77 lines
3.4 KiB
XML
77 lines
3.4 KiB
XML
<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xml:id="sec-language-java">
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<title>Java</title>
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<para>
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Ant-based Java packages are typically built from source as follows:
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<programlisting>
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stdenv.mkDerivation {
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name = "...";
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src = fetchurl { ... };
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nativeBuildInputs = [ jdk ant ];
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buildPhase = "ant";
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}
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</programlisting>
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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>.
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</para>
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<para>
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JAR files that are intended to be used by other packages should be installed in <filename>$out/share/java</filename>. JDKs have a stdenv setup hook that add any JARs in the <filename>share/java</filename> directories of the build inputs to the <envar>CLASSPATH</envar> environment variable. For instance, if the package <literal>libfoo</literal> installs a JAR named <filename>foo.jar</filename> in its <filename>share/java</filename> directory, and another package declares the attribute
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<programlisting>
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buildInputs = [ libfoo ];
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nativeBuildInputs = [ jdk ];
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</programlisting>
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then <envar>CLASSPATH</envar> will be set to <filename>/nix/store/...-libfoo/share/java/foo.jar</filename>.
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</para>
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<para>
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Private JARs should be installed in a location like <filename>$out/share/<replaceable>package-name</replaceable></filename>.
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</para>
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<para>
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If your Java package provides a program, you need to generate a wrapper script to run it using a JRE. You can use <literal>makeWrapper</literal> for this:
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<programlisting>
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nativeBuildInputs = [ makeWrapper ];
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installPhase =
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''
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mkdir -p $out/bin
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makeWrapper ${jre}/bin/java $out/bin/foo \
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--add-flags "-cp $out/share/java/foo.jar org.foo.Main"
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'';
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</programlisting>
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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 <package>jre</package> package is the full JDK. When building a minimal system/image, you can override the <literal>modules</literal> parameter on <literal>jre_minimal</literal> to build a JRE with only the modules relevant for you:
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<programlisting>
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let
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my_jre = pkgs.jre_minimal.override {
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modules = [
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# The modules used by 'something' and 'other' combined:
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"java.base"
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"java.logging"
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];
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};
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something = (pkgs.something.override { jre = my_jre; });
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other = (pkgs.other.override { jre = my_jre; });
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in
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...
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</programlisting>
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</para>
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<para>
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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>:
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<programlisting>
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--set JAVA_HOME ${jdk.home}
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</programlisting>
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</para>
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<para>
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It is possible to use a different Java compiler than <command>javac</command> from the OpenJDK. For instance, to use the GNU Java Compiler:
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<programlisting>
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nativeBuildInputs = [ gcj ant ];
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</programlisting>
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Here, Ant will automatically use <command>gij</command> (the GNU Java Runtime) instead of the OpenJRE.
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</para>
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</section>
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