Modules with platform specific parts

[Johan Vos]

Hi,

There have been discussions in the past about how to deal with
platform-specific parts (java code, native code, resources) in modules.
There is no standard for this, and afaik no recommendation. In the OpenJFX
project, we upload jars with module info to maven central, and we have
plugins for maven and gradle to deal with them at compiletime and at
runtime.

However, the lack of a standard recommendation forces us to change the
internal behavior every now and then. The latest change we made (removing
automatic module names from empty modules [1]) caused issues in the 17
release, when developers compile JavaFX modular apps [2]

Before we make many other changes, I would like to have a better view on
what would be the recommended approach, so here is my summary. Suggestions
are highly appreciated.

Let's assume we have a component that contains some
platform-independent Java code, some platform-dependent Java code, and
some platform-dependent native code. To make the example concrete,
let's use the javafx.graphics "module" from the OpenJFX project, which
contains exactly that.

There are a number of options, and before thinking about the best way
for tools do deal with this situation, it would be good to have a
recommended approach for those "hybrid" modules.

1. There is a single module (i.e. only one Module). All code, no
matter on what platform or in which layer, will report the same value
for class.getModule().getName(). This would lead to a single
"javafx.graphics" conceptual module, but there will be a number of
physical module files that are different (with different class files
and native code). Those different modules should obvisouly be mutually
exclusive in a runtime.

2. There are 2 modules: the platform-independent Java code goes into
one module (let's call that javafx.graphics.api) and a second module
is named javafx.graphics.platform and contains the platform-dependent
Java code and the native code.
In this approach, developers use the javafx.graphics.api module to
compile against, and at runtime the javafx.graphics.platform module is
required. Again, that second module will have a number of different
physical implementations. As an extension to this, we might add the
Service Provider Interface approach for loading platform-specific
modules/bits at runtime.

3. We create one module for each platform. There is no
"javafx.graphics" module in this case, but there is a
"javafx.graphics.linux.aarch64" module for example.
Doing so, there is a tight coupling between one conceptual module and
one physical module. A clear drawback of this is that this is a real
challenge at compiletime. Developers (who are only using generic
API's) need to compile against a platform-specific module.

4. We use 2 artifacts: an "empty" one and then a number of
implementation specific ones. The difference with option 2 is that the
empty "module" exists solely for the purpose of tools, which can
detect what implementation specific module(s) need to be loaded at
compile/runtime.

We currently use option 4, but in my opinion, option 2 would be the
better approach.

Thanks,

- Johan

[1] https://bugs.openjdk.java.net/browse/JDK-8264998
[2]
https://mail.openjdk.java.net/pipermail/openjfx-dev/2021-September/031934.html