Lambda expressions for generic target types

I am using InvokeDynamic.lambda(InDefinedShape, TypeDescription) to create the equivalent of these two methods:

    static interface StringMeasure
    {
        Integer measure(String string);
    }
    
    StringMeasure stringMeasure()
    {
        return String::length;
    }

    Function<String, Integer> stringLengthFunction()
    {
        return String::length;
    }

This works as expected for the stringMeasure() method, but I’m having a hard time getting Byte Buddy to generate correct code for the stringLengthFunction() method. So far, my efforts have yielded NoSuchMethodErrors, AbstractMethodErrors, or LambdaConversionException: Invalid receiver type.

I noticed that there is a slight difference in the bootstrap methods generated for the two cases. For stringMeasure() the Java compiler produces the following:

  fava.tests.ByteBuddyTest$StringMeasure stringMeasure();
    Code:
       0: invokedynamic #16,  0             // InvokeDynamic #0:measure:()Lfava/tests/ByteBuddyTest$StringMeasure;
       5: areturn

...

BootstrapMethods:
  0: #38 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:
      #40 (Ljava/lang/String;)Ljava/lang/Integer;
      #47 REF_invokeVirtual java/lang/String.length:()I
      #48 (Ljava/lang/String;)Ljava/lang/Integer;

For the other method, the Java compiler’s output looks a little different:

  java.util.function.Function<java.lang.String, java.lang.Integer> stringLengthFunction();
    Code:
       0: invokedynamic #23,  0             // InvokeDynamic #1:apply:()Ljava/util/function/Function;
       5: areturn

...

  1: #38 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:
      #50 (Ljava/lang/Object;)Ljava/lang/Object;
      #51 REF_invokeVirtual java/lang/String.length:()I
      #52 (Ljava/lang/String;)Ljava/lang/Integer;

The difference is the first argument to the bootstrap method: when targeting the (non-parameterized) StringMeasure interface we see the input and output types as expected: (Ljava/lang/String;)Ljava/lang/Integer;. When targeting the java.util.function.Function interface, the first argument corresponds to the type erasure of the interface, i.e., (Ljava/lang/Object;)Ljava/lang/Object;, whereas the third argument appears to be the fully reified signature.

Both of these examples were collected from the output of the Java compiler. I can generate the code for the first example in Byte Buddy, but the second example is giving me trouble: I end up with (Ljava/lang/Object;)Ljava/lang/Object; for both the first and the third bootstrap method argument. This even makes somewhat sense: the method description is passed as a MethodDescription.InDefinedShape, which does not resolve type variables, and the functional interface type is passed as a TypeDescription, which again can only represent non-generic types.

It seems to me that one would need the capability to pass the functional interface as a Generic type, to address this case. Is this a current limitation of Byte Buddy, or is there a different way to generate lambda expressions for generic interfaces?

[raphw]

This week, I think.

[raner]

Will there be another Byte Buddy release this month? I’m just curious when I can expect this to be available in Maven Central. Thanks!