public synchronized Service ipAddress(String ipAddress) {
        if (initialized) {
            throwBeforeRouteMappingException();
        }
        this.ipAddress = ipAddress;

        return this;
    }
Set the IP address that Spark should listen on. If not called the default
address is '0.0.0.0'. This has to be called before any route mapping is
done.

@param ipAddress The ipAddress
@return the object with IP address set
src/main/java/spark/Service.java