<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>