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<section xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="sec-language-java">
<title>Java</title>
<para>
Ant-based Java packages are typically built from source as follows:
<programlisting>
stdenv.mkDerivation {
name = "...";
src = fetchurl { ... };
nativeBuildInputs = [ jdk ant ];
buildPhase = "ant";
}
</programlisting>
Note that <varname>jdk</varname> is an alias for the OpenJDK (self-built where available, or pre-built via Zulu). Platforms with OpenJDK not (yet) in Nixpkgs (<literal>Aarch32</literal>, <literal>Aarch64</literal>) point to the (unfree) <literal>oraclejdk</literal>.
</para>
<para>
JAR files that are intended to be used by other packages should be installed in <filename>$out/share/java</filename>. JDKs have a stdenv setup hook that add any JARs in the <filename>share/java</filename> directories of the build inputs to the <envar>CLASSPATH</envar> environment variable. For instance, if the package <literal>libfoo</literal> installs a JAR named <filename>foo.jar</filename> in its <filename>share/java</filename> directory, and another package declares the attribute
<programlisting>
buildInputs = [ libfoo ];
nativeBuildInputs = [ jdk ];
</programlisting>
then <envar>CLASSPATH</envar> will be set to <filename>/nix/store/...-libfoo/share/java/foo.jar</filename>.
</para>
<para>
Private JARs should be installed in a location like <filename>$out/share/<replaceable>package-name</replaceable></filename>.
</para>
<para>
If your Java package provides a program, you need to generate a wrapper script to run it using the OpenJRE. You can use <literal>makeWrapper</literal> for this:
<programlisting>
nativeBuildInputs = [ makeWrapper ];
installPhase =
''
mkdir -p $out/bin
makeWrapper ${jre}/bin/java $out/bin/foo \
--add-flags "-cp $out/share/java/foo.jar org.foo.Main"
'';
</programlisting>
Note the use of <literal>jre</literal>, which is the part of the OpenJDK package that contains the Java Runtime Environment. By using <literal>${jre}/bin/java</literal> instead of <literal>${jdk}/bin/java</literal>, you prevent your package from depending on the JDK at runtime.
</para>
<para>
Note all JDKs passthru <literal>home</literal>, so if your application requires environment variables like <envar>JAVA_HOME</envar> being set, that can be done in a generic fashion with the <literal>--set</literal> argument of <literal>makeWrapper</literal>:
<programlisting>
--set JAVA_HOME ${jdk.home}
</programlisting>
</para>
<para>
It is possible to use a different Java compiler than <command>javac</command> from the OpenJDK. For instance, to use the GNU Java Compiler:
<programlisting>
nativeBuildInputs = [ gcj ant ];
</programlisting>
Here, Ant will automatically use <command>gij</command> (the GNU Java Runtime) instead of the OpenJRE.
</para>
</section>
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