public static PreparedStatement prepareStatement(Connection conn,String sql){
if(conn==null)
   System.out.println("--------------conn==null");
  
PreparedStatement ps=null;
try {
ps=conn.prepareStatement(sql);
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return ps;
}

------解决方案--------------------
Connection=null