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So far we have seen how to create POJOs and declare instances of them together with names in an XML deployment descriptor. However, individual POJO instances can only provide relatively simple behaviour. Things really get interesting when we combine POJOs together to perform more complex tasks. In our example we know that we can choose different salary strategy implementations for the HRManager so how do we go about wiring the POJOs together?
The answer is to use the XML deployment descriptor again as follows:
<?xml version="1.0" encoding="UTF-8"?>
<deployment xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="urn:jboss:bean-deployer:2.0 bean-deployer_2_0.xsd"
xmlns="urn:jboss:bean-deployer:2.0">
<bean name="HRService" class="org.jboss.example.service.HRManager">
<property name="salaryStrategy"><inject bean="AgeBasedSalary"/></property>
</bean>
<bean name="AgeBasedSalary"
class="org.jboss.example.service.util.AgeBasedSalaryStrategy"/>
</deployment>
We first need to create an instance of our chosen salary strategy implementation by including an additional <bean> element. Here we have chosen the AgeBasedSalaryStrategy. Next we need to inject a reference to this bean into the instance of HRManager created using the HRService bean. Injection is possible as the HRManager class contains a setSalaryStrategy(SalaryStrategy strategy) method. Behind the scenes JBoss Microcontainer will call this method on the newly created HRManager instance and pass in a reference to the AgeBasedSalaryStrategy instance.
In other words the XML deployment descriptor causes the same sequence of events to occur as if you had written the following code:
HRManager hrService = new HRManager();
AgeBasedSalaryStrategy ageBasedSalary = new AgeBasedSalaryStrategy();
hrService.setSalaryStrategy(ageBasedSalary);
In addition to performing injection via property setter methods JBoss Microcontainer can also perform injection via constructor parameters if necessary. For more details please see the 'Injection' chapter in Part II 'POJO Development'.
Although we can create instances of classes using the <bean> element in the deployment descriptor it is not always appropriate to do so. For example we do not need to create instances of the Employee and Address classes since these will be created by the client in response to input from the user. As such they remain part of the service but are not mentioned in the deployment descriptor.
Also note that it is possible to define multiple beans within a deployment descriptor providing that each has a unique name. The names are required in order to perform injection as shown above. However this does not mean to say that all of the beans represent services. While a service could be implemented using a single bean it is most often the case that multiple beans are used together as in our example. In these cases there is usually one bean that represents the service entry point containing the public methods intended for the clients to call. In our example this is the HRService bean. Notice that there is nothing in the XML deployment descriptor to say which beans represent a service or indeed which bean if any is the service entry point (a service may run autonomously in which case it is its own client). Care must therefore be taken when creating deployment descriptors to ensure that sufficient comments are included to describe what the beans are used for. Alternatively a naming convention such as ending each bean name that represents a service entry point with 'Service' can be used instead, e.g. HRService.