The baby and the bathwater

[Peter Levart]

On 03/28/2018 11:18 AM, Stephen Colebourne wrote:
> On 28 March 2018 at 08:28, Peter Levart <peter.levart at gmail.com> wrote:
>> That's easy to enforce in runtime. Just take a "victim" class from your
>> library that is most often needed when your library is being used (or take a
>> couple of them) and add a class initialization block like the following to
>> them:
>>
>> public class Whatever {
>>
>>      static {
>>          if (Whatever.class.getModule().getName() == null) {
>>              throw new Error("Can only use this library as a module");
>>          }
>>      }
> Agreed that this has always been possible, but it is code not
> metadata. Really, it should be a startup JPMS error if the module
> isn't running in the expected mode. That way tools like Maven and
> Gradle can also take decisions based on the metadata.
>
> Stephen

Hm, that would be hard to enforce, since what you're asking is for JPMS 
to issue an error when the library in question is not part of its search 
scope. Class-Path class loader could do that though (by searching for pa 
particular "sign" in the jars on the class path even before the JVM 
transfers control to MainClass.main()). But modulepath and classpath are 
not the only ways a particular jar can be deployed. It can be part of a 
.war or .ear. It can be deployed into a system that manages its own 
class loader(s), etc. All those mechanisms would have to agree on a 
"standard" way of specifying such metadata.

Regards, Peter