title: How Field Injection in Spring Boot works internally?
body: <pre><code>@Autowired
  UserService userService;
  </code></pre>
  
  <p>What happens exactly inside `@Autowired annotation whether it uses <strong>Constructor Injection</strong> or <strong>Setter Injection</strong>. I know that it is field Injection.</p>
  
  <p>I'm not asking How IOC or DI works, I'm asking How Field Injection in Spring Boot works internally?</p>