Velocity Plugin Basics

Now we will lay the groundwork for your first plugin. We will cover how to create your very first Velocity plugin.

Create the plugin class

Create a new class (let's say com.example.velocityplugin.VelocityTest) and paste this in:

package com.example.velocityplugin;

import com.google.inject.Inject;
import com.velocitypowered.api.plugin.Plugin;
import com.velocitypowered.api.proxy.ProxyServer;
import org.slf4j.Logger;

@Plugin(id = "myfirstplugin", name = "My First Plugin", version = "0.1.0-SNAPSHOT",
        url = "https://example.org", description = "I did it!", authors = {"Me"})
public class VelocityTest {

    private final ProxyServer server;
    private final Logger logger;

    @Inject
    public VelocityTest(ProxyServer server, Logger logger) {
        this.server = server;
        this.logger = logger;

        logger.info("Hello there! I made my first plugin with Velocity.");
    }
}

What did you just do there? There's quite a bit to unpack, so let's focus on the Velocity-specific bits:

@Plugin(id = "myfirstplugin", name = "My First Plugin", version = "0.1.0-SNAPSHOT",
        url = "awesome.org", description = "I did it!", authors = {"Me"})
public class VelocityTest {

This tells Velocity that this class contains your plugin (myfirstplugin) so that it can be loaded once the proxy starts up. Velocity will detect where the plugin will reside when you compile your plugin.

Moving on, what's this?

@Inject
public VelocityTest(ProxyServer server, Logger logger) {
    this.server = server;
    this.logger = logger;

    logger.info("Hello there, it's a test plugin I made!");
}

This looks like magic! How is Velocity doing this? The answer lies in the @Inject, which indicates that Velocity should inject a ProxyServer and the Logger when constructing your plugin. These two interfaces will help you out as you begin working with Velocity. We won't talk too much about dependency injection: all you need to know is that Velocity will do this.

All you need to do is build your plugin, put it in your plugins/ directory, and try it! Isn't that nice? In the next section you'll learn more about how to use the API.

Choosing @Plugin Information

Choose your plugin's ID wisely. Other plugins will use this ID to depend on yours. If you change it, you could break compatibility.

The plugin name is somewhat less important. It will be shown to users as the display name of your plugin, but tweaking it will not be catastrophic.

For the version, we recommend sticking to semantic versioning - you can read more about this concept at semver.org. Basically, use 3 numbers in your version, such as 1.4.25. Increment the major number when you make a backwards-incompatible breaking change, increment the minor number when you add functionality that is backwards compatible, and increment the patch number when you fix a bug or make an otherwise unnoticeable change in the implementation.

You can also describe your plugin's URL, authors, and description in your @Plugin annotation. Plugin dependencies are also be specified there, but we'll get to that later.

A word of caution

In Velocity, plugin loading is split into two steps: construction and initialization. The code in your plugin's constructor is part of the construction phase. There is very little you can do safely during construction, especially as the API does not specify which operations are safe to run during construction. Notably, you can't register an event listener in your constructor, because you need to have a valid plugin registration, but Velocity can't register the plugin until the plugin has been constructed, causing a "chicken or the egg" problem.

To break this vicious cycle, you should always wait for initialization, which is indicated when Velocity fires the ProxyInitializeEvent. We can do things on initialization by adding a listener for this event, as shown below. Note that Velocity automatically registers your plugin main class as a listener.

@Subscribe
public void onProxyInitialization(ProxyInitializeEvent event) {
    // Do some operation demanding access to the Velocity API here.
    // For instance, we could register an event:
    server.getEventManager().register(this, new PluginListener());
}

Getting your Plugin's Directory

At some point you may need your plugin's directory. To do this, add @DataDirectory Path dataDirectory to your plugin's constructor parameters:

private final Path dataDirectory;

@Inject
public VelocityTest(ProxyServer server, Logger logger, @DataDirectory Path dataDirectory) {
    this.server = server;
    this.logger = logger;
    this.dataDirectory = dataDirectory;
}

This will get you a java.nio.file.Path of your plugin directory. If you absolutely need a java.io.File, you may use Path#toFile(). However, Velocity usually works with Path.