Microsoft AJAX Library - Introducing JSON ...

In AJAX applications, client-server communication is usually packed in XML documents, or in the JSON (JavaScript Object Notation) format. Interestingly enough, JSON's popularity increased together with the AJAX phenomenon, although the AJAX acronym includes XML.

JSON is the format used by the Microsoft AJAX Library and the ASP.NET AJAX Framework to exchange data between the AJAX client and the server, which is why it deserves a quick look here. As you will learn, the Microsoft AJAX Library handles JSON data packaging through Sys.Serialization.JavaScriptSerializer, which is described in the Appendix—but more on this later.

Perhaps the best short description of JSON is the one proposed by its official website, http://www.json.org: "JSON (JavaScript Object Notation) is a lightweight data-interchange format. It is easy for humans to read and write. It is easy for machines to parse and generate."

If you are new to JSON, a fair question you could ask would be: why another data exchange format? JSON, just like XML, is a text-based format that it is easy to write and to understand for both humans and computers. The key word in the definition above is "lightweight". JSON data structures occupy less bandwidth than their XML versions.

To get an idea of how JSON compares to XML, let is take the same data structure and see how we would represent it using both standards:

    < ?xml version="1.0" encoding="UTF-8" standalone="yes"? >
    < response >
    < clear >false< /clear >
    < messages >
    < message >
    < id >1< /id >
    < color >#000000< /color >
    < time >2006-01-17 09:07:31< /time >
    < name >Guest550< /name >
    < text >Hello there! What is up?< /text >
    < /message >
    < message >
    < id >2< /id >
    < color >#000000< /color >
    < time >2006-01-17 09:21:34< /time >
    < name >Guest499< /name >
    < text >This is a test message< /text >
    < /message >
    < /messages >
    < /response >
    The same message, written in JSON this time, looks like this:
    [
    {"clear":"false"},
    "messages":
    [
    {"message":
    {"id":"1",
    "color":"#000000",
    "time":"2006-01-17 09:07:31",
    "name":"Guest550",
    "text":"Hello there! What is up?"}
    },
    {"message":
    {"id":"2",
    "color":"#000000",
    "time":"2006-01-17 09:21:34",
    "name":"Guest499",
    "text":"This is a test message"}

    }
    ]
    }
    ]

As you can see, they aren very different. If we disregard the extra formatting spaces that we added for better readability, the XML message occupies 396 bytes while the JSON message has only 274 bytes.

JSON is said to be a subset of JavaScript because it is based on the associative array-nature of JavaScript objects. JSON is based on two basic structures:

    * Object: This is defined as a collection of name/value pairs. Each object begins with a left curly brace ({) and ends with a right curly brace (}). The pairs of names/values are separated by a comma. A pair of name/value has the following form: string:value.
      .
    * Array: This is defined as a list of values separated by a coma (,).

We have mentioned strings and values. A value can be a string, a number, an object, an array, true or false, or null. A string is a collection of Unicode characters surrounded by double quotes. For escaping, we use the backslash ().

It is obvious that if you plan to use JSON, you need to be able to parse and generate JSON structures in both JavaScript and ASP.NET, at least if the communication is bidirectional. JSON libraries are available for most of today is programming languages: ActionScript, C, C++, C#, VB.NET, Delphi, E, Erlang, Java, JavaScript, Lisp,Lua, ML and Ruby, Objective CAML, OpenLazslo, Perl, PHP, Python, Rebol, Ruby, and Squeak. When we said almost every programming language we were right, weren we!
source: exforsys

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