<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 { ... }; buildInputs = [ jdk ant ]; buildPhase = "ant"; } </programlisting> Note that <varname>jdk</varname> is an alias for the OpenJDK.</para> <para>JAR files that are intended to be used by other packages should be installed in <filename>$out/share/java</filename>. The OpenJDK has a stdenv setup hook that adds any JARs in the <filename>share/java</filename> directories of the build inputs to the <envar>CLASSPATH</envar> environment variable. For instance, if the package <literal>libfoo</literal> installs a JAR named <filename>foo.jar</filename> in its <filename>share/java</filename> directory, and another package declares the attribute <programlisting> buildInputs = [ jdk libfoo ]; </programlisting> then <envar>CLASSPATH</envar> will be set to <filename>/nix/store/...-libfoo/share/java/foo.jar</filename>.</para> <para>Private JARs should be installed in a location like <filename>$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> buildInputs = [ 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>It is possible to use a different Java compiler than <command>javac</command> from the OpenJDK. For instance, to use the Eclipse Java Compiler: <programlisting> buildInputs = [ jre ant ecj ]; </programlisting> (Note that here you don’t need the full JDK as an input, but just the JRE.) The ECJ has a stdenv setup hook that sets some environment variables to cause Ant to use ECJ, but this doesn’t work with all Ant files. Similarly, you can use the GNU Java Compiler: <programlisting> buildInputs = [ gcj ant ]; </programlisting> Here, Ant will automatically use <command>gij</command> (the GNU Java Runtime) instead of the OpenJRE.</para> </section>