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Contributing to Paper

PaperMC is happy you're willing to contribute to our projects. We are usually very lenient with all submitted PRs, but there are still some guidelines you can follow to make the approval process go more smoothly.

Use a Personal Fork and not an Organization

Paper will routinely modify your PR, whether it's a quick rebase or to take care of any minor nitpicks we might have. Often, it's better for us to solve these problems for you than make you go back and forth trying to fix them yourself.

Unfortunately, if you use an organization for your PR, it prevents Paper from modifying it. To avoid this, please do not use repositories on organizations for PRs.

Requirements

To get started with making changes, you'll need the following software, most of which can be obtained in (most) package managers such as apt (Debian / Ubuntu; you will most likely use this for WSL), homebrew (macOS / Linux), and more:

  • git (package git everywhere);
  • A Java 21 or later JDK (packages vary, use Google/DuckDuckGo/etc.).
    • Adoptium has builds for most operating systems.
    • Paper requires JDK 21 to build, however, makes use of Gradle's Toolchains feature to allow building with only JRE 11 or later installed. (Gradle will automatically provision JDK 21 for compilation if it cannot find an existing install).

If you're on Windows, check the section on WSL.

If you're compiling with Docker, you can use Adoptium's eclipse-temurin images like so:

# docker run -it -v "$(pwd)":/data --rm eclipse-temurin:21.0.5_11-jdk bash
Pulling image...

root@abcdefg1234:/# javac -version
javac 21.0.5

Understanding Patches

Unlike the API and its implementation, modifications to Minecraft source files are done through patches. These patches/extensions are split into different three different sets, which are:

  • sources: Per-file patches to Minecraft classes;
  • resources: Per-file patches to Minecraft data files;
  • features: Larger feature patches that modify multiple Minecraft classes.

Because this entire structure is based on patches and git, a basic understanding of how to use git is required. A basic tutorial can be found here: https://git-scm.com/docs/gittutorial.

Assuming you have already forked the repository:

  1. Clone your fork to your local machine;
  2. Type ./gradlew applyPatches in a terminal to apply the patches. On Windows, remove the ./ the beginning of gradlew commands;
  3. cd into paper-server for server changes, and paper-api for API changes. Only changes made in paper-server/src/minecraft have to deal with the patch system.

paper-server/src/minecraft is not a git repositories in the traditional sense. Its initial commits are the decompiled and deobfuscated Minecraft source files. The per-file patches are applied on top of these files as a single, large commit, which is then followed by the individual feature-patch commits.

Modifying (per-file) Minecraft patches

This is generally what you need to do when editing Minecraft files. Updating our per-file patches is as easy as:

  1. Making your changes;
  2. Running ./gradlew fixupSourcePatches in the root directory;
  3. If nothing went wrong, rebuilding patches with ./gradlew rebuildPatches;

Resolving rebase conflicts

If you run into conflicts while running fixupSourcePatches, you need to go a more manual route:

This method works by temporarily resetting your HEAD to the desired commit to edit it using git rebase.

  1. If you have changes you are working on, type git stash to store them for later;
    • You can type git stash pop to get them back at any point.
  2. cd into paper-server/src/minecraft/java and run git rebase -i base;
    • It should show something like this in the text editor you get.
    • If your editor does not have a "menu" at the bottom, you're using vim.
      If you don't know how to use vim and don't want to learn, enter :q! and press enter. Before redoing this step, do export EDITOR=nano for an easier editor to use.
  3. Replace pick with edit for the commit/patch you want to modify (in this case the very first commit, paper File Patches), and "save" the changes;
  4. Make the changes you want to make to the patch;
  5. Run git add . to add your changes;
  6. Run git commit --amend to commit;
  7. Run git rebase --continue to finish rebasing;
  8. Run ./gradlew rebuildPatches in the root directory;

Adding larger feature patches

Feature patches are exclusively used for large-scale changes that are hard to track and maintain and that can be optionally dropped, such as the more involved optimizations we have. This makes it easier to update Paper during Minecraft updates, since we can temporarily drop these patches and reapply them later.

There is only a very small chance that you will have to use this system, but adding such patches is very simple:

  1. Modify paper-server/src/minecraft with the appropriate changes;
  2. Run git add . inside that directory to add your changes;
  3. Run git commit with the desired patch message;
  4. Run ./gradlew rebuildPatches in the root directory.

Your commit will be converted into a patch that you can then PR into Paper.

❗ Please note that if you have some specific implementation detail you'd like to document, you should do so in the patch message or in comments.

Modifying larger feature patches

One way of modifying feature patches is to reset to the patch commit and follow the instructions from the rebase section. If you are sure there won't be any conflicts from later patches, you can also use the fixup method.

