minecraft-assets/data/minecraft/recipes/barrel.json

{
  "type": "minecraft:crafting_shaped",
  "category": "misc",
  "key": {
    "P": {
      "tag": "minecraft:planks"
    },
    "S": {
      "tag": "minecraft:wooden_slabs"
    }
  },
  "pattern": [
    "PSP",
    "P P",
    "PSP"
  ],
  "result": {
    "item": "minecraft:barrel"
  }
}