206

Scenarios for Web Services Based Client Applications

...

...

J2EE Container

Submit

Purchase Order

 Web Service

orderId

 EJB Endpoint

...

JMS 

...

Queue

Credit Card

 Web Service

Synchronous 

Process

call

Purchase Order

Message Driven Bean

Shipping

Asynchronous 

call

 Web Service

Figure 5.3

EJB Components and Web Services in a Workflow

In this example, a Web component calls the order tracking service and, when

it receives a response from the service, it puts the results in a Java Beans compo 

nent (

OrderDetailsBean

) that is within the request scope. The Web component

uses a JSP to generate an HTML response, which the container returns to the

browser that made the original request. 

It is also possible to write J2EE clients using EJB components. (See Figure

5.3.) These EJB components may themselves be Web service endpoints as well as

clients of other Web services. Often, EJB components are used in a workflow to

provide Web services with the additional support provided by an EJB container 

that is, declarative transactional support, declarative security, and life cycle man 

agement. 

Figure 5.3 demonstrates a workflow scenario: a Web service endpoint used in

combination with a message driven bean component to provide a workflow oper 

ation that runs asynchronously once the initial service starts. The Web service

endpoint synchronously puts the purchase order in a JMS message queue and

returns an 

orderId

 to the calling application. The message driven bean listens for