MSc Enterprise Systems Development Guest Lecture at UniS (2/12/09)

Enterprise Systems 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)
Design Patterns
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…
Server
Client

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 (not OO)
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 (‘engine.js’)
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 (eg we can say user.firstname)

• 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 or Spring Controllers
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…

Design Patterns
• “A design pattern in architecture and computer science is a formal way of documenting a solution
to a design problem in a particular field of expertise.”
(Wikipedia, http://en.wikipedia.org/wiki/Design_patterns, accessed 30/11/2009)

• Already covered Model, View, Controller (MVC) – utilised within Spring “MVC”

• Difficult to use MVC with Ajax/DWR - Sacrificing some benefits
MVC isolates business logic from input and presentation,
MVC permits independent development, testing and maintenance of each
With MVC we can have multiple views...

Spring Dispatcher-Servlet

Your Business Objects (User, Address Location)
and Business Logic (Application Services?)

JSP

Design Patterns (continued...)
• DWR based on other Design Patterns - We gain simplicity of implementation
Business Objects – basic POJOs, beans
Application Service - our business logic classes. Note, these can/should be Unit Tested
View is determined by calling HTML/JSP and JavaScript
Front Controller taken care of – the DWRServlet

• When combining Spring MVC and DWR we have to be careful when managing state shared
between the two (such as security info).
We can overcome this using other patterns, such as the Intercepting Filter
Could be implemented using Aspect Oriented Programming e.g. Spring AOP?
• Images taken from http://www.corej2eepatterns.com/ (accessed 30/11/2009)

DWRServlet

Javascript in browser
(engine.js)
Your Application Services
and Business Objects
Intercepting Filter
Would be implemented here

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>dispatcher</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-
class>
<load-on-startup>2</load-on-startup>
</servlet>
<servlet-mapping>
<url-pattern>*.htm</url-pattern>
</servlet-mapping>
<servlet-mapping>
<url-pattern>/dwr/*</url-pattern>
</servlet-mapping>

…

Cooking the Spring Beans…


<bean id="dwrController"
class="org.springframework.web.servlet.mvc.ServletWrappingController">
<property name="servletClass" value="org.directwebremoting.servlet.DwrServlet">
</property>
<property name="initParameters">
<props>

<prop key="debug">
true
</prop>
<!—Location of annotated classes that DWR should scan for use -->
<prop key="classes">
esd.model.Location,
esd.service.LocService
</prop>
</props>
</property>
</bean>

Create the Model (Business Objects)

package esd.model;

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

@DataTransferObject
public class Location {

@RemoteProperty
private String address1;
@RemoteProperty
private String address2;
@RemoteProperty
private String city;
@RemoteProperty
private String county;
@RemoteProperty
private String country;
@RemoteProperty
private String postcode;

public Location(String address1, String address2, String city,
String county, String country, String postcode) {
…

Create your Application Services…

import …

@RemoteProxy
public class LocService {

private List<Location> initialLocs = new ArrayList<Location>();
//
// -------------- Constructors -----------------
//
public LocService() {
…
}
@RemoteMethod
public List<Location> findLocs(String postcode) {
List<Location> results = new ArrayList<Location>();
if (postcode == null || postcode.equals("")) {
results = initialLocs;
} else {
//perform search
//TODO
}
return results;
}
}

Create your client-side functions…
import …

@RemoteProxy
public class LocService {
<script src='dwr/interface/LocService.js'></script>
private List<Location> initialLocs = new
<script src='dwr/engine.js'></script> ArrayList<Location>();
//
// -------------- Constructors -----------------
<script> //
public LocService() {
function searchForLocs() { …
}
LocService.findLocs("", { @RemoteMethod
callback:function(dataFromServer) { public List<Location> findLocs(String postcode) {
List<Location> results = new ArrayList<Location>();
updateResults(dataFromServer); if (postcode == null || postcode.equals("")) {
results = initialLocs;
}, } else {
errorHandler:function(errorString, exception) { //perform search
//TODO
alert("Error: " + errorString); }
return results;
} }
}); }

}

function updateResults(locList) {
for (var i = 0, l = locList.length; i < l; i++) {
var txt = document.createTextNode(locList[i].address1 + "," + locList[i].postcode);
document.getElementById("results").appendChild(txt);
}
var br = document.createElement("br");
document.getElementById("results").appendChild(br);
}
</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 MSc Dissertations 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...

• We will learn more about DWR in the lab session
Sorry, but I can’t promise to answer individual emails...

• Feedback, comments, constructive criticism...

MSc Enterprise Systems Development Guest Lecture at UniS (2/12/09)

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