Introducing dwr (direct web remoting)

Introducing DWR (Direct Web
Remoting)
BOOBUN Ashish – 20th November 2009

Agenda
 Ajax
 Introduction to DWR
 Steps to create a DWR web application
 DWR configurations
 Callback Mechanism
 Converters
 DWR .js file
 Comet with DWR
 Advantages

AJAX
 Asynchronous JavaScript and XML
 Collection of technologies used on client site to develop interactive
web application
 Document Object Model (DOM)
 XML, JSON, HTML, plain text
 XMLHttpRequest (XHR) – for asynchronous
communication
 JavaScript
 Retrieve data asynchronously from server while preserving the
current state of the user interface
 Services on server had to be accessed via URL, with request
parameters. For e.g. http://www.google.co.uk/search?q=ajax

AJAX in Action

Google is intelligent

DWR Explained
 An open source library that resides between Java and JavaScript
code.
 An implementation of AJAX in Java.
 Enables Java code on the server and JavaScript code in a browser
to interact and call each other as simply as possible.
 Making the process resemble as a “Remote Procedure call”.
 Handles the low-level communication using the
XMLHttpRequest object.
 DWR consists of two parts:
 A Java Servlet on the server to capture DWR’s request and
sends response back to the browser
 JavaScript on browser side that handles request and response
and dynamically update part of the web page accordingly.

Why DWR - NO to boiler-plate code

Steps to create a DWR-enable AJAX
application
 Add maven dependency for DWR
 Add DWR servlet and DWR servlet mapping in web.xml
 Write Java service class and its corresponding methods
 Include DWR configuration (in spring bean configuration
file, if using DWR v2.x. Else specify in dwr.xml if using
v1.x) to state what classes and method of the backend
will be exposed.
 Note that we will be integrating DWR with Spring
 Write client-side JavaScript code and invoke methods
that has been exposed in an RPC style.

Requirements
 Maven Dependency

 Web.xml

DWR Configuration

 The tag <dwr:configuration> must be present (if defining
dwr configurations in the spring bean configuration file
and not in dwr.xml file), even if it’s empty.

Client-side JavaScript code

 DWR includes 2 plain JavaScript files: engine.js (compulsory) and util.js
(optional).
 OutboundService.js is a dynamically generated .js file that interfaces with
engine.js to invoke methods exposed on the server.

 Notice the RPC-style syntax – OutboundService.getOutboundList()

Handling Asynchronous Calls
 Data is available some time after the initial call was made.
 How to combine the asynchronous nature of Ajax with the
synchronous nature of normal Java method calls?
 DWR provides call-back method mechanism.
 Call-back function is called when data is received from the server.

Converters
 Marshall data when it is sent between client and server
 DWR provides the following converters
 Basic and Date Converters (By default it is enable)
 Bean and Object converters - convert Java Beans and
POJOs to and from JavaScript objects
 Array Converters
 Collection Converters
 DOM Converters

 E.g. <dwr:convert type="bean
class="com.tnt.express.mww.walldisplay.dto
.Outbound" />

DWR .js files
 Engine.js
 Used to marshal calls from the dynamically generated interface
JavaScript function, so it is needed wherever DWR is used.
 Also contain set option methods related to timeout, errorHandler,
warningHandler.

 Util.js
 Contains a number of utility functions to help you update your
web pages with JavaScript data
 Can use it outside of DWR scope

 E.g. of utility methods
 $(id) – document.getElementById(id)
 DWRUtil.getValue(id) – get the value of the HTML element with id.

Comet with DWR
 Long-lived HTTP
 Reversing the usual communication flow
 Allow the server to start answering the browser’s
request for information very slowly by pushing data to
the browser without the latter ever explicitly requesting it
 Disadvantage:
 Server can be overloaded with too many connection
 Server takes too much time to push data in IE 6 browser

Advantages of using DWR
 DWR uses an “RPC-style mechanism” whereas almost all other forms
of AJAX take a URL-based approach.
 DWR eliminate the needs of having each method (service) on the
server to be addressable via URL.
 No need to map parameters and return value to HTML input form and
responses respectively.
 DWR is packaged with JavaScript utility functions that allows you to
dynamically modify HTML tags.
 The code for the service class is a simple POJO, and not tied to HTTP
at all. Easier to unit test.
You can write AJAX web app at
the same rate as Jack Bauer
works at CTU
 Codes is simpler, less verbose.

Introducing dwr (direct web remoting)

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