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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. This requires us to manually merge your PR, resulting in us closing the PR instead of marking it as merged.

We much prefer to have PRs show as merged, so please do not use repositories on organizations for PRs.

See isaacs/github#1681 for more information on the issue.

Requirements

To get started with PRing 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:21.0.3_9-jdk bash
Pulling image...

root@abcdefg1234:/# javac -version
javac 21.0.3

Understanding Patches

Paper is mostly patches and extensions to Spigot. These patches/extensions are split into different directories which target certain parts of the code. These directories are:

  • Paper-API - Modifications to Spigot-API/Bukkit;
  • Paper-Server - Modifications to Spigot/CraftBukkit.

Because the 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 changes from upstream. On Windows, replace the ./ with .\ at the beginning for all gradlew commands;
  3. cd into Paper-Server for server changes, and Paper-API for API changes.

Paper-Server and Paper-API aren't git repositories in the traditional sense:

  • base points to the unmodified source before Paper patches have been applied.
  • Each commit after base is a patch.

Adding Patches

Adding patches to Paper is very simple:

  1. Modify Paper-Server and/or Paper-API with the appropriate changes;
  2. Type git add . inside these directories to add your changes;
  3. Run git commit with the desired patch message;
  4. Run ./gradlew rebuildPatches in the main directory to convert your commit into a new patch;
  5. PR the generated patch file(s) back to this repository.

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 Patches

Modifying previous patches is a bit more complex. Similar to adding patches, the methods to modify a patch are applied inside the Paper-Server and/or Paper-API folders.

Method 1

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

❗ While in the middle of an edit, you will not be able to compile unless you also reset the opposing module(s) to a related commit. In the API's case, you must reset the Server, and reset the API if you're editing the Server. Note also that either module may not compile when doing so. This is not ideal nor intentional, but it happens. Feel free to fix this in a PR to us!

  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. Type 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, and "save" the changes;
    • Only do this for one commit at a time.
  4. Make the changes you want to make to the patch;
  5. Type git add . to add your changes;
  6. Type git commit --amend to commit;
    • Make sure to add --amend or else a new patch will be created.
    • You can also modify the commit message and author here.
  7. Type git rebase --continue to finish rebasing;
  8. Type ./gradlew rebuildPatches in the root directory;
    • This will modify the appropriate patches based on your commits.
  9. PR your modified patch file(s) back to this repository.

Method 2 - Fixup commits

If you are simply editing a more recent commit or your change is small, simply making the change at HEAD and then moving the commit after you have tested it may be easier.

This method has the benefit of being able to compile to test your change without messing with your HEADs.

Manual method

  1. Make your change while at HEAD;
  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. Type ./gradlew rebuildPatches in the root directory;
    • This will modify the appropriate patches based on your commits.
  6. PR your modified patch file(s) back to this repository.

Automatic method

  1. Make your change while at HEAD;
  2. Make a fixup commit. git commit -a --fixup <hashOfPatchToFix>;
    • 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. Type ./gradlew rebuildPatches in the root directory;
    • This will modify the appropriate patches based on your commits.
  5. PR your modified patch file(s) back to this repository.

Rebasing PRs

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

  1. Pull the latest changes from upstreams master: git checkout master && git pull upstream master.
  2. Checkout feature/fix branch and rebase on master: git checkout patch-branch && git rebase master.
  3. Apply updated patches: ./gradlew applyPatches.
  4. If there are conflicts, fix them.
    • If there are conflicts within Paper-API, after fixing them run ./gradlew rebuildApiPatches before re-running ./gradlew applyPatches.
    • The API patches are applied first, so if there are conflicts in the API patches, the server patches will not be applied.
  5. If your PR creates new patches instead of modifying existing ones, in both the Paper-Server and Paper-API directories, 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.

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 non-Paper files should be marked. The one exception to this is when modifying javadoc comments, which should not have these markers.

  • 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.

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

entity.getWorld().dontBeStupid(); // Paper - Was beStupid(), which is bad
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 heavily 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 class in a file not created by the current patch, 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 its only used a couple of times, the FQN is preferred to prevent future patch conflicts in the import section of the file.

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 classes added by upstream: Keep using both @Nullable and @NotNull from org.jetbrains.annotations. These will be replaced later.

import org.bukkit.event.Event;
// don't add import here, use FQN like below

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

Access Transformers

Sometimes, vanilla or CraftBukkit 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.

