some small refactors

src/main/java/com/alibaba/druid/pool/ha/node/PoolUpdater.java

@@ -49,16 +49,9 @@ public class PoolUpdater implements Observer {
     private Lock lock = new ReentrantLock();
     private ScheduledExecutorService executor;
     private int intervalSeconds = DEFAULT_INTERVAL;
-    private boolean inited = false;
+    private volatile boolean inited = false;
 
-    public PoolUpdater() {
-    }
-
-    /**
-     * @see #PoolUpdater()
-     */
     public PoolUpdater(HighAvailableDataSource highAvailableDataSource) {
-        this();
         setHighAvailableDataSource(highAvailableDataSource);
     }
 
@@ -73,6 +66,10 @@ public class PoolUpdater implements Observer {
             if (inited) {
                 return;
             }
+            if (intervalSeconds < 10) {
+                LOG.warn("CAUTION: Purge interval has been set to " + intervalSeconds
+                        + ". This value should NOT be too small.");
+            }
             if (intervalSeconds <= 0) {
                 intervalSeconds = DEFAULT_INTERVAL;
             }

src/main/java/com/alibaba/druid/pool/ha/node/ZookeeperNodeListener.java

@@ -37,7 +37,7 @@ import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
 
 /**
- * A NodeListner that watches a Zookeeper Path.
+ * A NodeListener that watches a Zookeeper Path.
  * e.g.
  * Path to watch:
  * <pre>