Using constructor injection

For the DI pattern with the construction injection, Spring provides you two basic options as the <constructor-arg> element and c-namespace introduced in Spring 3.0. c-namespace has less verbosity in the application, which is the only difference between them--you can choose any one. Let's inject the collaborating beans with the construction injection as follows:

    <bean id="transferService"   
     class="com.packt.patterninspring.chapter4.
     bankapp.service.TransferServiceImpl"> 
     <constructor-arg ref="accountRepository"/> 
     <constructor-arg ref="transferRepository"/> 
    </bean> 
    <bean id="accountRepository"    
     class="com.packt.patterninspring.chapter4.
     bankapp.repository.jdbc.JdbcAccountRepository"/> 
    <bean id="transferRepository"       
     class="com.packt.patterninspring.chapter4.
     bankapp.repository.jdbc.JdbcTransferRepository"/> 

In the preceding configuration, the <bean> element of TransferService has two <constructor-arg>. This tells it to pass a reference to the beans whose IDs are accountRepository and transferRepository to the constructor of TransferServiceImpl.

As of Spring 3.0, the c-namespace, similarly, has a more succinct way of expressing constructor args in XML. For using this namespace, we have to add its schema in the XML file, as follows:

    <?xml version="1.0" encoding="UTF-8"?> 
    <beans xmlns="http://www.springframework.org/schema/beans" 
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
     xmlns:c="http://www.springframework.org/schema/c" 
     xsi:schemaLocation="http://www.springframework.org/schema/beans
     http://www.springframework.org/schema/beans/spring-beans.xsd"> 
    
    <bean id="transferService"    
     class="com.packt.patterninspring.chapter4.
     bankapp.service.TransferServiceImpl"  
     c:accountRepository-ref="accountRepository" c:transferRepository-
     ref="transferRepository"/> 
    <bean id="accountRepository"    
     class="com.packt.patterninspring.chapter4.
     bankapp.repository.jdbc.JdbcAccountRepository"/> 
    <bean id="transferRepository"    
     class="com.packt.patterninspring.chapter4.
     bankapp.repository.jdbc.JdbcTransferRepository"/> 
    
     <!-- more bean definitions go here --> 
    
    </beans> 

Let's see how to set up these dependencies with the setter injection.