问题解决：Class log does not exist

Laravel进不去，报服务器错误。那我就查看了一下我的nginx日志，如下所示：

# /var/log/nginx/error.log
PHP Fatal error: Uncaught ReflectionException: 
Class log does not exist in /var/www/html/Acme/vendor/laravel/framework/src/Illuminate/Container/Container.php:736 
Stack trace: 
#0/var/www/html/Acme/vendor/laravel/framework/src/Illuminate/Container/Container.php(736): ReflectionClass->__construct('log') #1/var/www/html/Acme/vendor/laravel/framework/src/Illuminate/Container/Container.php(631): Illuminate\Container\Container->build('log', Array) 
#2/var/www/html/Acme/vendor/laravel/framework/src/Illuminate/Foundation/Application.php(674): Illuminate\Container\Container->make('log', Array) 
#3/var/www/html/Acme/vendor/laravel/framework/src/Illuminate/Container/Container.php(845): Illuminate\Foundation\Application->make('log') 
#4/var/www/html/Acme/vendor/laravel/framework/src/Illuminate/Container/Container.php(800): Illuminate\Container\Container->resolveClass(Object(ReflectionParameter))
#5/var/www/html/Acme/vendor/laravel/framework/src/Illuminate/Container/Container.php(769): Illuminate\Container\Container->getDependenc in /var/www/html/Acme/vendor/laravel/framework/src/Illuminate/Container/Container.php on line 736

这问题查询了半天，依然没解决，各种权限、重新生成文件，依然没效果，搞得我头都大了。
最后我使用composer update，更新一下项目依赖时候， 还是狂报错；于是我又使用composer install，看看是不是什么依赖包丢失了，给我了下面的提示：

Problem 1
....
the requested PHP extension mbstring is missing from your system.
....

我擦，折磨了我几天的东西终于可以找到源头了，于是，我安装了mbstring的扩展。

apt-get install php7.0-mbstring

然后，问题完美解决了！