Lookup.defineAnonymousClass() vs indy

[forax at univ-mlv.fr]

> De: "mandy chung" <mandy.chung at oracle.com>
> À: "Remi Forax" <forax at univ-mlv.fr>, "core-libs-dev"
> <core-libs-dev at openjdk.java.net>
> Envoyé: Lundi 30 Novembre 2020 19:16:25
> Objet: Re: Lookup.defineAnonymousClass() vs indy

> The implementation method to be invoked for this lambda is a static bridge
> method
> taking this hidden class's instance as the parameter, i.e. a descriptor
> referencing
> a hidden class, which will fail to resolve.

> 0: #104 REF_invokeStatic
>  java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
>     Method arguments:
>       #111 ()V
>       #112 REF_invokeStatic HiddenClassWithIndy.lambda$test$0:(LHiddenClassWithIndy;)V
>       #111 ()V

> Now that a hidden class is a nestmate, should/can this bridge method be an
> instance method if this class is defined as a hidden class ?
Here the parameter of the lambda bridge is a captured local variable, it feels wrong to me to have s special case if the first captured local variable as the same type as the owning/enclosing class. 
And when you compile a class and generate the lambda bridge method, the compiler has no idea if the class will be used as an hidden class or not. 

The real question is why the descriptor is not rewritten to replace the owning class by the hidden class when lookup.defineHiddenClass is called, i believe this is the behavior of unsafe.defineAnonymousClass. 

> Mandy
Rémi 

> On 11/29/20 7:34 AM, Remi Forax wrote:

>> Hi Mandy, hi all,
>> it seems that when defineAnonymousClass rewrites the currentClass, it doesn't
>> work if there is an invokedynamic in the classfile, so defineHiddenClass fails
>> with a VerifyError when the hidden class is verified.

>> Here is an example showing the issue
>> ---
>> import java.io.IOException;
>> import java.lang.invoke.MethodHandles;

>> public class HiddenClassWithIndy {
>>   public void test() {
>>     var a = new HiddenClassWithIndy();
>>     Runnable r = () -> System.out.println(a);
>>   }

>>  public static void main(String[] args) throws IOException,
>>   IllegalAccessException {
>>     byte[] bytecode;
>>    try(var input =
>>    HiddenClassWithIndy.class.getClassLoader().getResourceAsStream(HiddenClassWithIndy.class.getName().replace('.',
>>     '/') + ".class")) {
>>       if (input == null) {
>>         throw new AssertionError();
>>       }
>>       bytecode = input.readAllBytes();
>>     }
>>     var hiddenLookup = MethodHandles.lookup().defineHiddenClass(bytecode, true);
>>   }
>> }

>> ---
>> The error message:
>> Exception in thread "main" java.lang.VerifyError: Bad type on operand stack
>> Exception Details:
>>   Location:
>>    fr/umlv/transmogrif/HiddenClassWithIndy+0x0000000801002400.test()V @9:
>>     invokedynamic
>>   Reason:
>>    Type 'fr/umlv/transmogrif/HiddenClassWithIndy+0x0000000801002400' (current
>>     frame, stack[0]) is not assignable to 'fr/umlv/transmogrif/HiddenClassWithIndy'
>>   Current Frame:
>>     bci: @9
>>     flags: { }
>>    locals: { 'fr/umlv/transmogrif/HiddenClassWithIndy+0x0000000801002400',
>>     'fr/umlv/transmogrif/HiddenClassWithIndy+0x0000000801002400' }
>>     stack: { 'fr/umlv/transmogrif/HiddenClassWithIndy+0x0000000801002400' }
>>   Bytecode:
>>     0000000: bb00 0759 b700 094c 2bba 000a 0000 4db1
>>     0000010:

>> 	at java.base/java.lang.ClassLoader.defineClass0(Native Method)
>> 	at java.base/java.lang.System$2.defineClass(System.java:2193)
>>	at
>> 	java.base/java.lang.invoke.MethodHandles$Lookup$ClassDefiner.defineClass(MethodHandles.java:2235)
>>	at
>> 	java.base/java.lang.invoke.MethodHandles$Lookup$ClassDefiner.defineClassAsLookup(MethodHandles.java:2216)
>>	at
>> 	java.base/java.lang.invoke.MethodHandles$Lookup.defineHiddenClass(MethodHandles.java:1952)
>> 	at fr.umlv.transmogrif.HiddenClassWithIndy.main(HiddenClassWithIndy.java:20)

>> regards,
>> Rémi