Simplified Nix integration with Gradle

build.nix

{ lib
, stdenv
, jdk
, gradle
, mavenRepo
}:

stdenv.mkDerivation {
  pname = "built-with-gradle";
  version = "0.0";
  
  nativeBuildInputs = [ gradle ];
  
  JDK_HOME = "${jdk.home}";
  
  buildPhase = ''
    runHook preBuild
    gradle build \
      --offline --no-daemon --no-build-cache --info --full-stacktrace \
      --warning-mode=all --parallel --console=plain \
      -DnixMavenRepo=file://${mavenRepo}
    runHook postBuild
  '';
  
  installPhase = ''
    runHook preInstall
    mkdir -p $out
    cp -r build/dist/* $out
    runHook postInstall
  '';
  
  dontStrip = true;
}

default.nix

{ pkgs ? import <nixpkgs> { } }:

with pkgs;

lib.makeScope newScope (self: with self; {
  gradle = callPackage ./gradle.nix { };

  updateLocks = callPackage ./update-locks.nix {
    inherit (haskellPackages) xml-to-json;
  };
    
  buildMavenRepo = callPackage ./maven-repo.nix { };
  
  mavenRepo = buildMavenRepo {
    name = "nix-maven-repo";
    repos = [
      "https://plugins.gradle.org/m2"
      "https://repo1.maven.org/maven2"
    ];
    deps = builtins.fromJSON (builtins.readFile ./deps.json);
  };
  
  builtWithGradle = callPackage ./build.nix { };
})

gradle.nix

{ gradleGen, fetchurl }:

gradleGen rec {
  name = "gradle-7.0-milestone-2";
  nativeVersion = "0.22-milestone-10";
  src = fetchurl (
    url = "https://services.gradle.org/distributions/${name}-bin.zip";
    sha256 = "10a2zhr7yhj7a9pi2rn1jaqdf1nxnxxpljvfnnz0v3il4p6v2wd9";
  };
}

maven-repo.nix

{ lib
, stdenv
, buildEnv
, fetchurl
, writeTextDir
}:

{ name ? "maven-deps"
, repos ? [ ]
, deps ? [ ]
, extraPaths ? [ ]
}:

with lib;

let
  mavenize = sep: replaceStrings ["."] [sep];

  fetch =
    { group
    , name
    , version
    , file
    , sha256
    }:
    fetchurl {
      name = file;
      urls = map (repo: "${repo}/${mavenize "/" group}/${name}/${version}/${file}") repos;
      inherit sha256;
      meta.platforms = platforms.all;
    };

  fetchDependency =
    { group
    , name
    , version
    , artifacts
    }:
    let
      fetchArtifact = file: sha256:
        fetch { inherit group name version file sha256; };

      # Each artifact uses the filename in the Gradle cache, which doesn't
      # correspond to the filename in the Maven repo. The mapping of name to URL
      # is provided by Gradle module metadata, so we fetch that first. See
      # https://github.com/gradle/gradle/blob/master/subprojects/docs/src/docs/design/gradle-module-metadata-latest-specification.md
      # for the file format.
      isModule = hasSuffix ".module";
      moduleArtifacts = filterAttrs (file: _: isModule file) artifacts;
      otherArtifacts = filterAttrs (file: _:  !isModule file) artifacts;

      modules = mapAttrsToList fetchArtifact moduleArtifacts;

      replacements = listToAttrs (flatten (map (module:
        let
          json = builtins.fromJSON (builtins.readFile module);
          variants = json.variants or [ ];
          files = flatten (map (v: v.files or [ ]) variants);
        in
        map ({ name, url, ... }: nameValuePair name url) files
      ) modules));

      replaced = mapAttrs' (file: sha256:
        nameValuePair (replacements.${file} or file) sha256
      ) otherArtifacts;
    in
      if moduleArtifacts == { }
      then mapAttrsToList fetchArtifact artifacts
      else modules ++ (mapAttrsToList fetchArtifact replaced);

  mkDep =
    { group
    , name
    , version
    , artifacts
    }@dep:
    stdenv.mkDerivation {
      pname = "${mavenize "-" group}-${name}";
      inherit version;

      srcs = fetchDependency dep;

      sourceRoot = ".";

      phases = "installPhase";

      enableParallelBuilding = true;
      preferLocalBuild = true;

      installPhase = ''
        dest=$out/${mavenize "/" group}/${name}/${version}
        mkdir -p $dest
        for src in $srcs; do
          cp $src $dest/$(stripHash $src)
        done
      '';
    };

