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Adding New Classes
Phabricator Contributor Documentation (Developer Guides)

Guide to adding new classes to extend Phabricator.


Phabricator is highly modular, and many parts of it can be extended by adding new classes. This document explains how to write new classes to change or expand the behavior of Phabricator.

IMPORTANT: The upstream does not offer support with extension development.


Phabricator primarily discovers functionality by looking at concrete subclasses of some base class. For example, Phabricator determines which applications are available by looking at all of the subclasses of PhabricatorApplication. It discovers available workflows in arc by looking at all of the subclasses of ArcanistWorkflow. It discovers available locales by looking at all of the subclasses of PhutilLocale.

This pattern holds in many cases, so you can often add functionality by adding new classes with no other work. Phabricator will automatically discover and integrate the new capabilities or features at runtime.

There are two main ways to add classes:

  • Extensions Directory: This is a simple way to add new code. It is less powerful, but takes a lot less work. This is good for quick changes, testing and development, or getting started on a larger project.
  • Creating Libraries: This is a more advanced and powerful way to organize extension code. This is better for larger or longer-lived projects, or any code which you plan to distribute.

The next sections walk through these approaches in greater detail.

Extensions Directory

The easiest way to extend Phabricator by adding new classes is to drop them into the extensions directory, at phabricator/src/extensions/.

This is intended as a quick way to add small pieces of functionality, test new features, or get started on a larger project. Extending Phabricator like this imposes a small performance penalty compared to using a library.

This directory exists in all libphutil libraries, so you can find similar directories in arcanist/src/extensions/ and libphutil/src/extensions/.

For example, to add a new application, create a file like this one and add it to phabricator/src/extensions/.


final class ExampleApplication extends PhabricatorApplication {

  public function getName() {
    return pht('Example');


If you load Applications in the web UI, you should now see your new application in the list. It won't do anything yet since you haven't defined any interesting behavior, but this is the basic building block of Phabricator extensions.

Creating Libraries

A more powerful (but more complicated) way to extend Phabricator is to create a libphutil library. Libraries can organize a larger amount of code, are easier to work with and distribute, and have slightly better performance than loose source files in the extensions directory.

In general, you'll perform these one-time setup steps to create a library:

  • Create a new directory.
  • Use arc liberate to initialize and name the library.
  • Configure Phabricator or Arcanist to load the library.

Then, to add new code, you do this:

  • Write or update classes.
  • Update the library metadata by running arc liberate again.

Initializing a Library

To create a new libphutil library, create a directory for it and run arc liberate on the directory. This documentation will use a conventional directory layout, which is recommended, but you are free to deviate from this.

$ mkdir libcustom/
$ cd libcustom/
libcustom/ $ arc liberate src/

Now you'll get a prompt like this:

No library currently exists at that path...
The directory '/some/path/libcustom/src' does not exist.

  Do you want to create it? [y/N] y
Creating new libphutil library in '/some/path/libcustom/src'.
Choose a name for the new library.

  What do you want to name this library?

Choose a library name (in this case, "libcustom" would be appropriate) and it you should get some details about the library initialization:

Writing '__phutil_library_init__.php' to
Using library root at 'src'...
Mapping library...
Verifying library...
Finalizing library map...
  OKAY   Library updated.

This will write three files:

  • src/.phutil_module_cache This is a cache which makes "arc liberate" faster when you run it to update the library. You can safely remove it at any time. If you check your library into version control, you can add this file to ignore rules (like .gitignore).
  • src/__phutil_library_init__.php This records the name of the library and tells libphutil that a library exists here.
  • src/__phutil_library_map__.php This is a map of all the symbols (functions and classes) in the library, which allows them to be autoloaded at runtime and dependencies to be statically managed by arc liberate.

Linking with Phabricator

If you aren't using this library with Phabricator (e.g., you are only using it with Arcanist or are building something else on libphutil) you can skip this step.

But, if you intend to use this library with Phabricator, you need to define its dependency on Phabricator by creating a .arcconfig file which points at Phabricator. For example, you might write this file to libcustom/.arcconfig:

  "load": [

For details on creating a .arcconfig, see Arcanist User Guide: Configuring a New Project. In general, this tells arc liberate that it should look for symbols in Phabricator when performing static analysis.

NOTE: If Phabricator isn't located next to your custom library, specify a path which actually points to the phabricator/ directory.

You do not need to declare dependencies on arcanist or libphutil, since arc liberate automatically loads them.

Finally, edit your Phabricator config to tell it to load your library at runtime, by adding it to load-libraries:

'load-libraries' => array(
  'libcustom' => 'libcustom/src/',

Now, Phabricator will be able to load classes from your custom library.

Writing Classes

To actually write classes, create a new module and put code in it:

libcustom/ $ mkdir src/example/
libcustom/ $ nano src/example/ExampleClass.php # Edit some code.

Now, run arc liberate to regenerate the static resource map:

libcustom/ $ arc liberate src/

This will automatically regenerate the static map of the library.

What You Can Extend And Invoke

libphutil, Arcanist and Phabricator are strict about extensibility of classes and visibility of methods and properties. Most classes are marked final, and methods have the minimum required visibility (protected or private). The goal of this strictness is to make it clear what you can safely extend, access, and invoke, so your code will keep working as the upstream changes.

IMPORTANT: We'll still break APIs frequently. The upstream does not support extension development, and none of these APIs are stable.

When developing libraries to work with libphutil, Arcanist and Phabricator, you should respect method and property visibility.

If you want to add features but can't figure out how to do it without changing Phabricator code, here are some approaches you may be able to take:

  • Use Composition: If possible, use composition rather than extension to build your feature.
  • Find Another Approach: Check the documentation for a better way to accomplish what you're trying to do.
  • File a Feature Request: Let us know what your use case is so we can make the class tree more flexible or configurable, or point you at the right way to do whatever you're trying to do, or explain why we don't let you do it. Note that we do not support extension development so you may have mixed luck with this one.

These approaches are discouraged, but also possible:

  • Fork: Create an ad-hoc local fork and remove final in your copy of the code. This will make it more difficult for you to upgrade in the future, although it may be the only real way forward depending on what you're trying to do.
  • Use Reflection: You can use Reflection to remove modifiers at runtime. This is fragile and discouraged, but technically possible.
  • Remove Modifiers: Send us a patch removing final (or turning protected or private into public). We will almost never accept these patches unless there's a very good reason that the current behavior is wrong.

Next Steps

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