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Access Web services from wireless devices

Handle SOAP messages on MIDP devices using kSOAP

By Michael Juntao Yuan, JavaWorld.com, 08/23/02

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As I discussed in the first Wireless Java column, "Java Readies Itself for Wireless Web Services," XML processing capability represents one of the key requirements for wireless Web services applications. However, the standard J2ME/MIDP (Java 2 Platform, Micro Edition/Mobile Information Device Profile) specification lacks standard XML APIs, which aren't included in the upcoming MIDP 2.0 specification either. Thus, we need third party J2ME/CLDC (Connected Limited Device Configuration) libraries that can handle XML, especially those Web services-specific XML protocols.

In this article, I discuss how to process Web services messages using the open source kSOAP package on the J2ME/MIDP platform. As do many other enterprise computing architectures, Web services involve both clients and servers. As do many discussions focus on how to use J2EE (Java 2 Platform, Enterprise Edition) to develop and deploy Web services on the server side, I focus on only the J2ME client side in this article.

The SOAP advantage

An important XML protocol for accessing Web services is SOAP (Simple Object Access Protocol). Compared with competing technologies, SOAP has the following advantages:

  1. SOAP defines more than 40 standard data types through XML Schema and allows users to custom-define complex data types. Such sophisticated data-type support makes SOAP a powerful and rich language for exchanging information among today's widely deployed object-oriented systems.
  2. In addition to strong data-type support, SOAP also supports various messaging schemes. Those schemes include synchronous remote procedure calls (RPC), asynchronous messaging, multicast messaging (subscription), and complex message routes with multiple intermediaries.
  3. Since SOAP has gained mainstream support as a Web services messaging standard, most other Web services protocols must interoperate or bind with SOAP. For example, WSDL (Web Services Description Language), UDDI (Universal Description, Discovery, and Integration), and most XML registries support SOAP; XML Digital Signature, XML Encryption, SAML (Security Assertion Markup Language), and other secure XML protocols all provide standard binding with SOAP. Each binding protocol provides syntax of its own special element inside SOAP messages. SOAP's full support for XML namespaces has made it easy to bind with other protocols.


Because of the above advantages, SOAP is already the most widely used communication protocol for Web services. So, a core requirement for a wireless Web service application is the ability to understand SOAP messages. Now let's look at some simple SOAP examples. Listing 1 illustrates a simple, generic SOAP message:

Listing 1. Hello World SOAP message

<SOAP-ENV:Envelope
  xmlns:SOAP-ENV="http://www.w3.org/2001/12/soap-envelope"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <SOAP-ENV:Body>
       <message xsi:type="xsd:string">Hello World</message>
    </SOAP-ENV:Body>
</SOAP-ENV:Envelope>


SOAP is most widely used in Web services RPCs. A SOAP response message from a Web services RPC usually contains the return values inside a Result element under the SOAP Body element. Listing 2 shows a simple SOAP RPC response message:

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