Fixup method

Manual method

  1. Make your changes;
  2. Make a temporary commit. You don't need to make a message for this;
  3. Type git rebase -i base, move (cut) your temporary commit and move it under the line of the patch you wish to modify;
  4. Change the pick to the appropriate action:
    1. f/fixup: Merge your changes into the patch without touching the message.
    2. s/squash: Merge your changes into the patch and use your commit message and subject.
  5. Run ./gradlew rebuildPatches in the root directory;
    • This will modify the appropriate patches based on your commits.

Automatic method

  1. Make your changes;
  2. Make a fixup commit: git commit -a --fixup <hash of patch to fix>;
    • If you want to modify a per-file patch, use git commit -a --fixup file
    • You can also use --squash instead of --fixup if you want the commit message to also be changed.
    • You can get the hash by looking at git log or git blame; your IDE can assist you too.
    • Alternatively, if you only know the name of the patch, you can do git commit -a --fixup "Subject of Patch name".
  3. Rebase with autosquash: git rebase -i --autosquash base. This will automatically move your fixup commit to the right place, and you just need to "save" the changes.
  4. Run ./gradlew rebuildPatches in the root directory. This will modify the appropriate patches based on your commits.

Rebasing PRs

Steps to rebase a PR to include the latest changes from main.
These steps assume the origin remote is your fork of this repository and upstream is the official PaperMC repository.

  1. Fetch the latest changes from upstream's main: git fetch upstream.
  2. Checkout your feature/fix branch and rebase on main: git switch patch-branch && git rebase upstream/main.
  3. Apply updated patches: ./gradlew applyPatches.
  4. If there are conflicts, fix them.
  5. If your PR creates new feature patches instead of modifying existing ones, ensure your newly-created patch is the last commit by either:
    • Renaming the patch file with a large 4-digit number in front (e.g. 9999-Patch-to-add-some-new-stuff.patch), and re-applying patches.
    • Running git rebase --interactive base and moving the commits to the end.
  6. Rebuild patches: ./gradlew rebuildPatches.
  7. Commit modified patches.
  8. Force push changes: git push --force. Make sure you're not deleting any of your commits or changes here!

PR Policy

We'll accept changes that make sense. You should be able to justify their existence, along with any maintenance costs that come with them. Using obfuscation helpers aids in the maintenance costs. Remember that these changes will affect everyone who runs Paper, not just you and your server.

While we will fix minor formatting issues, you should stick to the guide below when making and submitting changes.

Formatting

All modifications to Vanilla files should be marked. For historical reasons, API and API-implementation contain a lot of these too, but they are no longer required.

  • You need to add a comment with a short and identifiable description of the patch: // Paper start - <COMMIT DESCRIPTION>
    • The comments should generally be about the reason the change was made, what it was before, or what the change is.
    • After the general commit description, you can add additional information either after a ; or in the next line.
  • Multi-line changes start with // Paper start - <COMMIT DESCRIPTION> and end with // Paper end - <COMMIT DESCRIPTION>.
  • One-line changes should have // Paper - <COMMIT DESCRIPTION> at the end of the line.

[!NOTE] These comments are incredibly important to be able to keep track of changes across files and to remember what they are for, even a decade into the future.

Here's an example of how to mark changes by Paper:

entity.getWorld().dontBeStupid(); // Paper - Move away from beStupid()
entity.getFriends().forEach(Entity::explode);
entity.updateFriends();

// Paper start - Use plugin-set spawn
// entity.getWorld().explode(entity.getWorld().getSpawn());
Location spawnLocation = ((CraftWorld) entity.getWorld()).getSpawnLocation();
entity.getWorld().explode(new BlockPosition(spawnLocation.getX(), spawnLocation.getY(), spawnLocation.getZ()));
// Paper end - Use plugin-set spawn

We generally follow the usual Java style (aka. Oracle style), or what is programmed into most IDEs and formatters by default. There are a few notes, however:

  • It is fine to go over 80 lines as long as it doesn't hurt readability.
    There are exceptions, especially in Spigot-related files
  • When in doubt or the code around your change is in a clearly different style, use the same style as the surrounding code.
  • Usage of the var keyword is discouraged, as it makes reading patch files a lot harder and can lead to confusion during updates due to changed return types. The only exception to this is if a line would otherwise be way too long/filled with hard to parse generics in a case where the base type itself is already obvious.

Imports

When adding new imports to a Vanilla class, use the fully qualified class name instead of adding a new import to the top of the file. If you are using a type a significant number of times, you can add an import with a comment. However, if it's only used a couple of times, the FQN is preferred to prevent future patch conflicts in the import section of the file.

import net.minecraft.server.MinecraftServer;
// don't add import here, use FQN like below

public class SomeVanillaClass {
    public final org.bukkit.Location newLocation; // Paper - add location
}

Nullability annotations

We are in the process of switching nullability annotation libraries, so you might need to use one or the other:

For classes we add: Fields, method parameters and return types that are nullable should be marked via the @Nullable annotation from org.jspecify.annotations. Whenever you create a new class, add @NullMarked, meaning types are assumed to be non-null by default. For less obvious placing such as on generics or arrays, see the JSpecify docs.

