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State the design goals
Describe the architecture

One of the good practices of software development is to start with a set of basic requirements and then express these requirements more formally through a set the interfaces. So let us begin by listing the requirements for OpenTrader sample.

  • a trader can offer to buy or sell stocks at a certain price
  • an offer to sell(buy) a stock can be matched with matching offer(s) to buy(sell)
  • a pair of matching buy/sell offer can be committed as a trade

Besides the functional requirements, let us define some non-functional goals

  • the trader will access the service from a browser
  • the service will use a relational database to record the committed trades

and then some purely technical goals

  • the browser-based client will be implemented using Google Web Toolkit
  • the service will be implemented using Java Persistence API

GWT takes a position that is a significantly unique among the multitude of frameworks available to build a browser based client. GWT framework is based on several key concepts:

  • an asynchronous RPC protocol for communication between web client and server. Such asynchronous RPC is popularized by AJAX (Asynchronous JavaScript and XML) since last several years.
  • a cross-compiler that converts Java to JavaScript. This is the most critical component in the framework as it allows the client be written completely in Java. Besides adding a comfort layer for Java developers, the cross-compiler solves one of the most complex issue of browser based clients namely cross-browser compatibility. The cross-compiler generates separate JavaScript targeted to prominent browsers and the framework knows to activate the appropriate JavaScript based on the particular browser a client is using.
  • a library of interactive visual widgets such as dialog boxes, combo-boxes, tables etc. This Java based library realizes the widget as the elements of a Document Object Model (DOM) for a browser to render them in HTML. For example, an instance of com.google.gwt.user.client.ui.FlexTable urns into a <table> in the displayed HTML page by the framework. The widget library not only provides rendering support, it also comes with a event dispatching model for the client application to handle the user interaction.

OpenTrader - the sample example described in these pages - demonstrates how to develop a GWT client for a transactional, server application based on Java Persistence API (JPA). This example is somewhat more involved than a typical AddressBook example in terms of the complexity of the domain model, the transactional functions of the server as well as the interaction between multiple widgets in the client. Also the sample application covers a realistic use case where the core server application is defined independent of both GWT and JPA – and then demonstrates how these two technologies are used to implement an end-to-end service running inside a Tomcat Servlet Container.

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