L2 Web App Development Guest Lecture At University of Surrey 20/11/09

L2: Web Application Development
Direct Web Remoting (DWR): Ajax made easy…

Daniel Bryant
Department of Computing, FEPS (d.bryant@surrey.ac.uk)
Tai-Dev Ltd, (daniel.bryant@tai-dev.co.uk)

Today’s roadmap...
• My life story (in under 3 minutes)…

• Quick review - so, what is Ajax? (Old school vs new school)

• DWR
Introduction
Looking deeper into DWR (client-side/server-side)
Implementation
Demo (and debugging)

• Quick case study – TriOpsis Ltd

• DWR is awesome!! But are there any disadvantages?

• Review

My life story (abridged)…

• Studying at Surrey for 8 years
BSc Computing and IT - Placement at DTI (now called BERR, DBIS etc. etc...)
MSc Internet Computing

• PhD Student within the Department of Computing
Argumentation “how humans reason”
Software Agents “mobile and/or intelligent code”

• JEE, Web 2.0, J2ME & RDBMS Consultant
Working freelance for the past 5 years
Started Tai-Dev Ltd 1 year ago (http://tai-dev.blog.co.uk/)
J2EE, JEE 5, JSE, J2ME
Spring, Hibernate, MySQL, GlassFish v2
HTML, CSS, Javascript
Prototype, Script.aculo.us, JQuery
Direct Web Remoting (DWR)…

So, just what is Ajax?

• “Asynchronous JavaScript and XML”
“…group of interrelated web development techniques used for creating interactive web
applications or rich Internet applications.” (Wikipedia, 2008)

• Building block for “Web 2.0” applications
Facebook, Google Mail and many more (auto-complete forms)

• Applications can retrieve data from the server asynchronously in the background
without interfering with the display and behaviour of the existing page
No browser plugins (a’la Flash, Flex, SilverLight)

• The use of JavaScript, XML, or its asynchronous use is not required…

Ajax - the old school way…
Client

Server

http://java.sun.com/developer/technicalArticles/J2EE/AJAX

Old school, not so cool…

• Client-side
Browser incompatibilities (Microsoft, and then the rest of the world...)
Long-winded
Error prone
Responsible for parsing return data, often XML-based
Responsible for handling application errors (response codes?)
Large amount of repeated “boiler plate” code

• Server-side
Create Servlets (no abstraction, and limited chance to allow design patterns)
Construct XML document of data
Responsible for “flattening” Objects and Collections
Set content-type of return data manually
Manual error handing (convert Exceptions into response codes?)

Introducing the alternatives…

• JavaScript Libraries/Frameworks
dojo, JQuery, Prototype
Greatly simplify client-side code
Not so helpful on server-side…

• JSP Taglibs/JSF Components
jMaki, Ajax4jsf
Very easy to utilise
Limited server-side configuration (majority of focus on existing widgets and services)

• Proxy-based Frameworks
Direct Web Remoting (DWR), Rajax
Best of both worlds
Language specific on backend (Java)

• Tip: Always new stuff coming out – check blogs and news sites...

Direct Web Remoting (DWR)
Overview

• DWR allows easy implementation of Ajax functionality
Homepage @ http://directwebremoting.org/
Open source
JavaScript “client-side”
Java “server-side”

• Proxy-based framework
Client-side code can call Java server-side methods as if they were local
JavaScript functions.
Converts or “marshalls” parameters and return variable to/from Java/JavaScript

• DWR generates the intermediate code (“piping” or boilerplate code)

• Also provides Utility classes

DWR in pictures

Image from http://directwebremoting.org/dwr/overview/dwr

Client-side

• Core components
DWR JavaScript engine
JavaScript “interface” definitions of remote methods
JavaScript Object Notation (JSON) used instead of XML

• Call Java methods as if local JavaScript functions
Albeit with callbacks…

• Hides browser incompatibilities (via “engine.js”)
XMLHttpRequest Object
Maps function calls to URLs

• Converts or “marshalls” data
Java ArrayLists into JavaScript arrays
Java Objects into JavaScript object

• Simplifies error-handling
Maps Java Exceptions to JavaScript errors

Server-side

• Core components
DWR JAR Library
Proxy generator
DWRServlets

• Easy framework configuration
XML or Annotations (Java 5+)
Care needed…

• Not tied to writing Servlets
Promotes good OO coding and design patterns

• Simply expose (existing) Application Services
Specify order and types of parameter
Can return any type of Collection or Object
Can utilise Spring, Struts, JSF…

Implementation in 5 (easy) steps…

1. Copy DWR Library files into project

2. Configure your existing framework to handle DWR requests

3. Create your Data Model (Business Objects) and Application Services

4. Inform DWR of these classes and their required exposure client-side
1. dwr.xml configuration file
2. Annotations (Java 5+)

5. Create your client-side functions

Handling http requests (web.xml)….

