Why is spring.Main.service" is null

Question

Please help ;) main can't find bean Personserviceimp ???????????????????????????????????

Main

package spring;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.ComponentScan;

@ComponentScan
public class Main {
    @Autowired
    static PersonServiceImpl service;
    public static void main(String[] args){

       System.out.println(service.findByName("Valera").getAge());

    }
}

PersonService

package spring;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class PersonServiceImpl implements PersonService{

    private final PersonDao dao;
    @Autowired
    public PersonServiceImpl(PersonDao dao) {
        this.dao = dao;
    }

    @Override
    public Person findByName(String name) {
        return dao.findByName(name);
    }
}

PersonDao

package spring;

import org.springframework.stereotype.Repository;

@Repository
public class PersonDao implements IPersonDAO{

    public Person findByName(String name) {
        return new Person(name, 18);
    }


}

Person

package spring;

public class Person {

    private String name;
    private int age;

    public Person(String name, int age){
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }
}

Maven pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>spring2</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>15</maven.compiler.source>
        <maven.compiler.target>15</maven.compiler.target>
    </properties>
    <dependencies>
        <!-- https://mvnrepository.com/artifact/org.springframework/spring-context -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>5.3.9</version>
        </dependency>

    </dependencies>

</project>

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "spring.RersonServiceImpl.findByName(String)" because "spring.Main.service" is null
    at spring.Main.main(Main.java:12)

I did this but I still get null

@ComponentScan
@Component("Main")
public class Main {
    private static PersonServiceImpl service;
    @Autowired
    public Main(PersonServiceImpl service) {
        Main.service = service;
    }
    public static void main(String[] args){

       System.out.println(service.findByName("Valera").getAge());
    }
}

and so

public class Main {
    private static RersonServiceImpl service_;
    @Autowired
    private RersonServiceImpl service;
    @PostConstruct
    private void init() {
        service_ = this.service;
    }
    public static void main(String[] args){

       System.out.println(service_.findByName("Valera").getAge());
    }
}

Answer 1

Remove static and call the service in a controller. If you need autostartup just use

    @PostConstruct 

on the method that you need to run once at startup. You can use just one @PostConstruct per class.

Answer 2

Try simply like below. Remove static as it wouldn't help to populate spring components. I also see some typos, it needs to be PersonServiceImpl, but not RersonServiceImpl.

 public class Main {
    @Autowired
    private PersonServiceImpl service;
        
    public static void main(String[] args){
       System.out.println(service.findByName("Valera").getAge());
    }
}

Answer 3

@SpringBootApplication
public class Demo2Application implements CommandLineRunner {
    private static final Logger log = LoggerFactory.getLogger(Demo2Application.class);
    @Autowired
    PersonDaoJdbc dao;

    public static void main(String[] args) {
        SpringApplication.run(Demo2Application.class, args);

    }

    @Override
    public void run(String... args) {
        log.info(String.valueOf(dao.count()));
        log.info("StartApplication...");


    }