在某些情况下，如果找不到对象，则手动引发异常很有用。如果未找到实体，则可以通过向参数添加默认值来告诉操作跳过抛出异常。

In some cases it's useful to throw exception manually if object not found. You can tell action skip throw exception if entity not found by adding default value to param.

示例：

public function showUser(User $user = null) {
    if (empty($user)) {
        throw new CustomExceptionYouWant();
    }
    ...
}