Module API usage questions

[Alan Bateman]

On 19/07/2016 21:16, Paul Benedict wrote:

>
> Can you into more about packages "not be package of your module"? I 
> apologize for not understanding. If I assemble myself a jar file and 
> my jar file has 10 packages and I have an "empty" module-info.java 
> file (i.e., just a name and curly braces), don't I automatically have 
> 10 concealed packages? So that's what I don't understand -- why I need 
> to tell anyone what I have hiding.
>
> And what if I don't list out my concealed packages? What happens to my 
> 10 packages at runtime?
>
If you use the jar tool to create the modular JAR then it will scan the 
contents (at packaging time) and add the ConcealedPackages attribute 
with names of the 10 packages.

If you use some other tool to create the JAR file then it will work too, 
it just means that the JAR file contents will be scanned when the JAR 
file is opened in order to get the list of packages.

On the other hand, if you are rolling ModuleDescriptors yourself and are 
ignoring concealed packages then there will be problems. For starters, 
it's not clear if the classes in these packages will be loaded, it all 
depends on how your class loader works. Assuming the classes can be 
loaded then they won't be in module M, they will instead be in the 
unnamed module of their defining loader. Accessibility will likely be an 
issue. Module M may not read the unnamed module and so types in the 
unnamed module won't be accessible. Other scenarios are possible too, I 
can think of cases where a type is loaded (in the unnamed module) but it 
can't link to its super type in M. In summary, lots of potential issues 
if you drop the concealed packages.

-Alan