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根据提供的文件信息，本文将对“design-strategies.pdf”中的关键知识点进行详细的解读与扩展，主要聚焦于异步JavaScript和XML（AJAX）的设计策略及其与服务器端Java技术的结合。 ### 概述随着互联网的发展，用户对于网站响应速度及交互体验有了更高的要求。这些需求催生了新一代的网络应用——Web 2.0。这类应用的特点在于能够快速响应用户的操作，并且支持用户在创建和分享网站内容方面进行更多的互动。文章中提到了一些典型的Web 2.0网站，如Google Maps和Flickr，它们提供了高度动态且响应迅速的用户界面。为了实现这一目标，一种重要的技术手段就是AJAX。 ### 什么是AJAX？ AJAX是一种允许网页在无需重新加载整个页面的情况下更新部分数据的技术。它通过在后台与服务器交换少量数据来实现这一点，从而提供了一种更为流畅和交互式的用户体验。AJAX的核心优势在于其支持异步通信的能力，这意味着浏览器可以向服务器发送请求并在不阻塞其他操作的情况下接收响应。这样的设计让Web应用能够像桌面应用一样工作，极大地提升了用户体验。 ### AJAX技术 AJAX技术主要包括以下几个组成部分： - **JavaScript**：用于处理客户端逻辑、动态更新DOM（文档对象模型）以及发送异步请求。 - **XMLHttpRequest**：用于发送和接收异步请求。 - **CSS (Cascading Style Sheets)**：用于控制网页样式。 - **DOM (Document Object Model)**：用于动态更新网页内容。 ### AJAX与服务器端Java技术 AJAX作为一种客户端技术，通常需要与服务器端技术相结合才能发挥出最大效能。在服务器端，Java是一种广泛使用的语言，特别是在企业级应用开发中。Java与AJAX的结合使得开发者能够构建出更加灵活、高效的应用程序。例如，Java Server Faces (JSF) 是一种基于Java的框架，它可以很好地与AJAX技术集成，以提高应用程序的性能和用户体验。 ### 设计策略文章中提到了六种不同的设计策略，下面是对这些策略的详细解读： #### 设计策略1：自行实现这种方式意味着开发者完全手动地实现所有AJAX功能，包括发送请求、处理响应等。虽然这种方式给予了开发者最大的灵活性和控制权，但它也带来了较高的复杂性和开发成本。 #### 设计策略2：使用客户端侧JavaScript技术库利用现有的JavaScript库（如jQuery、Prototype等）可以大大简化AJAX的实现过程。这些库通常提供了一系列工具和方法，可以方便地处理异步请求、DOM操作等任务。 #### 设计策略3：使用客户端-服务器框架这种方法涉及到使用专门为AJAX设计的框架，如DWR (Direct Web Remoting) 或者 Dojo Toolkit。这些框架不仅提供了强大的客户端功能，还包含了一些服务器端组件，使得开发者能够轻松地在客户端和服务器之间传递数据。 #### 设计策略4：封装现有组件对于已经存在的服务器端组件，可以通过添加一层封装来使其支持AJAX功能。这种方式可以在不改变原有系统架构的基础上，快速提升应用程序的交互性。 #### 设计策略5：远程调用这种方法利用了远程过程调用 (RPC) 的概念，允许客户端直接调用服务器端的方法或服务，从而减少往返时间并提高效率。 #### 设计策略6：全Java技术栈还有可能选择使用全Java技术栈来构建AJAX应用。这意味着客户端和服务器端都使用Java相关的技术。这种方式的优势在于可以充分利用Java生态系统中的各种资源和技术支持，同时保持代码的一致性和可维护性。 ### 总结通过上述分析可以看出，AJAX技术为Web 2.0应用提供了强有力的支持，使开发者能够构建出更加动态、响应迅速的网络应用。然而，如何有效地将AJAX与现有的服务器端技术结合起来，则需要开发者根据实际情况选择合适的设计策略。无论是自行实现还是利用现有的框架和库，合理的设计都能够显著提升应用程序的性能和用户体验。

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http://java.sun.com/developer/technicalArticles/J2EE/AJAX/DesignStrategies/ Oct 18, 2006

Article

AJAX Design Strategies

By Ed Ort and Mark Basler, October 2006

Download PDF

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Contents

Overview

What is AJAX?