Important

ATs should be included in the patch file which requires them within the commit message. Do not commit any changes to the build-data/paper.at file, just use it to initially change the visibility of members until you have finalized what you need. Then, in the commit message for the patch which requires the ATs, add a header at the bottom of the commit message before any co-authors. It should look like the following after you ./gradlew rebuildPatches.

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Jake Potrebic <jake.m.potrebic@gmail.com>
Date: Wed, 8 Jun 2022 22:20:16 -0700
Subject: [PATCH] Paper config files

This patch adds Paper configuration files.
Access transformers for this patch are below, but before the co-authors.

== AT ==
public org.spigotmc.SpigotWorldConfig getBoolean(Ljava/lang/String;Z)Z
public net.minecraft.world.level.NaturalSpawner SPAWNING_CATEGORIES

Co-authored-by: Jason Penilla <11360596+jpenilla@users.noreply.github.com>

diff --git a/build.gradle.kts b/build.gradle.kts
...

Patch Notes

When submitting patches to Paper, we may ask you to add notes to the patch header. While we do not require it for all changes, you should add patch notes when the changes you're making are technical, complex, or require an explanation of some kind. It is very likely that your patch will remain long after we've all forgotten about the details of your PR; patch notes will help us maintain it without having to dig back through GitHub history looking for your PR.

These notes should express the intent of your patch, as well as any pertinent technical details we should keep in mind long-term. Ultimately, they exist to make it easier for us to maintain the patch across major version changes.

If you add a message to your commit in the Paper-Server/Paper-API directories, the rebuild patches script will handle these patch notes automatically as part of generating the patch file. If you are not extremely careful, you should always just squash or amend a patch (see the above sections on modifying patches) and rebuild.

Editing messages and patches by hand is possible, but you should patch and rebuild afterwards to make sure you did it correctly. This is slower than just modifying the patches properly after a few times, so you will not really gain anything but headaches from doing it by hand.

Underneath is an example patch header/note:

From 02abc033533f70ef3165a97bfda3f5c2fa58633a Mon Sep 17 00:00:00 2001
From: Shane Freeder <theboyetronic@gmail.com>
Date: Sun, 15 Oct 2017 00:29:07 +0100
Subject: [PATCH] revert serverside behavior of keepalives

This patch intends to bump up the time that a client has to reply to the
server back to 30 seconds as per pre 1.12.2, which allowed clients
more than enough time to reply potentially allowing them to be less
temperamental due to lag spikes on the network thread, e.g. that caused
by plugins that are interacting with netty.

We also add a system property to allow people to tweak how long the server
will wait for a reply. There is a compromise here between lower and higher
values, lower values will mean that dead connections can be closed sooner,
whereas higher values will make this less sensitive to issues such as spikes
from networking or during connections flood of chunk packets on slower clients,
 at the cost of dead connections being kept open for longer.

diff --git a/src/main/java/net/minecraft/server/PlayerConnection.java b/src/main/java/net/minecraft/server/PlayerConnection.java
index a92bf8967..d0ab87d0f 100644
--- a/src/main/java/net/minecraft/server/PlayerConnection.java
+++ b/src/main/java/net/minecraft/server/PlayerConnection.java

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;

Committing changes

All changes to the GlobalConfiguration and WorldConfiguration files should be done in the commit that created them. So do an interactive rebase or fixup to apply just those changes to that commit, then add a new commit that includes the logic that uses that option in the server somewhere.

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

I can't find the NMS file I need!

By default, Paper (and upstream) only import files we make changes to. If you would like to make changes to a file that isn't present in Paper-Server's source directory, you just need to add it to our import script ran during the patching process.

  1. Save (rebuild) any patches you are in the middle of working on! Their progress will be lost if you do not;
  2. Identify the name(s) of the file(s) you want to import.
    • A complete list of all possible file names can be found at ./Paper-Server/.gradle/caches/paperweight/mc-dev-sources/net/minecraft/. You might find MappingViewer useful if you need to translate between Mojang and Spigot mapped names.
  3. Open the file at ./build-data/dev-imports.txt and add the name of your file to the script. Follow the instructions there;
  4. Re-patch the server ./gradlew applyPatches;
  5. Edit away!

❗ This change is temporary! DO NOT COMMIT CHANGES TO THIS FILE!
Once you have made your changes to the new file, and rebuilt patches, you may undo your changes to dev-imports.txt.

Any file modified in a patch file gets automatically imported, so you only need this temporarily to import it to create the first patch.

To undo your changes to the file, type git checkout build-data/dev-imports.txt.

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

Well, 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