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.

Currently maintained by dmulloy2 on behalf of Spigot.

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ProtocolLib is built with Maven and requires Spigot and SpigotAPI, which can be found here.

ProtocolLib attempts to solve this problem by providing a event API, much like Bukkit, that allows 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!

To use this library, first add ProtocolLib.jar to your Java build path. Then, add ProtocolLib as a dependency or soft dependency to your plugin.yml file like any other plugin:

depend: [ProtocolLib]

You can also add ProtocolLib as a Maven dependency:

<repositories>
  <repository>
    <id>dmulloy2-repo</id>
    <url>http://repo.dmulloy2.net/nexus/repository/public/</url>
  </repository>
  ...
</repositories>

<dependencies>
  <dependency>
    <groupId>com.comphenix.protocol</groupId>
    <artifactId>ProtocolLib-API</artifactId>
    <version>4.4.0</version>
  </dependency>
</dependencies>

Or use the maven dependency with gradle:

repositories {
    maven { url "http://repo.dmulloy2.net/nexus/repository/public/" }
}

dependencies {
    compileOnly group: "com.comphenix.protocol", name: "ProtocolLib", version: "4.4.0";
}

Then get a reference to ProtocolManager in onLoad() or onEnable() 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, ListenerPriority.NORMAL, 
          PacketType.Play.Server.NAMED_SOUND_EFFECT) {
    @Override
    public void onPacketSending(PacketEvent event) {
        // Item packets (id: 0x29)
        if (event.getPacketType() == 
                PacketType.Play.Server.NAMED_SOUND_EFFECT) {
            event.setCancelled(true);
        }
    }
});

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,
        ListenerPriority.NORMAL, 
        PacketType.Play.Client.CHAT) {
    @Override
    public void onPacketReceiving(PacketEvent event) {
        if (event.getPacketType() == PacketType.Play.Client.CHAT) {
            PacketContainer packet = event.getPacket();
            String message = packet.getStrings().read(0);

            if (message.contains("shit")
                    || message.contains("damn")) {
                event.setCancelled(true);
                event.getPlayer().sendMessage("Bad manners!");
            }
        }
    }
});

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 with this version:

PacketContainer fakeExplosion = new PacketContainer(PacketType.Play.Server.EXPLOSION);
fakeExplosion.getDoubles().
    write(0, player.getLocation().getX()).
    write(1, player.getLocation().getY()).
    write(2, player.getLocation().getZ());
fakeExplosion.getFloat().write(0, 3.0F);

try {
    protocolManager.sendServerPacket(player, fakeExplosion);
} catch (InvocationTargetException e) {
    throw new RuntimeException(
        "Cannot send packet " + fakeExplosion, e);
}

One of the main goals of this project was to achieve maximum compatibility with CraftBukkit. And the end result is quite flexible. Aside from netty package changes, it should be resilient against future changes. It's likely that I won't have to update ProtocolLib for anything but bug fixes and new features.

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.