Maven

Maven is a well-known build tool for the Java ecosystem however it has some challenges when integrating into the Nix build system.

The following provides a list of common patterns with how to package a Maven project (or any JVM language that can export to Maven) as a Nix package.

Building a package using maven.buildMavenPackage

Consider the following package:

{ lib, fetchFromGitHub, jre, makeWrapper, maven }:

maven.buildMavenPackage rec {
  pname = "jd-cli";
  version = "1.2.1";

  src = fetchFromGitHub {
    owner = "intoolswetrust";
    repo = pname;
    rev = "${pname}-${version}";
    hash = "sha256-rRttA5H0A0c44loBzbKH7Waoted3IsOgxGCD2VM0U/Q=";
  };

  mvnHash = "sha256-kLpjMj05uC94/5vGMwMlFzLKNFOKeyNvq/vmB6pHTAo=";

  nativeBuildInputs = [ makeWrapper ];

  installPhase = ''
    mkdir -p $out/bin $out/share/jd-cli
    install -Dm644 jd-cli/target/jd-cli.jar $out/share/jd-cli

    makeWrapper ${jre}/bin/java $out/bin/jd-cli \
      --add-flags "-jar $out/share/jd-cli/jd-cli.jar"
  '';

  meta = with lib; {
    description = "Simple command line wrapper around JD Core Java Decompiler project";
    homepage = "https://github.com/intoolswetrust/jd-cli";
    license = licenses.gpl3Plus;
    maintainers = with maintainers; [ majiir ];
  };
}:

This package calls maven.buildMavenPackage to do its work. The primary difference from stdenv.mkDerivation is the mvnHash variable, which is a hash of all of the Maven dependencies.

Stable Maven plugins

Maven defines default versions for its core plugins, e.g. maven-compiler-plugin. If your project does not override these versions, an upgrade of Maven will change the version of the used plugins, and therefore the derivation and hash.

When maven is upgraded, mvnHash for the derivation must be updated as well: otherwise, the project will simply be built on the derivation of old plugins, and fail because the requested plugins are missing.

This clearly prevents automatic upgrades of Maven: a manual effort must be made throughout nixpkgs by any maintainer wishing to push the upgrades.

To make sure that your package does not add extra manual effort when upgrading Maven, explicitly define versions for all plugins. You can check if this is the case by adding the following plugin to your (parent) POM:

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-enforcer-plugin</artifactId>
  <version>3.3.0</version>
  <executions>
    <execution>
      <id>enforce-plugin-versions</id>
      <goals>
        <goal>enforce</goal>
      </goals>
      <configuration>
        <rules>
          <requirePluginVersions />
        </rules>
      </configuration>
    </execution>
  </executions>
</plugin>

Manually using mvn2nix

For the purposes of this example let's consider a very basic Maven project with the following pom.xml with a single dependency on emoji-java.

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>io.github.fzakaria</groupId>
  <artifactId>maven-demo</artifactId>
  <version>1.0</version>
  <packaging>jar</packaging>
  <name>NixOS Maven Demo</name>

  <dependencies>
    <dependency>
        <groupId>com.vdurmont</groupId>
        <artifactId>emoji-java</artifactId>
        <version>5.1.1</version>
      </dependency>
  </dependencies>
</project>

Our main class file will be very simple:

import com.vdurmont.emoji.EmojiParser;

public class Main {
  public static void main(String[] args) {
    String str = "NixOS :grinning: is super cool :smiley:!";
    String result = EmojiParser.parseToUnicode(str);
    System.out.println(result);
  }
}

You find this demo project at https://github.com/fzakaria/nixos-maven-example.

Solving for dependencies

buildMaven with NixOS/mvn2nix-maven-plugin

buildMaven is an alternative method that tries to follow similar patterns of other programming languages by generating a lock file. It relies on the maven plugin mvn2nix-maven-plugin.

First you generate a project-info.json file using the maven plugin.

This should be executed in the project's source repository or be told which pom.xml to execute with.

# run this step within the project's source repository
❯ mvn org.nixos.mvn2nix:mvn2nix-maven-plugin:mvn2nix

❯ cat project-info.json | jq | head
{
  "project": {
    "artifactId": "maven-demo",
    "groupId": "org.nixos",
    "version": "1.0",
    "classifier": "",
    "extension": "jar",
    "dependencies": [
      {
        "artifactId": "maven-resources-plugin",

This file is then given to the buildMaven function, and it returns 2 attributes.

repo: A Maven repository that is a symlink farm of all the dependencies found in the project-info.json

build: A simple derivation that runs through mvn compile & mvn package to build the JAR. You may use this as inspiration for more complicated derivations.

