348 lines
13 KiB
Markdown
348 lines
13 KiB
Markdown
# Maven {#maven}
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Maven is a well-known build tool for the Java ecosystem however it has some challenges when integrating into the Nix build system.
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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.
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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).
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```xml
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<?xml version="1.0" encoding="UTF-8"?>
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<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<groupId>io.github.fzakaria</groupId>
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<artifactId>maven-demo</artifactId>
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<version>1.0</version>
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<packaging>jar</packaging>
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<name>NixOS Maven Demo</name>
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<dependencies>
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<dependency>
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<groupId>com.vdurmont</groupId>
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<artifactId>emoji-java</artifactId>
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<version>5.1.1</version>
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</dependency>
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</dependencies>
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</project>
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```
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Our main class file will be very simple:
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```java
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import com.vdurmont.emoji.EmojiParser;
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public class Main {
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public static void main(String[] args) {
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String str = "NixOS :grinning: is super cool :smiley:!";
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String result = EmojiParser.parseToUnicode(str);
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System.out.println(result);
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}
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}
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```
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You find this demo project at https://github.com/fzakaria/nixos-maven-example
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## Solving for dependencies
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### buildMaven with NixOS/mvn2nix-maven-plugin
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> ⚠️ Although `buildMaven` is the "blessed" way within nixpkgs, as of 2020, it hasn't seen much activity in quite a while.
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`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).
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First you generate a `project-info.json` file using the maven plugin.
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> This should be executed in the project's source repository or be told which `pom.xml` to execute with.
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```bash
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# run this step within the project's source repository
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❯ mvn org.nixos.mvn2nix:mvn2nix-maven-plugin:mvn2nix
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❯ cat project-info.json | jq | head
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{
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"project": {
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"artifactId": "maven-demo",
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"groupId": "org.nixos",
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"version": "1.0",
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"classifier": "",
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"extension": "jar",
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"dependencies": [
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{
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"artifactId": "maven-resources-plugin",
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```
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This file is then given to the `buildMaven` function, and it returns 2 attributes.
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**`repo`**:
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A Maven repository that is a symlink farm of all the dependencies found in the `project-info.json`
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**`build`**:
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A simple derivation that runs through `mvn compile` & `mvn package` to build the JAR. You may use this as inspiration for more complicated derivations.
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Here is an [example](https://github.com/fzakaria/nixos-maven-example/blob/main/build-maven-repository.nix) of building the Maven repository
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```nix
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{ pkgs ? import <nixpkgs> { } }:
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with pkgs;
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(buildMaven ./project-info.json).repo
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```
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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.
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```bash
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❯ tree $(nix-build --no-out-link build-maven-repository.nix) | head
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/nix/store/g87va52nkc8jzbmi1aqdcf2f109r4dvn-maven-repository
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├── antlr
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│ └── antlr
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│ └── 2.7.2
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│ ├── antlr-2.7.2.jar -> /nix/store/d027c8f2cnmj5yrynpbq2s6wmc9cb559-antlr-2.7.2.jar
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│ └── antlr-2.7.2.pom -> /nix/store/mv42fc5gizl8h5g5vpywz1nfiynmzgp2-antlr-2.7.2.pom
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├── avalon-framework
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│ └── avalon-framework
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│ └── 4.1.3
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│ ├── avalon-framework-4.1.3.jar -> /nix/store/iv5fp3955w3nq28ff9xfz86wvxbiw6n9-avalon-framework-4.1.3.jar
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```
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### Double Invocation
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> ⚠️ This pattern is the simplest but may cause unnecessary rebuilds due to the output hash changing.
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The double invocation is a _simple_ way to get around the problem that `nix-build` may be sandboxed and have no Internet connectivity.
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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.
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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).
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> Traditionally the Maven repository is at `~/.m2/repository`. We will override this to be the `$out` directory.
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```nix
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{ lib, stdenv, maven }:
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stdenv.mkDerivation {
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name = "maven-repository";
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buildInputs = [ maven ];
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src = ./.; # or fetchFromGitHub, cleanSourceWith, etc
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buildPhase = ''
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mvn package -Dmaven.repo.local=$out
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'';
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# keep only *.{pom,jar,sha1,nbm} and delete all ephemeral files with lastModified timestamps inside
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installPhase = ''
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find $out -type f \
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-name \*.lastUpdated -or \
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-name resolver-status.properties -or \
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-name _remote.repositories \
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-delete
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'';
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# don't do any fixup
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dontFixup = true;
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outputHashAlgo = "sha256";
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outputHashMode = "recursive";
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# replace this with the correct SHA256
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outputHash = lib.fakeSha256;
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}
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```
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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.
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> Some additional files are deleted that would cause the output hash to change potentially on subsequent runs.
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```bash
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❯ tree $(nix-build --no-out-link double-invocation-repository.nix) | head
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/nix/store/8kicxzp98j68xyi9gl6jda67hp3c54fq-maven-repository
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├── backport-util-concurrent
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│ └── backport-util-concurrent
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│ └── 3.1
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│ ├── backport-util-concurrent-3.1.pom
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│ └── backport-util-concurrent-3.1.pom.sha1
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├── classworlds
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│ └── classworlds
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│ ├── 1.1
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│ │ ├── classworlds-1.1.jar
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```
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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`.
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## Building a JAR
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Regardless of which strategy is chosen above, the step to build the derivation is the same.
