为什么不用410呢?它表示所请求的资源不再存在，客户端希望永远不会对该资源发出请求，在您的例子中是users/9。

你可以在这里找到更多关于410的详细信息:https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html

我强烈反对404，而支持204或200的空数据。或者至少应该使用带有404的响应实体。

请求被接收并被正确处理——它确实触发了服务器上的应用程序代码，客户机可能没有犯任何错误，因此整个客户机错误代码(4xx)类可能不合适。

更重要的是，404的发生有很多技术原因。例如，应用程序在服务器上被暂时停用或卸载，代理连接问题等等。

当然，这种情况下存在5xx错误类，但实际上，受影响的中间件组件通常无法知道错误在它们这一边，然后只是假设错误在客户端，然后响应404而不是500/503。

因此，仅根据状态代码，客户端无法区分404(表示“您正在寻找的东西不存在”)和404(表示“有严重错误，请将此错误报告给运维团队”)。

This can be fatal: Imagine an accounting service in your company that lists all the employees that are due to an annual bonus. Unfortunately, the one time when it is called it returns a 404. Does that mean that no-one is due for a bonus, or that the application is currently down for a new deployment and the 404 is actually coming from the tomcat that it's supposed to be installed into, instead of from the application itself? These two scenarios yield the same status code, but they are fundamentally different in their meaning.

对于需要知道所请求的资源不存在而不是暂时不可访问的应用程序来说，没有响应实体的404几乎是行不通的。

此外，许多客户端框架通过抛出异常来响应404，而不询问进一步的问题。这迫使客户端开发人员捕获异常，对其进行评估，然后基于此决定是否将其记录为由监视组件捕获的错误，或者是否忽略它。这对我来说也不太好。

The advantage of 404 over 204 is that it can return a response entity that may contain some information about why the requested resource was not found. But if that really is relevant, then one may also consider using a 200 OK response and design the system in a way that allows for error responses in the payload data. Alternatively, one could use the payload of the 404 response to return structured information to the caller. If he receives e.g. a html page instead of XML or JSON that he can parse, then that is a good indicator that something technical went wrong instead of a "no result" reply that may be valid from the caller's point of view. Or one could use a HTTP response header for that.

尽管如此，我还是更喜欢204或200的空白回复。这样，请求的技术执行状态就与请求的逻辑结果分开了。2xx的意思是“技术执行ok，这就是结果，处理它”。

我认为在大多数情况下，应该让客户来决定一个空的结果是否可以接受。通过返回404而不返回响应实体(尽管技术执行正确)，客户端可能决定将根本不是错误的情况视为错误。

Another perspective: From an operations point of view a 404 may be problematic. Since it can indicate a connectivity/middleware problem rather than a valid service response, i would not want a fluctuating number of "valid" 404s in my metrics/dashboards that might conceal genuine technical issues (e.g. a misconfigured proxy somewhere in the request routing) that should be investigated and fixed. This is further excarbated by some APIs even using 404 instead of 401/403 (e.g. gitlab does such a thing), to conceal the information that the request URI would have been valid but the request lacked authorization to access it. In this case too a 404 should be treated as a technical error and not as a valid "resource not found" result.

Edit: Wow, this has caused a lot of controversy. Here is another argument against 404: Strictly from a HTTP spec (RFC7231) point of view, 404 does not even mean that a resource does not exist. It only means that the server has no current representation of the requested resource available, and this even may be only temporary. So strictly by HTTP spec, 404 is inherently unreliable regarding the nonexistence of a requested thing. If you want to communicate that the requested thing positively does not exist, do not use 404.

根据w3的帖子，

200好了

请求成功。随响应返回的信息取决于请求中使用的方法

202年接受

请求已接受处理，但处理尚未完成。

204无内容

服务器已经完成了请求，但不需要返回实体主体，并且可能希望返回更新后的元信息。

400错误请求

由于语法错误，服务器无法理解请求。客户不应该在没有修改的情况下重复请求

401年未经授权

请求需要用户身份验证。响应必须包含一个WWW-Authenticate报头字段

404未找到

服务器没有发现任何与Request-URI匹配的内容。没有说明这种情况是暂时的还是永久的

根据微软:控制器动作返回类型在ASP。NET Core web API，向下滚动几乎到底部，你会发现下面关于404的简介，与数据库中没有的对象有关。在这里，他们建议404适用于空数据。

在以前的项目中，我使用了404。如果没有用户9，则没有找到对象。因此404 Not Found是合适的。

如果对象存在，但没有数据，则为204 no Content。我认为在你的例子中，这个物体并不存在。

使用公共枚举对响应内容进行编码，以允许客户端打开它并相应地派生逻辑。我不知道你的客户如何区分“数据未找到”404和“网络资源未找到”404之间的区别?您不希望有人浏览到userZ/9，并让客户端怀疑请求是否有效，但没有返回数据。