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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.

Table of contents:

Use a Personal Fork and not Organization
Requirements
Understanding Patches
Adding Patches
Modifying Patches
- Method 1
  - Using the Paper tool
  - Manual method: Stashing
- Method 2 - Fixup commits
  - Manual method
  - Automatic method
PR Policy
Rebasing PRs
Formatting
Patch Notes
Obfuscation Helpers
Configuration files
- PaperConfig example
- PaperWorldConfig example
Frequently Asked Questions

Use a Personal Fork and not 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 it 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 https://github.com/isaacs/github/issues/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);
patch (often package patch);
A Java 8 or later JDK (packages vary, use Google/DuckDuckGo/etc.).
If you need one, you can find them on AdoptOpenJDK.
maven (often package maven; can be found on Apache's site too);
curl (package curl everywhere).

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

If you're compiling with Docker, you can use the adoptopenjdk images like so:

# docker run -it -v "$(pwd)":/data --rm adoptopenjdk:8-jdk-hotspot bash
Pulling image...

root@abcdefg1234:/# javac -version
javac 1.8.0_252

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-MojangAPI - An API for Mojang's Brigadier;
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:

Clone your fork to your local machine;
Type ./paper patch in a terminal to apply the changes from upstream;
cd into Paper-Server for server changes, and Paper-API for API changes.
You can also run ./paper server or ./paper api for these same directories respectively.
You can also run ./paper setup, which allows you to type paper <command> from anywhere in the Paper structure in most cases.

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

Every single commit in Paper-Server/Paper-API is a patch;
origin/master points to a directory similar to Paper-Server/Paper-API but for Paper;
Typing git status should show that we are 10 or 11 commits ahead of master, meaning we have 10 or 11 patches Spigot and CraftBukkit don't.
- If it says something like 212 commits ahead, 207 commits behind, cd into the root directory of the cloned repository and type git fetch to update your upstream. Setting up a remote for the upstream Paper repository might be necessary.

Adding Patches

Adding patches to Paper is very simple:

Modify Paper-Server and/or Paper-API with the appropriate changes;
Type git add . inside these directories to add your changes;
Run git commit with the desired patch message;
Run ./paper rebuild in the main directory to convert your commit into a new patch;
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:

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.

Using the Paper tool

The PaperMC build tool provides a handy command to automatically do this type of patch modification.

Type ./paper edit server or ./paper edit api depending on which project you want to edit;
- 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.
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.
Make the changes you want to make to the patch;
Type ./paper edit continue in the root directory to finish and rebuild patches;
PR your modified patch file(s) back to this repository.

Manual method: Stashing

In case you need something more complex or want more control, these step-by-step instruction do exactly what the above slightly automated system above does.

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.
Type git rebase -i upstream/upstream;
- 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.
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.
Make the changes you want to make to the patch;
Type git add . to add your changes;
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.
Type git rebase --continue to finish rebasing;
Type ./paper rebuild in the root directory;
- This will modify the appropriate patches based on your commits.
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

Make your change while at HEAD;
Make a temporary commit. You don't need to make a message for this;
Type git rebase -i upstream/upstream, move (cut) your temporary commit and move it under the line of the patch you wish to modify;
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.
Type ./paper rebuild in the root directory;
- This will modify the appropriate patches based on your commits.
PR your modified patch file(s) back to this repository.

Automatic method

Make your change while at HEAD;
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".
Rebase with autosquash: git rebase --autosquash -i upstream/upstream. This will automatically move your fixup commit to the right place, and you just need to "save" the changes.
Type ./paper rebuild in the root directory;
- This will modify the appropriate patches based on your commits.
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.

Pull latest changes from upstream's master: git checkout master && git pull upstream master.
Checkout feature/fix branch and rebase on master: git checkout patch-branch && git rebase master.
Apply updated patches: ./paper patch.
If there are conflicts, fix them.
If your PR creates new patches instead of modifying exist 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)
- Run git rebase --interactive upstream/upstream and move the commits to the end.
Rebuild patches: ./paper rebuild.
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.

