ProtocolLib

Certain tasks are impossible to perform with the standard Bukkit API, and may require working with and even modify Minecraft directly. A common technique is to modify incoming and outgoing packets, or inject custom packets into the stream. This is quite cumbersome to do, however, and most implementations will break as soon as a new version of Minecraft has been released, mostly due to obfuscation.

Critically, different plugins that use this approach may hook into the same classes, with unpredictable outcomes. More than often this causes plugins to crash, but it may also lead to more subtle bugs.

Resources

• JavaDoc

A new API

ProtocolLib attempts to solve this problem by providing a event API, much like Bukkit, that allow plugins to monitor, modify or cancel packets sent and received. But more importantly, the API also hides all the gritty, obfuscated classes with a simple index based read/write system. You no longer have to reference CraftBukkit!

Using ProtocolLib

To use the library, first add ProtocolLib.jar to your Java build path. Then, add ProtocolLib as a dependency (or soft-dependency, if you can live without it) to your plugin.yml file:

depends: [ProtocolLib]

Then get a reference to ProtocolManager in onLoad() and you're good to go.

private ProtocolManager protocolManager;

public void onLoad() {
    protocolManager = ProtocolLibrary.getProtocolManager();
}

To listen for packets sent by the server to a client, add a server-side listener:

// Disable all sound effects
protocolManager.addPacketListener(
  new PacketAdapter(this, ConnectionSide.SERVER_SIDE, ListenerPriority.NORMAL, 0x3E) {
    @Override
    public void onPacketSending(PacketEvent event) {
        // Item packets
        switch (event.getPacketID()) {
        case 0x3E: // Sound effect
            event.setCancelled(true);
            break;
        }
    }
});

It's also possible to read and modify the content of these packets. For instance, you can create a global censor by listening for Packet3Chat events:

// Censor
protocolManager.addPacketListener(
  new PacketAdapter(this, ConnectionSide.CLIENT_SIDE, ListenerPriority.NORMAL, 0x03) {
    @Override
    public void onPacketReceiving(PacketEvent event) {
        if (event.getPacketID() == 0x03) {
            try {
                PacketContainer packet = event.getPacket();
                String message = packet.getSpecificModifier(String.class).read(0);
                
                if (message.contains("shit") || message.contains("damn")) {
                    event.setCancelled(true);
                    event.getPlayer().sendMessage("Bad manners!");
                }
                		
            } catch (FieldAccessException e) {
                getLogger().log(Level.SEVERE, "Couldn't access field.", e);
            }
        }
    }
});

Sending packets

Normally, you might have to do something ugly like the following:

Packet60Explosion fakeExplosion = new Packet60Explosion();
	
fakeExplosion.a = player.getLocation().getX();
fakeExplosion.b = player.getLocation().getY();
fakeExplosion.c = player.getLocation().getZ();
fakeExplosion.d = 3.0F;
fakeExplosion.e = new ArrayList<Object>();

((CraftPlayer) player).getHandle().netServerHandler.sendPacket(fakeExplosion);

But with ProtocolLib, you can turn that into something more manageable. Notice that you don't have to create an ArrayList this version:

PacketContainer fakeExplosion = protocolManager.createPacket(60);

fakeExplosion.getSpecificModifier(double.class).
    write(0, player.getLocation().getX()).
    write(1, player.getLocation().getY()).
    write(2, player.getLocation().getZ());
fakeExplosion.getSpecificModifier(float.class).
    write(0, 3.0F);

protocolManager.sendServerPacket(player, fakeExplosion);

Compatiblity

One of the main goals of this project was to achive maximum compatibility with craftBukkit. And the end result is quite flexible - in tests I successfully ran an unmodified ProtocolLib on CraftBukkit 1.8.0, and it should be resiliant against future changes. It's likely that I won't have to update ProtocolLib for anything but bug and performance fixes.

How is this possible? It all comes down to reflection in the end. Essentially, no name is hard coded - every field, method and class is deduced by looking at field types, package names or parameter types. It's remarkably consistent across different versions.

Incompatiblity

The following plugins (to be expanded) are not compatible with ProtocolLib: