85 lines
2.8 KiB
XML
85 lines
2.8 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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buildInputs = [ 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
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where available, or pre-built via Zulu). Platforms with OpenJDK not (yet) in
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Nixpkgs (<literal>Aarch32</literal>, <literal>Aarch64</literal>) point to the
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(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
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in <filename>$out/share/java</filename>. JDKs have a stdenv setup hook that
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add any JARs in the <filename>share/java</filename> directories of the build
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inputs to the <envar>CLASSPATH</envar> environment variable. For instance, if
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the package <literal>libfoo</literal> installs a JAR named
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<filename>foo.jar</filename> in its <filename>share/java</filename>
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directory, and another package declares the attribute
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<programlisting>
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buildInputs = [ jdk libfoo ];
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</programlisting>
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then <envar>CLASSPATH</envar> will be set to
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<filename>/nix/store/...-libfoo/share/java/foo.jar</filename>.
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</para>
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<para>
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Private JARs should be installed in a location like
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<filename>$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
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script to run it using the OpenJRE. You can use
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<literal>makeWrapper</literal> for this:
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<programlisting>
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buildInputs = [ 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 $out/share/java/foo.jar org.foo.Main"
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'';
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</programlisting>
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Note the use of <literal>jre</literal>, which is the part of the OpenJDK
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package that contains the Java Runtime Environment. By using
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<literal>${jre}/bin/java</literal> instead of
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<literal>${jdk}/bin/java</literal>, you prevent your package from depending
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on the JDK at runtime.
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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
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requires environment variables like <envar>JAVA_HOME</envar> being set, that
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can be done in a generic fashion with the <literal>--set</literal> argument
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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>
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from the OpenJDK. For instance, to use the GNU Java Compiler:
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<programlisting>
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buildInputs = [ gcj ant ];
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</programlisting>
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Here, Ant will automatically use <command>gij</command> (the GNU Java
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Runtime) instead of the OpenJRE.
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</para>
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</section>
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