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Leading-Edge Java
Recent JDK Features Ease Web Service Development
A Brief Review of JAX-RPC 2.0
by Frank Sommers
May 23, 2005

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XML-based remote calls are a key Web services technology. The Java API for XML-based RPC (JAX-RPC) 2.0 is the second-generation XML-based RPC Java standard. Currently in early draft review in the JCP as JSR 224, JAX-RPC 2.0 promises to vastly simplify RPC-based Web service development. Part of that ease results from new JDK 1.5 features, such as annotations and the concurrency API. This article reviews key JAX-RPC 2.0 features, and highlights how taking advantage of recent additions to Java make JAX-RPC 2.0 a more flexible API.

With a myriad of new Java features currently winding their way through the Java Community Process (JCP), many proposed technologies and APIs may appear at first as extra "fluff," threatening to make Java overly complex. After all, you've been writing Java code for years—how could learning those new APIs possibly make your life easier?

A deeper look at some new JSRs, however, reveals that sometimes more is less: Some new and proposed APIs enable us to write less code, chiefly by automating a lot of tedious work. The effort of learning those new Java features pays dividends by making us more productive. In this article, I will show how new JDK 1.5 features, such as annotations and the concurrency API, can make writing Java web services much simpler.

Simplifying Web services programming

When it comes to tedious work, writing Web services seem destined to occupy an especially painful spot. Since a Web service is a combination of Java code, configuration files, and deployment-time options, managing all those artifacts takes time and effort away from focusing on code that provides the business benefits, often the Web service's sole raison d'etre.

Last month, the JAX-RPC 2.0 project on released an early access version [see Resources] of the reference implementation of the Java API for XML-Based RPC (JAX-RPC) 2.0 specification (JSR 224), currently in early draft review stage in the JCP. Among JAX-RPC 2.0's key aims: vastly simplify Web service development.

An exciting feature of JAX-RPC 2.0 is that it achieves a simpler Web services programming model by taking advantage of new J2SE 1.5 capabilities, especially metadata and annotations. JAX-RPC 2.0 also aligns the Java Web services programming model with related Java standards that have emerged since the initial JAX-RPC 1.0 release.

While this article focuses on new JAX-RPC 2.0 features, a companion article, "Three Minutes to a Web Service," illustrates JAX-RPC 2.0's annotation-based Web services programming model, and shows how to create a Web service with just 15 lines of code.

Note of caution: JAX-RPC 2.0 is still in early draft release. Both the specification and reference implementation are subject to change. When this article refers to JAX-RPC 2.0, those references mean the early draft specification available at the time of this writing.

Standards alignment

With the advent of XML-based Web services in the late nineties, Java needed an API for XML-based remote method calls, mainly to facilitate Java interoperability with SOAP-based Web services. To provide a standard Java XML RPC API, JSR 101 was accepted into the JCP in January 2001, and work on JAX-RPC 1.0 started shortly thereafter [see Resources]. While JAX-RPC version 1.0 delivered initial XML Web services support in Java, the version 1.1 maintenance release incorporated the WS-I Basic Profile into JAX-RPC [see Resources].

A key requirement of any XML-based remote method call system is to translate elements of a programming language's type system into an on-the-wire XML-representation, and in the reverse direction, into a language-specific data binding. A standard for Java-XML data binding had been in the works since 1999 (JSR 31), but would not produce an accepted specification until March 2003, due in part to the complexity of defining a general-purpose XML-Java mapping for a large schema [see Resources].

Since data binding was a necessary component of JAX-RPC, the initial JAX-RPC expert group decided to devise Java-XML bindings specific to the JAX-RPC specs, supporting a smaller XML schema needed to process Web service WSDL definitions. That allowed JAX-RPC to become an approved Java standard by mid-summer, 2002. JAX-RPC was subsequently incorporated into the Java Web services stack, and earned its way into a key part of J2EE 1.4 [see Resources]. But that decision also meant, in effect, two standards for turning Java data into XML.

JAX-RPC 2.0 remedies that duplication, and delegates Java-XML data binding to the newly emerging JAXB 2.0 standard (JSR 222) developed concurrently with JSR 224 [see Resources]. "The WSDL-based data binding worked well in the early stages," says Roberto Chinnici, a Sun Microsystems Senior Staff Engineer and JAX-RPC 2.0 spec co-lead. "Independently, JAXB defined data bindings for a much larger set of schemas. We now integrate with JAXB, and are a JAXB client. [On top of JAXB], we add support to do remote invocation. These two JSRs are now aligned."

Service annotations and customization

When creating a Web service, a developer works with a handful of classes and configuration files:

JAX-RPC 2.0 significantly reduces the complexity of maintaining these artifacts with the help of the JDK 1.5 annotations capability. (Annotations are defined in JSR 175, A Metadata Facility for the Java Programming Language, and JSR 181, Web Services Metadata for the Java Platform [see Resources].) With JAX-RPC 2.0, you need only define a service implementation class, apply annotations to it, and run the class through tools provided in the JAX-RCP 2.0 reference implementation to generate other required Web service artifacts. The result is a dramatically simplified Web service programming model that lets you focus almost exclusively on a Web service's business logic, and relegates other Web service development chores to infrastructure-level tools. (See the companion article, "Three Minutes to a Web Service," for an example.)

In addition to facilitating developer ease, annotations provide more control over how Java-to-WSDL mapping occurs. Prior versions of JAX-RPC tools mapped Java constructs to WSDL in an implementation-specific way, often resulting in awkward class and method names, for instance. You can now associate specific annotations with Java code to affect the resulting WSDL.

While annotations govern the Java-to-WSDL translation, customizations give a developer control over WSDL-to-Java conversion. Such customizations are specified via binding declarations that mimic a similar customization capability of JAXB 2.0. JAX-RPC 2.0 binding declarations let you specify the package, class, and method names created from a WSDL document, and may be specified either inside a WSDL document inline, or via external files.

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