Field-based DI

In the preceding sections, we saw how we can use constructor-based and setter-based dependencies in our application. In the following example, we will see field-based DI. Actually, field-based DI is easy to use, and it has clean code compared to the other two types of injection method; however, it has several serious trade-offs, and should generally be avoided.

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

@Component
public class BankingService {

  //Field based Dependency Injection
  @Autowired
  private CustomerService customerService;

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

}

As we discussed, this type of DI has the benefit of removing clutter code over setter-or constructor-based dependencies, but it has many drawbacks, such as dependencies are invisible from the outside. In constructor-based and setter-based dependencies, classes clearly expose these dependencies using the public interface or setter method. In a field-based DI, the class is inherently hiding the dependencies from the outside world. Another difficulty is that field injection cannot be used to assign dependencies to final/immutable fields, as these fields must be instantiated at class instantiation.

Generally, Spring discourages the use of the field-based dependency.

Here is a diagram with the different types of DI that we have learned about so far: