Building jar targeting multiple Java versions, including 9

[Alan Bateman]

On 26/08/2016 11:49, Robert Scholte wrote:

> Hi,
>
> I'm struggling with this issue too.
> I would have liked to see all the files under src/main/java, but since 
> module-info cannot be compiled at the same time as the other files we 
> need to do some extra things:
>
> Possible solutions:
> - Keep them all in the same folder but let Maven do 2 javac 
> executions, auto-selecting the right files. I don't like this kind of 
> magic and when the configuration of the maven-compiler-plugin becomes 
> complex, there's a chance the magic is doing it wrong.
> - Introduce an extra sourcefolder. This is by far the most clean 
> solution and with good documentation we should be able to explain this.
> I've created MCOMPILER-275[1] to implement this new feature.
The intention is that the module-info.java be in the root directory and 
I could imagine it confusing developers if they have to edit it in a 
separate source tree. I also worry that it would give the impression 
that a separate source tree is the right way to structure the source 
code. I'm also not sure how it would work in the IDE. Then we have the 
issue that the src and output tree no longer matching, that could be 
confusing too. Then we have the question of projects targeting >= JDK 9 
where I assume you would have the module-info.java in the root directory.

How difficult would it be for the compiler plugin to special case 
module-info.java when compiling for an older release? Would there be 
something in the POM to indicate whether the project produces a module, 
as a modular JAR?

-Alan.