tjh.dev
1# Java {#sec-language-java}
2
3Ant-based Java packages are typically built from source as follows:
4
5```nix
6stdenv.mkDerivation {
7 pname = "...";
8 version = "...";
9
10 src = fetchurl { /* ... */ };
11
12 nativeBuildInputs = [
13 ant
14 jdk
15 stripJavaArchivesHook # removes timestamp metadata from jar files
16 ];
17
18 buildPhase = ''
19 runHook preBuild
20 ant # build the project using ant
21 runHook postBuild
22 '';
23
24 installPhase = ''
25 runHook preInstall
26
27 # copy generated jar file(s) to an appropriate location in $out
28 install -Dm644 build/foo.jar $out/share/java/foo.jar
29
30 runHook postInstall
31 '';
32}
33```
34
35Note that `jdk` is an alias for the OpenJDK (self-built where available,
36or pre-built via Zulu).
37
38Also note that not using `stripJavaArchivesHook` will likely cause the
39generated `.jar` files to be non-deterministic, which is not optimal.
40Using it, however, does not always guarantee reproducibility.
41
42JAR files that are intended to be used by other packages should be
43installed in `$out/share/java`. JDKs have a stdenv setup hook that add
44any JARs in the `share/java` directories of the build inputs to the
45`CLASSPATH` environment variable. For instance, if the package `libfoo`
46installs a JAR named `foo.jar` in its `share/java` directory, and
47another package declares the attribute
48
49```nix
50{
51 buildInputs = [ libfoo ];
52 nativeBuildInputs = [ jdk ];
53}
54```
55
56then `CLASSPATH` will be set to
57`/nix/store/...-libfoo/share/java/foo.jar`.
58
59Private JARs should be installed in a location like
60`$out/share/package-name`.
61
62If your Java package provides a program, you need to generate a wrapper
63script to run it using a JRE. You can use `makeWrapper` for this:
64
65```nix
66{
67 nativeBuildInputs = [ makeWrapper ];
68
69 installPhase = ''
70 mkdir -p $out/bin
71 makeWrapper ${jre}/bin/java $out/bin/foo \
72 --add-flags "-cp $out/share/java/foo.jar org.foo.Main"
73 '';
74}
75```
76
77Since the introduction of the Java Platform Module System in Java 9,
78Java distributions typically no longer ship with a general-purpose JRE:
79instead, they allow generating a JRE with only the modules required for
80your application(s). Because we can't predict what modules will be
81needed on a general-purpose system, the default jre package is the full
82JDK. When building a minimal system/image, you can override the
83`modules` parameter on `jre_minimal` to build a JRE with only the
84modules relevant for you:
85
86```nix
87let
88 my_jre = pkgs.jre_minimal.override {
89 modules = [
90 # The modules used by 'something' and 'other' combined:
91 "java.base"
92 "java.logging"
93 ];
94 };
95 something = (pkgs.something.override { jre = my_jre; });
96 other = (pkgs.other.override { jre = my_jre; });
97in
98 <...>
99```
100
101You can also specify what JDK your JRE should be based on, for example
102selecting a 'headless' build to avoid including a link to GTK+:
103
104```nix
105{
106 my_jre = pkgs.jre_minimal.override {
107 jdk = jdk11_headless;
108 };
109}
110```
111
112Note all JDKs passthru `home`, so if your application requires
113environment variables like `JAVA_HOME` being set, that can be done in a
114generic fashion with the `--set` argument of `makeWrapper`:
115
116```bash
117--set JAVA_HOME ${jdk.home}
118```
119
120It is possible to use a different Java compiler than `javac` from the
121OpenJDK. For instance, to use the GNU Java Compiler:
122
123```nix
124{
125 nativeBuildInputs = [ gcj ant ];
126}
127```
128
129Here, Ant will automatically use `gij` (the GNU Java Runtime) instead of
130the OpenJRE.