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DWR Java AJAX Applications
DWR Java AJAX Applications

DWR Java AJAX Applications: A step-by-step example-packed guide to learning professional application development with Direct Web Remoting

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DWR Java AJAX Applications

Chapter 2. DWR Features

This chapter describes some main features of DWR: Reverse AJAX, security, DWR JavaScript libraries, and DWR's integration with other projects.

Reverse AJAX is basically a method to call client-side JavaScript from server-side Java classes. The security section discusses what has been done in DWR in order to prevent unwanted access to the server by exploiting DWR features.

DWR includes a couple of useful JavaScript libraries that are explained here together with how to use DWR with other projects, such as Struts or JSF.

This chapter covers the following sections:

  • Ease of Use—A high-level view of how DWR makes a developer's life easier

  • Reverse AJAX—Describes what Reverse AJAX means

  • DWR JavaScript Libraries—Describes the JavaScript libraries that come with DWR: engine.js, util.js, and gi.js

  • Converters—Presents how DWR marshals/unmarshals objects

  • Creators, Filters, and Signatures—Introduces important DWR configuration elements

  • Integration with Other Projects—Briefly describes...

Ease of Use


The main feature of DWR is its ease of use. DWR hides a lot of details from developers. This means that we can use AJAX functionality and we don't need to know about XmlHttpRequest for example, or how to send a Java object to a browser and so on. DWR has its own framework for performing the required marshaling/unmarshaling of Java objects to JavaScript and vice versa.

The setup for DWR consists of copying the dwr.jar file to the WEB-INF | lib directory in the application WAR file, and installing the application in the server before starting to use it. There are no special interfaces to implement in our own Java classes and it is even possible to develop a Java object completely transparently, so that the object doesn't know any DWR-specific classes. DWR provides well-documented APIs for us to use, and we can take advantage of it when developing, for example, Reverse AJAX applications.

And finally we can leverage the existing Java skills because DWR does not force us to replace...

Reverse AJAX


The term Reverse AJAX is used when a server is used to query and/or control a client-browser behavior. This may cause some questions because it sounds like our browsers are now vulnerable to attack while we visit the web pages of the world.

Luckily that is not the case, because it is not possible for a server to open a connection to a browser. A browser must always be the initiator of the connection. So, the question about security is actually valid, but a problem would mean that the website in question is designed and implemented for causing harm.

DWR supports three different methods to do Reverse AJAX in applications: Piggyback, Polling (by the client), and Comet (server push).

Piggyback

The piggyback method works so that whenever a server has an update to be sent to the client, it waits until the client opens a connection and requests some content from the server. When that happens, the server includes a new update with the response, and it is delivered to the client where DWR...

DWR JavaScript Libraries


DWR includes a few JavaScript libraries that are required for DWR to work and that are also helpful for developers: engine.js, util.js, and gi.js.

The first JavaScript library, engine.js, is the core of DWR's browser-side functionality, and is required for all the pages that use AJAX and DWR. The second library, util.js, contains useful utility functions but is not required for DWR. And the third library, gi.js, is used to integrate DWR with TIBCO General Interface (GI) for AJAX Applications.

engine.js

The engine.js library is mandatory for DWR applications since it has functions to marshal calls from the dynamically generated JavaScript functions to remote Java classes in the server. The following fragment must be present in all HTML pages that need DWR functionality:

<script type='text/javascript' src='/<web app name>/dwr/engine.js'>
</script>

There are a number of options and methods in the engine.js library. The functions in the library are prefixed...

Converters


Converters are DWR's means of marshaling data back and forth from the client to the server. There are many basic converters that are enabled by default, and it is also possible to create new converters.

Basic converters include converters for all primitive types, strings, and the following objects:

  • From java.lang package: Boolean, Byte, Short, Integer, Long, Float, Double, Character

  • From java.math package: BigInteger and BigDecimal

  • DateConverter for java.util.Date, and classes from java.sql package: Date, Times, and Timestamp

DWR also has converters for arrays (of all aforementioned objects), Java Beans and Objects, collections (Map and Collection), DOM objects, Enums, and others such as the Servlet objects and Hibernate beans.

The DWR API has a Converter interface that can be implemented for custom converters. This is not used very often because BeanConverter works for many custom objects if they are coded according to the JavaBean specification.

Creators, Filters, and Signatures


Creators are used by DWR to instantiate our remote objects on the server side. Chapter 3 has a section about Creators, and how to use them.

Filters are used to intercept calls to remote objects. Interception can occur before or after the call, and filters can be used for various purposes such as logging, security, parameter checking, or even adding extra latency to the DWR calls. The following samples shows simple filter code, and how it is configured in dwr.xml for a single class:

public class NotifyRestrictedAccessFilter {
public Object doFilter(Object obj, Method method, Object[] params, AjaxFilterChain chain) throws Exception {
//if params include monitored sentence
//then send mail to security officials
… code here …
return chain.doFilter(obj, method, params);
}
}
<allow>
<create creator="new"javascript="GetAreaDetails">
<param name="class" value="org.area.NumberedArea"/>
<filter class="org.filters.NotifyRestrictedAccessFilter"/&gt...

