openjdk: add derivation to generate bespoke minimal JRE's

Co-Authored-By: Robert Hensing <robert@roberthensing.nl>

doc/languages-frameworks/java.xml

@@ -32,7 +32,7 @@ nativeBuildInputs = [ jdk ];
  </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:
+  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:
 <programlisting>
 nativeBuildInputs = [ makeWrapper ];
 
@@ -43,7 +43,21 @@ installPhase =
       --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.
+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:
+<programlisting>
+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
+  ...
+</programlisting>
  </para>
 
  <para>

pkgs/development/compilers/openjdk/jre.nix

@@ -0,0 +1,19 @@
+{ jdk
+, runCommand
+, patchelf
+, lib
+, modules ? [ "java.base" ]
+}:
+
+let
+  jre = runCommand "${jdk.name}-jre" {
+    nativeBuildInputs = [ patchelf ];
+    buildInputs = [ jdk ];
+    passthru = {
+      home = "${jre}";
+    };
+  }   ''
+      jlink --module-path ${jdk}/lib/openjdk/jmods --add-modules ${lib.concatStringsSep "," modules} --output $out
+      patchelf --shrink-rpath $out/bin/* $out/lib/jexec $out/lib/jspawnhelper $out/lib/*.so $out/lib/*/*.so
+  '';
+in jre

pkgs/top-level/all-packages.nix

@@ -9267,10 +9267,19 @@ in
   /* default JDK */
 
   jdk = jdk14;
-  # jdk14 lacks a jre output. See https://github.com/NixOS/nixpkgs/issues/79490
+
+  # Since the introduction of the Java Platform Module System in Java 9, Java
+  # no longer ships a separate JRE package.
+  #
+  # If you are building a 'minimal' system/image, you are encouraged to use
+  # 'jre_minimal' to build a bespoke JRE containing only the modules you need.
+  #
+  # For a general-purpose system, 'jre' defaults to the full JDK:
   jre = jdk14;
   jre_headless = jdk14_headless;
 
+  jre_minimal = callPackage ../development/compilers/openjdk/jre.nix { };
+
   openjdk = openjdk14;
   openjdk_headless = openjdk14_headless;