What is JSONP, detailed explanation of the connection and difference between JSON and JSONP

When it comes to AJAX, we will inevitably face two questions.

The first is what format does AJAX use to exchange data?

The second is how to solve cross-domain requirements?

There are currently different solutions to these two problems. For example, data can be described with custom strings or XML, and cross-domain issues can be solved through server-side proxies.

But so far, the most recommended or preferred solution is to use JSON to transmit data and rely on JSONP to cross domains. And that’s what this article is about.

Although there is only one letter difference between JSON and JSONP, they are actually not the same thing at all: JSON is a data exchange format, while JSONP is an unofficial cross-domain format created by the ingenuity of developers. Data exchange protocol. Let's use the recent popular spy movie as an analogy. JSON is the "code" used by underground parties to write and exchange information, while JSONP is the connection method used to transmit information written in code to their comrades. did you see? One is to describe the format of the information, and the other is the method agreed upon by both parties for transmitting the information.

What is JSON?

As I briefly mentioned before, JSON is a text-based data exchange method, or a data description format. Whether you should choose it or not, you must first pay attention to its advantages.

Advantages of JSON:

Based on plain text, cross-platform transmission is extremely simple;

Javascript native support, almost all backend languages ​​are supported;

Lightweight data format, occupying very few characters, especially suitable for Internet transmission;

Highly readable, although not as clear as XML, it is still easy to identify after reasonable indentation;

Easy to write and parse, of course, provided you know the data structure;

Disadvantages of JSON Of course there is, but in the author's opinion it is really insignificant, so I won't explain it separately.

The format or rules of JSON:

JSON can describe the data structure in a very simple way. It can do everything XML can do, so the two are completely equal in terms of cross-platform.

JSON has only two data type descriptors, curly brackets {} and square brackets []. The remaining English colons are mapping characters, English commas are delimiters, and English double quotes "" are defining characters.

Braces {} are used to describe a set of "unordered key-value pairs of different types" (each key-value pair can be understood as an attribute description of OOP), and square brackets [] are used to describe a set of "unordered key-value pairs of different types". "Ordered data collection" (an array that can correspond to OOP).

If there are multiple sub-items in the above two sets, they should be separated by commas.

Key-value pairs are separated by English colon:, and it is recommended that the key names be added with English double quotes "" to facilitate parsing in different languages.

The commonly used data types inside JSON are nothing more than strings, numbers, Boolean, dates, and null. Strings must be enclosed in double quotes, and the rest are not used. The date type is quite special, so I won’t go into details here. It is recommended that if the client does not require the function of sorting by date, then just pass the date and time directly as a string, which can save a lot of trouble.

JSON Example

// 描述一个人
var person = {
    "Name": "Bob",
    "Age": 32,
    "Company": "IBM",
    "Engineer": true
}
// 获取这个人的信息
var personAge = person.Age;
// 描述几个人
var members = [
    {
        "Name": "Bob",
        "Age": 32,
        "Company": "IBM",
        "Engineer": true
    },
    {
        "Name": "John",
        "Age": 20,
        "Company": "Oracle",
        "Engineer": false
    },
    {
        "Name": "Henry",
        "Age": 45,
        "Company": "Microsoft",
        "Engineer": false
    }
]
// 读取其中John的公司名称
var johnsCompany = members[1].Company;
// 描述一次会议
var conference = {
    "Conference": "Future Marketing",
    "Date": "2012-6-1",
    "Address": "Beijing",
    "Members": 
    [
        {
            "Name": "Bob",
            "Age": 32,
            "Company": "IBM",
            "Engineer": true
        },
        {
            "Name": "John",
            "Age": 20,
            "Company": "Oracle",
            "Engineer": false
        },
        {
            "Name": "Henry",
            "Age": 45,
            "Company": "Microsoft",
            "Engineer": false
        }
    ]
}
// 读取参会者Henry是否工程师
var henryIsAnEngineer = conference.Members[2].Engineer;

That’s all about JSON. For more details, please refer to the information for in-depth study during the development process.

What is JSONP?

First let’s talk about how JSONP came into being:

In fact, there are many explanations about JSONP on the Internet, but they are all the same and confusing. It is a bit difficult for many people who are new to it to understand. It is not a small thing, try it. Try to explain the problem in your own way and see if it helps.

