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AJAX and REST

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Revision as of 18:40, 7 October 2011 by imported>Aeric (→‎Learn More)

AJAX is the name given to a set of JavaScript functions that let connect to a server, get back data, and parse the data into something usable. It originally stood for "Asynchronous JavaScript and XML", but it can also use the JSON format for data interchange.

In fact, an arbitrary string can be passed to or from the server. It's just a matter of having a functions that can deal with the data in the string. The two most popular formats by far, however, are XML and JSON. The platform understands both, so you can choose the flavor you prefer.

Perhaps the best summary of AJAX comes from the w3c.schools tutorial:

"AJAX is the art of exchanging data with a server, and updating parts of a web page, without reloading the whole page."

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Note: When you create a Page in the platform, you are creating a JSP page. You can embed Java code in that page, and use it to directly interact with the platform. Or you can use AJAX and the REST APIs, whichever is easiest.

Basic AJAX Syntax

In this syntax description, xmlhttp is the communications object. You'll learn how to make one in the example that follows. For now, the focus is on the methods you use to interact with it. 

xmlhttp.open(method, resource_url, async);
xmlhttp.send();                   // GET or DELETE
xmlhttp.send("request data");     // POST or PUT
where:
  • method is GET, POST, PUT, or DELETE
  • resource_url is the resource you are accessing
  • async is a boolean.
  • If true, you put processing code into an onreadystatechange function, and the page continues displaying itself while the request is being processed. Whenever the response from the server is received, it is integrated into the page. (That is the preferred way to do things.)
  • If false, you put processing code after the send() statement. (You don't need a function, but the page may appear hang while it is being rendered, so this procedure is not recommended.)
Considerations
  • Most often, you'll want to make asynchronous requests. They're slightly harder to write, because you have to add the onreadystatechange function. But the page won't hang while waiting for the server to respond.
  • The platform's REST APIs return XML, by default. To get back JSON data, you append "?alt=JSON" to the URL.
  • When a resource that responds to a GET request takes additional data, the data is included by appending a query string to the URL, in order to Specify Query Parameters.
  • Platform GET (read), PUT (update), and DELETE requests return 200 on success.
  • Platform POST (create) requests return 201 on success.

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Tip: In Firefox, the Firebug plugin makes it possible to debug JavaScript. You can set breakpoints, see the content of variables, and review the structure of the internal DOM after elements have been dynamically added.

Example: Retrieving Data with a GET Request

This example shows AJAX code in a JSP page being used to get login status.

Of course, the example is trivial, since login status will always be "true". (Otherwise, you couldn't see the page at all.) But even this simple example displays a number of interesting characteristics. 

<html>                                                           // Note #1
<head>
<script type="text/javascript">
function getInfo()
{
    // Create a communications object.
    var xmlhttp;
    if (window.XMLHttpRequest) {
        // code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp=new XMLHttpRequest();
    }
    else {
        // code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }

    // Configure the comms object with the function
    // that runs when a response is received.
    xmlhttp.onreadystatechange=function() {                       
        if (xmlhttp.readyState==4 && xmlhttp.status==200) {
            // Success. Insert returned data into the page.
            text = "<pre>" + xmlhttp.responseText + "</pre>";
            var reply = eval('(' + xmlhttp.responseText + ')');  // Note #2
            result = reply.platform.user.is_session_valid;       
            text += "Result: " + result;
            document.getElementById("myDiv").innerHTML=text;
        }
    }
    
    // Set up the request and send it to the server
    resource = "/networking/rest/user/isSessionValid?alt=json";  // Note #3
    async = true;
    xmlhttp.open("GET", resource, async);    
    xmlhttp.send();               
}
</script>
</head>

<body>
    <div id="myDiv"><h2>Click the button to check status.</h2></div>
    <button type="button" onclick="getInfo()">Ok</button>
</body>

</html>

Visiting the page and clicking the button echoes the response returned by server: 

{"platform": {                                                   // Note #2
  "message": {
    "code": "0",
    "description": "Success"
  },
  "user": {"is_session_valid": "true"}                           // Note #4
}}

Result: true
Notes
  1. Although this is a plain HTML page doesn't contain any JSP code, it must have a .jsp extension, to be stored on the platform. (JSP code can then be added whenever desired.)
  2. As shown by the sample response, the data is returned in JSON format. That format works well in JavaScript, because the eval() function can be used to turn it into a set of objects, making it easy to retrieve the nested is_session_valid value in the next line.
  3. Here's where we tell the platform to return the data in JSON format.
  4. Here's the value we're interested in. The is_session_valid value is in user, and user is in platform. So the path to access it in the code is platform.user.is_session_valid.

POST/PUT Syntax

When there is too much data to send using Query Parameters, a REST API will require you to send the data in a POST request (to add something new) or in a PUT request (to update something existing). You'll specify add a request header that tells the platform what form the data is in: 

xmlhttp.open(method,resource_url,async);
xmlhttp.setRequestHeader("Content-type","application/xml");  // or "application/json"
xmlhttp.send("request data");

Example: Sending Data with a POST Request

This example adds a new entry to the ultra-simple Customers object in the Sample Order Processing System. To keep the example as simple as possible, the data is hard-wired into the script.

Considerations
  • In this case, we'll use a synchronous request, because we don't want the rest of the page to render until after the attempted POST has completed.
  • A unique key has been defined for the Customer object, so only the first POST should succeed. Subsequent POSTs should fail.
  • POST requests return 201 when successful (unlike other REST requests, which return 200).
<html>
<head>
<script type="text/javascript">
function createNewCustomer()
{
    // Create the communications object.
    var xmlhttp;
    if (window.XMLHttpRequest) {
        // code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp=new XMLHttpRequest();
    }
    else {
        // code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
    
    // Set up the request to send XML data to the server.
    // Make it synchronous, so we know how page rendering should proceed
    async = false;
    resource = "/networking/rest/record/customer?alt=json";
    data = 
      "<platform>                                      \
          <record>                                     \
              <customer_name>Ian</customer_name>       \
              <company_address>SoHo</company_address>  \
          </record>                                    \
      </platform>";
    
    xmlhttp.open("POST", resource, async);    
    xmlhttp.setRequestHeader("Content-type", "application/xml"); 
                                       // or "application/json"
    xmlhttp.send(data); 
    
    var text = "";
    //text = "<pre>" + xmlhttp.responseText + "</pre>";  //For debugging
        
    // Echo the return value.  Success = 201
    var reply = eval('(' + xmlhttp.responseText + ')');
    if (xmlhttp.readyState==4) {
      if (xmlhttp.status==201) {
        text += "Customer added successfully.<br/>";
        text += "Record ID: " + reply.platform.message.id;
      }
      else {
        text += "Customer add failed.<br/>";
        text += "HTTP Status = " + xmlhttp.status + "<br/>"; 
        text += "Platform Status = " + reply.platform.message.code + "<br/>";
        text += reply.platform.message.description; 
      }      
    }
    document.getElementById("myDiv").innerHTML=text;
}
</script>
</head>

<body>
    <div id="myDiv"><h2>Click the button to create a new customer.</h2></div>
    <button type="button" onclick="createNewCustomer()">Ok</button>
</body>

</html>

The result of a successful POST looks like this: 

Customer added successfully.
Record ID: 313913951

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