How to get a header value in a RestController and pass it to the parent controller

I'm new to spring-boot and quite new to web services.

I would like to implement an authorization process in an (abstract) generic RestController which should get the content of the basic-auth. header from the extending controllers and then perform the authorization.

Something like that:

public class GenericController
{
  // Constructor 
  protected GenericController(String basicAuth) throws MalformedURLException, ProtocolException, IOException
  {
    // check user can execute action
  }
}


@RestController
@RequestMapping( "/users" )
public class UserController extends GenericController
{
  private static final Logger logger = LoggerFactory.getLogger(UserController.class);
  
  // Constructor
  protected UserController() throws MalformedURLException, ProtocolException, IOException
  {
	// somehow get the basic auth information from header and pass it to the parent's constructor
	basicAuth = ???
    
	super(basicAuth);
  }

  @GetMapping( "/user/{cn}" )
  public String getUser( @PathVariable String cn )
  {
    logger.error("Start getUser(...): cn=" + cn);

  }

How can I do that? (is that even possible?)

ADDED INFORMATION:
My web services are themselves consumers of other web services.

Once the calling user is authorized to "use" my web services I have to forward/set the Basic Authentication into the request for the web services I call.

So, how/where can I "intercept" and get the headers when my services are called?

You can use HttpServletRequest to get the header from the request.

 protected UserController(HttpServletRequest httpServletRequest) throws MalformedURLException, ProtocolException, IOException
    {

        System.out.println( httpServletRequest.getHeaders("key name"));
        // somehow get the basic auth information from header and pass it to the parent's constructor
        basicAuth = ???

        super(basicAuth);
    }

I would recommend to take a look at Spring Security framework. There are many tutorials showing how to configure it to perform basic authentication, e.g.: here.

The main idea is decoupling authentication / authorization concerns from controllers and business logic code. To implement this filters are used which intercept every request and validate it against different conditions. When implementing stateless REST-service (each request independent from others) it's not so difficult. When handling sessions, logic will become much more complicated. Spring Security implements filter chains, consisting of multiple filters, each performing its own checks.

Spring Security is used very often in spring-based projects and is definitely worth to look at, although it may seem a bit difficult when meeting it for the first time.