84

Designing a Service's Interaction Layer

extends another exception, such as 

RootException

, then when the service throws

CityNotFoundException,

 methods and properties inherited from 

RootException

are 

not

 passed to the client.

E

The exception stack trace is not passed to the client.

The stack trace for an exception is relevant only to the current execution envi 

ronment and is meaningless on a different system. Hence, when a service throws

an exception to the client, the client does not have the stack trace explaining the

conditions under which the exception occurred. Thus, you should consider

passing additional information in the message for the exception.

Web service standards make it easier for a service to pass error conditions to a

client in a platform independent way. While the following discussion may be of

interest, it is not essential that developers know these details about the J2EE plat 

form's error handling mechanisms for Web services.

As noted previously, error conditions are included within the SOAP messages

that a service returns to clients. The SOAP specification defines a message type,

called 

fault

, that enables error conditions to be passed as part of the SOAP

message yet still be differentiated from the request or response portion. Similarly,

the WSDL specification defines a set of operations that are possible on an end 

point. These operations include 

input

 and 

output

 operations, which represent the

request and response respectively, and an operation called 

fault

. 

A SOAP 

fault

 defines system level exceptions, such as 

RemoteException

,

which are irrecoverable errors. The WSDL 

fault

 denotes service specific excep 

tions, such as 

CityNotFoundException

, and these are recoverable application error

conditions. Since the WSDL 

fault

 denotes a recoverable error condition, the plat 

form can pass it as part of the SOAP response message. Thus, the standards

provide a way to exchange fault messages and map these messages to operations

on the endpoint.

Code Example 3.13 shows the WSDL code for the same weather Web service

example. This example illustrates how service specific exceptions are mapped

just like input and output messages are mapped.

...