java program: Init.

Mostly just provides a shell hook for the jdk's setup-hook.
Tested with openjdk and jre.

nixos/modules/module-list.nix

@@ -70,6 +70,7 @@
   ./programs/fish.nix
   ./programs/freetds.nix
   ./programs/info.nix
+  ./programs/java.nix
   ./programs/kbdlight.nix
   ./programs/light.nix
   ./programs/man.nix

nixos/modules/programs/java.nix

@@ -0,0 +1,57 @@
+# This module provides JAVA_HOME, with a different way to install java
+# system-wide.
+
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.programs.java;
+in
+
+{
+
+  options = {
+
+    programs.java = {
+
+      enable = mkEnableOption "java" // {
+        description = ''
+          Install and setup the Java development kit.
+          <note>
+          <para>This adds JAVA_HOME to the global environment, by sourcing the
+            jdk's setup-hook on shell init. It is equivalent to starting a shell
+            through 'nix-shell -p jdk', or roughly the following system-wide
+            configuration:
+          </para>
+          <programlisting>
+            environment.variables.JAVA_HOME = ''${pkgs.jdk.home}/lib/openjdk;
+            environment.systemPackages = [ pkgs.jdk ];
+          </programlisting>
+          </note>
+        '';
+      };
+
+      package = mkOption {
+        default = pkgs.jdk;
+        description = ''
+          Java package to install. Typical values are pkgs.jdk or pkgs.jre.
+        '';
+        type = types.package;
+      };
+
+    };
+
+  };
+
+  config = mkIf cfg.enable {
+
+    environment.systemPackages = [ cfg.package ];
+
+    environment.shellInit = ''
+      test -e ${cfg.package}/nix-support/setup-hook && source ${cfg.package}/nix-support/setup-hook
+    '';
+
+  };
+
+}