@Required annotation

BeanPostProcessor is used by the framework heavily. One of the best example is RequiredAnnotationBeanPostProcessor class. In spring we require @Required annotation to make it a mandatory dependency, that thas to be injected.

Just by using annotation in the code will not work, since someone has to check whether the requirement has been met or not and reposrt an error it not.

Example

//The service
public interface BankService {

    public BillPaymentService getBillPaymentService();
    public CustomerService getCustomerService();
}

//The impl class
public class BankServiceImpl implements BankService {

    private CustomerService customerService;
    private BillPaymentService billPaymentService;
    
    @Required
    public void setCustomerService(CustomerService customerService) {
        this.customerService = customerService;
    }
    
    @Required
    public void setBillPaymentService(BillPaymentService billPaymentService) {
        this.billPaymentService = billPaymentService;
    }    

    public BillPaymentService getBillPaymentService() {
        return billPaymentService;
    }
    
    public CustomerService getCustomerService() {
        return customerService;
    }
}

Now here we have mentioned what is required, but we need RequiredAnnotationBeanPostProcessor to check whether these required dependency are injected or not.

The configuration

<!--  According the code, we need to set both the dependencies.
    Comment out one or both the property tag and see the error -->
    <bean id="bankService" class="com.xxxx.BankServiceImpl">
        <property name="billPaymentService" ref="billPaymentService" />
        <property name="customerService" ref="customerService" />
    </bean>
    
    <!-- Just by using @Required will not work. Someone has to parse it
    and that's the role of this class -->
    <bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor" />