I have this inventory:

package tlhpoe.fs.inventory;

import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.inventory.InventoryBasic;
import net.minecraft.item.ItemStack;
import net.minecraft.nbt.NBTTagCompound;
import net.minecraft.nbt.NBTTagList;

public class InventoryPOOP extends InventoryBasic {
private EntityPlayer player;
private String n;

public InventoryPOOP(EntityPlayer player) {
	super("container.poop", false, 27);
	this.player = player;
	this.n = player.getDisplayName() + "POOP";
}

@Override
public void openInventory() {
	NBTTagCompound nbt = player.worldObj.getWorldInfo().getNBTTagCompound();
	NBTTagList pNBT = (NBTTagList) nbt.getTag(n);
	if(pNBT != null) {
		System.err.println("NBT found, loading");
		loadInventoryFromNBT(pNBT);
	} else {
		System.err.println("NBT not found, creating & loading");
		loadInventoryFromNBT(saveInventoryToNBT());
	}
}

@Override
public void closeInventory() {
	NBTTagCompound nbt = player.worldObj.getWorldInfo().getNBTTagCompound();
	nbt.setTag(n, saveInventoryToNBT());
}

public void loadInventoryFromNBT(NBTTagList par1NBTTagList) {
	int i;
	for(i = 0; i < this.getSizeInventory(); ++i) {
		this.setInventorySlotContents(i, (ItemStack) null);
	}
	for(i = 0; i < par1NBTTagList.tagCount(); ++i) {
		NBTTagCompound nbttagcompound = par1NBTTagList.getCompoundTagAt(i);
		int j = nbttagcompound.getByte("Slot") & 255;
		if(j >= 0 && j < this.getSizeInventory()) {
			this.setInventorySlotContents(j, ItemStack.loadItemStackFromNBT(nbttagcompound));
		}
	}
}

public NBTTagList saveInventoryToNBT() {
	NBTTagList nbttaglist = new NBTTagList();
	for(int i = 0; i < this.getSizeInventory(); ++i) {
		ItemStack itemstack = this.getStackInSlot(i);
		if(itemstack != null) {
			NBTTagCompound nbttagcompound = new NBTTagCompound();
			nbttagcompound.setByte("Slot", (byte) i);
			itemstack.writeToNBT(nbttagcompound);
			nbttaglist.appendTag(nbttagcompound);
		}
	}
	return nbttaglist;
}
}

 

Problem is, it won't save the nbt tag. Every time I open it, it prints out "NBT not found, creating & loading".

 

 

Edit:

I wrote to the player's NBT instead.

Your save function doesn't save the NBT back to player.worldObj.getWorldInfo().getNBTTagCompound() in any fashion.

@Override
public void closeInventory() {
	NBTTagCompound nbt = player.worldObj.getWorldInfo().getNBTTagCompound();
	nbt.setTag(n, saveInventoryToNBT());
}

 

It returns a NBTTagList and then I set it.

After looking through the source for net.minecraft.world.storage.WorldInfo class, I can say for certain that this function

player.worldObj.getWorldInfo().getNBTTagCompound();

creates and returns a new NBT compound object (loaded with current world info, and the {player: NBTCompound} added), if does not in any way modify the worldInfo object no matter what is done to it, nor is it saved at any point.

 

However, any changes made to the original playerTag should be saved in the course of world saves.