Nix shell for building OpenJDK on macOS

Fri Mar 7 13:29:47 UTC 2025

Hi,

The dependencies required to build JDK vary depending on which JDK version
you are building. I've been building my own set of scripts that allows me
to easily manage this, to the extent that I can do it on the given base OS
I run. E.g. if I'm building a mainline I'd use X as boot JDK but if I'm
building a JDK 21 backport I'd use Y as boot JDK. These scripts are
cumbersome and they can't control all tooling dependencies, so their use is
limited.

In my spare time I've been experimenting with Nix, which offers a
declarative way for package management and system configuration. Its
package manager can be installed on either Linux or Mac and enables you to
start shell instances with a set of isolated dependencies.

I've been wanting to move from my cumbersome scripts to a setup that
allows me to build my local JDKs within a Nix isolated shell. However,
although Nix packages exist that build OpenJDK [1], these only support
Linux and I'm often developing on a mac. After some exploration, I've been
able to craft a Nix descriptor file that allows me to build JDK in an
isolated Nix shell on a mac. I wanted to share this in this mailing list in
case it is useful to other JDK developers.

The starting point is having a mac devkit. Since I'm not a Nix expert I
built it outside of Nix, e.g.

```
$ cd jdk/make/devkit
$ bash ./createMacosxDevkit.sh /Applications/Xcode.app
```

Then I added the devkit manually to the Nix store:

```
$ cd jdk/build/devkit
$ nix-store --add-fixed --recursive sha256 Xcode16.2-MacOSX15
/nix/store/vhsix1jn849mpxggwbw2zh1nbxpy0grc-Xcode16.2-MacOSX15
```

Then I created a shell.nix file that declares the dependencies and sets
environment variables to build the JDK successfully. Something that is odd
is that if you depend on pkgs.jdk23 on mac, it doesn't throw an error
because [1] is not supported there, but it downloads Azul's Zulu JDK. I've
asked how this happens [2] but didn't get a satisfactory answer but this is
ok for now for a boot jdk. The DEVKIT env variable points to the devkit I
added to the nix store and MIGCC is set so that the JDK build uses the
clang from the package dependency.

```
{ pkgs ? import <nixpkgs> {} }:

let
  devkit = "/nix/store/vhsix1jn849mpxggwbw2zh1nbxpy0grc-Xcode16.2-MacOSX15";
in
pkgs.mkShell {
  packages = [
    pkgs.autoconf
    pkgs.jdk23
    pkgs.clang

    devkit
  ];

  shellHook = ''
    echo "Setting DEVKIT_ROOT to path of the devkit in the Nix store."
    export DEVKIT_ROOT=${devkit}

    echo "Setting MIGCC to clang compiler cc binary."
    export MIGCC="${pkgs.clang}/bin/cc"
  '' ;
}
```

>From the directory where the shell.nix file is located you can start a Nix
shell that will start the isolated environment:

```
nix-shell
```

In that nix-shell you can now configure the JDK pointing the boot JDK and
the devkit:

```
[nix-shell: jdk] $ bash configure \
    --with-boot-jdk=$(dirname $(dirname $(readlink -f $(which java)))) \
    --with-devkit=$DEVKIT_ROOT
```

The configure output looks like this:

```
A new configuration has been successfully created in
/Users/galder/1/colata/nix-darwin/jdk/build/macosx-aarch64-server-release
using configure arguments
'--with-boot-jdk=/nix/store/wm5rma6x2527qmypzj7rwml8vf9vprgj-zulu-ca-jdk-23.0.0/zulu-23.jdk/Contents/Home
--with-devkit=/nix/store/vhsix1jn849mpxggwbw2zh1nbxpy0grc-Xcode16.2-MacOSX15'.

Configuration summary:
* Name:           macosx-aarch64-server-release
* Debug level:    release
* HS debug level: product
* JVM variants:   server
* JVM features:   server: 'cds compiler1 compiler2 dtrace epsilongc g1gc
jfr jni-check jvmci jvmti management parallelgc serialgc services
shenandoahgc vm-structs zgc'
* OpenJDK target: OS: macosx, CPU architecture: aarch64, address length: 64
* Version string: 25-internal-adhoc.galder.jdk (25-internal)
* Source date:    315532800 (1980-01-01T00:00:00Z)

Tools summary:
* Boot JDK:       openjdk version "23" 2024-09-17 OpenJDK Runtime
Environment Zulu23.28+85-CA (build 23+37) OpenJDK 64-Bit Server VM
Zulu23.28+85-CA (build 23+37, mixed mode, sharing) (at
/nix/store/wm5rma6x2527qmypzj7rwml8vf9vprgj-zulu-ca-jdk-23.0.0/zulu-23.jdk/Contents/Home)
* Toolchain:      clang (clang/LLVM from Xcode 16.2)
* Devkit:         Xcode 16.2 (devkit)
(/nix/store/vhsix1jn849mpxggwbw2zh1nbxpy0grc-Xcode16.2-MacOSX15)
* C Compiler:     Version 16.0.0 (at
/nix/store/vhsix1jn849mpxggwbw2zh1nbxpy0grc-Xcode16.2-MacOSX15/Xcode/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang)
* C++ Compiler:   Version 16.0.0 (at
/nix/store/vhsix1jn849mpxggwbw2zh1nbxpy0grc-Xcode16.2-MacOSX15/Xcode/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++)

Build performance summary:
* Build jobs:     14
* Memory limit:   49152 MB
```

Then you can just run make and verify the JDK version:

```
[nix-shell: jdk] $ make
...
Finished building target 'default (exploded-image)' in configuration
'macosx-aarch64-server-release'

[nix-shell: jdk] $ ./build/macosx-aarch64-server-release/jdk/bin/java
--version
openjdk 25-internal 2025-09-16
OpenJDK Runtime Environment (build 25-internal-adhoc.galder.jdk)
OpenJDK 64-Bit Server VM (build 25-internal-adhoc.galder.jdk, mixed mode)
```

The above is sufficient for my own use case, but it can be enhanced further
to add capstone dependency...etc.

I don't know who is taking care of [1] but while working on this a member
in Nix macos discord asked me to create an issue in Nix packages so that
others can maybe take this further and enhance [1] so that it also supports
darwin-aarch64. So I created [3].

XCode is tricky to handle, but it would be great if eventually there would
be Nix packages for JDK devkits for macos. If those were available, it
would enable OpenJDK developers to build earlier JDK versions even of the
very latest macos environments, by depending on earlier XCode version based
JDK devkits. You could also pick whichever clang, capstone...etc version
that matches that...etc.

Thanks
Galder

[1]
https://github.com/NixOS/nixpkgs/blob/nixos-24.11/pkgs/development/compilers/openjdk/generic.nix
[2] https://github.com/NixOS/nixpkgs/issues/377908
[3] https://github.com/NixOS/nixpkgs/issues/387516
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