doc/java: convert to markdown

doc/languages-frameworks/index.xml

@@ -17,7 +17,7 @@
  <xi:include href="haskell.section.xml" />
  <xi:include href="idris.section.xml" />
  <xi:include href="ios.section.xml" />
- <xi:include href="java.xml" />
+ <xi:include href="java.section.xml" />
  <xi:include href="lua.section.xml" />
  <xi:include href="maven.section.xml" />
  <xi:include href="node.section.xml" />

doc/languages-frameworks/java.section.md

@@ -0,0 +1,91 @@
+# Java {#sec-language-java}
+
+Ant-based Java packages are typically built from source as follows:
+
+```nix
+stdenv.mkDerivation {
+  name = "...";
+  src = fetchurl { ... };
+
+  nativeBuildInputs = [ jdk ant ];
+
+  buildPhase = "ant";
+}
+```
+
+Note that `jdk` is an alias for the OpenJDK (self-built where available,
+or pre-built via Zulu). Platforms with OpenJDK not (yet) in Nixpkgs
+(`Aarch32`, `Aarch64`) point to the (unfree) `oraclejdk`.
+
+JAR files that are intended to be used by other packages should be
+installed in `$out/share/java`. JDKs have a stdenv setup hook that add
+any JARs in the `share/java` directories of the build inputs to the
+`CLASSPATH` environment variable. For instance, if the package `libfoo`
+installs a JAR named `foo.jar` in its `share/java` directory, and
+another package declares the attribute
+
+```nix
+buildInputs = [ libfoo ];
+nativeBuildInputs = [ jdk ];
+```
+
+then `CLASSPATH` will be set to
+`/nix/store/...-libfoo/share/java/foo.jar`.
+
+Private JARs should be installed in a location like
+`$out/share/package-name`.
+
+If your Java package provides a program, you need to generate a wrapper
+script to run it using a JRE. You can use `makeWrapper` for this:
+
+```nix
+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"
+'';
+```
+
+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 jre package is the full
+JDK. When building a minimal system/image, you can override the
+`modules` parameter on `jre_minimal` to build a JRE with only the
+modules relevant for you:
+
+```nix
+let
+  my_jre = pkgs.jre_minimal.override {
+    modules = [
+      # The modules used by 'something' and 'other' combined:
+      "java.base"
+      "java.logging"
+    ];
+  };
+  something = (pkgs.something.override { jre = my_jre; });
+  other = (pkgs.other.override { jre = my_jre; });
+in
+  ...
+```
+
+Note all JDKs passthru `home`, so if your application requires
+environment variables like `JAVA_HOME` being set, that can be done in a
+generic fashion with the `--set` argument of `makeWrapper`:
+
+```bash
+--set JAVA_HOME ${jdk.home}
+```
+
+It is possible to use a different Java compiler than `javac` from the
+OpenJDK. For instance, to use the GNU Java Compiler:
+
+```nix
+nativeBuildInputs = [ gcj ant ];
+```
+
+Here, Ant will automatically use `gij` (the GNU Java Runtime) instead of
+the OpenJRE.