Modify the appearance of a dropped item stack on the client side.

ModifyDroppedItemStack.java

package com.comphenix.example;

import org.bukkit.Material;
import org.bukkit.entity.Item;
import org.bukkit.inventory.ItemStack;
import org.bukkit.plugin.java.JavaPlugin;
import com.comphenix.protocol.Packets;
import com.comphenix.protocol.ProtocolLibrary;
import com.comphenix.protocol.events.ConnectionSide;
import com.comphenix.protocol.events.PacketAdapter;
import com.comphenix.protocol.events.PacketContainer;
import com.comphenix.protocol.events.PacketEvent;
import com.comphenix.protocol.wrappers.WrappedDataWatcher;

public class ModifyDroppedItemStack extends JavaPlugin {
    @Override
    public void onEnable() {
        ProtocolLibrary.getProtocolManager().addPacketListener(
          new PacketAdapter(this, ConnectionSide.SERVER_SIDE, Packets.Server.ENTITY_METADATA) {
            @Override
            public void onPacketSending(PacketEvent event) {
                PacketContainer packet = event.getPacket();
                
                // See if we are modifying an item stack
                if (packet.getEntityModifier(event).read(0) instanceof Item) {
                    WrappedDataWatcher watcher = new WrappedDataWatcher(packet.getWatchableCollectionModifier().read(0));
                    ItemStack stack = watcher.getItemStack(10);
                    
                    if (stack != null) {
                        // You have to clone the watcher, unfortunately
                        watcher = watcher.deepClone();
                        watcher.removeObject(10); // Not needed after build #148
                        watcher.setObject(10, processItemStack(stack));
                        
                        // Save the modification
                        packet.getWatchableCollectionModifier().write(0, watcher.getWatchableObjects());
                    }
                }
            }
        });
    }
    
    private ItemStack processItemStack(ItemStack stack) {
        // Otherwise you'll modify the stack on the server
        ItemStack cloned = stack.clone();
    
        cloned.setType(Material.STONE);
        return cloned;
    }
}