項目基本目錄結構：

1. global.css ,設置 html,body,以及 #app區域所有的高度占比都是 100%

/* 全局樣式表 */

html,
body,
#app {
  height: 100%;
  margin: 0;
  padding: 0;
}

2. router 的配置

import Vue from 'vue'
import Router from 'vue-router'

import LoginView from "@/views/login/LoginView";


import NavLeft from "@/views/tabbar/NavLeft";
import NavExample from "@/views/tabbar/NavExample";
import LoginPage from "@/views/login/LoginPage";
import Home from "../views/home/Home";


Vue.use(Router)

const router = new Router({
  routes: [
    {
      path: '/',
      name: 'LoginPage',
      component: LoginPage
    },
    {
      path: '/login',
      component: LoginPage
    },
    {
      path:'/home',
      component: Home
    },
    {
      path: '/nav',
      name: 'NavLeft',
      component: NavLeft
    },
    {
      path: '/nav2',
      name: 'NavExample',
      component: NavExample
    }



  ]
})

router.beforeEach((to,from,next) => {
  //to: 將要訪問的路徑
  //next 放行函數，強制跳轉路徑 next(url)
  if(to.path=='/login') return next()

  let token = window.sessionStorage.getItem('token')
  if(!token) {
    return next('/login')
  }
  next()


})

export default router

3. main.js的配置

// The Vue build version to load with the `import` command
// (runtime-only or standalone) has been set in webpack.base.conf with an alias.
import Vue from 'vue'
import App from './App'
import router from './router'
//需要再 main這里引入全局樣式表 ，并且注冊
import '@/assets/css/global.css'


import 'element-ui/lib/theme-chalk/index.css';

import {
  Button,Form,FormItem,Input,Message,Container,Header,Aside,Footer,Main

} from "element-ui";



Vue.use(Button)
Vue.use(Form)
Vue.use(FormItem)
Vue.use(Input)
Vue.use(Container)
Vue.use(Header)
Vue.use(Aside)
Vue.use(Footer)
Vue.use(Main)


Vue.prototype.$message = Message


import axios from 'axios'
import el from "element-ui/src/locale/lang/el";
axios.defaults.baseURL='localhost:8080/api/';
Vue.prototype.$http = axios;


Vue.config.productionTip = false

/* eslint-disable no-new */
new Vue({
  el: '#app',
  router,
  components: { App },
  template: '<App/>'
})

4. 登錄界面：

<template>
  <div class="login_container">
    <div class="login_box">
      <div class="avater_box">
        <img src="@/assets/logo.png" alt="">
      </div>
      <!-- 登錄組件      -->
      <el-form label-width="0px" class="login_form"
               comment="這個是登錄框，:model 用來雙向綁定數據"
               :model="loginInfo"
               :rules="inputRules"
               ref="loginFormRef"
      >
        <el-form-item comment="用戶名" prop="username">
          <!-- 如果沒有提供的話，就去阿里圖標庫下載          -->
          <el-input prefix-icon="el-icon-user" v-model="loginInfo.username"></el-input>
        </el-form-item>
        <el-form-item comment="密碼"  prop="password">
          <el-input prefix-icon="el-icon-warning-outline"  v-model="loginInfo.password" type="password"></el-input>
        </el-form-item>

        <el-form-item class="btn-line">
          <el-button type="success" @click="doPost">登錄</el-button>
          <el-button type="info" @click="resetFields" >重置</el-button>

        </el-form-item>


      </el-form>
    </div>
  </div>
</template>

<script>
  export default {
    name: "LoginPage",
    data() {
      return {
        //用戶輸入信息校驗
        loginInfo: {
          username: '',
          password: ''
        },
      //  用戶輸入校驗
        inputRules:{
          username:[
            {required:true,message:'請輸入登錄賬號',trigger:'blur'},
            {min:3,max:20,message: "長度在3到20個字符",trigger: 'blur'}
          ],
          password: [
            {required:true,message:'請輸入登錄密碼',trigger:'blur'},
            {min:5,max:20,message: "長度在5到20個字符",trigger: 'blur'}
          ]
        }
      }
    },
    methods:{
      doPost() {
        this.$refs.loginFormRef.validate(async valid => {
          // console.log(123)
          console.log(valid);
          if(!valid) return;
          try{
            const {data:result} = await this.$http.post("login",this.loginInfo);
            console.log(result)
          }catch (e) {
            console.log('異常')
            // this.$message.error("網絡有問題")
            this.$message.success("登錄成功")
            let token = '123'
            window.sessionStorage.setItem('token',token)
            this.$router.push('/home')
          }
          //將 result.data 拿出來，并且打印
        })
      },
      resetFields() {
        this.$refs.loginFormRef.resetFields();
      }
    }
  }
</script>

<style scoped lang="less">
  .login_container {
    background-color: #111821;
    height: 100%;
    width: 100%;


    .login_box {
      width: 450px;
      height: 300px;
      background-color: white;
      border-radius: 3px;
      position: absolute;
      left: 50%;
      top: 50%;
      transform: translate(-50%, -50%);

      .avater_box {
        height: 130px;
        width: 133px;
        border: 1px solid #eeeeee;
        border-radius: 50%; /*    父節點圓角          */
        box-shadow: 0 0 10px #ddd; /* 設置同心圓陰影 */
        left: 50%;
        right: 50%;
        position: absolute;
        transform: translate(-50%, -50%); /* 使用絕對定位，所有根據該盒子的高度移動*/
        background-color: #fff;

        img {
          width: 100%;
          height: 100%;
          border-radius: 50%; /*   圖片圓角       */
          background-color: #eeeeee;
        }

      }

      .btn-line {
        display: flex;
        justify-content: flex-end;
      }

      .login_form {
        position: absolute;
        bottom: 0;
        width: 100%;
        padding: 0 10px;
        box-sizing: border-box; /* form 不設置border-sizing 會撐開父元素 */
      }
    }

  }


</style>

界面如圖所示：