Defining anonymous classes

[Remi Forax]

On 08/17/2014 12:55 PM, Florian Weimer wrote:
> On 08/16/2014 12:39 AM, John Rose wrote:
>
>> Do you have a use case in mind that could be expressed as a more 
>> tightly focused API?
>
> My use-case is run-time class generation and avoiding unnecessary 
> retention of classes which are referred to by the generated class.
>
> I do not need constant pool patching.  It would be interesting to 
> inject static final values of arbitrary types, but I could use a 
> thread-local variable for that if necessary.  Being able to pass 
> parameters to <clinit> would be a much cleaner way to achieve that, 
> but it doesn't seem strictly necessary.

As john said, you can also use one classloader by class and then for the 
static values either use a ClassValue or store them as field of the 
classloader
(as I've done in the prototype)

>
> > Or, most or all of dAC could be simulated using regular class
> > loading, into a single-use ClassLoader object.  The nominal bytecodes
> > would have to be rewritten to use invokedynamic to manage the
> > linking, at least to host-class names.
>
> Why is that?  Because you cannot directly reference to a class by name 
> if you don't use the class loader which was used to define the class 
> to look up the name?
>

An anonymous loaded class has a fulll access to the members of the host 
class even if the members is declared private,
to simulate this all method and field instructions that access to that 
host class are re-written to an invokedynamic
which at runtime will use a lookup object that can see the private 
member of the host class instead of the lookup
object corresponding to the anonymous class which is pass as first 
parameter of the bootstrap method.

lines 69 and 78 of the prototype,
https://gist.github.com/forax/0c25deac1867d1ef3247#file-gistfile1-java-L69,
rewrite the bytecode to use invokedynamic and line 180
https://gist.github.com/forax/0c25deac1867d1ef3247#file-gistfile1-java-L180
the bootstrap method which is called at runtime uses the lookup object 
stored
in the classloader instead of the lookup object provided by the 
invokedynamic mechanism
to access the methods and fields of the host class to trump the 
classical accessibility checks of the JVM.

cheers,
Rémi