3.4. Connecting POJOs Together

Individual POJO instances can only provide relatively simple behavior. The true power of POJOs comes from connecting them together to perform complex tasks. How can you wire POJOs together to choose different salary strategy implementations?
The following XML deployment descriptor does just that: 
<?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>
This XML creates an instance of the chosen salary strategy implementation by including an additional <bean> element. This time, the AgeBasedSalaryStrategy is chosen. Next the code injects a reference to this bean into the instance of HRManager created using the HRService bean. Injection is possible because the HRManager class contains a setSalaryStrategy(SalaryStrategy strategy) method. Behind the scenes, JBoss Microcontainer calls this method on the newly created HRManager instance and passes a reference to the AgeBasedSalaryStrategy instance.
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.'

3.4.1. Special Considerations

Although creating instances of classes using the <bean> element in the deployment descriptor is possible, it is not always the best way. For example, creating instances of the Employee and Address classes is unnecessary, because the client creates these in response to input from the user. They remain part of the service but are not referenced in the deployment descriptor.
Comment Your Code

You can define multiple beans within a deployment descriptor as long as each has a unique name, which is used to perform injection as shown above. However all of the beans do not necessarily represent services. While a service can be implemented using a single bean, multiple beans are usually used together. One bean typically represents the service entry point, and contains the public methods called by the clients. In this example the entry point is the HRService bean. The XML deployment descriptor does not indicate whether a bean represents a service or whether a bean is the service entry point. It is a good idea to use comments and an obvious naming scheme to delineate service beans from non-service beans.