AJAX In Action [63]
Licensed to jonathan zheng 112 CHAPTER 3 Introducing order to Ajax DWR, JSON-RPC, and SAJAX We’ll begin by looking at three small server-side frameworks together, because they share a common approach, although they are written for different serverside languages. SAJAX works with a variety of server-side languages, including PHP, Python, Perl, and Ruby. DWR (which stands for Direct Web Remoting) is a Java-based framework with a similar approach, exposing methods of objects rather than standalone functions. JSON-RPC (JavaScript Object Notation-based Remote Procedure Calls) is also similar in design. It offers support for server-side JavaScript, Python, Ruby, Perl, and Java. All three allow objects defined on the server to expose their methods directly as Ajax requests. We will frequently have a server-side function that returns a useful result that has to be calculated on the server, say, because it looks up a value from a database. These frameworks provide a convenient way to access those functions or methods from the web browser and can be a good way of exposing the server-side domain model to the web browser code. Let’s look at an example using SAJAX, exposing functions defined on the server in PHP. We’ll use a straightforward example function that simply returns a string of text, as follows: function sayHello(name){ return("Hello! {$name} Ajax in Action!!!!"); ?> To export this function to the JavaScript tier, we simply import the SAJAX engine into our PHP and call the sajax_export function: require('Sajax.php'); sajax_init(); sajax_export("sayHello"); ?> When we write our dynamic web page, then, we use SAJAX to generate some JavaScript wrappers for the exported functions. The generated code creates a local JavaScript function with identical signatures to the server-side function: Licensed to jonathan zheng Third-party libraries and frameworks 113 When we call sayHello("Dave") in the browser, the generated JavaScript code will make an Ajax request to the server, execute the server-side function, and return the result in the HTTP response. The response will be parsed and the return value extracted to the JavaScript. The developer need not touch any of the Ajax technologies; everything is handled behind the scenes by the SAJAX libraries. These three frameworks offer a fairly low-level mapping of server-side functions and objects to client-side Ajax calls. They automate what could otherwise be a tedious task, but they do present a danger of exposing too much server-side logic to the Internet. We discuss these issues in greater detail in chapter 5. The remaining frameworks that we’ll look at in this section take a more sophisticated approach, generating entire UI layers from models declared on the server. Although they use standard Ajax technologies internally, these frameworks essentially provide their own programming model. As a result, working with these frameworks is quite different from writing generic Ajax, and we will be able to provide only a broad overview here. Backbase The Backbase Presentation Server provides a rich widget set that binds at runtime to XML tags embedded in the HTML documents generated by the server. The principle here is similar to the Rico behavior components, except that Backbase uses a custom set of XHTML tags to mark up the UI components, rather than standard HTML tags. Backbase provides server-side implementations for both Java and .NET. It is a commercial product but offers a free community edition. Echo2 NextApp’s Echo2 framework is a Java-based server engine that generates rich UI components from a model of the user interface that is declared on the server. Once launched in the browser,