code: @Override
      public ReqContext populateContext(ReqContext context, HttpServletRequest req) {
          String userName = getUserName(req);
  
          String doAsUser = req.getHeader("doAsUser");
          if (doAsUser == null) {
              doAsUser = req.getParameter("doAsUser");
          }
  
          if (doAsUser != null) {
              context.setRealPrincipal(new SingleUserPrincipal(userName));
              userName = doAsUser;
          }
  
          Set<Principal> principals = new HashSet<Principal>();
          if (userName != null) {
              Principal p = new SingleUserPrincipal(userName);
              principals.add(p);
          }
          Subject s = new Subject(true, principals, new HashSet(), new HashSet());
          context.setSubject(s);
  
          return context;
      }
docs: Populates a given context with a new Subject derived from the credentials in a servlet request.
  
  @param context the context to be populated
  @param req the servlet request
  @return the context