Here is an example of building the Maven repository

{ pkgs ? import <nixpkgs> { } }:
with pkgs;
(buildMaven ./project-info.json).repo

The benefit over the double invocation as we will see below, is that the /nix/store entry is a linkFarm of every package, so that changes to your dependency set doesn't involve downloading everything from scratch.

❯ tree $(nix-build --no-out-link build-maven-repository.nix) | head
/nix/store/g87va52nkc8jzbmi1aqdcf2f109r4dvn-maven-repository
├── antlr
│   └── antlr
│       └── 2.7.2
│           ├── antlr-2.7.2.jar -> /nix/store/d027c8f2cnmj5yrynpbq2s6wmc9cb559-antlr-2.7.2.jar
│           └── antlr-2.7.2.pom -> /nix/store/mv42fc5gizl8h5g5vpywz1nfiynmzgp2-antlr-2.7.2.pom
├── avalon-framework
│   └── avalon-framework
│       └── 4.1.3
│           ├── avalon-framework-4.1.3.jar -> /nix/store/iv5fp3955w3nq28ff9xfz86wvxbiw6n9-avalon-framework-4.1.3.jar

Double Invocation

The double invocation is a simple way to get around the problem that nix-build may be sandboxed and have no Internet connectivity.

It treats the entire Maven repository as a single source to be downloaded, relying on Maven's dependency resolution to satisfy the output hash. This is similar to fetchers like fetchgit, except it has to run a Maven build to determine what to download.

The first step will be to build the Maven project as a fixed-output derivation in order to collect the Maven repository -- below is an example.

{ lib, stdenv, maven }:
stdenv.mkDerivation {
  name = "maven-repository";
  buildInputs = [ maven ];
  src = ./.; # or fetchFromGitHub, cleanSourceWith, etc
  buildPhase = ''
    mvn package -Dmaven.repo.local=$out
  '';

  # keep only *.{pom,jar,sha1,nbm} and delete all ephemeral files with lastModified timestamps inside
  installPhase = ''
    find $out -type f \
      -name \*.lastUpdated -or \
      -name resolver-status.properties -or \
      -name _remote.repositories \
      -delete
  '';

  # don't do any fixup
  dontFixup = true;
  outputHashAlgo = "sha256";
  outputHashMode = "recursive";
  # replace this with the correct SHA256
  outputHash = lib.fakeSha256;
}

The build will fail, and tell you the expected outputHash to place. When you've set the hash, the build will return with a /nix/store entry whose contents are the full Maven repository.

❯ tree $(nix-build --no-out-link double-invocation-repository.nix) | head
/nix/store/8kicxzp98j68xyi9gl6jda67hp3c54fq-maven-repository
├── backport-util-concurrent
│   └── backport-util-concurrent
│       └── 3.1
│           ├── backport-util-concurrent-3.1.pom
│           └── backport-util-concurrent-3.1.pom.sha1
├── classworlds
│   └── classworlds
│       ├── 1.1
│       │   ├── classworlds-1.1.jar

If your package uses SNAPSHOT dependencies or version ranges; there is a strong likelihood that over-time your output hash will change since the resolved dependencies may change. Hence this method is less recommended then using buildMaven.

Building a JAR

Regardless of which strategy is chosen above, the step to build the derivation is the same.

{ stdenv, maven, callPackage }:
# pick a repository derivation, here we will use buildMaven
let repository = callPackage ./build-maven-repository.nix { };
in stdenv.mkDerivation rec {
  pname = "maven-demo";
  version = "1.0";

  src = builtins.fetchTarball "https://github.com/fzakaria/nixos-maven-example/archive/main.tar.gz";
  buildInputs = [ maven ];

  buildPhase = ''
    echo "Using repository ${repository}"
    mvn --offline -Dmaven.repo.local=${repository} package;
  '';

  installPhase = ''
    install -Dm644 target/${pname}-${version}.jar $out/share/java
  '';
}

❯ tree $(nix-build --no-out-link build-jar.nix)
/nix/store/7jw3xdfagkc2vw8wrsdv68qpsnrxgvky-maven-demo-1.0
└── share
    └── java
        └── maven-demo-1.0.jar

2 directories, 1 file

Runnable JAR

The previous example builds a jar file but that's not a file one can run.

You need to use it with java -jar $out/share/java/output.jar and make sure to provide the required dependencies on the classpath.

The following explains how to use makeWrapper in order to make the derivation produce an executable that will run the JAR file you created.

We will use the same repository we built above (either double invocation or buildMaven) to setup a CLASSPATH for our JAR.

The following two methods are more suited to Nix then building an UberJar which may be the more traditional approach.

