java.net.SocketTimeoutException vs java.net.ConnectException

Question

When connecting to a server with a Java client socket I had this two different connection timeout exceptions.

Caused by: java.net.SocketTimeoutException: con         


        

Answer 1

Also came here looking for the same answer, seemingly the docs can be easily misinterpreted:

Connects this socket to the server with a specified timeout value. A timeout of zero is interpreted as an infinite timeout. The connection will then block until established or an error occurs.

The key part that I overlooked in this is "an error"... going to the source I can see how Java's connect() is actually invoking the Linux connect():

if (timeout <= 0) {
    connect = connect(args...);
    if (connect == -1 && errno == EINPROGRESS) {
        connect = poll(args...);
        // try again on EINTR
    }
} else {
    // Go to non-blocking mode for a timeout.
    connect = connect(args...);

    if (connect!=0) {
        // not EINPROGRESS? -> throw ConnectException
        while (!connect || !error || timedout) {
            connect = poll(args...);
            // error / timedout handling
        }
        if (timedout) {
            // throw SocketTimeoutException
        }
    }
}

/* report the appropriate exception */
if (error) {
    //EINVAL; throw SocketException
    //EINTR; throw InterruptedIOException
    //EPROTO; throw ProtocolException
    //ECONNREFUSED;ETIMEDOUT; throw ConnectException
    //EHOSTUNREACH; throw NoRouteToHostException
    //EADDRNOTAVAIL; throw NoRouteToHostException
    //EISCONN, EBADF, other; throw SocketException
}

i.e. I think SocketTimeoutException is being thrown when the network is slow, or the host doesn't respond at all. Checking man connect, I can see that ECCONNREFUSED must be thrown when "No-one listening on the remote address", i.e. and an ICMP tells us so.

This means that if like me, you were trying to use the timeout to connect to a (localhost) socket that wasn't ready to be connected to, you are SOL'd.