无法在 Spring Boot 应用程序的 Main 方法中使用 @Autowired JPA Repository

Question

I've autowired a JPA repository that adds dummy data in my H2 database before the application starts. But is there a reason as to why I can't use it in the main () method but am able to use it in the runner() method?

@SpringBootApplication
public class FullstackApplication {
    
    @Autowired
    private CarRepository carRepository;
    
    private static final Logger logger = LoggerFactory.getLogger(FullstackApplication.class); 

    public static void main(String[] args) {
        carRepository. // Here I get a compilation error: Cannot make a static reference to a non-static field
        SpringApplication.run(FullstackApplication.class, args);

    }
    
    @Bean
    CommandLineRunner runner(){
        return args -> {
            // Save demo data to database
            carRepository.save(new Car("Ford", "Mustang", "Red",
            "ADF-1121", 2017, 59000));
            carRepository.save(new Car("Nissan", "Leaf", "White",
            "SSJ-3002", 2014, 29000));
            carRepository.save(new Car("Toyota", "Prius", "Silver",
            "KKO-0212", 2018, 39000));
        };
    }
}

Answer 1

You are accessing a non static field directly from static method which is not permitted in java

Also you cannot make static field @Autowired

so if you do this

@Autowired
  private static CarRepository carRepository;

it won't throw any error but it will be ignored.

Answer 2

Main method is marked static, which means, everything that is used there should be either static as well , or be manually instantiated.

You do not instantiate CarRepository manually in static body of main method, you are relying on Spring to instantiate it somewhere during its startup phase , which will happen after "carRepository. //...." this line.

That is why , you cannot use carRepository in this exact place, because it is not static by itself and in was not manually instantiated.

In CommandRunner though , at the time return is being called , instance of CarRepository is already created by Spring and autowired to field, because Spring startup already has finished , and it can be easily used.

Answer 3

Well main method is marked as static and you cannot access non static members from a static method.

To just solve that you have to mark the carRepository static. But since static field cannot be autowired it will ignore it and you will not get the object.

It will work in command runner because at that time the sprint start up is already finished and beans has been instantiated.