }
 
-  private void validateAllCollectionsExistAndNoDups(String collections, ZkStateReader zkStateReader) {
-    List<String> collectionArr = StrUtils.splitSmart(collections, ",", true);
-    if (new HashSet<>(collectionArr).size() != collectionArr.size()) {
+  private void validateAllCollectionsExistAndNoDups(List<String> collectionList, ZkStateReader zkStateReader) {
+    final String collectionStr = StringUtils.join(collectionList, ",");
+
+    if (new HashSet<>(collectionList).size() != collectionList.size()) {
       throw new SolrException(SolrException.ErrorCode.BAD_REQUEST,
-          String.format(Locale.ROOT,  "Can't create collection alias for collections='%s', since it contains duplicates", collections));
+          String.format(Locale.ROOT,  "Can't create collection alias for collections='%s', since it contains duplicates", collectionStr));
     }
     ClusterState clusterState = zkStateReader.getClusterState();
     Set<String> aliasNames = zkStateReader.getAliases().getCollectionAliasListMap().keySet();
-    for (String collection : collectionArr) {
+    for (String collection : collectionList) {
       if (clusterState.getCollectionOrNull(collection) == null && !aliasNames.contains(collection)) {
         throw new SolrException(SolrException.ErrorCode.BAD_REQUEST,
-            String.format(Locale.ROOT,  "Can't create collection alias for collections='%s', '%s' is not an existing collection or alias", collections, collection));
+            String.format(Locale.ROOT,  "Can't create collection alias for collections='%s', '%s' is not an existing collection or alias", collectionStr, collection));
       }
     }
   }
+  
+  /**
+   * The v2 API directs that the 'collection' parameter be provided as a JSON array (e.g. ["a", "b"]).  We also
+   * maintain support for the legacy format, a comma-separated list (e.g. a,b).
+   */
+  private List<String> parseCollectionsParameter(Object colls) {
+    if (colls instanceof List) return  (List) colls;
+    return StrUtils.splitSmart(colls.toString(), ",", true).stream()
+        .map(coll -> coll.trim())
+        .collect(Collectors.toList());
+  }
 
 }