Arduino is heating

hi , i'm using an arduino uno . and i supplied it with a 12V 1A Dc power adapter ...
why is my arduino heating around the power barrel ?
whereas if i supplied it with the parallel usb port to the computer, it does not heat .
is using 12V 1A dc power adapter ok ? tnx!

The linear regulator next to the barrel jack dissipates the difference between 12V and 5V as heat.
USB (which is serial, not parallel) only supplies 5V, it does not need any regulating (only a small current amount from 5V to 3.3V).
If you can use a 7.5V wallwart, or 9V, it will run much cooler.
When I make projects, I use a 5V supply so no power is wasted in the 5V regulator.