$ Injection在Interceptor中做了什么？ Angularjs

Question

我之前有一些同事写的代码。我无法得到 $ injector 在这里做什么？

$httpProvider.interceptors.push(function($q, $location, $injector) {
        return {
            'responseError': function(response) {
                if((response.status === 401 || response.status === 403) && $location.path().substring(1)!='login') {
                 var ModalService = $injector.get('modalService');
                 var modalOptions = {
                          headerText: 'Session Expired',
                          displayAction: true,
                          displayClose: false
                  };
                  if (!gotError){
                  gotError = true;
                  ModalService.showModal({}, modalOptions,'Your session has expired. Please login again to continue.').then(function(){
                            gotError = false;
                            window.location.reload();
                            var session = $injector.get('Auth');
                            session.clearSession(); //clearing cookies
                  });
                  }
                }
                return $q.reject(response);
            }
        };
    });