ValidJson.php

<?php

namespace App\Http\Middleware;

use Closure;

class ValidJson {

    /**
     * Handle an incoming request.
     * @param  \Illuminate\Http\Request $request
     * @param  \Closure                 $next
     * @return mixed
     */
    public function handle($request, Closure $next) {

        json_decode($request->getContent());
        if (json_last_error() != JSON_ERROR_NONE) {
            return response()->json('Request must be valid json', 422);
        }

        return $next($request);
    }
}