A well-known problem, Ajax direct request for ordinary files has the problem of cross-domain unauthorized access. Regardless of whether you are a static page, dynamic web page, web service, or WCF, as long as it is a cross-domain request, it is not allowed;

But we We also found that when calling js files on a Web page, it is not affected by whether it is cross-domain (not only that, we also found that all tags with the "src" attribute have cross-domain capabilities, such as 3f1c4e4b6b16bbbd69b2ee476dc4f83a, b364fee97caf20c42571523a0ce6d0a2);

So it can be judged that at the current stage, if you want to access data across domains through pure web (ActiveX controls, server-side proxies, future HTML5 Websockets, etc. are not included), there is only one possibility. , that is, trying to load the data into a js format file on the remote server for client calling and further processing;

恰巧我们已经知道有一种叫做JSON的纯字符数据格式可以简洁的描述复杂数据，更妙的是JSON还被js原生支持，所以在客户端几乎可以随心所欲的处理这种格式的数据；

这样子解决方案就呼之欲出了，web客户端通过与调用脚本一模一样的方式，来调用跨域服务器上动态生成的js格式文件（一般以JSON为后缀），显而易见，服务器之所以要动态生成JSON文件，目的就在于把客户端需要的数据装入进去。

客户端在对JSON文件调用成功之后，也就获得了自己所需的数据，剩下的就是按照自己需求进行处理和展现了，这种获取远程数据的方式看起来非常像AJAX，但其实并不一样。

为了便于客户端使用数据，逐渐形成了一种非正式传输协议，人们把它称作JSONP，该协议的一个要点就是允许用户传递一个callback参数给服务端，然后服务端返回数据时会将这个callback参数作为函数名来包裹住JSON数据，这样客户端就可以随意定制自己的函数来自动处理返回数据了。

那AJAX和JSONP有有什么联系呢?

ajax和jsonp这两种技术在调用方式上“看起来”很像，目的也一样，都是请求一个url，然后把服务器返回的数据进行处理，因此jquery和ext等框架都把jsonp作为ajax的一种形式进行了封装；

但ajax和jsonp其实本质上是不同的东西。ajax的核心是通过XmlHttpRequest获取非本页内容，而jsonp的核心则是动态添加3f1c4e4b6b16bbbd69b2ee476dc4f83a标签来调用服务器提供的js脚本。

所以说，其实ajax与jsonp的区别不在于是否跨域，ajax通过服务端代理一样可以实现跨域，jsonp本身也不排斥同域的数据的获取。

还有就是，jsonp是一种方式或者说非强制性协议，如同ajax一样，它也不一定非要用json格式来传递数据，如果你愿意，字符串都行，只不过这样不利于用jsonp提供公开服务。

总而言之，jsonp不是ajax的一个特例，哪怕jquery等巨头把jsonp封装进了ajax，也不能改变着一点！

如果对于callback参数如何使用还有些模糊的话，我们后面会有具体的实例来讲解。

JSONP的客户端具体实现：

不管jQuery也好，extjs也罢，又或者是其他支持jsonp的框架，他们幕后所做的工作都是一样的，下面我来循序渐进的说明一下jsonp在客户端的实现：

1、我们知道，哪怕跨域js文件中的代码（当然指符合web脚本安全策略的），web页面也是可以无条件执行的。

远程服务器remoteserver.com根目录下有个remote.js文件代码如下：

alert('我是远程文件');

本地服务器localserver.com下有个jsonp.html页面代码如下：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title></title>
    <script type="text/javascript" src="/uploads/201411/c0c96a45571dfe3f0049e31503fb0c401.js"></script>
</head>
<body>
</body>
</html>

毫无疑问，页面将会弹出一个提示窗体，显示跨域调用成功。

2、现在我们在jsonp.html页面定义一个函数，然后在远程remote.js中传入数据进行调用。jsonp.html页面代码如下：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title></title>
    <script type="text/javascript">
    var localHandler = function(data){
        alert(&#39;我是本地函数，可以被跨域的remote.js文件调用，远程js带来的数据是：&#39; + data.result);
    };
    </script>
    <script type="text/javascript" src="/uploads/201411/c0c96a45571dfe3f0049e31503fb0c401.js"></script>
</head>
<body>
</body>
</html>

remote.js文件代码如下：

localHandler({"result":"我是远程js带来的数据"});

