文件写入了userInfo的值，但readObject读取文件中的对象，值全都是Null

Question

    FileInputStream fileInputStream = null;
    ObjectInputStream objectInputStream = null;
    ArrayList<UserInfo> userList = null;
    try {
        fileInputStream = new FileInputStream(file);
        objectInputStream = new ObjectInputStream(fileInputStream);
        userList = new ArrayList<UserInfo>();
        boolean flag = true;
        while(flag){
            try {
                Object obj = objectInputStream.readObject(); //这里返回的对象的值全是null
                UserInfo userInfo = (UserInfo) obj;
                userList.add(userInfo);
                flag = true;
            } catch (Exception e) {
                flag = false;
            }
            if(!flag){
                break;
            }
        }

    } catch (FileNotFoundException e) {
        System.out.println("未找到文件，请检查文件路径");
        e.printStackTrace();
    } finally {
        objectInputStream.close();
        fileInputStream.close();
    }