pyrox.dev
1# Java {#sec-language-java}
2
3Ant-based Java packages are typically built from source as follows:
4
5```nix
6stdenv.mkDerivation {
7 name = "...";
8 src = fetchurl { ... };
9
10 nativeBuildInputs = [ jdk ant ];
11
12 buildPhase = "ant";
13}
14```
15
16Note that `jdk` is an alias for the OpenJDK (self-built where available,
17or pre-built via Zulu). Platforms with OpenJDK not (yet) in Nixpkgs
18(`Aarch32`, `Aarch64`) point to the (unfree) `oraclejdk`.
19
20JAR files that are intended to be used by other packages should be
21installed in `$out/share/java`. JDKs have a stdenv setup hook that add
22any JARs in the `share/java` directories of the build inputs to the
23`CLASSPATH` environment variable. For instance, if the package `libfoo`
24installs a JAR named `foo.jar` in its `share/java` directory, and
25another package declares the attribute
26
27```nix
28buildInputs = [ libfoo ];
29nativeBuildInputs = [ jdk ];
30```
31
32then `CLASSPATH` will be set to
33`/nix/store/...-libfoo/share/java/foo.jar`.
34
35Private JARs should be installed in a location like
36`$out/share/package-name`.
37
38If your Java package provides a program, you need to generate a wrapper
39script to run it using a JRE. You can use `makeWrapper` for this:
40
41```nix
42nativeBuildInputs = [ makeWrapper ];
43
44installPhase = ''
45 mkdir -p $out/bin
46 makeWrapper ${jre}/bin/java $out/bin/foo \
47 --add-flags "-cp $out/share/java/foo.jar org.foo.Main"
48'';
49```
50
51Since the introduction of the Java Platform Module System in Java 9,
52Java distributions typically no longer ship with a general-purpose JRE:
53instead, they allow generating a JRE with only the modules required for
54your application(s). Because we can't predict what modules will be
55needed on a general-purpose system, the default jre package is the full
56JDK. When building a minimal system/image, you can override the
57`modules` parameter on `jre_minimal` to build a JRE with only the
58modules relevant for you:
59
60```nix
61let
62 my_jre = pkgs.jre_minimal.override {
63 modules = [
64 # The modules used by 'something' and 'other' combined:
65 "java.base"
66 "java.logging"
67 ];
68 };
69 something = (pkgs.something.override { jre = my_jre; });
70 other = (pkgs.other.override { jre = my_jre; });
71in
72 ...
73```
74
75You can also specify what JDK your JRE should be based on, for example
76selecting a 'headless' build to avoid including a link to GTK+:
77
78```nix
79my_jre = pkgs.jre_minimal.override {
80 jdk = jdk11_headless;
81};
82```
83
84Note all JDKs passthru `home`, so if your application requires
85environment variables like `JAVA_HOME` being set, that can be done in a
86generic fashion with the `--set` argument of `makeWrapper`:
87
88```bash
89--set JAVA_HOME ${jdk.home}
90```
91
92It is possible to use a different Java compiler than `javac` from the
93OpenJDK. For instance, to use the GNU Java Compiler:
94
95```nix
96nativeBuildInputs = [ gcj ant ];
97```
98
99Here, Ant will automatically use `gij` (the GNU Java Runtime) instead of
100the OpenJRE.