How to make spider farm in minecraft?

I need string so I wanted to make a spider farm like a dark hole with an arrow slot and stuff but it kept spawning other mobs. Is there a way to only let spiders spawn there?
(I'm using PE not PC)

Guest

I can make a industrial farm but for you listen. Create a room around the spawned each with 2 blocks away from the spawned, then mine 2 blocks below the spawned and clear the room out from that. Now the spawned should be flouting. Were the spawned is below it mine 3 blocks below then 2 blocks in any direction this is your ghetto spider farm. Because spiders can fit through 1 wide gaps they are stuck

Kieran