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

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A guest lecture I presented to MSc Level Enterprise Systems Development students within the Department of Computing at the University of Surrey. This was a very similar presentation to the L2 lecture …

A guest lecture I presented to MSc Level Enterprise Systems Development students within the Department of Computing at the University of Surrey. This was a very similar presentation to the L2 lecture delivered the week earlier, but also included more advanced material.

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  • 1. 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)
  • 2. 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
  • 3. 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)…
  • 4. 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…
  • 5. Ajax - the old school way… Server Client http://java.sun.com/developer/technicalArticles/J2EE/AJAX
  • 6. 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?)
  • 7. 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...
  • 8. 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
  • 9. DWR in pictures Image from http://directwebremoting.org/dwr/overview/dwr
  • 10. 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
  • 11. 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…
  • 12. 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
  • 13. 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
  • 14. 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
  • 15. 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> <servlet-name>dispatcher</servlet-name> <url-pattern>*.htm</url-pattern> </servlet-mapping> <servlet-mapping> <servlet-name>dispatcher</servlet-name> <url-pattern>/dwr/*</url-pattern> </servlet-mapping> …
  • 16. Cooking the Spring Beans… <!-- DWR configuration (wrapping Spring ServletWrappingController around standard DWR Servlet, and specifying init-params (including the location of annotated classes --> <bean id="dwrController" class="org.springframework.web.servlet.mvc.ServletWrappingController"> <property name="servletClass" value="org.directwebremoting.servlet.DwrServlet"> </property> <property name="initParameters"> <props> <!-- remember to disable in production use --> <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>
  • 17. 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) { …
  • 18. 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; } }
  • 19. 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>
  • 20. 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…
  • 21. 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
  • 22. Real world case study... TriOpsis Ltd Screenshot of TriOpsis Flagship product – the ‘Asset Manager’ (implemented by yours truly!)
  • 23. 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
  • 24. 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
  • 25. 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...