जगदीश खोलिया: JSON overview

Saturday, June 25, 2011

JSON overview

JSON is a lightweight format for exchanging data between the client and server. It is often used in Ajax applications because of its simplicity and because its format is based on JavaScript object literals. We will start this lesson by learning JavaScript's object-literal syntax and then we will see how we can use JSON in an Ajax application.

Object Literals

We saw earlier how to create new objects in JavaScript with the Object() constructor. We also saw how we could create our constructors for our own objects. In this lesson, we'll examine JavaScript's literal syntax for creating arrays and objects.

Arrays

Array literals are created with square brackets as shown below:
var Beatles = ["Paul","John","George","Ringo"];
This is the equivalent of:
var Beatles = new Array("Paul","John","George","Ringo");

Objects

Object literals are created with curly brackets:
var Beatles = {
 "Country" : "England",
 "YearFormed" : 1959,
 "Style" : "Rock'n'Roll"
}
This is the equivalent of:
var Beatles = new Object();
Beatles.Country = "England";
Beatles.YearFormed = 1959;
Beatles.Style = "Rock'n'Roll";
Just as with all objects in JavaScript, the properties can be references using dot notation or bracket notation.
alert(Beatles.Style); //Dot Notation
alert(Beatles["Style"]); //Bracket Notation

Arrays in Objects

Object literals can contain array literals:
var Beatles = {
 "Country" : "England",
 "YearFormed" : 1959,
 "Style" : "Rock'n'Roll",
 "Members" : ["Paul","John","George","Ringo"]
}

Objects in Arrays

Array literals can contain object literals:
var Rockbands = [
 {
  "Name" : "Beatles",
  "Country" : "England",
  "YearFormed" : 1959,
  "Style" : "Rock'n'Roll",
  "Members" : ["Paul","John","George","Ringo"]
 },
 {
  "Name" : "Rolling Stones",
  "Country" : "England",
  "YearFormed" : 1962,
  "Style" : "Rock'n'Roll",
  "Members" : ["Mick","Keith","Charlie","Bill"]
 }
]

JSON

On the JSON website (http://www.json.org), JSON is described as:
  1. a lightweight data-interchange format
  2. easy for humans to read and write
  3. easy for machines to parse and generate
Numbers 1 and 3 are certainly true. Number 2 depends on the type of human. Experienced programmers will find that they can get comfortable with the syntax relatively quickly.

JSON Syntax

The JSON syntax is like JavaScript's object literal syntax except that the objects cannot be assigned to a variable. JSON just represents the data itself. So, the Beatles object we saw earlier would be defined as follows:
{
 "Name" : "Beatles",
 "Country" : "England",
 "YearFormed" : 1959,
 "Style" : "Rock'n'Roll",
 "Members" : ["Paul","John","George","Ringo"]
}

JSON Parsers

As JSON is just a string of text and not an object in and of itself, it needs to be converted to an object before to make it useful. Although this can be done in JavaScript with the eval() function, it is safer to use a JSON parser. You can download the JavaScript JSON parser at http://www.json.org/json.js. Including this file on a web page allows you to take advantage of the JSON object, which has the following very useful methods:
  • JSON.parse(strJSON) - converts a JSON string into a JavaScript object.
  • JSON.stringify(objJSON) - converts a JavaScript object into a JSON string.
The process for sending data between the browser and server with JSON is as follows:
  1. On the client-side:
    • Create a JavaScript object using the standard or literal syntax.
    • Use the JSON parser to stringify the object.
    • Send the URL-encoded JSON string to the server as part of the HTTP Request. This can be done using the HEAD, GET or POST method by assigning the JSON string to a variable. It can also be sent as raw text using the POST method, but this may create extra work for you on the server-side.
  2. On the server-side:
    • Decode the incoming JSON string and convert the result to an object using a JSON parser for the language of your choice. At http://www.json.org, you'll find JSON parsers for many modern programming languages. The methods available depend upon which parser you are using. See the parser's documentation for details.
    • Do whatever you wish with the object.
    • If you wish to send JSON back to the client:
      • Create a new object for storing the response data.
      • Convert the new object to a string using your JSON parser.
      • Send the JSON string back to the client as the response body (e.g, Response.Write(strJSON), echo $strJSON, out.write(strJSON) etc.).
  3. On the client-side:
    • Convert the incoming JSON string to an object using the JavaScript JSON parser.
    • Do whatever you wish with the object.
    • And so on...
The examples below show how to transfer data to the server using JSON. 

Code Sample: JSON/Demos/SendJson.html

<html>
<head>
<script type="text/javascript" src="../../prototype.js"></script>
<script type="text/javascript" src="../../json.js"></script>
<script type="text/javascript">
 function SendRequest(MSG)
 {
  document.getElementById("ResponseDiv").innerHTML="";
  var objJSON = {
   "msg" : MSG
  };
  var strJSON = encodeURIComponent(JSON.stringify(objJSON));
  new Ajax.Request("ReceiveJSON.jsp",
   {
    method: "post",
    parameters: "strJSON=" + strJSON,
    onComplete: Respond
   });
 }

 function Respond(REQ)
 {
  document.getElementById("ResponseDiv").innerHTML=REQ.responseText;
 }
</script>
<title>Using JSON</title>
</head>

<body>
<h1>Request</h1>
<form>
 <input type="button" value="Hi There!" onClick="SendRequest(this.value)"/>
 <input type="button" value="Good bye!" onClick="SendRequest(this.value)" />
</form>
<h1>Response</h1>
<div id="ResponseDiv">Waiting...</div>
</body>
</html>

Code Sample: JSON/Demos/ReceiveJSON.jsp

<%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1" import="java.net.*,org.json.simple.*"%>
<%
 try
 {
  URLDecoder decoder = new URLDecoder();
  String strJSON = decoder.decode(request.getParameter("strJSON"));
  JSONObject objJSON = (JSONObject) JSONValue.parse(strJSON);
  
  String Message = objJSON.get("msg").toString();
  if (Message.equals("Hi There!"))
  {
   out.write("And hi there to you!");
  }
  else
  {
   out.write("Later Gator!");
  }
 }
 catch (Exception e)
 {
  out.write(e.toString());
 }
%>
Code Explanation
This code is relatively simple. The client-side script:
  • creates a simple JSON object with one property holding the passed in MSG string.
  • converts the JSON object to a string and encodes it.
  • passes the string as a parameter of our Ajax request (using Prototype).
  • outputs the responseText to the page.
The server-side script:
  • decodes the passed in string and converts it to a JSON object.
  • outputs an appropriate response.
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