AJAX Technologies

AJAX and Server-Side Java Technologies

Design Strategies

Design Strategy 1: Do It Yourself

Design Strategy 2: Use a Client-Side JavaScript Technology

Library

Design Strategy 3: Use a Client-Server Framework

Design Strategy 4: Do the Wrap Thing

Design Strategy 5: Go Remote

Design Strategy 6: Go All Java Technology

Summary

For More Information

About the Authors

Overview

Web applications have entered a new era driven by web site goals such as fast response to user actions and

user collaboration in creating and sharing web site content. The popular term attributed to these highly

responsive and often collaborative sites is Web 2.0.

Some prime examples of Web 2.0 are web sites such as Google

Maps

and Flickr. Google Maps offers a highly responsive user

interface (UI). For instance, you can view a map, then move your

cursor across it to see adjacent areas almost immediately. Flickr is a

site on which users store and share photographs -- users manage

almost all the site's content.

Other Web 2.0 sites provide a similarly rich user experience by doing

things such as integrating services from other web sites or

incorporating a steady stream of new information. For example, the

Google map service can be brought into another web site, such as a

site for purchasing cars, to present a map that highlights the location

of auto dealerships that sell a particular car model. The term used for

these site integrations is "mashups." Or a sports-oriented site can

continually update scores without requiring the user to request a page

update.

This article is about the primary technique in use today for making Web 2.0 sites highly responsive:

Asynchronous JavaScript and XML (AJAX).

What is AJAX?

AJAX allows a web site to

be -- or appear to be --

highly responsive because

it supports asynchronous

and partial refreshes of a

web page.

A number of excellent articles that describe AJAX are available, for example,

Asynchronous JavaScript Technology and XML (AJAX) With Java 2 Platform,

Enterprise Edition

. In brief, AJAX is a set of technologies that together allows

a web site to be -- or appear to be -- highly responsive. AJAX enables this

because it supports asynchronous and partial refreshes of a web page.

A partial refresh means that when an interaction event fires -- for example, a

user enters information into a form on a web page and clicks a Submit button -- the server processes the

information and returns a limited response specific to the data it receives. Significantly, the server does not send

back an entire page, as is the case for conventional, "click, wait, and refresh" web applications. Instead, the

client then updates the page based on the response. Typically this means that only part of the page is updated.

In other words, the web page is treated like a template: The client and the server exchange data, and the client

updates parts of the template based on the data the client receives from the server. One way to think of this is

that web applications that use AJAX are driven by events and data, whereas conventional web applications are

driven by pages.

Asynchronous means that after sending data to the server, the client can continue processing while the server

does its processing in the background. This means that a user can continue interacting with the client without

noticing a lag in response. For example, a user can continue to move the mouse over a Google map and see a

smooth, uninterrupted change in the display. The client does not have to wait for a response from the server

before continuing, as is the case for the traditional, synchronous, approach.

Another important aspect of AJAX-enabled sites is that the events that trigger AJAX responses aren't limited to

submitting data in a form or clicking on a link. Moving a mouse over an area on a web page, typing part of an

entry in a field, or -- as in the Google Maps case -- dragging a map around with a cursor can be enough to

trigger an AJAX response. This dynamic interaction between the user and the web page moves web applications

closer to what users experience in highly responsive desktop applications, often termed rich desktop

applications. So Web 2.0 applications are often called rich Internet applications.

AJAX Technologies

The following technologies are typically included in AJAX:

● Cascading Style Sheets (CSS), a markup language for defining the presentation style of a page, such as

fonts and colors.

● JavaScript, a scripting language. One element of JavaScript technology that is key to AJAX is

XMLHttpRequest, an object that is used to exchange data between the web client and web server.

● Document Object Model (DOM), which provides a logical view of a web page as a tree structure.

● XML, the format for sending data from the web server to the client. However, you can use other formats,

such as HTML, JavaScript Object Notation (JSON), or plain text.

Like other web applications, an AJAX-enabled web application uses a markup language such as HTML or

XHTML to present web pages, or a server-side technology such as JavaServer Pages (JSP) technology to

generate web pages. In addition, server-side application systems play a key role in processing AJAX

applications. A server-side application system such as Java Platform, Enterprise Edition (Java EE), that includes

support for data validation, user identity management, and persistence fits very well with the AJAX methodology.

See the

AJAX and Server-Side Java Technologies section of this article.

