public synchronized Service port(int port) {
        if (initialized) {
            throwBeforeRouteMappingException();
        }
        this.port = port;
        return this;
    }
Set the port that Spark should listen on. If not called the default port
is 4567. This has to be called before any route mapping is done.
If provided port = 0 then the an arbitrary available port will be used.

@param port The port number
@return the object with port set
src/main/java/spark/Service.java