JEP-282: JLink two modules exporting the same package name

[Alan Bateman]

On 12/10/2016 19:17, Karl Heinz Marbaise wrote:

> Hi,
>
> i have a question concerning the jlink command in JDK 9..(EA +138 build)
>
> I have created two modules lets call them module A and module B 
> (module-info.java):
>
> module A {
>    requires java.base;
>    exports com.single.package;
> }
>
> module B {
>    requires java.base;
>    exports com.single.package;
> }
>
> both export the same package: com.single.package
>
> The question is: Shouldn't the jlink command fail in such cases cause 
> both modules export the same package ?
>
> Otherwise it would mean to fail at the time of starting the generated 
> JVM image which I think is too late...
You can resolve independent modules A and B without error (and get a 
configuration that contains both) because there isn't any module that 
reads both. If you had module C that requires both A and B then you'd 
get an error to say that A and B export com.single.package to C.

So this is why it succeeds at link time. There isn't any notion of class 
loader at link time and jlink cannot know how the modules will be mapped 
to loaders at run time. jlink could assume that all modules will likely 
be defined to the same class loader and thus reject the above but it 
doesn't.

As to whether the resulting run time image is useful or not then it 
depends on the initial module. If the initial module is C that only 
requires A then it will work, same thing if the initial module is D that 
only requires B. On the other hand, if you do `java -jar X.jar` then the 
initial module is the unnamed module, which by convention, will cause 
all modules in the runtime image with an API to be resolved (the module 
system does not know anything about what X needs). In that scenario when 
A and B will be resolved but will fail when attempting to map both to 
the application class loader. I will guess that is what promoted your mail.

-Alan