Figure 1 illustrates how these technologies work together to handle a user action that triggers an AJAX response

Figure 1: How AJAX Technologies Handle a User Action

1. A user generates an event on the client. This results in a JavaScript technology call.

2. A JavaScript technology function creates and configures an XMLHttpRequest object on the client, and

specifies a JavaScript technology callback function.

3. The XMLHttpRequest object makes a call -- an asynchronous HTTP request -- to the web server.

4. The web server processes the request and returns an XML document that contains the result.

5. The XMLHttpRequest object calls the callback function and exposes the response from the web server so

that the request can be processed.

6. The client updates the HTML DOM representing the page with the new data.

You can incorporate these technologies into a web application in a variety of ways. For example, you can write

JavaScript technology code in an HTML page, or you can use a library such as one in the Dojo toolkit to provide

part or all of the JavaScript technology functionality you need. In addition, you have server-related choices

regarding AJAX.

AJAX and Server-Side Java Technologies

All the technologies included in AJAX are either client technologies such as JavaScript technology, or -- like XML

-- are pertinent to the interchange of data between the client and server. The fact is that the request to the server

that the XMLHttpRequest object makes is an HTTP request. To the server, handling an AJAX request is no

different than handling an HTTP request from a conventional web application -- any server technology can be

used to handle the request, including server-side Java technologies in the Java EE Platform such as servlets,

JSP technology, and JavaServer Faces technology. In fact, Java EE technologies fit very well with the AJAX

methodology. JavaServer Faces technology and other Java EE technologies that include support for data

validation, user identity management, and persistence, are particularly well-suited for AJAX.

Java EE technologies fit

very well with the AJAX

methodology.

For example, you can use a servlet to process a request, manage state for

the client, access enterprise resources, and generate the XML for the

response. Or you can create custom JavaServer Faces components to handle

server-side processing as well as encapsulate the JavaScript technology and

CSS for client-side processing. In fact, a

library of AJAX-enabled custom

JavaServer Faces components

is already available as part of the Java Blueprints Solutions Catalog.

Design Strategies

As a developer, you have a variety of options in building AJAX into a web application. These options range from

a complete "do-it-yourself" strategy, where you do all the AJAX coding, to strategies that take advantage of

libraries and frameworks that provide some or all of the AJAX coding for you. And you can combine multiple

strategies. For example, you might use a client-server framework based on JavaServer Faces technology in

combination with a client-side JavaScript technology library such as a library in the Dojo toolkit.

The remainder of this article discusses some of these design strategies and identifies a few of their advantages

and disadvantages. These strategies assume that you're using a Java EE implementation on the server, such as

Sun Java Application Server.

Design Strategy 1: Do It Yourself

In this approach you do all the coding to build AJAX into a web application. This means you do all the JavaScript

technology, CSS, and DOM coding on the client, as well as the coding for page presentation. On the server, you

do the coding to handle the XMLHttpRequest call, and to return an appropriate response, such as XML.

Refer to the article

"Creating an AJAX-

Enabled Bookstore

Application, a Do-It-

Yourself Approach"

view the code for an AJAX-

enabled version of

bookstore2, one of the

Duke's Bookstore

examples documented in

the Java EE 5 Tutorial.

The "Do-It-Yourself" article

is part of a series of

articles titled

"Hands-On

Java EE5"

. The series

presents code that

implements various

approaches to enabling

Let's look at an example that adds AJAX functionality to a web application.

The example appears in the article "Creating an AJAX-Enabled Bookstore

Application, a Do-It-Yourself Approach"

. That example is based on

bookstore2, one of the Duke's Bookstore examples documented in the

Java

EE 5 Tutorial

. You can find the instructions for building the bookstore2 web

application in

The Example JSP Pages section of the Java EE 5 Tutorial.

The original bookstore2 web application lists books available for purchase.

You can see what the list looks like in here. The interaction between a user

and the current bookstore2 application is a conventional synchronous

interaction. A user clicks on a book title in the list to get further information

about that book. This requires the server to do a full refresh of the page.

The example that appears in the "Creating an AJAX-Enabled Bookstore

Application" article adds some AJAX functionality to enable pop-up balloons in

the web application. This added functionality provides for a more dynamic UI.

When a user moves the mouse cursor over a listed book, details about that

book appear in a pop-up balloon, as

Figure 2 shows. Because it's driven by

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