Java Web Services：探索SOAP, WSDL 和 UDDI

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"Java Web Services 是一种基于开放标准的技术，用于构建分布式应用程序和服务。通过使用SOAP（简单对象访问协议）进行远程方法调用和消息传递，开发者能够实现不同系统间的互操作性。WSDL（Web服务描述语言）则用于定义服务接口，使得其他开发者能够理解和使用这些服务。UDDI（统一描述、发现和集成）是服务注册和查找的关键工具，允许服务提供商在本地或全球范围内发布服务，并使消费者能够搜索和发现所需的服务。本书由David Chappell和Tyler Jewell撰写，是O'Reilly出版社于2002年首次出版，共计276页，深入探讨了Java Web Services的相关主题。内容涵盖了安全问题、与EJB（企业级JavaBean）等其他Java企业技术的集成、JAX-M和JAX-RPC包的工作进展，以及与微软.NET服务的互操作性。书中章节包括： 1. 引言，对Web Services进行了概述，解释了它们是什么，为什么会被采用，以及在J2EE（Java 2企业版）环境中的应用。 2. 描述了复合计算模型，特别是服务导向架构的概念，以及对等网络（P2P）模型如何与此相适应。 3. 探讨了SOAP的基础作用，作为跨平台交互的核心，解释了如何使用SOAP进行通信。 4. WSDL的详细使用，用于描述Web服务接口，让开发者能够理解和使用服务。 5. UDDI的角色，用于服务的发布和查找，以及在本地和全局服务注册中的应用。此外，书中还讨论了Java Web Services在安全性、互操作性方面的问题，以及JAX-M和JAX-RPC等新兴技术的发展，这些都是Java开发者在构建Web服务时必须考虑的关键点。同时，书中也涵盖了与.NET平台的集成，这对于多平台开发环境下的协作具有重要意义。" 这本书对于想要了解和使用Java技术构建Web服务的开发者来说是一份宝贵的资源，它不仅提供了基础理论，还深入到实际应用和技术细节，帮助读者掌握Web服务的核心技术和实践方法。

Java Web Services

machines, they became important for business-to-business communications. Today, fax

machines are nearly ubiquitous—you can fax in your pizza order. We expect to see the same

trend in web services. They will be used first for internal business communications before

they become part of everyday life. In all cases, though—the rapid adoption of the Web, the

slower adoption of the fax machine, and the current adoption of web services—the same

factor has enabled the paradigm shift. That factor is a standards communications mechanism.

Whether the standard be the phone line and FAX protocols, the TCP/IP stack and HTTP

(together with the phone line and modem protocols), or the web service protocols, standards

have been, and continue to be, the key factor in enabling the acceptance of new technologies.

1.2.1 Industry Drivers

Many tangible drivers make web services technology attractive, both from a business and a

technical perspective. Classic Enterprise Application Integration (EAI) problems require

applications to integrate and interoperate. Even within a particular business unit, there exist

islands of IT infrastructure. For example, a Customer Relationship Management (CRM)

system may have no knowledge of how to communicate with anything outside of its own

application suite. It may need to communicate with a third-party Sales Order system so it can

know about new customers as soon as they place their first order.

Corporate acquisitions and mergers are also an issue. Entire parallel business application

infrastructures have to be synchronized or merged. Business partners such as suppliers and

buyers need to collaborate across corporate boundaries.

These EAI and B2B problems exist in abundance and are increasing exponentially. Every new

deployed system becomes a legacy system, and any future integration with that system is an

EAI or B2B problem. As the growth of integration problems and projects accelerates over the

next couple of years, the standards-based approach that web services offer makes adopting

web services technology an attractive option for companies that need to cost-effectively

accomplish seamless system integration.

1.2.2 Lessons Learned from Recent History

Some industry analysts claim that the web service model is causing a paradigm shift that will

change the way distributed computing is done forever. Others say that this model is just a fad

that will go away soon. Currently, web services is still very much in the hype phase. Drawing

parallels to other new technologies can teach us important lessons.

Other distributed-computing models have had an opportunity to garner universal acceptance

and adoption, yet they have not. While these models offer great technical advantages for

solving real problems, none have achieved the massive widespread adoption that their

proponents had hoped for. This is largely due to their proprietary nature and the inevitable

vendor lock-in. Though COM/DCOM had a widespread following, it could not permeate an

enterprise because it was limited to Microsoft platforms. CORBA was controlled by the

OMG, a neutral standards body. However, software availability was a problem. There were

really only two robust vendor implementations: Iona and Visigenic.

Forcing middleware infrastructure down the throats of other departments and business

partners is not easy. Both CORBA and DCOM required that a piece of the vendor-supplied

middleware be installed at every node of the system. You can't always force a business

Java Web Services

partner to install a piece of your software at their site for them to be able to participate in

business transactions with your systems. Even within the four walls of an organization,

agreeing upon and rolling out an enterprise-wide middleware solution is a huge, concerted

effort. CORBA implementations eventually achieved cross-vendor interoperability, but by

then it was too late; the wave had already passed.

Crossing corporate boundaries in a secure, reliable fashion is key. If you go back only as far

as 1996 to 1997, you would have seen every trade magazine talking about a world of

distributed CORBA objects happily floating around on the Internet, discovering one another

dynamically and communicating through firewalls. Standards were proposed for firewall

communications, and IIOP was going to be adopted by all major firewall vendors as a

recognizable protocol. It just never happened—partly due to the aforementioned adoption

problems and partly due to widespread adoption and general acceptance of HTTP as a

firewall-friendly protocol.