Multi-line changes start with // Paper start and end with // Paper end;
You can put a comment with an explanation if it isn't obvious, like this: // Paper start - reason.
- The comments should generally be about the reason the change was made, what it was before, or what the change is.
- Multi-line messages should start with // Paper start and use /* Multi line message here */ for the message itself.
One-line changes should have // Paper or // Paper - reason.

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.a();
entity.b();
// 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

We generally follow 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.

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
tempermental 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

In an effort to make future updates easier on ourselves, Paper tries to use obfuscation helpers whenever possible. The purpose of these helpers is to make the code more readable and maintainable. These helpers should be be made as easy to inline as possible by the JVM whenever possible.

An obfuscation helper to access an obfuscated item may be as simple as something like this:

public final int getStuckArrows() { return this.bY(); } // Paper - OBFHELPER

Or it may be as complex as forwarding an entire method so that it can be overridden later:

public boolean be() {
    // Paper start - OBFHELPER
    return this.pushedByWater();
}

public boolean pushedByWater() {
    // Paper end
    return true;
}

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 entry in either the PaperConfig or PaperWorldConfig classes. Use PaperConfig 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.

PaperConfig example

public static boolean saveEmptyScoreboardTeams = false;
private static void saveEmptyScoreboardTeams() {
    // This is called automatically!
    // The name also doesn't matter.
    saveEmptyScoreboardTeams = getBoolean("settings.save-empty-scoreboard-teams", false);
}

Notice that the field is always public, but the setter is always private. This is important to the way the configuration generation system works. To access this value, reference it as you would any other static value:

if (!PaperConfig.saveEmptyScoreboardTeams) {

It is often preferred that you use the fully qualified name for the configuration class when accessing it, like so: com.destroystokyo.paper.PaperConfig.valueHere.
If this is not done, a developer for Paper might fix that for you before merging, but it's always nice if you make it a habit where you only need 1-2 lines changed.

PaperWorldConfig example

public boolean useInhabitedTime = true;
private void useInhabitedTime() {
    // This is called automatically!
    // The name also doesn't matter.
    useInhabitedTime = getBoolean("use-chunk-inhabited-timer", true);
}

Again, notice that the field is always public, but the setter is always private. To access this value, you'll need an instance of the net.minecraft.World object:

return this.world.paperConfig.useInhabitedTime ? this.w : 0;

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.

Save (rebuild) any patches you are in the middle of working on! Their progress will be lost if you do not;
Identify the name(s) of the file(s) you want to import.
- A complete list of all possible file names can be found at ./work/Minecraft/$MCVER/spigot/net/minecraft/server. You might find MiniMappingViewer very useful to find the file you need.
Open the file at ./scripts/importmcdev.sh and add the name of your file to the script. Skip to the 2nd last header and follow the instructions there;
Re-patch the server ./paper patch;
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 importmcdev.sh.

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 scripts/importmcdev.sh.

Where can I learn how to name method/field?

For most cases, it is preferred if you use yarn, as their license works with Paper's license. If you can't do that, MiniMappingViewer is always around to provide you with more of a general idea, but you cannot use the Mojang names for more than understanding the code.

yarn is in general more thorough than Mojang's own mappings, as they include method arguments as well, whereas Mojang's do not. If you need local variables to understand the code, you might be more lucky with ModCoderPack.

For more information on the Mojang name licensing issues, check this out: https://cpw.github.io/MinecraftMappingData.html

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 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 out of date, update your system with the Windows Update Assistant.

To setup WSL 2, follow the information here: https://docs.microsoft.com/en-us/windows/wsl/install-win10

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://www.howtogeek.com/426749/how-to-access-your-linux-wsl-files-in-windows-10/

I wrote some API, how do I use it in Paper-Server?

To install the API to your local maven repository, do the following:

Enter the API directory by running ./paper api;
Run mvn install.
- If you are working on a patch without much care for whether the tests pass, you can instead run mvn install -DskipTests. Do not PR changes without running tests first.
- If a test failed, you have to identify the failing tests by scrolling up a couple lines (i.e. around 50-200). You should find it fairly quickly.
- 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.

You can now use the API in your plugin to test it before PRing. You will also need to do this to build the Server with the implemented API.

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