manual: Document Java support

doc/language-support.xml

@@ -233,10 +233,83 @@ twisted = buildPythonPackage {
 </section>
 
 
-<section><title>Java</title>
+<section xml:id="ssec-language-java"><title>Java</title>
 
-<para>Java packages should install JAR files in
-<filename>$out/share/java</filename>.</para>
+<para>Ant-based Java packages are typically built from source as follows:
+
+<programlisting>
+stdenv.mkDerivation {
+  name = "...";
+  src = fetchurl { ... };
+
+  buildInputs = [ jdk ant ];
+
+  buildPhase = "ant";
+}
+</programlisting>
+
+Note that <varname>jdk</varname> is an alias for the OpenJDK.</para>
+
+<para>JAR files that are intended to be used by other packages should
+be installed in <filename>$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>