static symbol never referenced 变量定义未使用
Static routing implies that the routing rules never change.
静态的路由是指不能更改路由规则。
The other way to say it is, never define a global (static global) instance of a class whose constructor or destructor may throw exceptions.
换一种说法就是永远都不要为那些其构造函数和析构函数可能抛出异常的类定义全局(静态全局)实例。
For static components: implement "Never expire" policy by setting far future Expires header.
为静态的组件：通过设置远的将来实现永不过期策略过期标头。
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