作者：Bob Ippolito /原文链接
浏览器安全模型规定，XMLHttpRequest、框架（frame）等只能在一个域中通信。从安全角度考虑，这个规定很合理；但是，也确实给分布式（面向服务、混搭等等本周提到的概念）Web开发带来了麻烦。
为了实现跨域通信，通常的解决方案有3种：
本地代理：
需要一些硬件设施（没有服务器的客户端无法运行），并且带宽和潜伏时间也要加倍（远程服务器－代理服务器－客户端）。
Flash：
远程主机中需要部署一个crossdomain.xml文件，而且，Flash作为一门专有技术，其前途尚不明朗；换句话说，开发人员很可能要学习一种目标不确定的编程语言。
Script标签：
无法确切知道内容是否有效，没有标准的实现方法，又可能被认为是一种“安全风险”。
在此，我建议使用一种新技术，也是一种独立于标准的方法，即通过script标签来跨域获取数据，名为JSON with Padding，或者就叫
JSONP。JSONP的原理很简单，但需要服务器端给予相应配合。大致来说，JSONP的实现思路就是在客户端编程时作好使用JSON数据的准备，然后再通过圆括号将这些数据括起来以创建一条有效的JavaScript语句（可能是一次有效的函数调用）。
也就是说，客户端可以使用一个用于命名jsonp的查询参数来决定可以获取的数据。最简单的情况下，如果jsonp参数为空，则返回的数据就是被括在圆括号中的JSON。
下面，我们就以del.icio.us的JSON API为例，来说明JSONP的原理。该API有一个“script tag”变量（即，可以将下面的URL作为script标签的src属性值，用以加载del.icio.us这个API提供的数据。——译者注）如下所示：
http://del.icio.us/feeds/json/bob/mochikit+interpreter:
1 | if(typeof(Delicious) == 'undefined') Delicious = {}; |
4 | "d": "Interpreter - JavaScript Interactive Interpreter", |
6 | "mochikit","webdev","tool","tools", |
7 | "javascript","interactive","interpreter","repl" |
如果用JSONP的方式来表示，那么与此具有相同语义的URL应该是这样的：
http://del.icio.us/feeds/json/bob/mochikit+interpreter?jsonp=if(typeof(Delicious)%3D%3D%27undefined%27)Delicious%3D%7B%7D%3BDelicious.posts%3D
单纯看这个URL似乎没有什么，但我们可以要求服务器在数据有效时给出通知。因此，我可以编写一个用于跟踪数据的小系统：
01 | var delicious_callbacks = {}; |
02 | function getDelicious(callback, url) { |
03 | var uid = (new Date()).getTime(); |
04 | delicious_callbacks[uid] = function () { |
05 | delete delicious_callbacks[uid]; |
08 | url += "?jsonp=" + encodeURIComponent("delicious_callbacks[" + uid + "]"); |
根据以上假设，用于获取数据的URL应该如下所示：
http://del.icio.us/feeds/json/bob/mochikit+interpreter?jsonp=delicious_callbacks%5B12345%5D
1 | delicious_callbacks[12345]([{ |
3 | "d": "Interpreter - JavaScript Interactive Interpreter", |
5 | "mochikit","webdev","tool","tools", |
6 | "javascript","interactive","interpreter","repl" |
可见，由于使用圆括号括住了返回的数据，这就相当于把一个JSONP请求转化成了一次函数调用，或者得到了一个纯粹的JSON直接量。服务器所要配合做的，就是在JSON数据的开头添加一小段文本（即回调函数的名称。——译者注）并将JSON数据放在括号中！
当然，接下来最好是使用Mochikit、Dojo等框架来抽象JSONP，从而让自己省去动手编写DOM以插入script标签的麻烦。
没错，JSONP只是解决了标准化的问题。假如远程主机想通过script标签向页面中注入恶意代码，而不是返回JSON数据，那么页面安全可能会随时受到威胁。不过，一旦实现了JSONP，那么对开发人员来说肯定是一件省时省力的大好事，在此基础上各种一般化的抽象、教程及文档也会应运而生的。
注：缩写词 JSONP 由 Bob Ippolito 在一篇名为 “Remote JSON – JSONP” 的文章中提出。但许多支持以 JSONP 技术实现跨域通信的厂商没有称其为 JSONP。例如，雅虎公司就称这种技术为 “JSON with callbacks”。另外，原文发表于2005年12月5日。
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