> For the complete documentation index, see [llms.txt](https://openjs.wiki.gd/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://openjs.wiki.gd/openjs-components/interacting-with-external-plugins/protocollib-integration.md).

# ProtocolLib integration

[**ProtocolLib**](https://www.spigotmc.org/resources/1997/) is a powerful Minecraft plugin that allows other plugins to **intercept, read, and modify packets** sent between the **server and client**.

While it's technically possible to access ProtocolLib directly via raw Javascript in scripts, this is **not recommended**. Unofficial use results in:

* Memory leaks
* Script instability
* Limitations due to Java-JS translation

OpenJS provides **official support** for ProtocolLib, exposing a clean and stable API.

***

{% hint style="warning" %}

### Enabling ProtocolLib Support

At the **very top of your script**, include the following [feature flag](/openjs-components/fflags.md)

```javascript
//!ProtocolLib
```

**Without this flag, the ProtocolLib API is not accessible in the script.**
{% endhint %}

***

### API Overview

#### `ProtocolLib.registerListener(priority, handler, packetTypes)`

Registers a new packet listener.

* `priority`: `"LOW"`, `"NORMAL"`, `"HIGH"`, or `"MONITOR"`
* `handler`: JavaScript object containing `onSend(event)` and/or `onReceive(event)`
* `packetTypes`: Array of packet names, e.g., `"Play.Server.BLOCK_CHANGE"`

**Returns:** a `listener` object

```js
listener = ProtocolLib.registerListener("NORMAL", {
  onSend(event) {
    const packet = event.getPacket();
    // handle packet here
  }
}, ["Play.Server.BLOCK_CHANGE"]);
```

{% hint style="info" %}
**Additional info on `packetTypes:`** 

The `packetType` is specified relative to the `com.comphenix.protocol.PacketType` class. For example, `Play.Server.BLOCK_CHANGE` corresponds to `com.comphenix.protocol.PacketType.Play.Server.BLOCK_CHANGE`. All provided packet types follow this structure and are based on their original class path.
{% endhint %}

***

#### `ProtocolLib.unregisterListener(listener)`

Unregisters a previously registered packet listener.

```js
ProtocolLib.unregisterListener(listener);
```

***

### Packet Event Object

The `event` passed into your handler (`onSend` / `onReceive`) provides access to:

| Method            | Description                                           |
| ----------------- | ----------------------------------------------------- |
| `getPacket()`     | Returns the actual ProtocolLib packet                 |
| `getPacketType()` | Returns packet type as enum (e.g. `Play.Server.CHAT`) |

You can use:

* `read(index)` / `write(index, value)` to access fields
* Packet-specific methods like `getBlockData()` or `getBlockDataArrays()`

***

### Example Script: Block Spoofing

This script replaces all outgoing non-air blocks with a spoofed `OAK_LOG` block:

```js
//!import org.bukkit.Material
//!import com.comphenix.protocol.wrappers.WrappedBlockData
//!ProtocolLib

const Material = org.bukkit.Material;
const createDataMethod = getMethod(WrappedBlockData, "createData", ["org.bukkit.Material"]);

let SpoofedBlock = createDataMethod.invoke(null, [Material.OAK_LOG]);

const handler = {
  onSend(event) {
    const packet = event.getPacket();
    const type = event.getPacketType().name();

    if (type === "BLOCK_CHANGE") {
      const blockData = packet.getBlockData().read(0);
      if (!blockData.getType().equals(Material.AIR)) {
        packet.getBlockData().write(0, SpoofedBlock);
      }
    }

    if (type === "MULTI_BLOCK_CHANGE") {
      let blocks = packet.getBlockDataArrays().read(0);
      for (let i = 0; i < blocks.length; i++) {
        if (!blocks[i].getType().equals(Material.AIR)) {
          blocks[i] = SpoofedBlock;
        }
      }
      packet.getBlockDataArrays().write(0, blocks);
    }
  }
};

const listener = ProtocolLib.registerListener("NORMAL", handler, [
  "Play.Server.BLOCK_CHANGE",
  "Play.Server.MULTI_BLOCK_CHANGE"
]);
```

***

### Best Practices

* Unregister unused listeners with `ProtocolLib.unregisterListener(...)`
* Avoid modifying packets in `"MONITOR"` priority (read-only phase)
* Use `getMethod()` and `invoke()` carefully for wrapper class access

***

### Related Links

* [Feature Flags Reference](/openjs-components/fflags.md) – Enable/disable APIs via script headers
* [Event Handling](/openjs-components/events/events.md) – Learn how to listen to Bukkit and custom events


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