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Extensions

Summary:
​Writing your own extension for Minestom​
​How extensions are loaded​
​Dependencies​
​Callback order​
​Extension Isolation​
​Testing in a dev environment​
Writing your own extension for Minestom
To test in a dev environment, see last section.
Start by creating a new extension class:
package testextension;
​
import net.minestom.server.extensions.Extension;
​
public class TestExtension extends Extension {
    @Override
    public void initialize() {
        System.out.println("Hello from extension!");
    }
​
    @Override
    public void terminate() {
​
    }
}
Then, create a extension.json at the root of the resources folder (src/main/resources for instance) and fill it up:
{
  "entrypoint": "testextension.TestExtension",
  "name": "TestExtension",
  "version": "1.0.0",
  "dependencies": [
    "DependentExtension",
    "Extension2"
  ],
  "externalDependencies": {
    "repositories": [
      {"name": "Central", "url": "https://repo1.maven.org/maven2/"}
    ],
    "artifacts": [
      "com.squareup:javapoet:1.13.0"
    ]
  }
}
entrypoint: Fully qualified name of your extension class
name: Name to use to represent the extension to users. Must match regex [A-Za-z][_A-Za-z0-9]+
version: Version of your extension
dependencies (optional): List of extension names required for this extension to work.
externalDependencies (optional): List of external libraries used for this extension (see Dependencies)
How extensions are loaded
This section is purely informational and not required to work on extensions, but it is a good thing to know how extensions are loaded inside Minestom.
1. Discovery
At launch, Minestom inspects the extensions folder (resolved from the current working folder) for jar files. For each file found, it then checks if there is an extension.json file and attempts to parse it. If the file exists, and parsing succeeds, the extension is considered discovered.
Discovery can also be forced when using ExtensionManager#loadDynamicExtension(File) but works the same.
2. Load order generation / Dependency solving
Then, Minestom ensures all required dependencies for the extension are found. For external dependencies, it will download them if necessary. For extension dependencies, it simply checks if they are already loaded, or about to be loaded (because discovered in the current load-cycle).
4. Instanciation and callbacks
The extension is then instanciated from the class provided inside entrypoint, and the preInitialize, initialize and postInitialize callbacks are called. (see Callback order for more information)
Dependencies
Minestom extensions can have two types of dependencies:
1.Extension dependencies
2.External dependencies
Extension dependencies
Extensions can require other extensions to be present at runtime in order to be loaded. This is done via the dependencies array inside extension.json.
Extensions and their dependencies will be loaded in parent-first order: the root extensions of the dependency graph will always be loaded first, then extensions with one dependency, then extensions with two, and so on. If an extension is a dependency of at least two other, it is guaranteed that it will be loaded only once.
External dependencies
Your extension is free to depend on external libraries. For the moment, only maven-accessible libraries are supported.
To declare external dependencies, use the externalDependencies object inside extension.json:
"externalDependencies": {
    "repositories": [
      {"name": "Central", "url": "https://repo1.maven.org/maven2/"}
    ],
    "artifacts": [
      "com.squareup:javapoet:1.13.0"
    ]
  }
repositories is the list of repositories to contact to get the artifacts
name: Name of the repository, used to recognize the repository inside logs
url: URL of the repository to contact
artifacts is the list of Maven coordinates from the dependencies you want to use
Minestom will download and cache the libraries inside extensions/.libs/, so that it does not require to redownload them at each launch.
Callback order
During MinecraftServer#start, Minestom calls preInitialize on all extensions, then initialize on all extensions, and finally postInitialize on all extensions. Minestom does NOT guarantee the loading order of extensions, but it should be deterministic.
Extension Isolation
All extensions are completely isolated from each other by default, this means that you may not load a class from another dependency (without shading it, which has other concerns). If your extension depends on another, it must be specified in the extension.json, and then it will have access to the relevant classes.
Currently, if two extensions depend on the same library they will not share the same instance.
Testing in a dev environment
You may set the following vm arguments to load an extension from the classes and resources on disk (eg in your build directory), as opposed to a packaged jar file.
-Dminestom.extension.indevfolder.classes=<folder to compiled classes of your extension> Specifies the folder in which compiled classes of your extension are. With a default Gradle setup, build/classes/java/main/ should work.
-Dminestom.extension.indevfolder.resources=<folder to resources of your extension> Specifies the folder in which resources of your extension are. With a default Gradle setup, build/resources/main/ should work.
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