@RequiresSession
  public ExtensionEntity findById(long extensionId) {
    return entityManagerProvider.get().find(ExtensionEntity.class, extensionId);
  }
Gets a extension with the specified ID.

@param extensionId
         the ID of the extension to retrieve.
@return the extension or {@code null} if none exists.
ambari-server/src/main/java/org/apache/ambari/server/orm/dao/ExtensionDAO.java