he onslaught of Web services messaging and XML-based data transfer has led to a need to express these message exchanges in some structured way. The Web services Description Language (WSDL) deals with this need by defining an XML grammar for exposing services as sets of endpoints for exchanging related messages. WSDL (pronounced "wizdle") documents provide a standard means for describing services by outlining the data types consumed by the services, the URL for communicating with the services, and the names by which services are known.
WSDL documents are quickly becoming the key to successful B2B interoperability. A WSDL document provides a generic Web service description without regard for the specific programming language in which the services are written. Developers can use WSDL documents to generate client code that can communicate with the corresponding Web services. This generic form of service/component description has been attempted in the past by various binary forms of interface description languages (IDLs), but never has it seen such widespread use as with WSDL and Web services.
One reason for the success of WSDL-based service description is the pervasive use of XML and its ability to define virtually any data type for any programming language in a generic way. This article discusses how to handle custom WSDL data types in the Java programming language.
WSDL documents are XML documents that describe services as a set of message-enabled or procedure-oriented abstract endpoints. These operations and/or messages and their associated data types are described conceptually, and then bound concretely to a network protocol, message format, and programming language as needed.
A WSDL document defines the following elements for describing services:
WSDL and Data Types
- Typesdata type definitions
- Messagean abstract definition of the data being transferred
- Operationan abstract description of a service procedure
- Port Typean abstract set of operations supported by one or more endpoints
- Bindinga concrete protocol and data format for a given port type
- Porta single endpoint defined as a binding and a network address
- Servicea collection of related endpoints or ports
It's challenging for Web services providers to expose a data type system that can be understood by the broadest range of Web services consumers, yet that understanding must be in place for Web service consumers and providers to communicate effectively. Because different programming languages each have their own set of idiosyncrasies, creating a common data type system is of primary importance.
In contrast to previous attempts at a generic interface definition language (IDL) and a common data-type system, such as COM, DCOM, and CORBA, WSDL solves the common data-type problem by aiming for maximum flexibility rather than attempting to create an all-inclusive data-typing standard. Thus, WSDL is not bound to any one data type system. However, WSDL does recommend the W3C XML Schema specification as the canonical type system, thereby intrinsically treating the most widely used data type system as the default.