How do I create a blast-proof exp grinder in Minecraft?

Question

How does one make an efficient kill room in Minecraft, that can survive creeper explosions?

So, I contained the spawner; I can probably augment it with some spawn rooms that will transport the mobs to the shafts dropping them the needed 22 blocks down to what is to be the room where I kill them. Then I'll have a creeper blow up and set all consecutive survivors free.

I know the walls must be built from obsidian, but how do I make the gap in it to swing my sword through? The text tutorials are rather sketchy, and the video tutorials use either fenceposts (which are definitely not blast proof) or some kind of transparent grid which I failed to identify (a different tileset, maybe?).

Answer 1

If a creeper is standing in water, then the blast damage is restricted to the water. So you can have water for the mobs to stand in, and if one accidentally blows up then you're not totally screwed with a broken trap, but it will kill the other mobs in the trap.

This is mine:

Answer 2

A properly constructed XP grinder doesn't need to be made of obsidian, because the mobs will never be able to 'see' you for the purposes of attacking/blowing up.

The trick to constructing such a grinder is to have the mobs' feet on the same level as your head, with this level having the only opening and also having one block of horizontal space between you and the mobs.

Here's a side-on view of such an XP grinder

Mobs drop in here
 \/

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|||_ <- You stand here

For a video tutorial, check out this one by Monkeyfarm.

Answer 3

Creeper sight through glass will not trigger an explosion when it "sees" the player. Often above the 1x1 or 1x2 horizontal opening used in mob spawners is a row of glass. This is not necessary for functionality but may better suit player aesthetics.

Answer 4

I made a Creeper farm recently, and decided to make the sword swing gap with an enchanting table. I then was very surprised to find that they could see me and explode at certain angles, however, very happy to find that the explosion did not destroy any blocks as the explosion took place "inside" the enchanting table

{1.15 functional}