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```nix
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{ stdenv, maven, callPackage }:
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# pick a repository derivation, here we will use buildMaven
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let repository = callPackage ./build-maven-repository.nix { };
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in stdenv.mkDerivation rec {
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pname = "maven-demo";
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version = "1.0";
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src = builtins.fetchTarball "https://github.com/fzakaria/nixos-maven-example/archive/main.tar.gz";
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buildInputs = [ maven ];
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buildPhase = ''
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echo "Using repository ${repository}"
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mvn --offline -Dmaven.repo.local=${repository} package;
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'';
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installPhase = ''
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install -Dm644 target/${pname}-${version}.jar $out/share/java
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'';
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}
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```
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> We place the library in `$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.
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```bash
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❯ tree $(nix-build --no-out-link build-jar.nix)
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/nix/store/7jw3xdfagkc2vw8wrsdv68qpsnrxgvky-maven-demo-1.0
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└── share
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└── java
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└── maven-demo-1.0.jar
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2 directories, 1 file
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```
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## Runnable JAR
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The previous example builds a `jar` file but that's not a file one can run.
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You need to use it with `java -jar $out/share/java/output.jar` and make sure to provide the required dependencies on the classpath.
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The following explains how to use `makeWrapper` in order to make the derivation produce an executable that will run the JAR file you created.
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We will use the same repository we built above (either _double invocation_ or _buildMaven_) to setup a CLASSPATH for our JAR.
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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.
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### CLASSPATH
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> This is ideal if you are providing a derivation for _nixpkgs_ and don't want to patch the project's `pom.xml`.
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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.
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We make sure to provide this classpath to the `makeWrapper`.
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```nix
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{ stdenv, maven, callPackage, makeWrapper, jre }:
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let
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repository = callPackage ./build-maven-repository.nix { };
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in stdenv.mkDerivation rec {
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pname = "maven-demo";
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version = "1.0";
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src = builtins.fetchTarball
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"https://github.com/fzakaria/nixos-maven-example/archive/main.tar.gz";
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buildInputs = [ maven makeWrapper ];
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buildPhase = ''
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echo "Using repository ${repository}"
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mvn --offline -Dmaven.repo.local=${repository} package;
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'';
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installPhase = ''
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mkdir -p $out/bin
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classpath=$(find ${repository} -name "*.jar" -printf ':%h/%f');
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install -Dm644 target/${pname}-${version}.jar $out/share/java
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# create a wrapper that will automatically set the classpath
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# this should be the paths from the dependency derivation
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makeWrapper ${jre}/bin/java $out/bin/${pname} \
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--add-flags "-classpath $out/share/java/${pname}-${version}.jar:''${classpath#:}" \
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--add-flags "Main"
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'';
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}
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```
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### MANIFEST file via Maven Plugin
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> This is ideal if you are the project owner and want to change your `pom.xml` to set the CLASSPATH within it.
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Augment the `pom.xml` to create a JAR with the following manifest:
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```xml
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<build>
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<plugins>
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<plugin>
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<artifactId>maven-jar-plugin</artifactId>
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<configuration>
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<archive>
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<manifest>
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<addClasspath>true</addClasspath>
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<classpathPrefix>../../repository/</classpathPrefix>
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<classpathLayoutType>repository</classpathLayoutType>
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<mainClass>Main</mainClass>
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</manifest>
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<manifestEntries>
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<Class-Path>.</Class-Path>
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</manifestEntries>
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</archive>
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</configuration>
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</plugin>
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</plugins>
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</build>
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```
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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.
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```bash
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❯ unzip -q -c $(nix-build --no-out-link runnable-jar.nix)/share/java/maven-demo-1.0.jar META-INF/MANIFEST.MF
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Manifest-Version: 1.0
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Archiver-Version: Plexus Archiver
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Built-By: nixbld
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Class-Path: . ../../repository/com/vdurmont/emoji-java/5.1.1/emoji-jav
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a-5.1.1.jar ../../repository/org/json/json/20170516/json-20170516.jar
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Created-By: Apache Maven 3.6.3
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Build-Jdk: 1.8.0_265
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Main-Class: Main
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```
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We will modify the derivation above to add a symlink to our repository so that it's accessible to our JAR during the `installPhase`.
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```nix
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{ stdenv, maven, callPackage, makeWrapper, jre }:
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# pick a repository derivation, here we will use buildMaven
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let repository = callPackage ./build-maven-repository.nix { };
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in stdenv.mkDerivation rec {
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pname = "maven-demo";
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version = "1.0";
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src = builtins.fetchTarball
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"https://github.com/fzakaria/nixos-maven-example/archive/main.tar.gz";
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buildInputs = [ maven makeWrapper ];
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buildPhase = ''
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echo "Using repository ${repository}"
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mvn --offline -Dmaven.repo.local=${repository} package;
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'';
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installPhase = ''
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mkdir -p $out/bin
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# create a symbolic link for the repository directory
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ln -s ${repository} $out/repository
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install -Dm644 target/${pname}-${version}.jar $out/share/java
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# create a wrapper that will automatically set the classpath
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# this should be the paths from the dependency derivation
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makeWrapper ${jre}/bin/java $out/bin/${pname} \
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--add-flags "-jar $out/share/java/${pname}-${version}.jar"
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'';
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}
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```
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> Our script produces a dependency on `jre` rather than `jdk` to restrict the runtime closure necessary to run the application.
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This will give you an executable shell-script that launches your JAR with all the dependencies available.
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```bash
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❯ tree $(nix-build --no-out-link runnable-jar.nix)
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/nix/store/8d4c3ibw8ynsn01ibhyqmc1zhzz75s26-maven-demo-1.0
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├── bin
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│ └── maven-demo
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├── repository -> /nix/store/g87va52nkc8jzbmi1aqdcf2f109r4dvn-maven-repository
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└── share
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└── java
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└── maven-demo-1.0.jar
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❯ $(nix-build --no-out-link --option tarball-ttl 1 runnable-jar.nix)/bin/maven-demo
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NixOS 😀 is super cool 😃!
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```
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