diff --git a/solr/solrj/src/java/org/apache/solr/common/cloud/SolrZooKeeper.java b/solr/solrj/src/java/org/apache/solr/common/cloud/SolrZooKeeper.java
index e1c1f3952f8..82b86c364d5 100644
--- a/solr/solrj/src/java/org/apache/solr/common/cloud/SolrZooKeeper.java
+++ b/solr/solrj/src/java/org/apache/solr/common/cloud/SolrZooKeeper.java
@@ -30,7 +30,7 @@ import org.apache.zookeeper.ZooKeeper;
 
 // we use this class to expose nasty stuff for tests
 public class SolrZooKeeper extends ZooKeeper {
-  Set<Thread> spawnedThreads = new CopyOnWriteArraySet<Thread>();
+  final Set<Thread> spawnedThreads = new CopyOnWriteArraySet<Thread>();
   
   // for test debug
   //static Map<SolrZooKeeper,Exception> clients = new ConcurrentHashMap<SolrZooKeeper,Exception>();