Java simple code: java.net.SocketException: Unexpected end of file from server

"Unexpected end of file" implies that the remote server accepted and closed the connection without sending a response. It's possible that the remote system is too busy to handle the request, or that there's a network bug that randomly drops connections.

It's also possible there is a bug in the server: something in the request causes an internal error, and the server simply closes the connection instead of sending a HTTP error response like it should. Several people suggest this is caused by missing headers or invalid header values in the request.

With the information available it's impossible to say what's going wrong. If you have access to the servers in question you can use packet sniffing tools to find what exactly is sent and received, and look at logs to of the server process to see if there are any error messages.

Summary

This exception is encountered when you are expecting a response, but the socket has been abruptly closed.

Detailed Explanation

Java's HTTPClient, found here, throws a SocketException with message "Unexpected end of file from server" in a very specific circumstance.

After making a request, HTTPClient gets an InputStream tied to the socket associated with the request. It then polls that InputStream repeatedly until it either:

1. Finds the string "HTTP/1."
2. The end of the InputStream is reached before 8 characters are read
3. Finds a string other than "HTTP/1."

In case of number 2, HTTPClient will throw this SocketException if any of the following are true:

• The HTTP method is CONNECT
• The HTTP method is POST and the client is set to streaming mode

Why would this happen

This indicates that the TCP socket has been closed before the server was able to send a response. This could happen for any number of reasons, but some possibilities are:

• Network connection was lost
• The server decided to close the connection
• Something in between the client and the server (nginx, router, etc) terminated the request

_{Note: When Nginx reloads its config, it forcefully closes any in-flight HTTP Keep-Alive connections (even POSTs), causing this exact error.}