MR JAR Support in JBoss Parent

Table of Contents

Overview

Add support for building MR JARs to the JBoss parent POM for Maven builds, which all JBoss projects can seamlessly utilize.

Provide a means to automatically build MR (multi-release) JARs on demand as specified by JEP 238
Provide a simple, Maven-style structure for source overlay directories
- For example, src/main/java9, src/main/java10, etc.
- Each source overlay directory must correspond to an output directory e.g. META-INF/versions/9, META-INF/versions/10, etc.
Add Multi-Release to the JAR manifest when building MR JARs
Only enable MR JAR functionality when source overlay directories are present
Provide a means to automatically run unit tests on each supported JDK
Allow each layer to automatically depend on the layers below it
Do not require any additional configuration beyond upgrading the parent POM
Allow the "baseline" Java version to be Java 8 or any available Java version later than Java 8
- For example, a project may require Java 10 as its minimum version; in this case, src/main/java would contain Java 10 sources
Allow overlay Java versions to be Java 9 or any available Java version later than Java 9
- For example, a project requiring Java 10 as its minimum version may have a Java 11 src/main/java11 overlay
Only require overlay source directories for Java versions beyond the baseline version
Do not interfere with maven-compiler-plugin customized configurations
Provide basic developer documentation in support of the feature

Include overlay sources in the source JAR
- This is not presently supported in any reasonable way with the maven-source-plugin so either that plugin would have to be enhanced, or a custom plugin written to perform this task
Include a mechanism to prevent artifact deployment unless all layers are unit tested during the same build

It is not a requirement to support building variations of projects with a JDK whose version is earlier than the latest supported JDK of the project
It is not a requirement at this time to support multiple layers of test source directories (though this may be revisited in a future iteration)
It is not a requirement to provide any special support for JavaDoc
- By spec, only the baseline directory may contain API elements, i.e. it is not allowed to introduce classes which only "exist" when running under certain JDK versions
It is not a requirement to provide any special support for changes in a project’s minimum version
- For example, if a Java 8 project with a java9 overlay were to change to require Java 9 as a minimum version, it is the developer’s responsibility to flatten the Java 9 sources over the old Java 8 sources

Build above projects with new parent POM in place of existing per-project configuration; verify output