Write two more lines following your example.

public class Main{

  public static void main(String[] args){
    ClassPathXmlApplicationContext ctx = new FileSystemXMLApplicationContext("beans.xml");
    Hello hello = ctx.getBeans("hello");
    hello.say();
  }
}

class Hello {
  @Autowired
  private DbUtil db;

  public void say(){
    System.out.println(db.getWelcomeInfo());
  }
}

The layer that initially handles requests in the web project still needs to be generated through getBeans(). It's just that this step is done by tomcat/strtus/springMVC.
Spring will automatically inject classes managed by it. However, the main class started by your program is not generated by Spring management.

You need Autowired

To use Spring's dependency injection, you must first obtain the bean from Spring, so as to ensure that the bean's dependencies have been injected. If you create a new object yourself, how can Spring inject it for you?

You can encapsulate the getBeans method yourself

You have to treat your HELLO as a member variable, and then go to @autowired or @Resourse

Dependency injection is nothing more than two steps:

1. declare bean

2. Inject beans

How to declare a bean:

1. In xml

2. Add @Component, @Service, etc. to the class

3. Write the method with @Bean in the class with @Configuration

How to inject beans:

1. In xml

2. Annotations like @Autowire

It may not be all mentioned, but these should be the commonly used ones. For details, please refer to the spring documentation and ioc part

Let’s ask a question

1. There is no need to configure listener in web.xml in web applications (the prerequisite is servlet3.0+, such as tomcat7+)

2. For Java Application, why not try Spring boot?

Recommended topic owner can take a look @Chachage Servlet 3 + Spring MVC zero configuration: remove all xml