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.

## 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](https://adoptium.net/) has builds for most operating systems.
  - Paper requires JDK 21 to build, however, makes use of Gradle's
    [Toolchains](https://docs.gradle.org/current/userguide/toolchains.html)
    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](#patching-and-building-is-really-slow-what-can-i-do).

If you're compiling with Docker, you can use Adoptium's
[`eclipse-temurin`](https://hub.docker.com/_/eclipse-temurin/) images like so:

```console
# 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

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

- `sources` - Per-file patches to individual 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 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.
**Only changes made in `paper-server/src/vanilla` have to deal with the patch system.**

`paper-server/src/vanilla` is not a git repositories in the traditional sense. Its
initial commits are the decompiled and deobfuscated Vanilla 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) Vanilla patches

This is generally what you need to do when editing Vanilla files. Updating our
per-file patches is as easy as making your changes and then running
# TODO
in the root directory. If nothing went wrong, you can rebuild patches with
`./gradlew rebuildPatches` and finally commit and PR the patch changes.

### 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.

Adding such patches is very simple:

1. Modify `paper-server/src/vanilla` with the appropriate changes;
1. Type `git add .` inside these directories to add your changes;
1. Run `git commit` with the desired patch message;
1. Run `./gradlew rebuildPatches` in the main directory to convert your commit into a new
patch;
1. 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 larger feature patches

Modifying existing feature patches is slightly 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.
> 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.
1. Type `git rebase -i base`;
   - It should show something like
   [this](https://gist.github.com/zachbr/21e92993cb99f62ffd7905d7b02f3159) 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.
1. 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.
1. Make the changes you want to make to the patch;
1. Type `git add .` to add your changes;
1. 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.
1. Type `git rebase --continue` to finish rebasing;
1. Type `./gradlew rebuildPatches` in the root directory;
   - This will modify the appropriate patches based on your commits.
1. 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;
1. Make a temporary commit. You don't need to make a message for this;
1. Type `git rebase -i base`, move (cut) your temporary commit and
move it under the line of the patch you wish to modify;
1. Change the `pick` to the appropriate action:
   1. `f`/`fixup`: Merge your changes into the patch without touching the
  message.
   1. `s`/`squash`: Merge your changes into the patch and use your commit message
  and subject.
1. Type `./gradlew rebuildPatches` in the root directory;
   - This will modify the appropriate patches based on your commits.
1. PR your modified patch file(s) back to this repository.

##### Automatic method

1. Make your change while at HEAD;
1. 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"`.
1. 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.
1. Type `./gradlew rebuildPatches` in the root directory;
   - This will modify the appropriate patches based on your commits.
1. 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 switch main && git pull upstream main`.
1. Checkout feature/fix branch and rebase on master: `git checkout patch-branch && git rebase main`.
1. Apply updated patches: `./gradlew applyPatches`.
1. If there are conflicts, fix them.
1. 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.
1. Rebuild patches: `./gradlew rebuildPatches`.
1. Commit modified patches.
1. 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](#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.

- 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:

```java
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 Vanilla class (or if you're editing feature patches), 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.

```java
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](https://jspecify.dev/docs/user-guide/).

**For classes added by upstream**: 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](https://mcforge.readthedocs.io/en/latest/advanced/accesstransformers/#access-modifiers).

### 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 feature 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 Vanilla source directory,
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:

```patch
From 0000000000000000000000000000000000000000 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:
```java
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.
```java
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
```java
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
```java
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](https://docs.gradle.org/current/userguide/declaring_repositories.html#sec:case-for-maven-local)
  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?

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](https://www.microsoft.com/en-us/software-download/windows10) or [Windows 11 Update Assistant](https://www.microsoft.com/en-us/software-download/windows11).

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](#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>
