My phone is in a bootloop

The phone keeps turning on and off. I've tried multiple times but it will not get into recovery. I have used a cable for the attemps i made. What happens at startup is a fraction of a second the samsung logo comes in and than a black screen then it goes back to the samsung logo and stays there for a few seconds and then continues on to the samsung galaxy screen powerd by android. I think the battery is severly uderpowerd as it has been laying for a while, but even still without any power to the phone from any power source it does still keep restarting. I was able to access the download menu, but I do not want to lose any information on the phone.