XML和JSON两种数据交换格式的比较

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目前,在web开发领域,主要的数据交换格式有XML和JSON,对于XML相信每一个web developer都不会感到陌生; 相比之下,JSON可能对于一些新步入开发领域的新手会感到有些陌生,也可能你之前已经听说过,但对于XML和 JSON的不同之处可能会不怎么了解。对于在 Ajax开发中,是选择XML还是JSON,一直存在着争议,个人还是比较倾向于JSON的,虽然JSON才处于起步阶段,但我相信JSON最终会取代 XML成为Ajax的首选,到时Ajax可能要更名为Ajaj(Asynchronous JavaScript and JSON)了;

1.数据交换格式比较之关于XML和JSON:

XML:extensible markup language,一种类似于HTML的语言,他没有预先定义的标签,使用DTD(document type definition)文档类型定义来组织数据;格式统一,跨平台和语言,早已成为业界公认的标准。具体的可以问Google或百度。相比之JSON这种 轻量级的数据交换格式,XML可以称为重量级的了。

JSON : JavaScript Object Notation 是一种轻量级的数据交换格式。易于人阅读和编写。同时也易于机器解析和生成。它基于JavaScript Programming Language , Standard ECMA-262 3rd Edition – December 1999 的一个子集。 JSON采用完全独立于语言的文本格式,但是也使用了类似于C语言家族的习惯(包括C, C++, C#, Java, JavaScript, Perl, Python等)。这些特性使JSON成为理想的数据交换语言。

2.数据交换格式比较之关于轻量级和重量级:

轻量级和重量级是相对来说的,那么XML相对于JSON的重量级体现在哪呢?我想应该体现在解析上,XML目前设计了两种解析方式:DOM和SAX;

DOM是把一个数据交换格式XML看成一个DOM对象,需要把XML文件整个读入内存,这一点上JSON和XML的原理是一样的,但是XML要考虑 父节点和子节点,这一点上JSON的解析难度要小很多,因为JSON构建于两种结构:key/value,键值对的集合;值的有序集合,可理解为数组;

SAX不需要整个读入文档就可以对解析出的内容进行处理,是一种逐步解析的方法。程序也可以随时终止解析。这样,一个大的文档就可以逐步的、一点一点的展现出来,所以SAX适合于大规模的解析。这一点,JSON目前是做不到得。

所以,JSON和XML的轻/重量级的区别在于:JSON只提供整体解析方案,而这种方法只在解析较少的数据时才能起到良好的效果;而XML提供了对大规模数据的逐步解析方案,这种方案很适合于对大量数据的处理。

3.数据交换格式比较之关于数据格式编码及解析的难度:

在编码上,虽然XML和JSON都有各自的编码工具,但是JSON的编码要比XML简单,即使不借助工具,也可以写出JSON代码,但要写出好的 XML代码就有点困难;与XML一样,JSON也是基于文本的,且它们都使用Unicode编码,且其与数据交换格式XML一样具有可读性。

主观上来看,JSON更为清晰且冗余更少些。JSON网站提供了对JSON语法的严格描述,只是描述较简短。从总体来看,XML比较适合于标记文档,而JSON却更适于进行数据交换处理。

在解析上,在普通的web应用领域,开发者经常为XML的解析伤脑筋,无论是服务器端生成或处理XML,还是客户端用 JavaScript 解析XML,都常常导致复杂的代码,极低的开发效率。

实际上,对于大多数web应用来说,他们根本不需要复杂的XML来传输数据,XML宣称的扩展性在此就很少具有优势;许多Ajax应用甚至直接返回 HTML片段来构建动态web页面。和返回XML并解析它相比,返回HTML片段大大降低了系统的复杂性,但同时缺少了一定的灵活性。同XML或HTML 片段相比,数据交换格式JSON 提供了更好的简单性和灵活性。在web serivice应用中,至少就目前来说XML仍有不可动摇的地位。

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