When you create a block with block entity, you might want to place the block with predefined NBT data from an ItemStack (of your BlockItem), or save the BlockEntity data in the ItemStack after breaking the block.

In this tutorial, we assume you have created a block (and block item) and the block entity, and stored data for it.

For a block to drop an ItemStack with the Nbt from the BlockEntity of the broken block, we only need to change the loot table.

As data components have been introduced, you need to store the block entity data in the data components of the block item. This requires you to store nbt data as data components. See using_data_components about it.

If you'are using versions before 1.21, replace the word loot_table with loot_tables in the path.

src/main/resources/data/tutorial/loot_table/blocks/demo_block.json

{
  "pools": [
    {
      "rolls": 1.0,
      "bonus_rolls": 0.0,
      "entries": [
        {
          "name": "tutorial:demo_block",
          "type": "minecraft:item"
        }
      ],
      "conditions": [
        {
          "condition": "minecraft:survives_explosion"
        }
      ]
    }
  ],
  "functions": [
    {
      "source": "block_entity",
      "include": [
        "tutorial:number"
      ],
      "function": "minecraft:copy_components"
    }
  ]
}

where:

• "include": the IDs of the data component types.

Before version 1.20.5, data components were not introduced. Therefore, just copy nbt.

src/main/resources/data/tutorial/loot_tables/blocks/demo_block.json

{
  "type": "minecraft:block",
  "pools": [
    {
      "rolls": 1.0,
      "entries": [
        {
          "type": "minecraft:item",
          "name": "tutorial:demo_block",
          "functions": [
            {
              "function": "minecraft:copy_nbt",
              "source": "block_entity",
              "ops": [
                {
                  "source": "number",
                  "target": "BlockEntityTag.number",
                  "op": "replace"
                }
              ]
            }
          ]
        }
      ]
    }
  ]
}

where:

• "source" is the key of the nbt we used in the writeNbt and readNbt methods in our DemoBlockEntity class – "number"
• "target" is the hierarchy in the nbt of dropped ItemStack (source prefixed with the "BlockEntityTag" key, which is needed for placing the block back with the saved data) – "BlockEntityTag.number"

To save more fields, just add more replace operations (with source, target and op) to the %“ops”%% array.

For versions since 1.20.5, to get the BlockEntity's data stored in the item stack, we can use get its data component.

For versions since 1.20.5:

DemoBlock.class

public class DemoBlock extends BlockWithEntity {
 
    [...]
 
    @Override
    public void appendTooltip(ItemStack stack, Item.TooltipContext context, List<Text> tooltip, TooltipType options) {
        final Integer i = stack.get(TutorialDataComponentTypes.NUMBER);
        if (i == null) return;
 
        tooltip.add(Text.literal("Number: " + i));
    }
}

For versions before 1.20.5, we call getBlockEntityNbt, which internally calls getSubNbt.

For versions before 1.20.5:

DemoBlock.class

public class DemoBlock extends BlockWithEntity {
 
    [...]
 
    @Override
    public void appendTooltip(ItemStack stack, BlockView world, List<Text> tooltip, TooltipContext context) {
        NbtCompound nbt = BlockItem.getBlockEntityNbt(stack);
        if (nbt == null) return;
 
        tooltip.add(Text.literal("Number: " + nbt.getInt("number"))
    }
}

We can change our tooltip to always display some default value even if the stack does not have any BlockEntity data.

public class DemoBlock extends Block implements BlockEntityProvider {
 
    [...]
 
    @Override
    public void appendTooltip(ItemStack stack, BlockView world, List<Text> tooltip, TooltipContext context) {
        NbtCompound nbt = BlockItem.getBlockEntityNbt(stack);
        if (nbt == null){
            NbtCompound nbt = new NbtCompound();
            nbt.putInt("number", 0);
 
            BlockItem.setBlockEntityNbt(stack, ExampleMod.DEMO_BLOCK_ENTITY, nbt);
        }
 
        tooltip.add(Text.literal("Number: "+nbt.getInt("number"))
    }
}

Is automatic! As long as the data is in the “BlockEntityTag” subNbt (e.g. by using the BlockItem.getBlockEntityNbt and BlockItem.setBlockEntityNbt helper methods) under tag keys recognized by the readNbt method of the BlockEntity.

If something still isn't clear or you want more examples, I highly recommend looking at the Minecraft implementation of ShulkerBoxBlock and ShulkerBoxBlockEntity. That for example also implements Nameable interface and has code to save custom names through ItemStack and BlockEntity.

More info on Loot tables on Minecraft Wiki.

Vanilla loot tables: .minecraft\versions\[version]\[version].jar\data\minecraft\loot_tables\

Also, this Loot Table Generator might be useful.