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Axis-orizing objects for SOAP

Go from Java objects to a SOAP Web service with Apache Axis

By Mitch Gitman, JavaWorld.com, 04/11/03

It all starts innocently enough. You have an object-oriented Java interface you want to turn into a Web service. You come across Axis, the Apache framework for publishing and consuming Web services using the XML-based SOAP (Simple Object Access Protocol) specification. Soon enough you find yourself needing to become more of a SOAP expert than you had hoped, opening the Axis source more than you have patience to, and, well, spending more time than you have. At least that's what happened to me. This article explains how to transform your Java objects for the Web-service world with Axis—minus the hassle.

First steps

First, make sure you have installed:

• A servlet engine or J2EE (Java 2 Platform, Enterprise Edition) application server. The Axis server component is typically deployed as a Web application.
• A programmable build tool like Apache Ant. Creating a Web service takes you through a string of deployment operations besides just compiling, so it's critical that you automate this process as fully as possible.
• Axis 1.1 Release Candidate 2. Download the archive and expand it. Follow the installation instructions at least up to "Validate Axis with happyaxis."
• Optionally, an XML parser. For example, a DOM (Document Object Model) parser like JDOM or dom4j. At times, you will need to edit automatically generated XML files. If you're willing to write some code using a DOM (or SAX (Simple API for XML)) parser, you can automate this work.
• Optionally, JUnit. Later, I discuss testing an Axis Web service via a JUnit client.

SOAP and objects

While this article doesn't purport to be a general primer on SOAP best practices, some words on SOAP hygiene are in order.

SOAP originally stood for Simple Object Access Protocol. Yet, SOAP never mandated the use of objects per se, so the acronym eventually was cleansed of its origin. Don't let that fact throw you off the scent. Axis still converts your Java objects into XML for transmission over some transport, typically HTTP. In SOAP-formatted XML, these Java objects appear as data structures, which can be built through what are effectively class inheritance and object composition, to borrow a couple object-oriented terms.

Of course, when you get into sending complex, self-defined types over SOAP, you can always ask yourself the question, "Do I really need to send these structures?" You will find your answer with a Java interface.

Java interface as service contract

You must define whatever methods you want to provide through your Web service in a single interface or class. Interface is preferred over class, not only so that the service contract does not depend on its implementation, but also so that Axis and its tools don't inadvertently publish extra methods or types.

For a Web service-worthy interface, minimize the interaction between client and server, in terms of both frequency and bandwidth. Even if the client and service are on the same machine, you pay the minor performance price of having to marshal and unmarshal each request and response into relatively bulky XML text, as opposed to binaries.

 

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