setter injection vs constructor injection in Spring

What is setter injection?

In spring you can inject the concrete dependencies with setters.

Example

applicationContext.xml

<bean name="customerRepo" class="com.inadram.repo.hibernateRepositoryImplementation"/>
<bean name="customerService" class="com.inadram.service.CustomerServiceImplementation">
 <property name="customerRepository" ref="customerRepo"/>
</bean>

Therefore your customer service class can accept customer repository as an injection with setters

customer service

public class CustomerServiceImplementation implements CustomerService {
    private CustomerRepository customerRepository;
 
    public void setCustomerRepository(CustomerRepository customerRepository) {
        this.customerRepository = customerRepository;
    }
 
    @Override
    public List<Customer> findAll() {
        return customerRepository.findAll();
    }
 
}

Benefits

it is easy to adopt legacy code to spring by using setter injection

What is constructor injection

You can injection the dependencies by using the constructor.

Example

applicationContext.xml

<bean name="customerRepo" class="com.inadram.repo.hibernateRepositoryImplementation"/> 
<bean name="customerService" class="com.inadram.service.CustomerServiceImplementation">
	<constructor-arg index="0" ref="customerRepo"/>
</bean>

index: the constructor parameter index ref: the concrete class name that we wish to inject into customer service

We need to add a constructor to the customer service class to pass the injection into it.

customer service implementation

public class CustomerServiceImplementation implements CustomerService {
    private CustomerRepository customerRepository;
 
    public CustomerServiceImplementation(CustomerRepository customerRepository) {
        this.customerRepository = customerRepository;
    }
 
    @Override
    public List<Customer> findAll() {
        return customerRepository.findAll();
    }
 
}

Benefits

constructor injection force a contract between the classes, therefore consumers have to pass the required dependencies on creation time to allow using the class functionalities.

Download

Feel free to download the full source code of this example from my github.