服务器检查该ETag，并判断出该页面自上次客户端请求之后还未被修改，直接返回响应304(未修改——Not Modified)和一个空的响应体。

Server examines the ETag and determines that the page hasn't changed since last time the client requested it, so sends back a response of 304 (Not Modified) with an empty body.

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如果Accept标头为空，这意味着该客户端可接受任何类型，因此设置了缺省值。

If the accept header is null, this means that the client accepts any type, so the default is set.

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如果服务器端组件上的方法具有非空返回值，该值就会作为其惟一的实参传递进此回调函数。

If the method on the server-side component has a non-void return value, that value is passed into the callback function as its only argument.

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