在main.js中添加

router.beforeEach((to, from, next) => {
  if(to.path === '/login'){
    sessionStorage.removeItem('user');
  }
  var user = sessionStorage.getItem('user');
  if(!user && to.path !== '/login'){
    next({
      path: '/login'
    })
  }else{
    next();
  }
})