232

Developing Client Applications to Use a Web Service

    java.lang.Runtime

    /order_tracking_system_exception.jsp

Code Example 5.14

Using the Servlet Error Messaging System

Although they do not directly interact with the user, workflow clients imple 

mented as EJB components can benefit from transactional processing, particularly

if using container managed transactions. However, since you cannot assume that

the Web service is also transactional, you might have to manually back out some

of your changes if other Web services are involved in the processing. Keep in

mind that backing out changes can be difficult to accomplish and is prone to prob 

lems. 

An J2EE component that receives a 

RemoteException

 when accessing a

service may retry connecting to the service a set number of times. If none of the

retries are successful, then the client can log an error and quit the workflow. Or,

the client may forward the unit of work to another process in the workflow and let

that component access the same service at a later point or use a different service.

EJB component based clients using DII may locate an alternative service using

the JAXR API from a registry.

5.3.6.2

Service Exceptions

Service exceptions occur when a Web service call results in the service returning a

fault. A service throws such faults when the data presented to it does not meet the

service criteria. For example, the data may be beyond boundary limits, it may dupli 

cate other data, or it may be incomplete. These exceptions are defined in the ser 

vice's WSDL file as 

operation

 elements, and they are referred to as 

wsdl:fault

elements. These exceptions are checked exceptions in client applications. For exam 

ple, a client accessing the order tracking service may pass to the service an order

identifier that does not match orders kept by the service. The client may receive an

OrderNotFoundException

, since that is the error message defined in the WSDL

document: