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.
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).
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
> ⚠️ Although `buildMaven` is the "blessed" way within nixpkgs, as of 2020, it hasn't seen much activity in quite a while.
`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
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 $(nix-build --no-out-link build-maven-repository.nix) | head
> ⚠️ This pattern is the simplest but may cause unnecessary rebuilds due to the output hash changing.
The double invocation is a _simple_ way to get around the problem that `nix-build` may be sandboxed and have no Internet connectivity.
It treats the entire Maven repository as a single source to be downloaded, relying on Maven's dependency resolution to satisfy the output hash. This is similar to fetchers like `fetchgit`, except it has to run a Maven build to determine what to download.
The first step will be to build the Maven project as a fixed-output derivation in order to collect the Maven repository -- below is an [example](https://github.com/fzakaria/nixos-maven-example/blob/main/double-invocation-repository.nix).
> Traditionally the Maven repository is at `~/.m2/repository`. We will override this to be the `$out` directory.
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.
> Some additional files are deleted that would cause the output hash to change potentially on subsequent runs.
```bash
❯ tree $(nix-build --no-out-link double-invocation-repository.nix) | head
If your package uses _SNAPSHOT_ dependencies or _version ranges_; there is a strong likelihood that over-time your output hash will change since the resolved dependencies may change. Hence this method is less recommended then using `buildMaven`.
## Building a JAR
Regardless of which strategy is chosen above, the step to build the derivation is the same.
```nix
{ stdenv, lib, maven, callPackage }:
# pick a repository derivation, here we will use buildMaven
let repository = callPackage ./build-maven-repository.nix { };
> We place the library in `$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.
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
> This 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`.
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.