For other classes: Keep using both @Nullable and @NotNull from org.jetbrains.annotations. These will be replaced later.

Access Transformers

Sometimes, Vanilla code already contains a field, method, or type you want to access but the visibility is too low (e.g. a private field in an entity class). Paper can use access transformers to change the visibility or remove the final modifier from fields, methods, and classes. Inside the build-data/paper.at file, you can add ATs that are applied when you ./gradlew applyPatches. You can read about the format of ATs here.

Obfuscation Helpers

While rarely needed, obfuscation helpers are sometimes useful when it comes to unmapped local variables, or poorly named method parameters. In an effort to make future updates easier on ourselves, Paper tries to use obfuscation helpers wherever it makes sense. The purpose of these helpers is to make the code more readable and maintainable. These helpers should be made easy to inline by the JVM wherever possible.

An example of an obfuscation helper for a local variable:

double d0 = entity.getX(); final double fromX = d0; // Paper - OBFHELPER
// ...   
this.someMethod(fromX); // Paper

While they may not always be done in exactly the same way, the general goal is always to improve readability and maintainability. Use your best judgment and do what fits best in your situation.

Configuration files

To use a configurable value in your patch, add a new field in either the GlobalConfiguration or WorldConfiguration classes (inside the io.papermc.paper.configuration package). Use GlobalConfiguration if a value must remain the same throughout all worlds, or the latter if it can change between worlds. World-specific configuration options are preferred whenever possible.

Example

This is adding a new miscellaneous setting that doesn't seem to fit in other categories. Try to check and see if an existing category (inner class) exists that matches whatever configuration option you are adding.

public class GlobalConfiguration {
    // other sections
    public class Misc extends ConfigurationPart {
        // other settings
        public boolean lagCompensateBlockBreaking = true;
        public boolean useDimensionTypeForCustomSpawners = false;
        public int maxNumOfPlayers = 20; // This is the new setting
    }
}

You set the type of the setting as the field type, and the default value is the initial field value. The name of the setting defaults to the snake-case of the field name, so in this case it would be misc.max-num-of-players. You can use the @Setting annotation to override that, but generally just try to set the field name to what you want the setting to be called.

Accessing the value

If you added a new global config value, you can access it in the code just by doing

int maxPlayers = GlobalConfiguration.get().misc.maxNumOfPlayers;

Generally for global config values you will use the fully qualified class name, io.papermc.paper.configuration.GlobalConfiguration since it's not imported in most places.

If you are adding a new world config value, you must have access to an instance of the net.minecraft.world.level.Level which you can then access the config by doing

int maxPlayers = level.paperConfig().misc.maxNumOfPlayers;

Testing API changes

Using the Paper Test Plugin

The Paper project has a test-plugin module for easily testing out API changes and additions. To use the test plugin, enable it in test-plugin.settings.gradle.kts, which will be generated after running Gradle at least once. After this, you can edit the test plugin, and run a server with the plugin using ./gradlew runDev (or any of the other Paper run tasks).

Publishing to Maven local (use in external plugins)

To build and install the Paper APIs and Server to your local Maven repository, do the following:

  • Run ./gradlew publishToMavenLocal in the base directory.

If you use Gradle to build your plugin:

  • Add mavenLocal() as a repository. Gradle checks repositories in the order they are declared, so if you also have the Paper repository added, put the local repository above Paper's.
  • Make sure to remove mavenLocal() when you are done testing, see the Gradle docs for more details.

If you use Maven to build your plugin:

  • If you later need to use the Paper-API, you might want to remove the jar from your local Maven repository.
    If you use Windows and don't usually build using WSL, you might not need to do this.

Frequently Asked Questions

My commit doesn't need a build, what do I do?

Quite simple: You add [ci skip] to the start of your commit subject.

This case most often applies to changes to files like README.md, this very file (CONTRIBUTING.md), the LICENSE.md file, and so forth.

Patching and building is really slow, what can I do?

This only applies if you're running Windows. If you're running a prior Windows release, either update to Windows 10/11 or move to macOS/Linux/BSD.

In order to speed up patching process on Windows, it's recommended you get WSL 2. This is available in Windows 10 v2004, build 19041 or higher. (You can check your version by running winver in the run window (Windows key + R)). If you're using an out of date version of Windows 10, update your system with the Windows 10 Update Assistant or Windows 11 Update Assistant.

To set up WSL 2, follow the information here: https://docs.microsoft.com/en-us/windows/wsl/install

You will most likely want to use the Ubuntu apps. Once it's set up, install the required tools with sudo apt-get update && sudo apt-get install $TOOL_NAMES -y. Replace $TOOL_NAMES with the packages found in the requirements. You can now clone the repository and do everything like usual.

❗ Do not use the /mnt/ directory in WSL! Instead, mount the WSL directories in Windows like described here: https://docs.microsoft.com/en-us/windows/wsl/filesystems#view-your-current-directory-in-windows-file-explorer