XXXDaoImpl.java文件未添加返回值
创建时默认为null

    @Override
    public List<Book> findAllBook() {
        String sql = "select * from t_book";
        // 调用方法
        List<Book> bookList = jdbcTemplate.query(sql, new BeanPropertyRowMapper<Book>(Book.class));
        return null; 
        
        // 将 null 修改为 bookList =>>>
        return bookList;
    }