How to use autowired (@Autowired) references from main(String[] args) method?

Question

I am trying to use an autowired reference from main class and am facing :

Cannot make a static reference to the non-static field zipCodeLookupService.

This is obvious. But I want to know how to handle this situation. What is the correct way of autowiring when main class is involved. My code will be as below -

Interface class

package com.example.services;
public interface IZipCodeLookup {
    String retriveCityForZip(String zipCode);
}

Service Class

package com.example.services;

import org.springframework.stereotype.Service;

@Service
public class ZipCodeLookupService implements IZipCodeLookup {

    @Override
    public String retriveCityForZip(String zipCode) {

        //below is mock code. actual code does a db lookup using a DAO.
        if(zipCode=="94123") return "San Francisco";
        return "not found in DB";
    }
}

here is the main class that requires the service class

package com.example.demo;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

import com.example.services.IZipCodeLookup;

@SpringBootApplication
public class AutowireWithMainClassApplication {

    @Autowired
    IZipCodeLookup zipCodeLookupService;

    public static void main(String[] args) {
        SpringApplication.run(AutowireWithMainClassApplication.class, args);
        String city;
        //this will not work, compilation error
        //Cannot make a static reference to the non-static field zipCodeLookupService
        city=zipCodeLookupService.retriveCityForZip(args[0]);

        System.out.println("city for zipcode " + args[0] + " is " +city);       
    }
}

Could someone suggest - how or what is the correct way of using autowiring when main class is involved.

(As making the Autowired reference as static does not work anyway)
in AutowireWithMainClassApplication class, changing to -

@Autowired
static IZipCodeLookup zipCodeLookupService;

throws

Exception in thread "main" java.lang.NullPointerException

Answer 1

You can do one of the following:

1. Use the @Autowired object in a @PostConstruct method, which is executed after dependency injection is done, as davidxxx explained above

2. Use Spring's getBean() in your main() to explicitly ask Spring framework to return the object after the injection completes:

   public static void main(String[] args) {
       ...
       ConfigurableApplicationContext appContext = SpringApplication.run(StartApplication.class, args);
       IZipCodeLookup service = appContext.getBean(IZipCodeLookup.class);
       ...
   }
   

3. Use Spring's CommandLineRunner component (runs right after main), which will be responsible on autowiring your object:

   @Component
   public class MyRunner implements CommandLineRunner {
   
       @Autowired
       private IZipCodeLookup service;
   
       @Override
       public void run(String... args) throws Exception {
           ...
           service.doSomething();
           ... 
       }
   }
   

4. Implement Spring's ApplicationRunner's run method in your main:

   @SpringBootApplication
   public class StartApplication implements ApplicationRunner {
   
       @Autowired
       private IZipCodeLookup service;
   
       public static void main(String[] args) {
           ConfigurableApplicationContext appContext = SpringApplication.run(StartApplication.class, args);
       }
   
       @Override
       public void run(ApplicationArguments args) throws Exception {
           ...
           service.doSomething();
           ... 
       }
   }