Revisiting JDK-8161269

[David M. Lloyd]

On 09/16/2016 10:30 AM, Alan Bateman wrote:
> On 16/09/2016 07:21, David M. Lloyd wrote:
>
>> Hi Alan,
>>
>> In JDK-8161269 you said [1] that the "null" class loader has never
>> been specified to contain all Java SE types, using this as a
>> justification to reject this issue as "Not an Issue", regardless of
>> the compatibility impact (particularly the common case of a class
>> loader with a null parent).
> The context here is de-privileging non-core components so that they are
> no longer defined to the boot loader with all permissions. Modules such
> as java.corba, java.xml.ws, java.sql and many more have no business or
> need to be defined to the boot loader. It would be nice, if over the
> very long term, that we could get to the point where the only module
> defined to the boot loader is java.base. I don't know if we will ever
> get there.
>
> We of course acknowledge that there is potential compatibility impact
> with this change and this is why it is called out in the Risks and
> Assumption section of JEP 261 [1] as
>
> "Some Java SE types have been de-privileged and are now loaded by the
> platform class loader rather than the bootstrap class loader, as noted
> above. Existing custom class loaders that delegate directly to the
> bootstrap class loader might not work correctly; they should be updated
> to delegate to the platform class loader, which is easily available via
> the new ClassLoader::getPlatformClassLoader method."
>
> So far then there hasn't been a lot of feedback on this. The default
> loader for delegation is the system class loader so someone creating a
> class loader that delegates to the boot loader is probably an advanced
> case.
>
> As noted in the bug report, the ClassLoader javadoc has been updated for
> Java SE 9 to define the built-in class loaders. The most important part
> of this is specifying that all platform classes are visible via the
> platform class loader. That works for Java SE 8 and older if you replace
> "platform" with "extension". I mention this in case there is concerns
> about using a new API in code that needs to be compiled for JDK 8 and
> run on 8 or 9.
>
> I don't have cycles just now to get into the topic of radically changing
> the class loader hierarchy or how modules on the module path are mapped
> to loaders - that topic is an order of magnitude larger than the above,
> esp. once you get into all the scenarios around migration.

OK.  For this issue though, would it not make sense to look at the null 
parent class loader case in a specific and separate way: in the past, 
such class loaders had access to all platform classes, so as a 
compatibility factor it would not be unreasonable to take "null" in this 
*specific* context to mean "platform class loader" and do this 
translation inside the constructor?  Since the change that is really 
occurring (in any real, observable sense) is that the "null" parent is 
suddenly shrinking with respect to what it was going back far into 
history, then the _compatible_ change would appear to be to provide a 
new ClassLoader value that indicates "java.base (and optionally a few 
more modules that can decrease over time)", which in reality maps to the 
bootstrap class loader.  This way compatibility is maintained, and the 
new (in the observable sense) functionality of having a more limited 
parent CL is still available.

WDYT?
-- 
- DML