See https://openjdk.org/jeps/238

This commit adds basic support for building multi-release jar files.
A multi-release jar file has release-specific classes in directories
under META-INF/versions/ and its MANIFEST.MF contains a line with 'Multi-Release: true'.

The probably most common case of a multi-release jar file has only one
single such class which is 'META-INF/versions/9/module-info.class'.

To do so, we add JAVA_RELEASE_SRC_DIRS as a new eclass variable which
is also used as the condition to trigger the new functionality. A new
local variable 'multi_release' is added to the packaging section (the
part using the 'jar -create' command). Only when JAVA_RELEASE_SRC_DIRS
is set, additional actions take place:

- Compilation (those are the parts with 'ejavac') will additionally loop
over the release-specific directories listed in JAVA_RELEASE_SRC_DIRS
and compile the release-specific classes into corresponding directories
under target/versions/.

- Packaging (the part using the 'jar -create' command) will add the
details to the 'multi_release' variable so that the release-specific
directories under target/versions/ can be packaged into the jar file.

This commit also adds funtionality to generate 'module-info.java' files.
It is useful for packages where module-info.java is not provided in the
sources but needs to be generated by the build system. We use the built-in jdeps function with the --generate-module-info option which became available with Java 11.

It generates the module-info.java file based on an intermediate jar file
and places it in the "${JAVA_MODULE_INFO_OUT}/${JAVA_INTERMEDIATE_JAR_NAME}/" directory.

For this purpose we add three new eclass variables:
- JAVA_INTERMEDIATE_JAR_NAME
- JAVA_MODULE_INFO_OUT
- JAVA_MODULE_INFO_RELEASE

When both JAVA_MODULE_INFO_OUT and JAVA_INTERMEDIATE_JAR_NAME are defined in the
ebuild we
- compile the sources still without module-info
- package them as an intermediate {JAVA_INTERMEDIATE_JAR_NAME}.jar
- let java-pkg-simple_generate-module-info generate the module-info
- compile module-info.java with the --patch-module option
- package the final jar file including the module-info.class

When the JAVA_MODULE_INFO_RELEASE variable is set, module-info.java is generated into a release specific directory "${JAVA_MODULE_INFO_OUT}/${JAVA_INTERMEDIATE_JAR_NAME}/versions/{JAVA_MODULE_INFO_RELEASE}".

Bug: https://bugs.gentoo.org/900433
Signed-off-by: Volkmar W. Pogatzki <gentoo@pogatzki.net>
---
eclass/java-pkg-simple.eclass | 267 +++++++++++++++++++++++++++++++++-
1 file changed, 262 insertions(+), 5 deletions(-)

diff --git a/eclass/java-pkg-simple.eclass b/eclass/java-pkg-simple.eclass index ce4a62f048da..79b90b927db0 100644
--- a/eclass/java-pkg-simple.eclass
+++ b/eclass/java-pkg-simple.eclass
@@ -1,4 +1,4 @@
-# Copyright 2004-2024 Gentoo Authors
+# Copyright 2004-2025 Gentoo Authors
# Distributed under the terms of the GNU General Public License v2

# @ECLASS: java-pkg-simple.eclass
@@ -11,7 +11,9 @@
# @DESCRIPTION:
# This class is intended to build pure Java packages from Java sources
# without the use of any build instructions shipped with the sources.
-# There is no support for generating source files, or for controlling
+# It can generate module-info.java files and supports adding the Main-Class
+# and the Automatic-Module-Name attributes to MANIFEST.MF. There is no
+# further support for generating source files, or for controlling
# the META-INF of the res