Add releaseResource param to cron api methods

docs/usage/rest_api/notebook.md

@@ -1180,7 +1180,8 @@ Notebooks REST API supports the following operations: List, Create, Get, Delete,
     <col width="200">
     <tr>
       <td>Description</td>
-      <td>This ```POST``` method adds cron job by the given note id.
+      <td>This ```POST``` method adds cron job by the given note id. 
+          Default value of ```releaseResource``` is ```false```.
       </td>
     </tr>
     <tr>
@@ -1197,7 +1198,7 @@ Notebooks REST API supports the following operations: List, Create, Get, Delete,
     </tr>
     <tr>
       <td> sample JSON input </td>
-      <td><pre>{"cron": "cron expression of note"}</pre></td>
+      <td><pre>{"cron": "cron expression of note", "releaseResource": "false"}</pre></td>
     </tr>
     <tr>
       <td> sample JSON response </td>
@@ -1241,7 +1242,7 @@ Notebooks REST API supports the following operations: List, Create, Get, Delete,
     <tr>
       <td>Description</td>
       <td>This ```GET``` method gets cron job expression of given note id.
-          The body field of the returned JSON contains the cron expression.
+          The body field of the returned JSON contains the cron expression and ```releaseResource``` flag.
       </td>
     </tr>
     <tr>
@@ -1258,7 +1259,14 @@ Notebooks REST API supports the following operations: List, Create, Get, Delete,
     </tr>
     <tr>
       <td> sample JSON response </td>
-      <td><pre>{"status": "OK", "body": "* * * * * ?"}</pre></td>
+      <td><pre>
+{
+   "status": "OK", 
+   "body": {
+      "cron": "0 0/1 * * * ?", 
+      "releaseResource": true
+   }
+}</pre></td>
     </tr>
   </table>
 

zeppelin-server/src/main/java/org/apache/zeppelin/rest/NotebookRestApi.java

@@ -878,6 +878,7 @@ public class NotebookRestApi {
 
     Map<String, Object> config = note.getConfig();
     config.put("cron", request.getCronString());
+    config.put("releaseresource", request.getReleaseResource());
     note.setConfig(config);
     notebook.refreshCron(note.getId());
 
@@ -906,7 +907,8 @@ public class NotebookRestApi {
     checkIfNoteSupportsCron(note);
 
     Map<String, Object> config = note.getConfig();
-    config.put("cron", null);
+    config.remove("cron");
+    config.remove("releaseresource");
     note.setConfig(config);
     notebook.refreshCron(note.getId());
 
@@ -932,8 +934,11 @@ public class NotebookRestApi {
     checkIfNoteIsNotNull(note);
     checkIfUserCanRead(noteId, "Insufficient privileges you cannot get cron information");
     checkIfNoteSupportsCron(note);
+    Map<String, Object> response = new HashMap<>();
+    response.put("cron", note.getConfig().get("cron"));
+    response.put("releaseResource", note.getConfig().get("releaseresource"));
 
-    return new JsonResponse<>(Status.OK, note.getConfig().get("cron")).build();
+    return new JsonResponse<>(Status.OK, response).build();
   }
 
   /**

zeppelin-server/src/main/java/org/apache/zeppelin/rest/message/CronRequest.java

@@ -26,7 +26,8 @@ import org.apache.zeppelin.common.JsonSerializable;
 public class CronRequest implements JsonSerializable {
   private static final Gson gson = new Gson();
 
-  String cron;
+  private String cron;
+  private Boolean releaseResource;
 
   public CronRequest (){
   }
@@ -35,6 +36,13 @@ public class CronRequest implements JsonSerializable {
     return cron;
   }
 
+  public Boolean getReleaseResource() {
+    if (releaseResource == null) {
+      return Boolean.FALSE;
+    }
+    return releaseResource;
+  }
+
   public String toJson() {
     return gson.toJson(this);
   }