How to call database store procedure with return parameter from EJB3

@Resource(mappedName = "java:/myDatasource")
private DataSource ds;

public void callSetLoginUserId(String userId) throws Exception {
  Connection conn = null;
  CallableStatement st = null;
  try {
    String sql = "{ ? = call my_pkg.create_user(?) }";
    conn = ds.getConnection();
    st = conn.prepareCall(sql);
    st.registerOutParameter(1, Types.BIGINT);
    st.setString(2, "username");
    st.execute();
    Long userId = st.getLong(1);
  }
  catch(Exception e) {
    log.error("calling my_pkg.create_user has failed", e);
    throw e;
  }
  finally {
    try {
      if (st != null) {
      st.close();
    }
    if (conn != null) {
      conn.close();
    }
  } catch (SQLException e) {
    // ignore
  }
}