Home > Articles

Using RPC-Style Web Services with J2EE

So far, you have seen how to use existing J2EE technologies to build multitier applications. However, the world moves on. A key area of interest at the turn of the millennium is how to integrate application both within and between organizations. Web Services provide a flexible and powerful integration mechanism that can be used to expose existing functionality and components to other organizations or to new applications. Today and tomorrow, you will see how you can use Web Services to build bridges between J2EE application components and any other platforms that support Web Services.

Web Services are seen by many as the next wave of the Internet revolution. The vision is of a Web as rich with functionality as the current Web is with information. The challenge is to expose this functionality in a consistent and usable way.

Today, you will

  • Examine the concepts underlying Web Services and how Web Services fit with J2EE

  • Create a client for an RPC-style Web Service

  • Implement an RPC-style Web Service

  • Generate client code from Web Services Description Language (WSDL) documents and generate WSDL documents from your server implementations

  • Pass complex Java types between client and service

First, you need to understand why you would use Web Services.

The aim of the last two days is to describe how to use J2EE technologies to implement and access a Web Service. This chapter will give an overview of the Web Service architecture and show how to generate and consume SOAP messages based on a WSDL interface.

NOTE

Before proceeding further, please be aware that the subject of Web Services is in itself very large, and there are many books dedicated to this popular topic. Today and tomorrow are intended to give you a start into using Web Services in Java and with J2EE technologies. However, it is not possible to answer every question or to pursue every topic. If you would like to find out more about Java and Web Services after you have read through the material in this book, try the following URLs:

Web Service Overview

This first section provides the underlying information and concepts required to successfully implement Web Services. Before employing Web Services, you should understand what problems they are designed to solve and the motivation behind them. This should ensure that you apply Web Services in appropriate places within your application.

What Is a Web Service?

Web Services can be seen as the next stage in the evolution of software. Procedural programming evolved into object-oriented (OO) programming to improve the modelling of system elements and the encapsulation of data and functionality. Component-based development provides a standardized, service-rich framework in which OO functionality can be delivered and built into applications. Web Services takes advantage of common Web protocols to make component instances easily accessible both within and outside an organization.

A Web Service is essentially an application component that can be accessed using Web protocols and data encoding mechanisms, such as HTTP and XML. In some cases, this will be a third-party component hosted remotely. The difference between a Web Service and a traditional component lies not only in the protocols used to access it, but also in that the service can bring its own "live" data and "back-end" functionality with it. An example of this would be a currency conversion service. Under the component model, a currency conversion component could bring with it a file containing a fixed set of currency conversion rates that must be updated regularly. However, it would be up to you to ensure that this information is updated. On the other hand, a currency conversion service takes responsibility for this updating. Your application simply makes use of the conversion service and leaves the details of obtaining the required data and subsidiary services to those who implement and host the service.

Similarly, a Web Service may represent a courier service or a credit-card processing service. Again, you do not need to concern yourself with how the service is implemented, simply the results of using the service. There are many types of Web Services appearing that provide a sliding scale of functionality from low-level infrastructure to high-level business services.

Applications can be built from services in a similar way to building applications from components. You will combine standard services (such as credit-card authorization) with custom code to create your desired application.

As a software developer, you may write Web Services for others to use. In this case you would

  1. Have a good idea for a service.

  2. Implement the service being offered.

  3. Describe the service being offered.

  4. Publish the description.

  5. Wait for customers or consumers of your Web Service.

Alternatively, you may use Web Services as part of your application as follows:

  1. Discover an interesting service.

  2. Retrieve the description.

  3. Plug it into your application.

  4. Use the service as the application executes.

This all sounds very easy, but you need a ubiquitous framework for Web Services to stop this from sliding into chaos. The key factor in delivering such a framework is the widespread agreement to use common, Web-based protocols. In the first instance, this comes down to the use of the Simple Object Access Protocol (SOAP), which is a combination of XML and HTTP. SOAP provides the transport mechanism over which Web Services communicate. Other protocols are also required to deliver the full framework and you will encounter these protocols over the course of the next two days.

Why Use Web Services?

Web Services bring similar advantages to the use of components. Using a service allows you to take advantage of another organization's expertise in, say credit card processing, without you having to become a specialist in it yourself. The service model allows you to use the most powerful and up-to-date functionality by connecting to a remote running service.

Although a service-based approach to application development is not a new concept, it has traditionally presented difficult challenges:

  • Interoperability between different distribution mechanisms, such as CORBA, RMI, and DCOM.

  • Application integration, including legacy systems, cross-vendor, and cross-version.

