Constructor-based DI is a design pattern to resolve the dependencies of a dependent object. In a constructor-based DI, a constructor is used to inject a dependent object. It is accomplished when the container invokes a constructor with a number of arguments.

Let's look at the following example for a constructor-based DI. In the following code, we show how to use a constructor for injecting a CustomerService object in a BankingService class:

@Component
public class BankingService {

  private CustomerService customerService;

  // Constructor based Dependency Injection
  @Autowired
  public BankingService(CustomerService customerService) {
    this.customerService = customerService;
  }

  public void showCustomerAccountBalance() {
    customerService.showCustomerAccountBalance();
  }

}

The following is the content of another dependent class file, CustomerServiceImpl.java:

public class CustomerServiceImpl implements CustomerService {

  @Override
  public void showCustomerAccountBalance() {
    System.out.println("This is call customer services");
  }
}

The content for the CustomerService.java interface is as follows:

public interface CustomerService {  
  public void showCustomerAccountBalance(); 
}