SOLR-915 - adding docs to CloseHook and making the list a Collection

git-svn-id: https://svn.apache.org/repos/asf/lucene/solr/trunk@727122 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/solr/core/CloseHook.java

@@ -29,5 +29,16 @@ package org.apache.solr.core;
   */
 
  public interface CloseHook {
+   
+   /**
+    * Method called when the given SolrCore object is closing / shutting down.
+    * <br />
+    *  <b>Important:</b> Keep the method implementation as short as possible. If it were to use any heavy i/o , network connections -
+    *   it might be a better idea to launch in a separate Thread so as to not to block the process of 
+    *   shutting down a given SolrCore instance.   
+    *   
+    * 
+    * @param core SolrCore object that is shutting down / closing
+    */
    void close( SolrCore core );
  }

src/java/org/apache/solr/core/SolrCore.java

@@ -740,7 +740,7 @@ public final class SolrCore implements SolrInfoMBean {
     }
   }
 
-  private List<CloseHook> closeHooks = null;
+  private Collection<CloseHook> closeHooks = null;
 
    /**
     * Add a close callback hook