Java Web Services入门与核心技术指南

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"《Web Services Essentials》是一本由Ethan Cerami所著、O'Reilly出版社出版的专业书籍，于2002年2月首次发行，ISBN号为0-596-00224-6，共304页。该书旨在为程序员提供Web服务的全面介绍和实用参考，帮助他们理解和掌握这一新型分布式应用技术的发展。书中内容涵盖了Web服务的基础知识，如架构、XML消息传递、服务描述（WSDL）和服务发现（UDDI），以及四个关键的新兴技术：XML-RPC、SOAP、UDDI和WSDL。在第一部分“Web服务入门”中，作者首先介绍了Web服务的概念，强调了它们如何突破传统Web的局限，实现更高效的数据交换和远程调用。接着，作者详细解释了Web服务的架构，包括客户端-服务器模型和RESTful设计原则，以及XML在Web服务中的核心作用，它是数据交换的标准格式。第1章“Introduction”深入浅出地讲解了Web服务的基本原理，从网络服务的角度出发，探讨了Web服务与传统应用程序的区别。章节中还提到了WSDL（Web Services Description Language），这是定义和描述Web服务接口的标准，使得服务的自动发现和消费成为可能。同时，UDDI（Universal Description, Discovery, and Integration）被解释为服务目录，它有助于在互联网上查找和注册服务。安全问题在Web服务中至关重要，因此作者专门讨论了如何处理隐私、认证和授权等安全挑战，确保服务的可靠性和安全性。最后，作者提醒读者关注标准的统一性，因为这有助于不同系统间的互操作性和长期的可持续发展。通过本书，读者不仅能学习到Java等开源工具在构建Web服务中的实践，还能了解到XML-RPC作为一种轻量级的Web服务技术，其简洁性和易用性；SOAP（Simple Object Access Protocol）作为XML消息协议，提供了更强的数据类型支持和安全性；而UDDI和WSDL则构成了服务生命周期的核心组成部分，是实现服务自动化部署和管理的关键。《Web Services Essentials》是一本适合希望深入了解Web服务开发、尤其是Java开发者和架构师的实用指南，帮助他们在理解技术原理的同时，能够快速上手并应用于实际项目中，应对不断演进的技术趋势。"

Web Services Essentials

XML messaging

This layer is responsible for encoding messages in a common XML format so that

messages can be understood at either end. Currently, this layer includes XML-RPC

and SOAP.

Service description

This layer is responsible for describing the public interface to a specific web service.

Currently, service description is handled via the Web Service Description Language

(WSDL).

Service discovery

This layer is responsible for centralizing services into a common registry, and

providing easy publish/find functionality. Currently, service discovery is handled

via Universal Description, Discovery, and Integration (UDDI).

As web services evolve, additional layers may be added, and additional technologies may

be added to each layer. Figure 1-7 summarizes the current web service protocol stack.

Each layer is described in detail later in this book.

Figure 1-7. Web service protocol stack

1.2.3 Architectural Snapshot: The IBM Web Services Browser

To gain a high-level understanding of how the protocol stack actually works, try out the

IBM Web Services Browser. The browser enables you to search for existing services,

view their service descriptions, and automatically invoke those services. This lets you see

each layer within the protocol stack without actually writing any code.

To get started, open a browser and go to http://demo.alphaworks.ibm.com/browser/. You

should see the screen depicted in Figure 1-8.

In the righthand pane, you can search a centralized registry for existing web services. (The

registry actually uses UDDI, but don't get too caught up in the details just yet.) Within the

Search box, type "IBM Web Services" and click Search. IBM will search the centralized

directory for you and display all matching results in the left pane. Select the last folder,

entitled IBM Web Services TestArea, and you will see a list of available web services.

(See Figure 1-9.)

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Web Services Essentials

The IBM browser does a good job of illustrating web services in action and highlighting

the main layers within the protocol stack. It also does a good job of illustrating the

potential of "just-in-time" application integration. Each service basically acts as an

individual building block, and you can continue stacking more and more services to the

same page. Best of all, you can do so without writing a single line of code!

