在demo代码中，在初始化时首先创建一个以A和J开头的城市的列表。
In the demo code, a list of cities beginning with a and J is created on initialization.
只在初始化时使用少量的阻塞调用，尤其是在编写注重性能的网络服务时。
Use blocking calls sparingly and only for initialization, especially when writing network services where performance matters.
ServletConfig：容器用于在初始化时将配置信息传递给 Servlet 的机制。
ServletConfig: A mechanism used by the container to pass configuration information to the servlet on initialization.
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