1.2.3 Why Web Services, and Why Now?

What is so different about web services, and why are they poised for success, whereas other

preceding technologies have failed to achieve widespread adoption? The answer lies in the

challenge that every organization faces today: to create a homogeneous environment while

still leveraging its core abilities and existing applications. IT needs a simple, platform-neutral

way of communicating between applications.

For starters, XML is ideal for representing data. IT developers have had exposure to XML for

a few years and they understand what it's good for. Even though the average IT developer

hasn't yet become a walking XML parser, by now most developers understand the concepts

behind XML and how it can be used.

Also, the base technologies of SOAP, WSDL, and UDDI are not themselves very exciting;

they are just new dressings for the same old distributed-computing model. What draws people

to them is the promise of what they enable. Finally, we have a platform-neutral

communication protocol that provides interoperability and platform independence. A

bidirectional conversation may occur between a Biztalk server and a set of hand-rolled Perl

scripts. The Perl scripts may be simultaneously involved in a conversation with a set of

applications held together by a J2EE-based application server or a message-oriented

middleware (MOM) infrastructure. The minimum requirement is that each participant in the

multiparty collaboration knows how to construct and deconstruct SOAP messages and how to

send and receive HTTP transmissions.

The heavy involvement of the Microsoft camp and the J2EE camp in web services is good for

everyone. It's advantage is not about .NET versus J2EE or .NET versus SunONE; it's about

the fact that you no longer have to let that debate or choice get in the way of achieving

interoperability across the enterprise. The programming languages and associated

infrastructure of each respective camp will continue to coexist and will remain "camps" for a

long time.

1.2.3.1 Low barrier to entry means grass-roots adoption

The widespread adoption of web services can be predicted by drawing parallels to the CGI

phenomenon discussed earlier.

Java Web Services

Similar conditions exist today. The straightforward approach that SOAP takes—XML

messages sent over HTTP—means that anyone can grab Apache SOAP and start exchanging

data with the application owned by the guy down the hall. There isn't any overly complex,

mysterious alchemy involving a strategic architecture group that takes two years to figure out.

A corporate-wide infrastructure adoption shift doesn't need to occur for a company to start

working and benefiting from web services; companies can be selective about how and where

they adopt these technologies to get the best return on their investment.

1.3 Web Services in a J2EE Environment

A common thread found throughout various web services specifications is the regular

reference to web services "platforms" and "providers." A web services platform is an

environment used to host one or more web services. It includes one or more SOAP servers,

zero or more UDDI business registries, the security and transaction services used by the web

services hosted on it, and other infrastructure provisions. A web services provider is generally

considered a vendor-supplied piece of middleware infrastructure, such as an ORB, an

application server, or a MOM. The provider may fully supply a platform, or it may deliver

some base J2EE functionality plus some web service add-ons.

Web services are a new approach for exposing and advertising enterprise services that are

hosted on a platform. These platform services still have a variety of enterprise requirements,

such as security, transactions, pooling, clustering, and batch processing. Web services do not

provide these infrastructure capabilities, but expose the services that do. J2EE and .NET still

play an important role in the enterprise as platform definitions: they define the behavior of

core capabilities that every software program needs internally. Web services, however, offer a

standard way to expose the services deployed onto a platform.

An important question is, "What is being web service enabled?" If the answer is the business

systems that run the enterprise, then the role of J2EE in the whole web services picture

becomes abundantly clear. The core requirements of a web service enabled ecosystem are the

same as they have always been—scalability, reliability, security, etc. Web services provide

new ways of wrapping things at the edge of the enterprise, but if you poke your head through

the web services hype, the requirements for holding together your core systems don't change

that much. The implemention of the web services backbone should still be based on the J2EE

architecture. Web services and J2EE come together at multiple points. The use of each J2EE

component depends on the application's requirements, just as it did prior to the advent of web

services. If the nature of the web service is for lightweight, quick-and-dirty processing, then

use a web container and implement the web service directly as a JSP. If the solution requires

a distributed component model, then use EJB. If the solution requires a highly distributed,

highly reliable, loosely coupled environment, then use JMS. Naturally, any of these

combinations is allowed and encouraged, as illustrated in Figure 1-4.

Java Web Services

Figure 1-4. SOA based on a J2EE backbone

1.4 What This Book Discusses

This is a book on Java and web services. It is for developers who need to develop client- or

server-side programs that either use web services or are exposed as web services. Web

services are built on XML and have specifications that focus on the XML nature of the

technology. These specifications do not discuss how these technologies might be bound to a

particular programming language such as Java. As a result, a plethora of industry technologies

that facilitate Java/web service integration have been proposed.

This book introduces the basics of SOAP, WSDL, and UDDI, and then discusses some of the

different Java technologies available for using each of these platforms within a Java program.

The technologies we've chosen range from open source initiatives, such as the Apache

project, to big-ticket commercial packages. One reason for touching on so many different

packages is that the web services story is still developing; a number of important standards are

still in flux, and vendors are providing their own solutions to these problems. Of course, this

book looks at the standards efforts designed to consolidate and standardize how Java

programs interface with web services. Most notably, this book discusses Java/XML

technologies, such as JAXR, JAX-RPC, and JAXM, and how they can be used in a web

services environment.

These standards are still works in progress; their status may be clarified by the time we write a

second edition. In the meantime, we thought it was important (and even critical) to show you

how things look. Just be aware that changes are certain between now and the time when these

standards are finalized and actual products are released.

Additionally, for developers who are producing J2EE applications, this book discusses

different technologies that are being proposed to web service-enable standard J2EE

applications. This book discusses how a web service facade can integrate with a J2EE

infrastructure. It also introduces some of the standards efforts proposed for solidifying this

work.

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