1.3 XML Messaging

XML has exploded onto the computing scene in recent years. It has gained rapid

acceptance because it enables diverse computer systems to share data more easily,

regardless of operating system or programming language. There are dozens of XML tools,

including parsers and editors that are available for nearly every operating system and every

programming language, including Java, Perl, Python, C#, C, C++, and Ruby. When

developers decided to build a web service messaging system, XML was therefore a natural

choice. There are two main contenders for XML messaging: XML-RPC and SOAP. The

following sections provide descriptions of both protocols.

1.3.1 XML-RPC

XML-RPC is a simple protocol that uses XML messages to perform RPCs. Requests are

encoded in XML and sent via HTTP POST. XML responses are embedded in the body of

the HTTP response. Because XML-RPC is platform-independent, it allows diverse

applications to communicate. For example, a Java client can speak XML-RPC to a Perl

server.

To gain a high-level understanding of XML-RPC, consider a simple weather service. The

service expects a zip code and returns the current temperature for the area. Here is a

sample XML-RPC request to the weather service (HTTP headers omitted):

<?xml version="1.0" encoding="ISO-8859-1"?>

<methodName>weather.getWeather</methodName>

</params>

</methodCall>

The request consists of a simple methodCall element that specifies the method name and

any method parameters.

Here is a sample XML-RPC response from the weather service:

<?xml version="1.0" encoding="ISO-8859-1"?>

<param>

</param>

</params>

</methodResponse>

The response consists of a single methodResponse element that specifies the return value.

In this case, the return value is specified as an integer.

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Web Services Essentials

XML-RPC is the easiest way to get started with web services. In many ways, it is simpler

than SOAP and easier to adopt. However, unlike SOAP, XML-RPC has no corresponding

service description grammar. This prevents automatic invocation of XML-RPC services -

a key ingredient for enabling just-in-time application integration. More details of XML-

RPC are covered in Chapter 2.

1.3.2 SOAP

SOAP is an XML-based protocol for exchanging information between computers.

Although SOAP can be used in a variety of messaging systems, and can be delivered via a

variety of transport protocols, the main focus of SOAP is RPCs transported via HTTP.

Like XML-RPC, SOAP is platform-independent and therefore enables diverse applications

to communicate.

To gain a high-level understanding of SOAP, let's revisit our simple weather service. Here

is a sample SOAP request (HTTP headers omitted):

<?xml version='1.0' encoding='UTF-8'?>

<SOAP-ENV:Envelope

xmlns:SOAP-ENV="http://www.w3.org/2001/09/soap-envelope/"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xmlns:xsd="http://www.w3.org/2001/XMLSchema">

<SOAP-ENV:Body>

<ns1:getWeather

xmlns:ns1="urn:examples:weatherservice"

SOAP-ENV:encodingStyle="http://www.w3.org/2001/09/soap-encoding/">

</ns1:getWeather>

</SOAP-ENV:Body>

</SOAP-ENV:Envelope>

As you can see, the SOAP request is slightly more complicated than the XML-RPC

request. It makes use of both XML namespaces and XML Schemas. As in XML-RPC,

however, the body of the SOAP request specifies both a method name and a list of

parameters.

Here is a sample SOAP response from the weather service:

<?xml version='1.0' encoding='UTF-8'?>

<SOAP-ENV:Envelope

xmlns:SOAP-ENV="http://www.w3.org/2001/09/soap-envelope/"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xmlns:xsd="http://www.w3.org/2001/XMLSchema">

<SOAP-ENV:Body>

<ns1:getWeatherResponse

xmlns:ns1="urn:examples:weatherservice"

SOAP-ENV:encodingStyle="http://www.w3.org/2001/09/soap-encoding/">

</ns1:getWeatherResponse>

</SOAP-ENV:Body>

</SOAP-ENV:Envelope>

The response indicates a single integer return value. Full details of SOAP are discussed in

Chapter 3.

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