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# Java {#sec-language-java}
Ant-based Java packages are typically built from source as follows:
```nix
stdenv.mkDerivation {
pname = "...";
version = "...";
src = fetchurl { /* ... */ };
nativeBuildInputs = [
ant
jdk
stripJavaArchivesHook # removes timestamp metadata from jar files
];
buildPhase = ''
runHook preBuild
ant # build the project using ant
runHook postBuild
'';
installPhase = ''
runHook preInstall
# copy generated jar file(s) to an appropriate location in $out
install -Dm644 build/foo.jar $out/share/java/foo.jar
runHook postInstall
'';
}
```
Note that `jdk` is an alias for the OpenJDK (self-built where available,
or pre-built via Zulu). Platforms with OpenJDK not (yet) in Nixpkgs
(`Aarch32`, `Aarch64`) point to the (unfree) `oraclejdk`.
Also note that not using `stripJavaArchivesHook` will likely cause the
generated `.jar` files to be non-deterministic, which is not optimal.
Using it, however, does not always guarantee reproducibility.
JAR files that are intended to be used by other packages should be
installed in `$out/share/java`. JDKs have a stdenv setup hook that add
any JARs in the `share/java` directories of the build inputs to the
`CLASSPATH` environment variable. For instance, if the package `libfoo`
installs a JAR named `foo.jar` in its `share/java` directory, and
another package declares the attribute
```nix
{
buildInputs = [ libfoo ];
nativeBuildInputs = [ jdk ];
}
```
then `CLASSPATH` will be set to
`/nix/store/...-libfoo/share/java/foo.jar`.
Private JARs should be installed in a location like
`$out/share/package-name`.
If your Java package provides a program, you need to generate a wrapper
script to run it using a JRE. You can use `makeWrapper` for this:
```nix
{
nativeBuildInputs = [ makeWrapper ];
installPhase = ''
mkdir -p $out/bin
makeWrapper ${jre}/bin/java $out/bin/foo \
--add-flags "-cp $out/share/java/foo.jar org.foo.Main"
'';
}
```
Since the introduction of the Java Platform Module System in Java 9,
Java distributions typically no longer ship with a general-purpose JRE:
instead, they allow generating a JRE with only the modules required for
your application(s). Because we can't predict what modules will be
needed on a general-purpose system, the default jre package is the full
JDK. When building a minimal system/image, you can override the
`modules` parameter on `jre_minimal` to build a JRE with only the
modules relevant for you:
```nix
let
my_jre = pkgs.jre_minimal.override {
modules = [
# The modules used by 'something' and 'other' combined:
"java.base"
"java.logging"
];
};
something = (pkgs.something.override { jre = my_jre; });
other = (pkgs.other.override { jre = my_jre; });
in
<...>
```
You can also specify what JDK your JRE should be based on, for example
selecting a 'headless' build to avoid including a link to GTK+:
```nix
{
my_jre = pkgs.jre_minimal.override {
jdk = jdk11_headless;
};
}
```
Note all JDKs passthru `home`, so if your application requires
environment variables like `JAVA_HOME` being set, that can be done in a
generic fashion with the `--set` argument of `makeWrapper`:
```bash
--set JAVA_HOME ${jdk.home}
```
It is possible to use a different Java compiler than `javac` from the
OpenJDK. For instance, to use the GNU Java Compiler:
```nix
{
nativeBuildInputs = [ gcj ant ];
}
```
Here, Ant will automatically use `gij` (the GNU Java Runtime) instead of
the OpenJRE.
|