I’m trying to understand the following JWT Authentication Filter.

@Override
    protected void doFilterInternal(
        @NonNull HttpServletRequest request,
        @NonNull HttpServletResponse response,
        @NonNull FilterChain filterChain
    ) throws ServletException, IOException {
        
        
        
        
        final String authHeader = request.getHeader("Authorization");

        if (authHeader == null || !authHeader.startsWith("Bearer ")) {
            filterChain.doFilter(request, response);
            return;
        }

        try {
            final String jwt = authHeader.substring(7);
            final String username = jwtService.extractUsername(jwt);

            
            Authentication authentication = SecurityContextHolder.getContext().getAuthentication();

            if (username != null && authentication == null) {
                UserDetails userDetails = this.userDetailsService.loadUserByUsername(username);

                if (jwtService.isTokenValid(jwt, userDetails)) {
                    UsernamePasswordAuthenticationToken authToken = new UsernamePasswordAuthenticationToken(
                            userDetails,
                            null,
                            userDetails.getAuthorities()
                    );

                    authToken.setDetails(new WebAuthenticationDetailsSource().buildDetails(request));
                    SecurityContextHolder.getContext().setAuthentication(authToken);
                }
            }

            filterChain.doFilter(request, response);
        } catch (Exception exception) {
            handlerExceptionResolver.resolveException(request, response, null, exception);
        }
    }

The specific line which i’m not understanding is the following:

Authentication authentication = SecurityContextHolder.getContext().getAuthentication();

We have 2 users, User A & User B.

User A sends an authenticaton request to the server.
How does getAuthentication() knows which user we’re trying to check if it’s already authenticated?