Unexpected Socket Close

Answer:
When a socket endpoint closes its socket, a read on the InputStream at the other end will return -1, indicating end of file. If the process crashes, the same will happen because the operating system will close the socket when the process dies.

However, if the host that the process is running on crashes, there is no way to detect the endpoint termination. With native system programming, it is possible to set the SO_KEEPALIVE socket option to test the integrity of a connection on a more frequent basis than the default 2 hours. Java does not allow you to do this, and instead relies on the system default.