Integration with Other Projects


Often, DWR is not used just by itself. Perhaps, you have an on-going project where decision has been made to use a framework such as JSF or Spring for building the whole solution. In these cases, AJAX and DWR are just a part of the overall picture, and so DWR needs to integrate with other projects nicely. And that it does.

DWR integrates with the following:

Security


Security is always important, and the DWR project has thought about security very thoroughly. The DWR framework has taken into account many security issues, and there is lots of discussion about security on the DWR website, enough to fill several books about the subject.

Among the people for whom security is important are developers like you and me. Software does only what we instruct it to do, so we must be conscious about security during development and do our best to limit the possibilities to exploit our work.

While using DWR, we manually specify in the dwr.xml configuration (unless we have created some automatic code-generation software that does it for us) which Java classes and methods we want to remote to JavaScript. This way we can be sure that no attacker can exploit any other objects than our explicitly remoted Java objects (and we can concentrate to make those objects as secure as possible).

The configuration in dwr.xml also includes a create entry for each remoted Java...

Summary


This chapter presented an overview of DWR features including JavaScript libraries (util.js, engine.js, and gi.js), security, and Reverse AJAX and on basic operational elements of how DWR works.

In addition to the topics discussed in this chapter, DWR has lots of other useful features that may or may not be relevant to developers. It depends on the application. Topics in this chapter are intended to be high-level overviews, but they will give you a good starting point to learn more from the DWR website (http://directwebremoting.org/dwr/documentation) and even from the DWR source code.

Very often, most developers do not need all the features of a technology when they start working on a project. After the project has been going on for some time, we may find out that the solution has matured to include most if not all features of the technology. But that is typically not the initial case and, besides, we have to start somewhere and it is practical to start taking small steps at a time...

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What you will learn

  • Will make your development faster and simpler by teaching you the features of DWR and implementing it with Reverse AJAX. Teaches you how to use DWR JavaScript libraries, Convertors, Creators, Filters and Signatures. Integration with other projects, and Security factors which are essential to developing any DWR application. Specifies practical aspects such as supported browser and environments, configuration, setting up development environment, testing and debugging, error handling, packaging and deployment in a comprehensive manner, by demonstration where required. Provides a practical demonstration of creating a dynamic user interface, implementing tables and lists, and field completion for you to derive concepts from. Shows advanced elements of user interface through practical examples of creating forms, building a navigation tree, and map scrolling. Teaches back-end integration: integrating a database with DWR, integrating with web services, and integrating with a messaging system. Includes collaborative book authoring and chatroom applications, which show how easy it would be to enhance your applications to production grade using these samples as a starting point for development and as a source of ideas.

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Publication date : Oct 29, 2008
Length: 232 pages
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Publication date : Oct 29, 2008
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Table of Contents

7 Chapters
Introduction Chevron down icon Chevron up icon
DWR Features Chevron down icon Chevron up icon
Getting Started Chevron down icon Chevron up icon
User Interface: Basic Elements Chevron down icon Chevron up icon
User Interface: Advanced Elements Chevron down icon Chevron up icon
Backend Integration Chevron down icon Chevron up icon
Sample Applications Chevron down icon Chevron up icon

Customer reviews

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bbrockRailFan Jan 30, 2010
Full star icon Full star icon Full star icon Full star icon Full star icon 5
I have used this book in detail and tried a few code samples sucesfuly. I you are a hard core programmer And interested in combining J2EE and Ajax, this is the book for you. Even though its not a very thick soft cover book it is jam packed with information.
Amazon Verified review Amazon
RS Jan 14, 2009
Full star icon Full star icon Full star icon Full star icon Full star icon 5
ecently read "DWR Java AJAX Applications" by Sami Salkous-a PACKT Publication. At first, I was put off by the books short length and cover price of $39.99 US. However, after reading the book-to-cover twice I was pleasantly surprised to find that book not only adequately covers the subject of DWR, but covers it well. This is an excellent book if you are just starting to dabble with DWR. The book is jam packed with excellent code samples. It also provides solid coverage of deployment. I will recommend this book to any novice. Where I find the book falls short, but am not certain that it was ever meant to cover, is on the topics of performance and scalability. The book also only provides deployment examples for Apache Geronimo. It would be nice to add a few more examples for Jetty and JBoss. The development setup in the book is also specific to Eclipse. One additional nice to have would be an example of how-to structure a DWR project with Maven. Despite these minor shortcomings the book is a good read and the author should be proud of this publication. Nice work Sami! Now, if I can just get PACKT to lower their price a tad.
Amazon Verified review Amazon
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