  • Web-based business requires cross-organization development, high flexibility to accommodate a rapid rate of change, and safe operation through company firewalls.

Web Services can provide a consistent, cross-organization, cross-vendor framework that will speed up the integration of applications and application components. By selecting existing, widely-used standards, the Web Service framework removes many barriers to integration that existed when using other frameworks. The Web Service model is language- and platform-neutral, so developers anywhere can potentially build and consume Web Services.

Probably most important of all is the fact that all the major application, platform, and technology vendors have adopted the Web Service concept. This means that Web Services will form a large part of application development over the next few years.

Web Service Technologies and Protocols

The following are the central protocols, technologies, and standards in Web Services:

  • The Simple Object Access Protocol (SOAP) combines XML and Multipart Internet Mail Extensions (MIME) to create an extensible packaging format. The SOAP envelope can be used to contain either RPC-style or document-centric, message-style service invocations. A SOAP message can be carried over many transport mechanisms, including HTTP, SMTP, and traditional messaging transports. Although SOAP began its life outside the World Wide Web Consortium (W3C), ongoing work on SOAP can be found at http://www.w3.org/2002/ws/. This includes the latest working drafts of the 1.2 specifications, as well as a link to the version 1.1 specification.

  • The Web Services Description Language (WSDL) is an XML vocabulary used to describe Web Services. It defines operations, data types, and binding information. The WSDL specification can be found at http://www.w3.org/TR/wsdl.

  • Universal Description, Discovery, and Integration (UDDI) provides a model for organizing, registering and accessing information about Web Services. The UDDI specifications can be found at http://www.uddi.org/.

  • The Web Service Flow Language (WSFL) and Web Service Collaboration Language (WSCL) are concerned with describing the workflow between services so that their relationships can be encapsulated as part of an application. More information on WSFL can be found at http://xml.coverpages.org/wsfl.html.

  • Electronic Business XML (ebXML) provides a framework for e-commerce that includes the inter-application workflow, and the description and discovery of services. It uses SOAP as its transport mechanism but does not directly use WSDL, UDDI, or WSFL. ebXML is a joint initiative between OASIS and the United Nations CEFACT group. The set of ebXML specifications can be found at http://www.ebXML.org/.

Web Service Architecture

The interaction between a Web Service-based application and the Web Service itself is shown in Figure 20.1. The overall interaction is very similar to the way that a J2EE client uses an EJB. When a Web Service is created, information about its interface and its location are stored in a registry. The Web Service consumer can then retrieve this information and use it to invoke the Web Service.

Figure 20.1 Interaction between Web Service, registry, and service consumer.

Some of this consumer/service interaction takes place at design and development time. The interface and service contract information will can be registered, regardless of whether the service is active or not. This information is required by the application builder to create code that uses the Web Service in their application. At runtime, the application can look up the precise location of the Web Service to locate it, very much like a traditional RPC mechanism, such as RMI.

There are several variations on this interaction. A Web Service can be used entirely dynamically in that the service description is discovered and invoked dynamically. Alternatively, the location information discovered at design time as part of the service description can be bound into the client application so that it has no need of the registry at runtime.

Similarly, the way in which an application interacts with a Web Service will depend on the service. Some services may provide an RPC-style interface based on request/response operations while others may work in a massaging style by exchanging XML-based documents. In either case, the interaction can be synchronous or asynchronous. There is nothing to stop a Web Service from offering out its services in all four combinations.

Service developers will define an interface for their service using a description mechanism such as WSDL. This can be based on an existing service implementation, or the service can be developed after the interface is defined.

Application developers will take the service description and write code based on this. In many cases, a client-side proxy will be created for the services and the application will interact with this proxy. However, the precise details of this are left to the client-side developer.

The service implementations will take a variety of forms. On the server-side, an adapter and router will be required to accept inbound SOAP messages and dispatch them to the appropriate service implementation. This performs the role of the Object Request Broker (ORB) in CORBA and RMI or of the Service Control Manager (SCM) under DCOM.

The services being invoked can be of varying granularity. Web Service mechanisms can be used as a convenience to integrate existing, fine-grained components. Alternatively, the Web Service being accessed can represent a whole application, such as an ERP system.

Although there is much about the Web Service paradigm that will seem familiar to you, the use of Web Services, especially third-party Web Services, does bring some extra considerations for developers:

  • The fact that the service is hosted elsewhere will impact testing, security, availability, and scalability. There will be a need for Service-Level Agreements (SLAs) to be defined for all services used.

  • The providers of an external service will have to be paid somehow. There will be associated authentication requirements so that use of the service can be tracked by the providers.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020