depot/doc/languages-frameworks/gradle.section.md

Gradle

Gradle is a popular build tool for Java/Kotlin. Gradle itself doesn't currently provide tools to make dependency resolution reproducible, so nixpkgs has a proxy designed for intercepting Gradle web requests to record dependencies so they can be restored in a reproducible fashion.

Building a Gradle package

Here's how a typical derivation will look like:

stdenv.mkDerivation (finalAttrs: {
  pname = "pdftk";
  version = "3.3.3";

  src = fetchFromGitLab {
    owner = "pdftk-java";
    repo = "pdftk";
    rev = "v${finalAttrs.version}";
    hash = "sha256-ciKotTHSEcITfQYKFZ6sY2LZnXGChBJy0+eno8B3YHY=";
  };

  nativeBuildInputs = [ gradle ];

  # if the package has dependencies, mitmCache must be set
  mitmCache = gradle.fetchDeps {
    inherit (finalAttrs) pname;
    data = ./deps.json;
  };

  # this is required for using mitm-cache on Darwin
  __darwinAllowLocalNetworking = true;

  gradleFlags = [ "-Dfile.encoding=utf-8" ];

  # defaults to "assemble"
  gradleBuildTask = "shadowJar";

  # will run the gradleCheckTask (defaults to "test")
  doCheck = true;

  installPhase = ''
    mkdir -p $out/{bin,share/pdftk}
    cp build/libs/pdftk-all.jar $out/share/pdftk

    makeWrapper ${jre}/bin/java $out/bin/pdftk \
      --add-flags "-jar $out/share/pdftk/pdftk-all.jar"

    cp ${finalAttrs.src}/pdftk.1 $out/share/man/man1
  '';

  meta.sourceProvenance = with lib.sourceTypes; [
    fromSource
    binaryBytecode # mitm cache
  ];
})

To update (or initialize) dependencies, run the update script via something like $(nix-build -A <pname>.mitmCache.updateScript) (nix-build builds the updateScript, $(...) runs the script at the path printed by nix-build).

If your package can't be evaluated using a simple pkgs.<pname> expression (for example, if your package isn't located in nixpkgs, or if you want to override some of its attributes), you will usually have to pass pkg instead of pname to gradle.fetchDeps. There are two ways of doing it.

The first is to add the derivation arguments required for getting the package. Using the pdftk example above:

{ lib
, stdenv
# ...
, pdftk
}:

stdenv.mkDerivation (finalAttrs: {
  # ...
  mitmCache = gradle.fetchDeps {
    pkg = pdftk;
    data = ./deps.json;
  };
})

This allows you to override any arguments of the pkg used for the update script (for example, pkg = pdftk.override { enableSomeFlag = true };), so this is the preferred way.

The second is to create a let binding for the package, like this:

let self = stdenv.mkDerivation {
  # ...
  mitmCache = gradle.fetchDeps {
    pkg = self;
    data = ./deps.json;
  };
}; in self

This is useful if you can't easily pass the derivation as its own argument, or if your mkDerivation call is responsible for building multiple packages.

In the former case, the update script will stay the same even if the derivation is called with different arguments. In the latter case, the update script will change depending on the derivation arguments. It's up to you to decide which one would work best for your derivation.

Update Script

The update script does the following:

• Build the derivation's source via pkgs.srcOnly
• Enter a nix-shell for the derivation in a bwrap sandbox (the sandbox is only used on Linux)
• Set the IN_GRADLE_UPDATE_DEPS environment variable to 1
• Run the derivation's unpackPhase, patchPhase, configurePhase
• Run the derivation's gradleUpdateScript (the Gradle setup hook sets a default value for it, which runs preBuild, preGradleUpdate hooks, fetches the dependencies using gradleUpdateTask, and finally runs the postGradleUpdate hook)
• Finally, store all of the fetched files' hashes in the lockfile. They may be .jar/.pom files from Maven repositories, or they may be files otherwise used for building the package.

fetchDeps takes the following arguments:

• attrPath - the path to the package in nixpkgs (for example, "javaPackages.openjfx22"). Used for update script metadata.
• pname - an alias for attrPath for convenience. This is what you will generally use instead of pkg or attrPath.
• pkg - the package to be used for fetching the dependencies. Defaults to getAttrFromPath (splitString "." attrPath) pkgs.
• bwrapFlags - allows you to override bwrap flags (only relevant for downstream, non-nixpkgs projects)
• data - path to the dependencies lockfile (can be relative to the package, can be absolute). In nixpkgs, it's discouraged to have the lockfiles be named anything other deps.json, consider creating subdirectories if your package requires multiple deps.json files.

Environment

The Gradle setup hook accepts the following environment variables:

• mitmCache - the MITM proxy cache imported using gradle.fetchDeps
• gradleFlags - command-line flags to be used for every Gradle invocation (this simply registers a function that uses the necessary flags).
  • You can't use gradleFlags for flags that contain spaces, in that case you must add gradleFlagsArray+=("-flag with spaces") to the derivation's bash code instead.
  • If you want to build the package using a specific Java version, you can pass "-Dorg.gradle.java.home=${jdk}" as one of the flags.
• gradleBuildTask - the Gradle task (or tasks) to be used for building the package. Defaults to assemble.
• gradleCheckTask - the Gradle task (or tasks) to be used for checking the package if doCheck is set to true. Defaults to test.
• gradleUpdateTask - the Gradle task (or tasks) to be used for fetching all of the package's dependencies in mitmCache.updateScript. Defaults to nixDownloadDeps.
• gradleUpdateScript - the code to run for fetching all of the package's dependencies in mitmCache.updateScript. Defaults to running the preBuild and preGradleUpdate hooks, running the gradleUpdateTask, and finally running the postGradleUpdate hook.
• gradleInitScript - path to the --init-script to pass to Gradle. By default, a simple init script that enables reproducible archive creation is used.
  • Note that reproducible archives might break some builds. One example of an error caused by it is Could not create task ':jar'. Replacing an existing task that may have already been used by other plugins is not supported. If you get such an error, the easiest "fix" is disabling reproducible archives altogether by setting gradleInitScript to something like writeText "empty-init-script.gradle" ""
• enableParallelBuilding / enableParallelChecking / enableParallelUpdating - pass --parallel to Gradle in the build/check phase or in the update script. Defaults to true. If the build fails for mysterious reasons, consider setting this to false.
• dontUseGradleConfigure / dontUseGradleBuild / dontUseGradleCheck - force disable the Gradle setup hook for certain phases.
  • Note that if you disable the configure hook, you may face issues such as Failed to load native library 'libnative-platform.so', because the configure hook is responsible for initializing Gradle.