服务器A请求服务器B的接口，那么一般会出现跨域问题。全解跨域请求处理办法 

XMLHttpRequest cannot load http://api.console.vms3.com/api/user. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:8080' istherefore not allowed access.


意思就是服务器响应不允许跨域访问.

那我们就需要让服务器支持跨域访问, 也就是在响应头部中添加

```'Access-Control-Allow-Origin: *' ```

第一步: 创建中间件

创建 `app/Http/Middleware/AccessControlAllowOrigin.php` middleware 把 'Access-Control-Allow-Origin: *' 写入头部.
app/Http/Middleware/AccessControlAllowOrigin.php
<?php

namespace App\Http\Middleware;

use Closure;
use Illuminate\Support\Facades\Auth;

class AccessControlAllowOrigin
{
    /**
     *
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @return mixed
     */
    public function handle($request, Closure $next)
    {
        header('Access-Control-Allow-Origin: *');
        header("Access-Control-Allow-Credentials: true");
        header("Access-Control-Allow-Methods: *");
        header("Access-Control-Allow-Headers: Content-Type,Access-Token");
        header("Access-Control-Expose-Headers: *");

        return $next($request);
    }

}


第二步: 注册路由

注册这个 ​​middleware​​ 到 ​​kernel​​ 中.

分别在 ​​protected $middleware ​​数组中和​​ protected $routeMiddleware ​​数组中

添加我们刚才创建的那个文件​​class​​名, 使用 ​​cors​​ 这个别名.

第三步: 设置中间件保护接口

然后在设置它保护 api , 就是​​$middlewareGroups['api']​​ 的数组中添加它的别名, 本文中是 ​​'cors'​​

​​app/Http/Kernel.php​​

<?php

namespace App\Http;

use Illuminate\Foundation\Http\Kernel as HttpKernel;

class Kernel extends HttpKernel
{
    /**
     * The application's global HTTP middleware stack.
     *
     * These middleware are run during every request to your application.
     *
     * @var array
     */
    protected $middleware = [
        \Illuminate\Foundation\Http\Middleware\CheckForMaintenanceMode::class,
        \Illuminate\Foundation\Http\Middleware\ValidatePostSize::class,
        \App\Http\Middleware\TrimStrings::class,
        \Illuminate\Foundation\Http\Middleware\ConvertEmptyStringsToNull::class,
        \App\Http\Middleware\AccessControlAllowOrigin::class,
    ];

    /**
     * The application's route middleware groups.
     *
     * @var array
     */
    protected $middlewareGroups = [
        'web' => [
            \App\Http\Middleware\EncryptCookies::class,
            \Illuminate\Cookie\Middleware\AddQueuedCookiesToResponse::class,
            \Illuminate\Session\Middleware\StartSession::class,
            // \Illuminate\Session\Middleware\AuthenticateSession::class,
            \Illuminate\View\Middleware\ShareErrorsFromSession::class,
            \App\Http\Middleware\VerifyCsrfToken::class,
            \Illuminate\Routing\Middleware\SubstituteBindings::class,
        ],

        'api' => [
            'throttle:60,1',
            'bindings',
            'cors'
        ],
    ];

    /**
     * The application's route middleware.
     *
     * These middleware may be assigned to groups or used individually.
     *
     * @var array
     */
    protected $routeMiddleware = [
        'auth' => \Illuminate\Auth\Middleware\Authenticate::class,
        'auth.basic' => \Illuminate\Auth\Middleware\AuthenticateWithBasicAuth::class,
        'bindings' => \Illuminate\Routing\Middleware\SubstituteBindings::class,
        'can' => \Illuminate\Auth\Middleware\Authorize::class,
        'guest' => \App\Http\Middleware\RedirectIfAuthenticated::class,
        'throttle' => \Illuminate\Routing\Middleware\ThrottleRequests::class,
        'cors' => \App\Http\Middleware\AccessControlAllowOrigin::class,
    ];
}


第四步：在路由中添加路由

Route::middleware('cors')->group(function () {
    //
});