CLASSPATH

This method is ideal if you are providing a derivation for nixpkgs and don't want to patch the project's pom.xml.

We will read the Maven repository and flatten it to a single list. This list will then be concatenated with the CLASSPATH separator to create the full classpath.

We make sure to provide this classpath to the makeWrapper.

{ stdenv, maven, callPackage, makeWrapper, jre }:
let
  repository = callPackage ./build-maven-repository.nix { };
in stdenv.mkDerivation rec {
  pname = "maven-demo";
  version = "1.0";

  src = builtins.fetchTarball
    "https://github.com/fzakaria/nixos-maven-example/archive/main.tar.gz";
  nativeBuildInputs = [ makeWrapper ];
  buildInputs = [ maven ];

  buildPhase = ''
    echo "Using repository ${repository}"
    mvn --offline -Dmaven.repo.local=${repository} package;
  '';

  installPhase = ''
    mkdir -p $out/bin

    classpath=$(find ${repository} -name "*.jar" -printf ':%h/%f');
    install -Dm644 target/${pname}-${version}.jar $out/share/java
    # create a wrapper that will automatically set the classpath
    # this should be the paths from the dependency derivation
    makeWrapper ${jre}/bin/java $out/bin/${pname} \
          --add-flags "-classpath $out/share/java/${pname}-${version}.jar:''${classpath#:}" \
          --add-flags "Main"
  '';
}

MANIFEST file via Maven Plugin

This method is ideal if you are the project owner and want to change your pom.xml to set the CLASSPATH within it.

Augment the pom.xml to create a JAR with the following manifest:

<build>
  <plugins>
    <plugin>
        <artifactId>maven-jar-plugin</artifactId>
        <configuration>
            <archive>
                <manifest>
                    <addClasspath>true</addClasspath>
                    <classpathPrefix>../../repository/</classpathPrefix>
                    <classpathLayoutType>repository</classpathLayoutType>
                    <mainClass>Main</mainClass>
                </manifest>
                <manifestEntries>
                    <Class-Path>.</Class-Path>
                </manifestEntries>
            </archive>
        </configuration>
    </plugin>
  </plugins>
</build>

The above plugin instructs the JAR to look for the necessary dependencies in the lib/ relative folder. The layout of the folder is also in the maven repository style.

❯ unzip -q -c $(nix-build --no-out-link runnable-jar.nix)/share/java/maven-demo-1.0.jar META-INF/MANIFEST.MF

Manifest-Version: 1.0
Archiver-Version: Plexus Archiver
Built-By: nixbld
Class-Path: . ../../repository/com/vdurmont/emoji-java/5.1.1/emoji-jav
 a-5.1.1.jar ../../repository/org/json/json/20170516/json-20170516.jar
Created-By: Apache Maven 3.6.3
Build-Jdk: 1.8.0_265
Main-Class: Main

We will modify the derivation above to add a symlink to our repository so that it's accessible to our JAR during the installPhase.

{ stdenv, maven, callPackage, makeWrapper, jre }:
# pick a repository derivation, here we will use buildMaven
let repository = callPackage ./build-maven-repository.nix { };
in stdenv.mkDerivation rec {
  pname = "maven-demo";
  version = "1.0";

  src = builtins.fetchTarball
    "https://github.com/fzakaria/nixos-maven-example/archive/main.tar.gz";
  nativeBuildInputs = [ makeWrapper ];
  buildInputs = [ maven ];

  buildPhase = ''
    echo "Using repository ${repository}"
    mvn --offline -Dmaven.repo.local=${repository} package;
  '';

  installPhase = ''
    mkdir -p $out/bin

    # create a symbolic link for the repository directory
    ln -s ${repository} $out/repository

    install -Dm644 target/${pname}-${version}.jar $out/share/java
    # create a wrapper that will automatically set the classpath
    # this should be the paths from the dependency derivation
    makeWrapper ${jre}/bin/java $out/bin/${pname} \
          --add-flags "-jar $out/share/java/${pname}-${version}.jar"
  '';
}

This will give you an executable shell-script that launches your JAR with all the dependencies available.

❯ tree $(nix-build --no-out-link runnable-jar.nix)
/nix/store/8d4c3ibw8ynsn01ibhyqmc1zhzz75s26-maven-demo-1.0
├── bin
│   └── maven-demo
├── repository -> /nix/store/g87va52nkc8jzbmi1aqdcf2f109r4dvn-maven-repository
└── share
    └── java
        └── maven-demo-1.0.jar

❯ $(nix-build --no-out-link --option tarball-ttl 1 runnable-jar.nix)/bin/maven-demo
NixOS 😀 is super cool 😃!