How to get the player input from a custom anvil menu

Question

For my plugin I want to get the users input from a custom anvil UI. When a player enters a command, it opens a custom anvil UI where they can enter a string by renaming a item. When they press the item in the output slot I want to get that name so I can process the entered string in my code.

I have tried to do this with the spigot-API, but when I click on the item in the output slot the event is triggered but the whole anvil inventory is reset before I can get the name of the clicked item.

code

public final class Test extends JavaPlugin implements Listener
{
   @Override
    public void onEnable()
    {
        this.getServer().getPluginManager().registerEvents(this, this);
        getCommand("openGui").setExecutor(new Command(this));
    }
    
    @EventHandler 
    public void OnInventoryClick(InventoryClickEvent e) 
    {     
        if(e.getInventory().getHolder() instanceof MenuHolder)     
        {        
            //gets input from user from the display name of the clicked item
            String input = e.getCurrentItem().getItemMeta().getDisplayName().toString(); 
            e.getView().getPlayer().sendMessage("Users input: " + input);
    
            //cancels and closes the custom anvil UI
            e.setCancelled(true); 
            e.getView().getPlayer().closeInventory();
        } 
    }  
}

public class Command implements CommandExecutor
{
    @Override
    public boolean onCommand(CommandSender sender, Command command, String s, String[] args)
    {
        if(sender instanceof Player)
        {
            Player player = (Player)sender;

            //created a new Anvil Menu
            Inventory inv = Bukkit.createInventory(new MenuHolder(), InventoryType.ANVIL, "Custom menu");
        
            //adds a custom item with no displayName to the input slot of the new anvil menu
            ItemStack paper = new ItemStack(Material.PAPER);
            ItemMeta paperMeta = paper.getItemMeta();
            paperMeta.setDisplayName("");     
            paper.setItemMeta(paperMeta);          
            inv.setItem(0, paper);       
           
            player.openInventory(inv); 
        }
    }
}


//InventoryHolder class to easily identify custom Inventories
public class MenuHolder implements InventoryHolder
{
    public Inventory getInventory()
    {
        return  null;
    }
}

test-1

    @EventHandler
    public void onInventoryClick(InventoryClickEvent e)
    {
        if(e.getInventory().getHolder() instanceof MenuHolder)
        {

            //Test 1
            if(e.getInventory().getHolder() instanceof MenuHolder)
            {
                if(e.getCurrentItem() != null)
                {
                    ItemMeta input = e.getCurrentItem().getItemMeta();
                    if(input != null)
                    {
                        e.getView().getPlayer().sendMessage("Users input1: " + input.getDisplayName());
                    }
                    else
                    {
                        e.getView().getPlayer().sendMessage("CurrantItem ItemStack does not have a ItemMeta");
                    }
                }
                else
                {
                    e.getView().getPlayer().sendMessage("CurrentItem does not have a item");
                }

                //-----------------------------------------------

                //test 2
                if(e.getView().getCursor() != null)
                {
                    ItemMeta input2 = e.getView().getCursor().getItemMeta();
                    if(input2 != null)
                    {
                        e.getView().getPlayer().sendMessage("Users input: " + input2.getDisplayName());
                    }
                    else
                    {
                        e.getView().getPlayer().sendMessage("Curser ItemStack does not have a ItemMeta");
                    }
                }
                else
                {
                    e.getView().getPlayer().sendMessage("Curser does not have a item");
                }
            }
        }
    }

    when pressing the inputslot(slot 0):
    test1 = Users input: (The default set string)
    test2 = Curser ItemStack does not have a ItemMeta

    when pressing the outputslot(slot 2):
    test1 = CurrentItem does not have a item
    test2 = Curser ItemStack does not have a ItemMeta

test-2

public class MenuHolder implements InventoryHolder
{
    public MenuTypes menuType;
    private final Player player;

    public MenuHolder (MenuTypes menuType, Player player)
    {
        this.menuType = menuType;
        this.player = player;
    }

    public Player getPlayer() {
        return player;
    }

    public Inventory getInventory()
    {
        return  null;
    }
}

    @EventHandler
    public void onPrepareAnvilEvent(PrepareAnvilEvent e) {
        AnvilInventory inv = e.getInventory();

        if(e.getResult() == null)
        {
            e.getView().getPlayer().sendMessage("result was null");
            return;
        }

        if(!e.getResult().hasItemMeta())
        {
            e.getView().getPlayer().sendMessage("result does not have a item meta");
            return;
        }

        e.getView().getPlayer().sendMessage("Wrote: " + e.getResult().getItemMeta().getDisplayName());
    }

Answer 1

You can do firstly add player to holder, then register listener to event, like this:

//InventoryHolder class to easily identify custom Inventories
public class MenuHolder implements InventoryHolder {
    
    private final Player player;

    public MenuHolder(Player player) {
         this.player = player;
    }

    public Player getPlayer() {
        return player;
    }

    public Inventory getInventory() {
        return null;
    }
}

public class AnvilListener implements Listener {
    @EventHandler
    public void onPrepareAnvilEvent(PrepareAnvilEvent e) {
        AnvilInventory inv = e.getInventory();
        if(inv.getHolder() instanceof MenuHolder holder) {
             Player player = holder.getPlayer();
             player.sendMessage("Wrote: " + (e.getResult() != null && e.getResult().hasItemMeta() ? e.getResult().getItemMeta().getDisplayName() : ""));
        }
    }
}

And for the command:

public class Command implements CommandExecutor {
    @Override
    public boolean onCommand(CommandSender sender, Command command, String s, String[] args) {
        if(sender instanceof Player player) {
            //created a new Anvil Menu
            Inventory inv = Bukkit.createInventory(new MenuHolder(player), InventoryType.ANVIL, "Custom menu");
        
            //adds a custom item with no displayName to the input slot of the new anvil menu
            ItemStack paper = new ItemStack(Material.PAPER);
            ItemMeta paperMeta = paper.getItemMeta();
            paperMeta.setDisplayName("");     
            paper.setItemMeta(paperMeta);          
            inv.setItem(0, paper);       
           
            player.openInventory(inv); 
        }
    }
}

Don't forget to register the listener with Bukkit.getPluginManager().registerEvents(new AnvilListener(), this);.