Solve the problem of Failed to load resource: net::ERR_CONNECTION_REFUSED

The intuitive mistake in this problem is that the access is different. The first reaction is related to the firewall, but after thinking that I am visiting a local project, it has nothing to do with the firewall. Later, I thought it might be the wrong address. The IP number is 127.0.0.1, so it must be the wrong port. Sure enough, the front end was accessing port 8000, but the server was listening on port 9999. Just make it consistent.

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