@EventHandler
public void onPrepareAnvilEvent(PrepareAnvilEvent event) {
    AnvilInventory inventory = event.getInventory();
    inventory.setMaximumRepairCost(50);

    final Player player = ((Player) event.getView().getPlayer());

    ItemStack leftItem = inventory.getItem(0);
    ItemStack rightItem = inventory.getItem(1);

    //check if both item slots are filled
    if (leftItem != null && rightItem != null) {
        player.sendMessage("TEST 1");
        handleLevelCost(player,event,inventory);
    } else if(leftItem != null) { // Start for detect rename
        if(inventory.getRenameText() != null && !inventory.getRenameText().isEmpty() && event.getResult() != null) { //Only Rename
            handleLevelCost(player,event,inventory);
        }
    }
}

//Detect a player has click the item result in Anvil!
@EventHandler
public void onInventoryClick(InventoryClickEvent event) {
    // check whether the event has been cancelled by another plugin
    if (!event.isCancelled()) {
        HumanEntity humanEntity = event.getWhoClicked();

        if (humanEntity instanceof Player) {
            Player player = (Player) humanEntity;
            // Check if this event fired inside an anvil.
            if (event.getInventory() instanceof AnvilInventory) {
                final AnvilInventory anvilInventory = (AnvilInventory) event.getInventory();
                InventoryView view = event.getView();
                int rawSlot = event.getRawSlot();

                // check if we are in the upper inventory of the anvil
                if (rawSlot == view.convertSlot(rawSlot)) {
                    // check if we are talking about the result slot
                    if (rawSlot == 2) {
                        // get all 3 items in the anvil
                        ItemStack[] items = anvilInventory.getContents();

                        if (player.getLevel() < anvilInventory.getRepairCost() && !player.getGameMode().equals(GameMode.CREATIVE)) { // Handle level and vanilla feature for creative
                            player.sendMessage("CANT PAY " + event.getResult()); // This return ALLOW
                        }
                    }
                }
            }
        }
    }
}

private void handleLevelCost(Player player, PrepareAnvilEvent event, AnvilInventory inventory) {
    int finalRepairCost = inventory.getRepairCost();
    player.sendMessage(ChatColor.RED + "Anvil Cost is: " + ChatColor.GREEN + finalRepairCost + ChatColor.RED + " levels.");
}