运行之后查看结果，页面成功弹出提示窗口，显示本地函数被跨域的远程js调用成功，并且还接收到了远程js带来的数据。很欣喜，跨域远程获取数据的目的基本实现了，但是又一个问题出现了，我怎么让远程js知道它应该调用的本地函数叫什么名字呢？毕竟是jsonp的服务者都要面对很多服务对象，而这些服务对象各自的本地函数都不相同啊？我们接着往下看。

3、聪明的开发者很容易想到，只要服务端提供的js脚本是动态生成的就行了呗，这样调用者可以传一个参数过去告诉服务端“我想要一段调用XXX函数的js代码，请你返回给我”，于是服务器就可以按照客户端的需求来生成js脚本并响应了。

看jsonp.html页面的代码：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title></title>
    <script type="text/javascript">
    // 得到航班信息查询结果后的回调函数
    var flightHandler = function(data){
        alert(&#39;你查询的航班结果是：票价 &#39; + data.price + &#39; 元，&#39; + &#39;余票 &#39; + data.tickets + &#39; 张。&#39;);
    };
    // 提供jsonp服务的url地址（不管是什么类型的地址，最终生成的返回值都是一段javascript代码）
    var url = "http://flightQuery.com/jsonp/flightResult.aspx?code=CA1998&callback=flightHandler";
    // 创建script标签，设置其属性
    var script = document.createElement(&#39;script&#39;);
    script.setAttribute(&#39;src&#39;, url);
    // 把script标签加入head，此时调用开始
    document.getElementsByTagName(&#39;head&#39;)[0].appendChild(script); 
    </script>
</head>
<body>
</body>
</html>

这次的代码变化比较大，不再直接把远程js文件写死，而是编码实现动态查询，而这也正是jsonp客户端实现的核心部分，本例中的重点也就在于如何完成jsonp调用的全过程。

我们看到调用的url中传递了一个code参数，告诉服务器我要查的是CA1998次航班的信息，而callback参数则告诉服务器，我的本地回调函数叫做flightHandler，所以请把查询结果传入这个函数中进行调用。

OK，服务器很聪明，这个叫做flightResult.aspx的页面生成了一段这样的代码提供给jsonp.html（服务端的实现这里就不演示了，与你选用的语言无关，说到底就是拼接字符串）：

flightHandler({
    "code": "CA1998",
    "price": 1780,
    "tickets": 5
});

我们看到，传递给flightHandler函数的是一个json，它描述了航班的基本信息。运行一下页面，成功弹出提示窗口，jsonp的执行全过程顺利完成！

4、到这里为止的话，相信你已经能够理解jsonp的客户端实现原理了吧？剩下的就是如何把代码封装一下，以便于与用户界面交互，从而实现多次和重复调用。

什么？你用的是jQuery，想知道jQuery如何实现jsonp调用？好吧，那我就好人做到底，再给你一段jQuery使用jsonp的代码（我们依然沿用上面那个航班信息查询的例子，假定返回jsonp结果不变）：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
 <html xmlns="http://www.w3.org/1999/xhtml" >
 <head>
     <title>Untitled Page</title>
      <script type="text/javascript" src=jquery.min.js"></script>
      <script type="text/javascript">
     jQuery(document).ready(function(){ 
        $.ajax({
             type: "get",
             async: false,
             url: "http://flightQuery.com/jsonp/flightResult.aspx?code=CA1998",
             dataType: "jsonp",
             jsonp: "callback",//传递给请求处理程序或页面的，用以获得jsonp回调函数名的参数名(一般默认为:callback)
             jsonpCallback:"flightHandler",//自定义的jsonp回调函数名称，默认为jQuery自动生成的随机函数名，也可以写"?"，jQuery会自动为你处理数据
             success: function(json){
                 alert(&#39;您查询到航班信息：票价： &#39; + json.price + &#39; 元，余票： &#39; + json.tickets + &#39; 张。&#39;);
             },
             error: function(){
                 alert(&#39;fail&#39;);
             }
         });
     });
     </script>
     </head>
  <body>
  </body>
 </html>

是不是有点奇怪？为什么我这次没有写flightHandler这个函数呢？而且竟然也运行成功了！哈哈，这就是jQuery的功劳了，jquery在处理jsonp类型的ajax时（还是忍不住吐槽，虽然jquery也把jsonp归入了ajax，但其实它们真的不是一回事儿），自动帮你生成回调函数并把数据取出来供success属性方法来调用，是不是很爽呀？