  mkMetadata = deps:
    let
      modules = groupBy'
        (meta: { group, name, version, ... }:
          let
            isNewer = versionOlder meta.latest version;
            isNewerRelease = versionOlder meta.release version;
          in {
            groupId = group;
            artifactId = name;
            latest = if isNewer then version else meta.latest;
            release = if isNewerRelease then version else meta.release;
            versions = meta.versions ++ [ version ];
          }
        )
        {
          latest = "";
          release = "";
          versions = [ ];
        }
        ({ group, name, ... }: "${mavenize "/" group}/${name}/maven-metadata.xml")
        deps;
    in
      attrValues (mapAttrs (path: { groupId, artifactId, latest, release, versions }:
        let
          versions' = sort versionOlder (unique versions);
        in
          writeTextDir path ''
            <?xml version="1.0" encoding="UTF-8"?>
            <metadata modelVersion="1.1">
              <groupId>${groupId}</groupId>
              <artifactId>${artifactId}</artifactId>
              <versioning>
                ${optionalString (latest != "") "<latest>${latest}</latest>"}
                ${optionalString (release != "") "<release>${release}</release>"}
                <versions>
                  ${concatMapStringsSep "\n    " (v: "<version>${v}</version>") versions'}
                </versions>
              </versioning>
            </metadata>
          ''
      ) modules);

  mkGradleRedirectionPoms = deps:
    let
      depsMissingPoms = filter ({ artifacts, ... }@dep:
        any (f: hasSuffix ".module" f) (attrNames artifacts) &&
        !(any (f: hasSuffix ".pom" f) (attrNames artifacts))
      ) deps;
    in
      map ({ group, name, version, ... }:
        writeTextDir "${mavenize "/" group}/${name}/${version}/${name}-${version}.pom" ''
          <project xmlns="http://maven.apache.org/POM/4.0.0"
                   xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
                   http://maven.apache.org/xsd/maven-4.0.0.xsd"
                   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
            <!-- This module was also published with a richer model, Gradle metadata,  -->
            <!-- which should be used instead. Do not delete the following line which  -->
            <!-- is to indicate to Gradle or any Gradle module metadata file consumer  -->
            <!-- that they should prefer consuming it instead. -->
            <!-- do_not_remove: published-with-gradle-metadata -->
            <modelVersion>4.0.0</modelVersion>
            <groupId>${group}</groupId>
            <artifactId>${name}</artifactId>
            <version>${version}</version>
          </project>
        ''
      ) depsMissingPoms;

in
buildEnv {
  inherit name;
  paths = map mkDep deps ++ mkMetadata deps ++ mkGradleRedirectionPoms deps ++ extraPaths;
}

settings.gradle.kts

val repoProperty = "nixMavenRepo"

val systemRepo: Provider<String> =
    providers.systemProperty(repoProperty).forUseAtConfigurationTime()
val gradleRepo: Provider<String> =
    providers.gradleProperty(repoProperty).forUseAtConfigurationTime()
val repo: Provider<List<String>> =
    systemRepos.orElse(gradleRepos)

pluginManagement.repositories {
  if (repo.isPresent) {
    clear()
    maven(repo.get())
  }
}

dependencyResolutionManagement {
  if (repo.isPresent) {
    clear()
    maven(repo.get())
  }
}

update-locks.nix

{ lib
, writeShellScriptBin
, gradle
, jq
, xml-to-json
}:

writeShellScriptBin "update-locks" ''
  set -eu -o pipefail

  ${gradle}/bin/gradle lock --write-locks

  ${gradle}/bin/gradle --write-verification-metadata sha256 dependencies

  ${xml-to-json}/bin/xml-to-json -sam -t components gradle/verification-metadata.xml \
    | ${jq}/bin/jq '[
        .[] | .component |
        { group, name, version,
          artifacts: [([.artifact] | flatten | .[] | {(.name): .sha256.value})] | add
        }
      ]' > deps.json

  rm gradle/verification-metadata.xml
''

I've used this to create a minimal example: https://github.com/tmcl/minimal-android-nix-example

good solution, thanks. Is there a lib where supports all the maven fetch depedencies nix functions?
To avoid re-write them every project?

Here's a follow-up on this chain of work, supporting Gradle 8 and packaged as a flake: https://github.com/owickstrom/minimal-kotlin-nix-example