Developing with POJOs: faster and easier - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 26

access the resources that it needs, such as a JDBC DataSource . The trouble with
JNDI is that it couples application code to the application server, which makes
development and testing more difficult. The Spring framework provides POJO s
with a much easier-to-use mechanism called dependency injection , which decouples
application components from one another and from the application server.
Dependency injection is another powerful feature of Spring's bean factory.
Spring beans can be configured to depend on other beans, and when Spring
instantiates a bean, it will pass to it any required beans, instantiating them if nec-
essary. Two main types of dependency injection are used with Spring: constructor
injection and setter injection. With constructor injection, the container passes the
required objects to a component's constructor; with setter injection, the con-
tainer passes the required objects by calling setters.
Dependency injection was used earlier to wire together the Spring beans—
TransactionInterceptor , PlatformTransactionManager , and BeanNameAutoProxy-
Creator —that provide transaction management. It can also be used to wire
together application components. In the money transfer example, we can config-
ure the TransferFacade bean to depend on TransferService and Transfer-
Service to depend on HibernateAccountRepository and HibernateBanking-
TransactionRepository :
<beans>
…
<bean id="TransferFacade"
class="TransferFacadeImpl">
<constructor-arg ref="TransferService"/>
</bean>
<bean id="TransferService"
class=" TransferServiceImpl">
<constructor-arg ref="AccountRepository"/>
<constructor-arg ref="BankingTransactionRepository"/>
</bean>
…
</beans>
The first bean definition specifies that TransferFacadeImpl 's constructor take a
TransferService as a parameter. The second bean definition specifies that Trans-
ferServiceImpl 's constructor be passed AccountRepository and BankingTrans-
actionRepository . When Spring instantiates TransferFacade , it will also instantiate
TransferService , HibernateAccountRepository , and HibernateBankingTrans-
actionRepository . See the online source code, which can be downloaded from