…

<servlet>
<servlet-name>dwr-invoker</servlet-name>
<servlet-class>org.directwebremoting.servlet.DwrServlet</servlet-class>
<init-param>
<param-name>classes</param-name>
<param-value>
uk.ac.surrey.wappdev.appservices.FeedbackService,
uk.ac.surrey.wappdev.model.FeedbackBean
</param-value>
</init-param>
</servlet>

<servlet-mapping>
<servlet-name>dwr-invoker</servlet-name>
<url-pattern>/dwr/*</url-pattern>
</servlet-mapping>
…

Create the Model (Business Objects)

package uk.ac.surrey.wappdev.model;

import org.directwebremoting.annotations.DataTransferObject;
import org.directwebremoting.annotations.RemoteProperty;

@DataTransferObject
public class FeedbackBean {

@RemoteProperty
private int id;
@RemoteProperty
private String feedbackValue;

public FeedbackBean() {
}

public FeedbackBean(int id, String feedbackValue) {
this.id = id;
this.feedbackValue = feedbackValue;
}

//getter and setter defined here...
…

Create your Application Services…

package uk.ac.surrey.wappdev.appservices;

import ...

@RemoteProxy
public class FeedbackService {

@RemoteMethod
public List<FeedbackBean> getFeedbackAvailable() {
List<FeedbackBean> results = new ArrayList<FeedbackBean>();

for (int i = 0; i < 5; i++) {
FeedbackBean fbBean = new FeedbackBean(i, "Feedback Value " + i);
results.add(fbBean);
}
return results;
}
...
}

Create your client-side functions…
package uk.ac.surrey.wappdev.appservices;

<script src='dwr/interface/LocService.js'></script> import ...
<script src='dwr/engine.js'></script>
@RemoteProxy
public class FeedbackService {
<script>
function updateFeedback() { @RemoteMethod
//alert("updateFeedback()"); public List<FeedbackBean>
getFeedbackAvailable() {
List<FeedbackBean> results =
FeedbackService.getFeedbackAvailable({ new ArrayList<FeedbackBean>();
callback:function(dataFromServer) {
cbUpdateFeedback(dataFromServer); for (int i = 0; i < 5; i++) {
}, FeedbackBean fbBean =
new FeedbackBean(i, "Feedback
errorHandler:function(errorString, exception) {
Value " + i);
alert("Error: " + errorString); results.add(fbBean);
} }
}); return results;
} }
...
}
function cbUpdateFeedback(feedbackBeanList) {
//alert("cbUpdateFeedback()");
for (var i = 0, l = feedbackBeanList.length; i < l; i++) {

var option = document.createElement("option");
option.setAttribute("value",feedbackBeanList[i].id);

var optText = document.createTextNode(feedbackBeanList[i].feedbackValue);
option.appendChild(optText);

document.getElementById("feedbackEl").appendChild(option);

}
}
</script>

Lights, camera, action...
(oh yes, and debugging)

• Quick demo of slide material

• Quick look at debugging
Client-side – Firefox’s Firebug
Server-side – Netbeans’ debugger

• Tip: If you want to be a professional software developer debugging
efficiently should become as natural as breathing…
Not emphasized enough in teaching (but this is just my opinion)
Probably a worthwhile skill for those final year projects as well…

Real world case study... TriOpsis Ltd

• Highly innovative start-up company based at the Research Park (STC)
• Check out www.triopsis.co.uk for more information
• Experts in the emerging field of Visual Business Information
• Specialising on ‘in the field’ data capture via mobile devices
• Images and associated metadata reporting relevant to target customer

Real world case study... TriOpsis Ltd

Screenshot of TriOpsis Flagship product – the ‘Asset Manager’ (implemented by yours truly!)

And finally…
There are some disadvantages with DWR…
• As with any framework that generates (blackbox) “piping”
Sometimes difficult to know what is happening “in the pipe”

• Potentially difficult to debug
Spans across client and server domain
Can use Netbeans debugger and FireFox’s Firebug

• Maintaining http session information
Hybrid of POSTed forms and Ajax

• Can cause unexpectedly large amounts of http traffic
Passing of complete object graphs (typically developer error ☺ )

• Potential security implications
Exposing incorrect methods etc.
Easy to pass sensitive data in plaintext (passwords etc.) without knowing

Conclusions

• We know what Ajax is…

• We examined old school/new school approaches to implementation

• We learned that DWR is a “proxy-based” framework
Providing (JavaScript) client and (Java) server-side Ajax support
Allows exposure of Java model (BOs) and services
DWR “handles the details”..

• We’ve seen how to implement DWR

• We’ve had a look at an often undervalued skill – debugging

• Seen real case study using this technology, TriOpsis, which is actively used within Industry

• And we are always aware of potential disadvantages
Beware of “black box” implementations…
Security, session and http traffic

Thanks for your attention…

• I’m happy to answer questions now or later...

• If you want to know more about DWR or debugging ask for a lab session
Sorry, but I can’t answer individual emails...

• Feedback, comments, constructive criticism...

L2 Web App Development Guest Lecture At University of Surrey 20/11/09

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L2 Web App Development Guest Lecture At University of Surrey 20/11/09