210

Developing Client Applications to Use a Web Service

Applications for J2ME devices have a standard, uniform set of GUI widgets

available for manipulating data, such as the liquid crystal display UI that is part of

MIDP 1.0 and MIDP 2.0. Plus, each device type may have its own set of widgets.

Such widgets have different abilities for validating data prior to accessing a

service.

J2ME devices, while capable of small computations and data validation, have

limited processing capabilities and memory constraints. Client applications need

to consider the types of data exchanged and any pre  and post processing

required. For example, J2ME devices have limited support for XML document

processing and are not required to perform XML document validation.

In summary, keep the following considerations in mind when designing J2ME

clients.

E

Connectivity and bandwidth limitations

 J2ME clients may have limited 

bandwidth, intermittent disconnects, and fees for connection usage.

E

Processing power

 The processing capabilities of a Web service client 

should be considered in its design.

E

State maintenance

 J2ME clients may operate in a connected or disconnect 

ed mode, requiring the maintenance of state and synchronization between cli 

ent and service.

5.3

Developing Client Applications to Use a Web Service

All client applications follow certain steps to use a Web service. In brief, they must

first look up or locate the service, make a call to the service, and process any

returned data. The choice of communication mode determines much of the details

for performing these steps. 

An application can communicate with a service using stubs, dynamic proxies,

or the dynamic invocation interface (DII) 

Call

 interface. (The next section

explains these modes.) With these modes, a developer relies on some form of

client side proxy class (stub, dynamic proxy, or a DII interface) representing a

Web service to access the service's functionality. The client developer decides

which representation to use based on the WSDL availability, the service's end 

point address, and the use cases for the service.

After deciding on a communication mode, developers need to do the follow 

ing when designing and developing client applications: