@edu.umd.cs.findbugs.annotations.SuppressWarnings(value = "MS_EXPOSE_REP",
      justification = "singleton pattern")
  public synchronized static RESTServlet getInstance(Configuration conf, UserProvider userProvider)
    throws IOException {
    if (INSTANCE == null) {
      INSTANCE = new RESTServlet(conf, userProvider);
    }
    return INSTANCE;
  }
@param conf         Existing configuration to use in rest servlet
@param userProvider the login user provider
@return the RESTServlet singleton instance
hbase-rest/src/main/java/org/apache/hadoop/hbase/rest/RESTServlet.java