# Maven {#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` {#maven-buildmavenpackage}

Consider the following package:

```nix
{ 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.

::: {.tip}
After setting `maven.buildMavenPackage`, we then do standard Java `.jar` installation by saving the `.jar` to `$out/share/java` and then making a wrapper which allows executing that file; see [](#sec-language-java) for additional generic information about packaging Java applications.
:::

### Stable Maven plugins {#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:

```xml
<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` {#maven-mvn2nix}
::: {.warning}
This way is no longer recommended; see [](#maven-buildmavenpackage) for the simpler and preferred way.
:::

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](https://github.com/vdurmont/emoji-java).

```xml
<?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:

```java
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](https://github.com/fzakaria/nixos-maven-example).

### Solving for dependencies {#solving-for-dependencies}

#### buildMaven with NixOS/mvn2nix-maven-plugin {#buildmaven-with-nixosmvn2nix-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](https://github.com/NixOS/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.

```bash
# 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](https://github.com/fzakaria/nixos-maven-example/blob/main/build-maven-repository.nix) of building the Maven repository

```nix
{ 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.

```bash
❯ 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 {#double-invocation}
::: {.note}
This pattern is the simplest but may cause unnecessary rebuilds due to the output hash changing.
:::

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](https://github.com/fzakaria/nixos-maven-example/blob/main/double-invocation-repository.nix).

::: {.note}
Traditionally the Maven repository is at `~/.m2/repository`. We will override this to be the `$out` directory.
:::

```nix
{ 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.

::: {.warning}
Some additional files are deleted that would cause the output hash to change potentially on subsequent runs.
:::

```bash
❯ 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 {#building-a-jar}

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

```nix
{ 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
  '';
}
```

::: {.tip}
We place the library in `$out/share/java` since JDK package has a _stdenv setup hook_ that adds any JARs in the `share/java` directories of the build inputs to the CLASSPATH environment.
:::

```bash
❯ 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 {#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](https://imagej.net/Uber-JAR) which may be the more traditional approach.

#### CLASSPATH {#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`.

```nix
{ 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 {#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:

```xml
<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.

```bash
❯ 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`.

```nix
{ 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"
  '';
}
```
::: {.note}
Our script produces a dependency on `jre` rather than `jdk` to restrict the runtime closure necessary to run the application.
:::

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

```bash
❯ 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 😃!
```