#+STARTUP: inlineimages

[[./assets/gradle2nix.png]]

Generate [[https://nixos.org/nix/][Nix]] expressions which build [[https://gradle.org/][Gradle]]-based projects.

** Why?

Nix is an OS-agnostic package manager, a language-agnostic build
system, and a bespoke programming language. One of its unique features
is that it is purely functional; a "package" is a function which
accepts inputs (source code, configuration, etc) and produces an
output (binaries, a Java JAR, documentation, really anything).

One benefit of a functional build system is [[https://reproducible-builds.org/][reproducibility]]. If you
specify your inputs precisely, and take care not to introduce
impurities—such as files retrieved over a network without tracking
their content—you will receive, byte-for-byte, the exact output as
someone else running the same function over the same inputs.

Gradle is not a functional build system. Most Gradle-based projects
will produce highly variable outputs depending on a host of impure
inputs, including:

- The JVM hosting the build
- The Gradle installation running the build
- Any usage of dynamic version constraints for dependencies
- SNAPSHOT dependencies
- Environment variables and command-line options
- Artifacts cached on the system hosting the build

=gradle2nix= helps to solve this problem by leveraging Nix to control
the most common inputs to a Gradle build. When run on a project, it
will record all dependencies for both the build environment (including
=plugins= and =buildscript= blocks) and the project, and provide a Nix
expression to run the build given these dependencies. The build itself
is then run in a sandbox, where only content-tracked network requests
are allowed to fetch dependencies, and a local Maven repository is
created on-the-fly to host the dependency artifacts somewhere Gradle
can resolve them without a network.

This tool is useful for both development and packaging. You can use
=gradle2nix= to:

- Create isolated and reproducible development environments that work
  anywhere Nix itself can run;
- Reduce or eliminate flakiness and maintenance headaches from CI/CD
  pipelines
- Distribute a recipe which can reliably build a Gradle project in
  repositories such as the [[https://nixos.org/nixpkgs/][Nix Package Collection]].

** Installation

A [[./default.nix][Nix expression]] (generated by =gradle2nix= itself) is provided for
convenience. The following expression will fetch and build the latest
version of this package:

#+begin_src nix
import (fetchTarball "https://github.com/tadfisher/gradle2nix/archive/master.tar.gz")  {}
#+end_src

If this expression is in, say, =gradle2nix.nix=, =gradle2nix= can be
built and placed in =.//result= with the following:

#+begin_example
nix build -f gradle2nix.nix
#+end_example

You can also use the following one-liners to build or install
=gradle2nix= in your user profile:

#+begin_example
# Build and place in ./result/
nix build -f "https://github.com/tadfisher/gradle2nix/archive/master.tar.gz"

# Build and install in the user profile
nix-env -if "https://github.com/tadfisher/gradle2nix/archive/master.tar.gz"
#+end_example

=gradle2nix= is not yet packaged in =nixpkgs= itself, but work is
[[https://github.com/NixOS/nixpkgs/pull/77422][in progress]].

*** Flake (experimental)

A [[./flake.nix][flake.nix]] is provided for those using [[https://nixos.wiki/wiki/Flakes][Nix flakes]]. For example, the
following will build and run =gradle2nix= with the arguments provided
after =--=:

#+begin_example
nix run github:tadfisher/gradle2nix -- --help
#+end_example

** Usage

#+begin_example
Usage: gradle2nix [OPTIONS] [PROJECT-DIR]

Options:
  -g, --gradle-version VERSION     Use a specific Gradle version
  -a, --gradle-args ARGS           Extra arguments to pass to Gradle
  -c, --configuration NAME         Add a configuration to resolve (default:
                                   all configurations)
  -i, --include DIR                Add an additional project to include
  -p, --project PATH               Only resolve these subproject paths, e.g.
                                   ':', or ':sub:project' (default: all
                                   projects)
  -o, --out-dir DIR                Path to write generated files (default:
                                   PROJECT-DIR)
  -e, --env FILENAME               Prefix for environment files (.json and
                                   .nix) (default: gradle-env)
  -b, --build-src / -nb, --no-build-src
                                   Include buildSrc project (default: true)
  -q, --quiet                      Disable logging
  -h, --help                       Show this message and exit

Arguments:
  PROJECT-DIR  Path to the project root (default: .)
#+end_example

Simply running =gradle2nix= in the root directory of a project should
be enough for most projects. This will produce two files, by default
called =gradle-env.json= and =gradle-env.nix=, which contain the
pinned dependencies for the project and a standard build expression
which can be imported or called by other Nix expressions. An example
of such an expression can be found in this project's [[./default.nix][default.nix]].

*** Specifying the Gradle version

By default, if the project has configured the Gradle wrapper, that
version will be detected and pinned; otherwise, the version of Gradle
installed on your system will be pinned. You can override this with
the =--gradle-version= argument, which also avoids the need to have
Gradle installed.

#+begin_example
gradle2nix -g 6.1
#+end_example

*** Multi-project builds

If you want to resolve only a subset of projects in a [[https://docs.gradle.org/current/userguide/intro_multi_project_builds.html][multi-project
build]], add the =--project= option for each project. For example, in a
project where you only want to build the subprojects =:app= and
=:widget=:

#+begin_example
gradle2nix -p :app -p :widget
#+end_example

Any [[https://docs.gradle.org/current/userguide/declaring_dependencies.html#sub:project_dependencies][project dependencies]] will be also be included when pinning
dependency artifacts.

** Contributing

Bug reports and feature requests are encouraged.

[[https://github.com/tadfisher/gradle2nix/issues/new][Create an issue]]

Code contributions are also encouraged. Please review the test cases
in the [[./fixtures][fixtures]] directory and create a new one to reproduce any fixes
or test new features. See the [[./plugin/src/compatTest/kotlin/org/nixos/gradle2nix][existing compatibility tests]] for
examples of testing with these fixtures.

** License

=gradle2nix= is licensed under the [[./COPYING][MIT License]].