In this blog post, we will see the @Autowired Annotation in the Constructor Injection in Spring Framework.

Spring’s relaxation on the Autowired Annotation

Till Spring version 4.3, the @Autowired annotation was mandatory in the Constructor injection. However, Spring 4.3 version has relaxed that a bit.

If a class has a single constructor, the @Autowired annotation can be ignored and Spring framework will supply that for you behind the scenes. However, if the class has more than one constructor, any one of them must be annotated with the @Autowired annotation for the parameters you want to injected. Otherwise, you get an error for the obvious fact and doubt that which one should be annotated.

Code 1 - Class with a Single Constructor

import com.example.springdatajpademo.repository.EmployeeRepository;
import org.springframework.boot.CommandLineRunner;
import org.springframework.stereotype.Component;

@Component
public class CheckDataRunner implements CommandLineRunner {

    private final EmployeeRepository employeeRepository;

    public CheckDataRunner(EmployeeRepository repository) {
        System.out.println("CheckDataRunner() - invoked, repository : " + repository);
        this.employeeRepository = repository;
    }

    @Override
    public void run(String... args) throws Exception {
        System.out.println("CheckDataRunner - executed");
        System.out.println(employeeRepository.findAll());
    }

The class has a single constructor and hence it gets the relaxation of omitting the @Autowired annotation.

Code 2 - Class with more than one constructor

import com.example.springdatajpademo.repository.EmployeeRepository;
import org.springframework.boot.CommandLineRunner;
import org.springframework.stereotype.Component;

@Component
public class CheckDataRunner implements CommandLineRunner {

    private final EmployeeRepository employeeRepository;

    public CheckDataRunner() {
        System.out.println("Default constructor invoked!");
    }

    public CheckDataRunner(EmployeeRepository repository) {
        System.out.println("CheckDataRunner() - invoked, repository : " + repository);
        this.employeeRepository = repository;
    }

    @Override
    public void run(String... args) throws Exception {
        System.out.println("CheckDataRunner - executed");
        System.out.println(employeeRepository.findAll());
    }

The class has more than one version of constructor and the parameterized constructor does NOT have the @Autowired annotation which causes a compiler error on the field - employeeRepository. The IDE will complain that the variable might not have been initialized.

Solution for the Code 2 - class with more than one constructors

You can solve this issue by doing the following:

  • Make the field employeeRepository non-final.
  • Add the @Autowired annotation for the parameterized constructor
// Imports are ignored for brevity.

@Component
public class CheckDataRunner implements CommandLineRunner, Ordered {

    private EmployeeRepository employeeRepository;

    private CheckDataRunner() {
    }

    @Autowired
    public CheckDataRunner(EmployeeRepository repository) {
        System.out.println("CheckDataRunner() - invoked, repository : " + repository);
        this.employeeRepository = repository;
    }

    @Override
    public void run(String... args) throws Exception {
        System.out.println("CheckDataRunner - executed");
        System.out.println(employeeRepository.findAll());
    }
}

Note: If you forgot to annotate the parameterized constructor with the @Autowired annotation, you get a NullPointerException due to the fact that field employeeRepository not being initalized and the reference invoked on the 2nd line of the run() method will be null.

References

Cheers,
RM…
Raghavan alias Saravanan Muthu
17 Jun 2021 | Thu | 00 32 00 AM IST